furnaces

Furnaces and Refractories

FURNACES AND REFRACTORIES

1. INTRODUCTION............................................................................................................... 1

2. TYPES OF FURNACES, REFRACTORIES AND INSULATION ................. 5

3. ASSESSMENT OF FURNACES................................................................................. 18

4. ENERGY EFFICIENCY OPPORTUNITES.......................................................... 27

5. OPTIONS CHECKLIST................................................................................................ 35

6. WORKSHEETS.............................................................................................................. 35

7. REFERENCES .................................................................................................................. 36

1. INTRODUCTION This section introduces furnaces and refractories and explains the various design and operation aspects. 1.1 What is a furnace? A furnace is an equipment used to melt metals for casting or to heat materials to change their shape (e.g. rolling, forging) or properties (heat treatment). Since flue gases from the fuel come in direct contact with the materials, the type of fuel chosen is important. For example, some materials will not tolerate sulphur in the fuel. Solid fuels generate particulate matter, which will interfere the materials placed inside the furnace. For this reason: Most furnaces use liquid fuel, gaseous fuel or electricity as energy input. Induction and arc furnaces use electricity to melt steel and cast iron. Melting furnaces for nonferrous materials use fuel oil. Oil-fired furnaces mostly use furnace oil, especially for reheating and heat treatment of

materials. Light diesel oil (LDO) is used in furnaces where sulphur is undesirable.

Furnace ideally should heat as much of material as possible to a uniform temperature with the least possible fuel and labor. The key to efficient furnace operation lies in complete combustion of fuel with minimum excess air. Furnaces operate with relatively low efficiencies (as low as 7 percent) compared to other combustion equipment such as the boiler (with efficiencies higher than 90 percent. This is caused by the high operating temperatures in the furnace. For example, a furnace heating materials to 1200 oC will emit exhaust gases at 1200 oC or more, which results in significant heat losses through the chimney.

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Chapter - 1


All furnaces have the following components as shown in Figure 1 (Carbon Trust, 1993): Refractory chamber constructed of insulating materials to retain heat at high operating

temperatures. Hearth to support or carry the steel, which consists of refractory materials supported by a

steel structure, part of which is water-cooled. Burners that use liquid or gaseous fuels to raise and maintain the temperature in the chamber.

Coal or electricity can be used in reheating furnaces. Chimney to remove combustion exhaust gases from the chamber Charging and discharging doors through which the chamber is loaded and unloaded. Loading

and unloading equipment include roller tables, conveyors, charging machines and furnace pushers.

Figure 1: Typical Furnace Components (The Carbon Trust, 1993)

1.2 What are refractories? Any material can be described as a ‘refractory,’ if it can withstand the action of abrasive or corrosive solids, liquids or gases at high temperatures. The various combinations of operating conditions in which refractories are used, make it necessary to manufacture a range of refractory materials with different properties. Refractory materials are made in varying combinations and shapes depending on their applications. General requirements of a refractory material are: Withstand high temperatures Withstand sudden changes of temperatures Withstand action of molten metal slag, glass, hot gases, etc Withstand load at service conditions

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Withstand load and abrasive forces Conserve heat Have low coefficient of thermal expansion Should not contaminate the material with which it comes into contact

Table 1 compares the thermal properties of typical high density and low density refractory materials. Table 1. Typical Refractory Properties (The Carbon Trust, 1993) Property High Thermal Mass

(High density refractories) Low Thermal Mass (Ceramic fiber)

Thermal conductivity (W/m K) 1.2 0.3 Specific heat (J/kg K) 1000 1000 Density (kg/m3) 2300 130 Depending on the area of application such as boilers, furnaces, kilns, ovens etc, temperatures and atmospheres encountered different types of refractories are used. Typical installations of refractories are shown in Figure 2.

Figure 2a. Refractory lining of a furnace

arch (BEE, 2005) Figure 2b. Refractory walls of a furnace interior with burner blocks (BEE, 2005)

Some of the important properties of refractories are: Melting point: Pure substances melt instantly at a specific temperature. Most refractory materials consist of particles bonded together that have high melting temperatures. At high temperatures, these particles melt and form slag. The melting point of the refractory is the temperature at which a test pyramid (cone) fails to support its own weight. Size: The size and shape of the refractories is a part of the design of the furnace, since it affects the stability of the furnace structure. Accurate size is extremely important to properly fit the refractory shape inside the furnace and to minimize space between construction joints. Bulk density: The bulk density is useful property of refractories, which is the amount of refractory material within a volume (kg/m3). An increase in bulk density of a given refractory increases its volume stability, heat capacity and resistance to slag penetration.

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Porosity: The apparent porosity is the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume of the refractory. This property is important when the refractory is in contact with molten charge and slag. A low apparent porosity prevents molten material from penetrating into the refractory. A large number of small pores is generally preferred to a small number of large pores. Cold crushing strength: The cold crushing strength is the resistance of the refractory to crushing, which mostly happens during transport. It only has an indirect relevance to refractory performance, and is used as one of the indicators of abrasion resistance. Other indicators used are bulk density and porosity. Pyrometric cones and Pyrometric cones equivalent (PCE): The ‘refractoriness’ of (refractory) bricks is the temperature at which the refractory bends because it can no longer support its own weight. Pyrometric cones are used in ceramic industries to test the refractoriness of the (refractory) bricks. They consist of a mixture of oxides that are known to melt at a specific narrow temperature range. Cones with different oxide composition are placed in sequence of their melting temperature alongside a row of refractory bricks in a furnace. The furnace is fired and the temperature rises. One cone will bends together with the refractory brick. This is the temperature range in oC above which the refractory cannot be used. This is known as Pyrometric Cone Equivalent temperatures. (Figure 3)

Figure 3: Pyrometric Cones (Bureau of Energy Efficiency, 2004)

Creep at high temperature: Creep is a time dependent property, which determines the deformation in a given time and at a given temperature by a refractory material under stress. Volume stability, expansion, and shrinkage at high temperatures: The contraction or expansion of the refractories can take place during service life. Such permanent changes in dimensions may be due to: The changes in the allotropic forms, which cause a change in specific gravity A chemical reaction, which produces a new material of altered specific gravity The formation of liquid phase Sintering reactions

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Fusion dust and slag or by the action of alkalies on fireclay refractories, to form alkali-alumina silicates. This is generally observed in blast furnaces.

Reversible thermal expansion: Any material expands when heated, and contracts when cooled. The reversible thermal expansion is a reflection on the phase transformations that occur during heating and cooling. Thermal conductivity: Thermal conductivity depends on the chemical and mineralogical composition and silica content of the refractory and on the application temperature. The conductivity usually changes with rising temperature. High thermal conductivity of a refractory is desirable when heat transfer though brickwork is required, for example in recuperators, regenerators, muffles, etc. Low thermal conductivity is desirable for conservation of heat, as the refractory acts as an insulator. Additional insulation conserves heat but at the same time increases the hot face temperature and hence a better quality refractory is required. Because of this, the outside roofs of open-hearth furnaces are normally not insulated, as this could cause the roof to collapse. Lightweight refractories of low thermal conductivity find wider applications in low temperature heat treatment furnaces, for example in batch type furnaces where the low heat capacity of the refractory structure minimizes the heat stored during the intermittent heating and cooling cycles. Insulating refractories have very low thermal conductivity. This is usually achieved by trapping a higher proportion of air into the structure. Some examples are: Naturally occurring materials like asbestos are good insulators but are not particularly good

refractories Mineral wools are available which combine good insulating properties with good resistance

to heat but these are not rigid Porous bricks are rigid at high temperatures and have a reasonably low thermal conductivity.

2. TYPES OF FURNACES, REFRACTORIES AND INSULATION This section describes the types of furnaces, refractories and insulation materials used in industry. It also gives criteria for selecting refractory types for optimum results. 2.1 Types of furnaces Furnaces are broadly classified into two types based on the heat generation method: combustion furnaces that use fuels, and electric furnaces that use electricity. Combustion furnaces can be classified in several based as shown in Table 2: type of fuel used, mode of charging the materials, mode of heat transfer and mode of waste heat recovery. However, it is not possible to use this classification in practice, because a furnace can be using different types of fuel, different ways to charge materials into the furnace etc. The most commonly used furnaces are described in the next sections

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Table 2. Classification of furnaces Classification method Types and examples Type of fuel used Oil-fired Gas-fired Coal-fired Mode of charging materials Intermittent / Batch Periodical

Forging Re-rolling (batch/pusher) Pot

Continuous Pusher Walking beam Walking hearth Continuous recirculating bogie furnaces Rotary hearth furnaces

Mode of heat transfer Radiation (open fire place) Convection (heated through medium) Mode of waste heat recovery Recuperative Regenerative 2.1.1 Forging furnaceThe forging furnace is used for preheating billets and ingots to attain a ‘forge’ temperature. The furnace temperature is maintained at around 1200 to 1250 oC. Forging furnaces use an open fireplace system and most of the heat is transmitted by radiation. The typical load is 5 to 6 ton with the furnace operating for 16 to 18 hours daily. The total operating cycle can be divided into (i) heat-up time (ii) soaking time and (iii) forging time. Specific fuel consumption depends upon the type of material and number of ‘reheats’ required. 2.1.2 Re-rolling mill furnace a) Batch type A box type furnace is used as a batch type re-rolling mill. This furnace is mainly used for heating up scrap, small ingots and billets weighing 2 to 20 kg for re-rolling. Materials are manually charged and discharged and the final products are rods, strips etc. The operating temperature is about 1200 oC. The total cycle time can be further categorized into heat-up time and re-rolling time. During heat-up time the material gets heated up-to the required temperature and is removed manually for re-rolling. The average output from these furnaces varies from 10 to 15 tons / day and the specific fuel consumption varies from 180 to 280 kg. of coal / ton of heated material. b) Continuous pusher type The process flow and operating cycles of a continuous pusher type is the same as that of the batch furnace. The operating temperature is about 1250 o C. Generally, these furnaces operate 8 to 10 hours with an output of 20 to 25 ton per day. The material or stock recovers a part of the

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heat in flue gases as it moves down the length of the furnace. Heat absorption by the material in the furnace is slow, steady and uniform throughout the cross-section compared with batch type. eat in flue gases as it moves down the length of the furnace. Heat absorption by the material in

the furnace is slow, steady and uniform throughout the cross-section compared with batch type. 2.1.3 Continuous reheating furnace 2.1.3 Continuous reheating furnace In continuous reheating, the steel stock forms a continuous flow of material and is heated to the desired temperature as it travels through the furnace. The temperature of a piece of steel is typically raised to between 900°C and 1250oC, until it is soft enough to be pressed or rolled into the desired size or shape. The furnace must also meet specific stock heating rates for metallurgical and productivity reasons.

In continuous reheating, the steel stock forms a continuous flow of material and is heated to the desired temperature as it travels through the furnace. The temperature of a piece of steel is typically raised to between 900°C and 1250

To ensure that the energy loss is kept to a minimum, the inlet and outlet doors should be minimal in size and designed to avoid air infiltration. Continuous reheating furnaces can be categorized by the two methods of transporting stock through the furnace:

To ensure that the energy loss is kept to a minimum, the inlet and outlet doors should be minimal in size and designed to avoid air infiltration. Continuous reheating furnaces can be categorized by the two methods of transporting stock through the furnace: Stock is kept together to form a stream of material that is pushed through the furnace. Such

furnaces are called pusher type furnaces. Stock is kept together to form a stream of material that is pushed through the furnace. Such

furnaces are called pusher type furnaces. Stock is placed on a moving hearth or supporting structure which transports the steel through

the furnace. The furnaces include walking beam, walking hearth, continuous recirculating bogie furnaces, and rotary hearth furnaces.

Stock is placed on a moving hearth or supporting structure which transports the steel through the furnace. The furnaces include walking beam, walking hearth, continuous recirculating bogie furnaces, and rotary hearth furnaces.

Table 3 compares the main types of continuous reheating furnaces used in industry. Table 3 compares the main types of continuous reheating furnaces used in industry.

oC, until it is soft enough to be pressed or rolled into the desired size or shape. The furnace must also meet specific stock heating rates for metallurgical and productivity reasons.

Figure 4. Pusher Furnace (The Carbon Trust, 1993)


Table 3. Comparison of Different Continuous Reheating Furnaces (Adapted from The Carbon Trust, 1993 and BEE, 2005) Type Description Advantages Disadvantages Pusher furnace (Figure 4)

The main features are: Furnaces may have solid hearth, but in most

cases pushers are used to charge and discharge stock, that move on “skids” (rails) with water-cooled supports.

These furnaces typically have a hearth sloping towards the discharge end of up to 35 meters divided into five zones in top-fired furnaces.

Firing of furnace by burners located at the discharge end of the furnace, or at top and/or bottom to heat stock from both top and/or bottom

The discharge ends of these furnaces have a chimney with a recuperator for waste heat recovery.

Low installation and maintenance costs (compared with moving hearth furnaces) Advantages of top and bottom firing: Faster heating of stock Lower temperature

differences within stock Reduced stock residence

time Shorter furnace lengths

(compared to solid hearth furnaces)

Water cooling energy losses from the skids and stock supporting structure in top and bottom fired furnaces

Discharge must be accompanied by charge

Stock sizes/weights and furnace length are limited by friction and possibility of stock pile-ups

Furnace needs facilities to be completely emptied

Quality reduction by (a) physical marking by skids or ‘skid marks’ (b) temperature differences along the stock length caused by the water cooled supports in top and bottom fired furnaces

Walking beam furnace (Figure 5)

These furnaces operate as follows: Stock is placed on stationary ridges Walking beams are raised from the bottom to

raise the stock Walking beams with the stock move forwards Walking beams are lowered at end of the

furnace to place stock on stationary ridges Stock is removed from furnace and walking

beams return to furnace entrance Initially temperatures were limited 1000 0C but new models are able to reach 1100 0C

Overcomes many of the problems of pusher furnaces (skid marks, stock pile-ups, charge/discharge)

Possible to heat bottom face of the stock resulting in shorter stock heating times and furnace lengths and thus better control of heating rates, uniform stock discharge temperatures and operational flexibility

High energy loss through water cooling (compared with walking hearth furnaces)

Much of the furnace is below the level of the mill; this may be a constraint in some applications

Sometimes when operating mechanism of beam make it necessary to fire from the sides, this results in non-uniform heating of the stock

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Type Description Advantages Disadvantages Walking hearth furnace (Figure 6)

These furnaces are designed so that the stock rests on fixed refractory blocks, which are extended through openings in the hearth. The stock is transported towards the discharge end in discrete steps by “walking the hearth”, similar to walking beam furnaces

Simplicity of design Ease of construction Ability to cater for different

stock sizes (within limits) Negligible water cooling

energy losses Can be emptied Minimal physical marking of

the stock

Temperatures across the stock are not uniform because the bottom of stock cannot be heated and small spaces between the stock limits heating of the sides. Large spaces between stocks can partially alleviate this. But this increases stock residence time to up to several hours, which affects furnace flexibility and yield

Continuous recirculating bogie furnace (Figure 7)

The furnace has the shape of a long and narrow tunnel with rails inside and works as follows: Stock is placed on a bogie (cart with wheels)

with a refractory hearth Several bogies move like a train over the

entire furnace length through the furnace Stock is removed at the discharge end and the

bogie returns to the charge end of the furnace

Suitable for compact stock of variable size and geometry

The stock in the bogie has to undergo a cycle of heating and cooling then again heating

Heat storage loss through heating and cooling of the bogies

Inadequate sealing of the gap between the bogies and furnace shell, difficulties in removing scale, and difficulties in firing across a narrow hearth width caused by the narrow and long furnace shape

Rotary hearth furnace (Figure 8)

More recent developed furnace type that is overtaking the bogie furnace. The walls and the roof of the furnace remains stationery while the hearth moves in a circle on rollers, carrying the stock. Heated gas moves in opposite direction of the hearth and flue gases are discharged near the charging door. The temperature can reach 1300 oC

Suitable for stock of variable size and geometry

Reduced heat storage loss compared to bogie furnace

More complex design with an annular shape and revolving hearth

Possible logistical problems in layout of some rolling mills and forges because of close location of charge and discharge positions

Figure 5. Walking Beam Furnace (The Carbon Trust 1993)

Figure 6. Walking Hearth Furnace (The Carbon Trust, 1993)

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Figure 7. Continuous Re-circulating Bogie Furnace (The Carbon Trust, 1993)

Figure 8. Rotary Hearth Furnace (The Carbon Trust, 1993)

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2.2 Types of refractories Refractories can be classified on the basis of chemical composition, end use and methods of manufacture as shown below. Table 4. Classification of refractories based on chemical composition (Adapted from Gilchrist) Classification method Examples Chemical composition ACID, which readily combines with bases

Silica, Semisilica, Aluminosilicate

BASIC, which consists mainly of metallic oxides that resist the action of bases

Magnesite, Chrome-magnesite, Magnesite-chromite, Dolomite

NEUTRAL, which does not combine with acids nor bases

Fireclay bricks, Chrome, Pure Alumina

Special Carbon, Silicon Carbide, Zirconia End use Blast furnace casting pit Method of manufacture Dry press process, fused cast, hand moulded, formed normal,

fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying)

2.2.1 Fireclay refractories Firebrick is the most common form of refractory material. It is used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many others. Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminum silicates with varying silica (SiO2) content of up to 78 percent and Al2O3 content of up to 44 percent. Table 5 shows that the melting point (PCE) of fireclay brick decreases with increasing impurity and decreasing Al2O3. This material is often used in furnaces, kilns and stoves because the materials are widely available and relatively inexpensive. Table 5. Properties of typical fireclay bricks (BEE, 2005) Brick type Percentage

SiO2 Percentage Al2O3

Percentage other constituents

PCE oC

Super Duty 49-53 40-44 5-7 1745-1760 High Duty 50-80 35-40 5-9 1690-1745 Intermediate 60-70 26-36 5-9 1640-1680 High Duty (Siliceous) 65-80 18-30 3-8 1620-1680 Low Duty 60-70 23-33 6-10 1520-1595

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2.2.2 High alumina refractories Alumina silicate refractories containing more than 45 percent alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100 percent. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals. 2.2.3 Silica brick Silica brick (or Dinas) is a refractory that contains at least 93 percent SiO2. The raw material is quality rocks. Various grades of silica brick have found extensive use in the iron and steel melting furnaces and the glass industry. In addition to high fusion point multi-type refractories, other important properties are their high resistance to thermal shock (spalling) and their high refractoriness. The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached. This behavior contrasts with that of many other refractories, for example alumina silicate materials, which begin to fuse and creep at temperatures considerably lower than their fusion points. Other advantages are flux and stag resistance, volume stability and high spalling resistance. 2.2.4 Magnesite Magnesite refractories are chemically basic materials, containing at least 85 percent magnesium oxide. They are made from naturally occurring magnesite (MgCO3). The properties of magnesite refractories depend on the concentration of silicate bond at the operating temperatures. Good quality magnesite usually results from a CaO-SiO2

ratio of less than two with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions. The slag resistance is very high particularly to lime and iron rich slags. 2.2.5 Chromite refractories Two types of chromite refractories are distinguished: Chrome-magnesite refractories, which usually contain 15-35 percent Cr2O3 and 42-50

percent MgO. They are made in a wide range of qualities and are used for building the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness.

Magnesite-chromite refractories, which contain at least 60 percent MgO and 8-18 percent Cr2O3. They are suitable for service at the highest temperatures and for contact with the most basic slags used in steel melting. Magnesite-chromite usually has a better spalling resistance than chrome-magnesite.

2.2.6 Zirconia refractories Zirconium dioxide (ZrO2) is a polymorphic material. It is essential to stabilize it before application as a refractory, which is achieved by incorporating small quantities of calcium, magnesium and cerium oxide, etc. Its properties depend mainly on the degree of stabilization, quantity of stabilizer and quality of the original raw material. Zirconia refractories have a very high strength at room temperature, which is maintained up to temperatures as high as 15000C. They are therefore useful as high temperature construction materials in furnaces and kilns. The thermal conductivity of zirconium dioxide is much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory. Zirconia exhibits very

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low thermal losses and does not react readily with liquid metals, and is particularly useful for making refractory crucibles and other vessels for metallurgical purposes. Glass furnaces use zirconia because it is not easily wetted by molten glasses and does not react easily with glass. 2.2.7 Oxide refractories (Alumina) Alumina refractory materials that consist of aluminium oxide with little traces of impurities are known as pure alumina. Alumina is one of the most chemically stable oxides known. It is mechanically very strong, insoluble in water, super heated steam, and most inorganic acids and alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodium carbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and reducing atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is used for lining furnaces operating up to 1850oC. 2.2.8 Monolithics Monolithic refractories are single piece casts in the shape of equipment, such as a ladle as shown in Figure 9. They are rapidly replacing the conventional type fired refractories in many applications including industrial furnaces. The main advantages of monolithics are: Elimination of joints which is an inherent weakness Faster application method Special skill for installation not required Ease of transportation and handling Better scope to reduce downtime for repairs Considerable scope to reduce inventory and eliminate special shapes Heat savings Better spalling resistance Greater volume stability

Monolithics are put into place using various methods, such as ramming, casting, gunniting, spraying, and sand slinging. Ramming requires proper tools and is mostly used in cold applications where proper consolidation of the material is important. Ramming is also used for air setting and heat setting materials. Because calcium aluminate cement is the binder, it will have to be stored properly to prevent moisture absorption. Its strength starts deteriorating after 6 to 12 months.

Figure 9. A Monolithic Lining for Ladel

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2.3 Insulating materials Insulating materials greatly reduce the heat losses through walls. Insulation is achieved by providing a layer of material with low heat conductivity between the internal hot surface of a furnace and the external surface, thus keeping the temperature of the external surface low. Insulating materials may be classified into the following groups: Insulating bricks Insulating castables Ceramic fiber Calcium silicate Ceramic coating

Insulating materials owe their low conductivity to their pores while their heat capacity depends on the bulk density and specific heat. Air insulating materials consist of minute pores filled with air, which have a very low thermal conductivity. Excessive heat affects all insulation material adversely, but at what temperatures this takes place varies widely. Therefore the choice of an insulating material must be based on its ability to resist heat conductivity and on the highest temperature it will withstand. One of the most widely used insulating materials is diatomite, also known as kiesel guhr, which consists of a mass of skeletons of minute aquatic plants deposited thousands of years ago on the beds of seas and lakes. Its chemical composition is silica contaminated with clay and organic matter. A wide range of insulating refractories with wide combinations of properties is now available. Table 6 shows important physical properties of some insulating refractories.

Table 6. Physical properties of insulating materials (BEE, 2005) Type Thermal

conductivity at 400oC

Max. safe temperature (oC)

Cold crushing strength (kg/cm2)

Porosity percent

Bulk density (kg/m3)

Diatomite Solid Grade

0.025 1000 270 52 1090

Diatomite Porous Grade

0.014 800 110 77 540

Clay 0.030 1500 260 68 560 High Alumina 0.028 1500-1600 300 66 910 Silica 0.040 1400 400 65 830 2.3.1 Castables and concretes Monolithic linings of furnace sections can be constructed by casting refractory insulating concretes, and stamping lightweight aggregates into place that are suitably bonded. Other applications include the bases of tunnel kiln cars used in the ceramic industry. The ingredients are similar to those insulation materials used for making piece refractories, except that concretes contain either Portland or high-alumina cement.

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2.3.2 Ceramic fiber Ceramic fiber is a low thermal mass insulation material, which has revolutionized furnace design lining systems. Ceramic fiber is manufactured by blending and melting alumina and silica at a temperature of 1800 – 2000oC, and breaking the molten stream by blowing compressed air or dropping the molten stream on a spinning disc to form loose or bulk ceramic fiber. The bulk fiber is used to produce various insulation products including blankets, strips, veneering and anchored modules, paper, vacuum formed boards and shapes, ropes, wet felt, mastic cement etc. Fibers are usually produced in two temperature grades based on Al2O3 content. A new product is ZrO2 added alumino-silicate fiber, which helps to reduce shrinkage levels and thereby making the fiber suitable for higher temperatures. Continuous recommended operating temperature for fibers are given in the Table 7.

Table 7. Continuous recommended operating temperature for fibers (BEE, 2005)

Al2O3 SiO2 ZrO2

1150oC 43 – 47 percent 53 – 57 percent - 1250oC 52 – 56 percent 44 – 48 percent - 1325oC 33 – 35 percent 47 – 50 percent 17 – 20 percent Ceramic fibers are generally produced in bulk wool form and needled into a blanket mass of various densities ranging from 64 to 190 kg/m3. Converted products and over 40 different forms are made from blankets to suit various requirements. The characteristics of ceramic fibers are a remarkable combination of the properties of refractories and traditional insulation material. a) Lower thermal conductivity Because of the low thermal conductivity (0.1 kCal/m per hour per oC at 600 oC for a blanket with 128 kg/m3 density) it is possible to construct thinner linings with the same thermal efficiency as conventional refractories. As a result of thinner lining, the furnace volume is higher. It is 40 percent more effective than good quality insulation brick and 2.5 times better than asbestos. Ceramic fiber is a better insulator than calcium silicate. b) Light weight The average density of ceramic fiber is 96 kg/m3. It is one tenth of the weight of insulating brick and one third of the weight of asbestos / calcium silicate boards. For new furnaces structural supports can be reduced by 40 percent. c) Lower heat storage Ceramic fiber linings absorb less heat because of their lower density. Furnaces can therefore be heated and cooled at faster rates. Typically the heat stored in a ceramic fiber lining system is in the range of 2700 - 4050 kCal/m2 (1000 – 1500 Btu/Ft2) as compared to 54200-493900 kCal/m2 (20000 – 250000 Btu/Ft2) for conventionally lined systems.

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d) Thermal shock resistant Ceramic fiber linings resist thermal shock due to their resilient matrix. This also allows for faster heat up and cool down cycles, thereby improving furnace availability and productivity. e) Chemical resistance Ceramic fiber resist most of the chemical attack and is unaffected by hydrocarbons, water and steam present in flue gases. f) Mechanical resilience The high mechanical resilience of ceramic fiber makes it possible to manufacture fiber-lined furnaces off-site, transport them to the site in assembled form without the risk of damage. g) Low installation cost As the application of ceramic fibers is a standardized process, no special skills are required. Fiber linings require no dry out or curing times and there is no risk of cracking or spalling when they are heated after installation.

h) Ease of maintenance In case of physical damage, the section of damaged ceramic fiber can be quickly removed and replaced with a new piece. Entire panel sections can be prefabricated for fast installation with minimal down time.

i) Ease of handling All product forms are easily handled and most can be quickly cut with a knife or scissors. Vacuum formed products may require cutting with a band saw. j) Thermal efficiency Thermal efficiency of a furnace lined with ceramic fiber is improved in two ways. First, the low thermal conductivity of ceramic fiber allows the lining to be thinner and therefore the furnace can be smaller. Second, the fast response of ceramic fiber to temperature changes also allows for more accurate control and uniform temperature distribution within the furnace. Other advantages offered by ceramic fiber are: Lightweight furnace Simple steel fabrication work Low down time Increased productivity Additional capacity Low maintenance cost Longer service life Higher thermal efficiency Faster response

2.3.3 High emissivity coatings Emissivity (i.e. the measure of a material’s ability to both absorb and radiate heat) is often considered as an inherent physical property that does not normally change (other examples are density, specific heat and thermal conductivity). However, the development of high emissivity

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coatings allows the surface emissivity of materials to be increased. High emissivity coatings are applied on the interior surface of furnaces. Figure 10 shows that the emissivity of various insulating materials reduces with increasing process temperatures. The advantage of high emissivity coatings is that the emissivity remains more or less constant. The emissivity of furnaces that operate at high temperatures is 0.3. By using high emissivity coatings this can go up to 0.8, resulting in an increase of heat transfer through radiation. Other benefits of high emissivity coatings in furnace chambers are uniform heating and extended life of refractories and metallic components such as radiant tubes and heating elements. For intermittent furnaces or where rapid heating is required, use of such coatings was found to reduce fuel or power by 25 - 45 percent.

Figure 10. Emissivity of Refractory Materials at Different Temperatures (BEE, 2005)

3. ASSESSMENT OF FURNACES This section describes the various methods and techniques used to quantify the losses from the furnace and the methods to carry out performance assessment of typical furnaces. 3.1 Heat losses affecting furnace performance Ideally, all heat added to the furnaces should be used to heat the load or stock. In practice, however, a lot of heat is lost in several ways as shown in Figure 11.

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FURNACE

Flue gas

Moisture in fuel

Openings in furnace

Furnace surface/skin

Other losses

Heat inputHeat in stock

Hydrogen in fuel

FURNACE

Flue gas

Moisture in fuel

Openings in furnace

Furnace surface/skin

Other losses

Heat inputHeat in stock

Hydrogen in fuel

Figure 11. Heat Losses in a Furnace These furnace heat losses include (BEE, 2005 and US DOE, 2004): Flue gas losses: part of the heat remains in the combustion gases inside the furnace. This

loss is also called waste-gas loss or stack loss. Loss from moisture in fuel: fuel usually contains some moisture and some of the heat is used

to evaporate the moisture inside the furnace Loss due to hydrogen in fuel which results in the formation of water Loss through openings in the furnace: radiation loss occurs when there are openings in the

furnace enclosure and these losses can be significant, especially for furnaces operating at temperatures above 540°C. A second loss is through air infiltration because the draft of furnace stacks/chimneys cause a negative pressure inside the furnace, drawing in air through leaks or cracks or when ever the furnace doors are opened.

Furnace skin / surface losses, also called wall losses: while temperatures inside the furnace are high, heat is conducted through the roof, floor and walls and emitted to the ambient air once it reaches the furnace skin or surface.

Other losses: there are several other ways in which heat is lost from a furnace, although quantifying these is often difficult. Some of these include − Stored heat losses: when the furnace is started the furnace structure and insulation is also

heated, and this heat only leaves the structure again when the furnace shuts down. Therefore this type of heat loss increases with the number of times the furnace is turned on and off

− Material handling losses: the equipment used to move the stock through the furnace, such as conveyor belts, walking beams, bogies etc, also absorb heat. Every time equipment leave the furnace they loose their heat, therefore heat loss increases with the amount of equipment and the frequency by which they enter and leave the furnace

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− Cooling media losses: water and air are used to cool down equipment, rolls, bearing and rolls, but heat is lost because these media absorb heat

− Incomplete combustion losses: heat is lost if combustion is incomplete because unburnt fuel or particles have absorbed heat but this heat has not been put to use

− Loss due to formation of scales 3.2 Instruments to assess furnace performance Furnace efficiency is calculated after subtracting the various heat losses. In order to find out furnace efficiency using the indirect method, various parameters must be measured, such as hourly furnace oil consumption, material output, excess air quantity, temperature of flue gas, temperature of furnace at various zones, and others. Date for some of these parameters can be obtained from production records while others must be measured with special monitoring instruments. Table 8 lists the instruments that are needed to measure these parameters.

Table 8. Instruments for Measuring Furnace Performance Parameter (BEE, 2005) Parameters

to be measured Location of

measurement Instrument

required Required

Value 1200-1300oC Furnace soaking zone

temperature (reheating furnaces)

Soaking zone and side wall

Pt/Pt-Rh thermocouple with indicator and recorder

700oC max. Flue gas temperature In duct near the discharge end, and entry to recuperator

Chromel Alummel Thermocouple with indicator

300oC (max) Flue gas temperature After recuperator Hg in steel thermometer

Furnace hearth pressure in the heating zone

Near charging end and side wall over the hearth

Low pressure ring gauge +0.1 mm of Wc

Oxygen in flue gas In duct near the discharge end

Fuel efficiency monitor for oxygen and temperature

5% O2

Billet temperature Portable Infrared pyrometer or optical pyrometer

-

3.3 Calculating furnace performance A furnace’s efficiency increases when the percentage of heat that is transferred to the stock or load inside the furnace increases. The efficiency of the furnace can be calculated in two ways, similar to that of the boiler: direct method and indirect method. Both methods are explained below.

20


3.3.1 Direct method The efficiency of a furnace can be determined by measuring the amount heat absorbed by the stock and dividing this by the total amount of fuel consumed.

Heat in the stock Thermal efficiency of the furnace = Heat in the fuel consumed for heating the stock

The quantity of heat (Q) that will be transferred to stock can be calculated with this equation:

Q = m x Cp

(t1 – t2) Where, Q = Quantity of heat of stock in kCal

m = Weight of the stock in kg Cp= Mean specific heat of stock in kCal/kg oC t1 = Final temperature of stock in oC t2 = Initial

temperature of the stock before it enters the furnace in oC

An example calculation is given in section 3.3.3. 3.3.2 Indirect method The furnace efficiency can also be determined through the indirect method, similar to the evaluation of boiler efficiency. The principle is simple: the heat losses are substracted from the heat supplied to the furnace. Different types of heat losses are illustrated in Figure 11. Typical thermal efficiencies for common industrial furnaces are given in the Table 9.

Table 9. Thermal Efficiencies for Common Industrial Furnaces (BEE 2005)

Furnace type Typical thermal efficiencies (percent) 1) Low Temperature furnaces a. 540 – 980 oC (Batch type) 20-30 b. 540 – 980 oC (Continous type) 15-25 c. Coil Anneal (Bell) radiant type 5-7 d. Strip Anneal Muffle 7-12 2) High temperature furnaces a. Pusher, Rotary 7-15 b. Batch forge 5-10 3) Continuous Kiln a. Hoffman 25-90 b. Tunnel 20-80 4) Ovens a. Indirect fired ovens (20 oC –370

oC) 35-40 b. Direct fired ovens (20 oC –370 oC) 35-40

21


An example calculation using the indirect method is given in the next section. 3.3.3 Example calculation of furnace efficiency Calculate the efficiency of an oil-fired reheating furnace with the direct and indirect method using the data below. Operating temperature: 1340oC Exit flue gas temperature after preheater: 750oC Ambient temperature: 40oC Preheated air temperature: 190oC Specific gravity of fuel oil: 0.92 Average fuel oil consumption: 400 liters / hr = 400 x 0.92 =368 kg/hr Calorific value of oil 10000 kCal/kg Average O2 percentage in flue gas: 12 percent Moisture in 1 kg of fuel oil: 0.15 kg H2 in 1 kg of fuel oil: 0.1123 kg Theoretical air required to burn 1 kg of oil: 14 kg Weight of stock: 6000 kg/hr Specific heat of billet: 0.12 kCal/kg/0C Furnace wall thickness (D): 460 mm Billet extraction outlet (X): 1 m x 1 m Average surface temperature of heating + soaking zone: 122 oC Average surface temperature of area other than heating and soaking zone: 80 oC Area of heating + soaking zone: 70.18 m2

Area other than heating and soaking zone: 12.6 m2

Direct method calculation The heat input is 400 liters per hour. The specific gravity of fuel is used to convert this into kg. Therefore: 400 l/hr x 0.92 kg/l = 368 kg/hr The heat output is calculated as follows:

= m x Cp x ΔT = 6000 kg x 0.12 x (1340 – 40) = 936000 kCal

The efficiency is:

= (heat input / heat output) x 100 = [(936000 / (368 x 10000)] x 100 = 25.43 percent

The approximate heat loss is 100% – 25% = 75%

22


The different heat losses are calculated below. a) Heat loss in flue gas Excess air (EA)

= O2 percent / (21 – O2 percent) = 12 / (21 – 12)

= 133 % Mass of air supplied

= (1 + EA/100) x Theoretical air = (1+ 1.13) x 14

= 32.62 kg/kg fuel oil

m x Cp

x ΔT x 100 % Heat loss in flue gas = GCV of fuel

Where,

m = weight of flue gas (air + fuel) = 32.62 + 1.0 = 33.62 kg/kg oil Cp = specific heat ΔT = temperature difference

% Heat loss = 33.62 x 0.24 x (750 – 40) x 100 = 57.29% 10000 b) Heat loss from moisture in fuel

M x 584 + Cp (Tf – Tamb) x 100

% Heat loss from moisture in fuel = GCV of fuel

Where,

M = kg of moisture in 1 kg of fuel oil Tfg = Flue gas temperature, 0C Tamb = Ambient temperature, 0C GCV = Gross Calorific Value of fuel, kCal/kg

% Heat loss = 0.15 x 584 + 0.45 (750 – 40) x 100 = 1.36% 10000

23


Indirect method

c) Loss due to hydrogen in fuel

9 x H2 x 584 + Cp (Tf – Tamb) x 100 % Heat loss due to hydrogen in fuel = GCV of fuel

Where,

H2 = kg of H2 in 1 kg of fuel oil (= 0.1123 kg/kg of fuel oil) % Heat loss = 9 x 0.1123 x 584 + 0.45 (750 – 40) x 100 = 9.13% 10000 d) Heat loss due to openings in furnace

(Black body radiation factor x emissivity x factor of radiation x area of opening) x 100 % Heat loss from openings in

furnace = Quantity of oil x GCV of oil

The factor of radiation through openings and the black body radiation factor can be obtained from standard graphs as shown in Figure 12 and Figure 13. Factor of radiation (refer Figure 12) = 0.71 Black body radiation at 1340 0C (refer Figure 13) = 36 kCal/kg/cm2/hr The area of the opening is 100 cm x 100 cm = 10000 cm2 Emissivity = 0.8

% Heat loss from furnace openings = 36 x 0.8 x 0.71 x 10000 x 100 = 5.56% 368 x 10000

24


Figure 12. Radiation Factor for Heat Release through Openings relative to

the Quality of Heat Release from Perfect Black Body (BEE, 2005)

Figure 13. Black Body Radiation at Different Temperatures (BEE, 2005)

25


e) Heat loss through furnace skin To determine the heat loss through the furnace skin, first the heat loss through the roof and sidewalls and through other areas must be calculated separately.

i). Heat loss through roof/ceiling and sidewalls (= heating and soaking zone): Total average surface temperature = 122oC Heat loss at 122oC (Refer Figure 14) = 1252 kCal /m2 hr Total area of heating + soaking zone = 70.18 m2

Heat loss from roof and walls Heat loss through furnace roof = Area of roof and walls

Total heat loss = 1252 kCal / m2 hr x 70.18 m2 = 87865 kCal/hr

ii) Heat lost from area other than heating and soaking zone Total average surface temperature = 80 oC Heat loss at 80oC (Refer Figure 14) = 740 kCal / m2 hr Total area = 12.6 m2

Heat loss from roof other areas Heat loss through other areas = Area of other areas

Total heat loss = 740 kCal / m2

hr x 12.6 m2 = 9324 kCal/hr

(Heat loss i + heat loss ii) x 100 % Heat loss through furnace skin = GCV of oil x Quantity of oil per hour

% Heat loss through furnace skin = (87865 kCal/hr + 9324 kCal/hr) x 100 = 2.64%

10000 kCal/kg x 368 kg/hr f) Unaccounted losses The unaccounted losses cannot be calculated unless the other types of losses are known. Furnace efficiency Adding the losses a to f up gives the total losses:

a) Flue gas loss = 57.29 %

26


b) Loss due to moisture in fuel = 1.36 % c) Loss due to H2 in fuel = 9.13 % d) Loss due to openings in furnace = 5.56 % e) Loss through furnace skin = 2.64 %

Total losses = 75.98 % The furnace efficiency calculated through the indirect method = 100 – 75.98 = 24.02%

Figure 14. Heat Loss from the Ceiling, Sidewall and Hearth of Furnace (BEE, 2005)

4. ENERGY EFFICIENCY OPPORTUNITES This section explains the various energy saving opportunities in furnaces.6 Typical energy efficiency measures for an industry with furnace are:

1. Complete combustion with minimum excess air 2. Proper heat distribution 3. Operation at the optimum furnace temperature 4. Reducing heat losses from furnace openings 5. Maintaining correct amount of furnace draft 6. Optimum capacity utilization 7. Waste heat recovery from the flue gases 8. Minimum refractory losses 9. Use of ceramic coatings 10. Selecting the right refractories

27


4.1 Complete combustion with minimum excess air The amount of heat lost in the flue gases (stack losses) depends on the amount of excess air. To obtain complete combustion of fuel with the minimum amount of air, it is necessary to control air infiltration, maintain pressure of combustion air, fuel quality and monitor the amount excess air. Too much excess air will reduce flame temperature, furnace temperature and heating rate. Too little excess air will result in an increase in unburnt components in flue gases that are carried away through the stack and it also causes more scale losses. Optimizing combustion air is the most attractive and economical measure for energy conservation. Potential savings are higher when the temperature of furnace is high. The air ratio (= actual air amount / theoretical combustion air amount) gives an indication of excess air air. If a reheating furnace is not equipped with an automatic air/fuel ratio controller, it is necessary to periodically take a sample of gas in the furnace and measure its oxygen contents with a gas analyzer. 4.2 Proper heat distribution A furnace should be designed to ensure that within a given time the stock is heated uniformly to a desired temperature with the minimum amount of fuel. Where burners are used to fire the furnace, the following should be ensured for proper heat distribution: The flame should not touch or be obstructed by any solid object. Obstruction causes the fuel

particles to de-atomize, which affects combustion and causes black smoke. If the flame impinges on the stock scale losses will increase. If the flame impinges on refractories, products from incomplete combustion can settle and react with the refractory constituents at high temperatures.

The flames of different burners should stay clear of each other, as intersecting flames cause incomplete combustion. It is also desirable to stagger burners on opposite sides.

The burner flame has a tendency to travel freely in the combustion space just above the material. For this reason, the axis of the burner in small furnaces is never placed parallel to the hearth but always at an upward angle, but the flame should not hit the roof.

Large burners produce longer flames, which may be difficult to contain within the furnace walls. More burners of less capacity ensure a better heat distribution inside the furnace and also increase the furnace life.

In small furnaces that use furnace oil, a burner with a long flame with a golden yellow color improves uniform heating. But the flame should not be too long, because heat is lost of the flame reaches the chimney or the furnace doors.

4.3. Operation at the optimum furnace temperature It is important to operate the furnace at its optimum temperature. Operating temperatures of various furnaces are given in Table 10. Operating at too high temperatures causes heat loss, excessive oxidation, de-carbonization and stress on refractories. Automatic control of the furnace temperature is preferred to avoid human error.

28


Table 10. Operating Temperatures of Various Furnaces Slab Reheating furnaces 1200oC

1200oC Rolling Mill furnaces 800oC Bar furnace for Sheet Mill 650oC –750oC Bogie type annealing furnaces

4.4. Prevent heat loss through openings Heat can be lost by direct radiation through openings in the furnace, such as the charging inless, extracting outlet and the peephole in the wall or ceiling. Heat is also lost due to pressure differences between the inside of the furnace and the ambient environment causing combustion gases to leak through the openings. But most heat is lost if outside air infiltrates into the furnace, because in addition to heat loss this also causes uneven temperatures inside the furnace and stock and can even lead to oxidization of billets. It is therefore important to keep the openings as small as possible and to seal them. Another effective way to reduce the heat loss through furnace openings is by opening the furnace doors less frequent and for the shortest time period as possible (another option is described under item 4.5). This heat loss is about 1 percent of the total quantity of heat generated in the furnace, if furnace pressure is controlled properly. Section 3.3.3 already explained one way of calculating heat loss through openings. But an alternative way is calculating heat loss with the following equation:

Where,

Q = heat loss T = absolute temperature (K) a = factor for total radiation A = area of opening, m2

H = time (hours) For example, a reheating furnace with a temperature of 1340 oC, the wall thickness is 460 mm (X) and the door is 1 m high (D) by 1 m wide. D/X = 1/0.460 = 0.71, and in Figure 12 this corresponds with a factor for total radiation of 0.71. The heat loss from openings in therefore:

29


4.5. Control of furnace draft If negative pressures exist inside the furnace, air can infiltrate through cracks and openings and affect the air-fuel ratio control. This in turn can cause metal to not reach the desired temperature or non-uniform temperatures, which affects the next processes like forging and rolling. Fuel consumption and product rejection rates increase. Tests conducted on seemingly airtight furnaces have shown air infiltration up to 40 percent. To avoid this, slight positive pressure should be maintained inside the furnace (in addition to the measures mentioned under 4.4). But the pressure difference should not be too high because this will cause ex-filtration. While this is less of a problem than infiltration, it can still result in flames reaching out of the furnace, overheating of refractories leading to reduced brick life, increased furnace maintenance, and burnout of ducts and equipment. Proper management of the pressure difference between the inside and outside of the furnace is therefore important to minimize heat loss and adverse impacts on products. 4.6. Optimum capacity utilization One of the most vital factors affecting the furnace efficiency is the load. This includes the amount of material placed in the furnace, the arrangement inside the furnace and the residence time inside the furnace. a) Optimum load If the furnace is under loaded the proportion of total heat available that will be taken up by the load is smaller, resulting in a lower efficiency. Overloading can lead to the load not heated to the right temperature within a given period of time. There is a particular load at which the furnace will operate at maximum thermal efficiency, i.e. where the amount of fuel per kg of material is lowest. This load is generally obtained by recording the weight of material of each charge, the time it takes to reach the right temperature, and the amount of fuel used. The furnace should be loaded to the optimum load at all times, although in practice this may not always be possible. b) Optimum arrangement of the load The loading of materials on the furnace hearth should be arranged so that It receives the maximum amount of radiation from the hot surfaces of the heating chambers

and flames Hot gases are efficiently circulated around the heat receiving surfaces of the materials Stock is not placed in the following position: − In the direct path of the burners or where flame impingement is likely to occur − In an area that is likely to cause a blockage or restriction of the flue system of the furnace − Close to any door openings where cold spots are likely to develop

30


c) Optimum residence time of the load Fuel consumption is kept at a minimum and product quality is best if the load only remains inside the furnace until it has the required physical and metallurgical properties. Sometimes the charge and production schedule does not correspond with the capacity of the furnace. If this is the case, either the Load is higher or lower than the optimum load Residence time is longer or shorter than the ideal residence time. Excessive residence time

will increase oxidation of the material surface, which can result in rejection of products. The rate of oxidation is dependent upon time, temperature, as well as free oxygen content

Temperature is increased to make up for shorter residence time. The higher the working temperature, the higher is the loss per unit of time.

All three result in fuel wastage and sometimes in reduced product quality. Coordination between the furnace operator, production and planning personnel is therefore essential. Optimum utilization of furnace can be planned at design stage, by selecting the size and type (batch, continuous) that best matches the production schedule. The overall efficiency of a continuous type furnace will increase with heat recuperation from the waste gas stream. If only batch type furnace is used, careful planning of the loads is important. Furnace should be recharged as soon as possible to enable use of residual furnace heat. 4.7. Waste heat recovery from furnace flue gases In any industrial furnace the combustion products leave the furnace at a temperature higher than the stock temperature. Flue gases carry 35 to 55 percent of the heat input to the furnace with them through the chimney. The higher the amount of excess air and flue gas temperature, the higher the amount of waste heat that is available. However, the primary objective should be to minimize the amount of waste heat generated through energy conservation measures. Waste heat recovery should only be considered when further energy conservation is not possible or practical. Waste heat in flue gases can be recovered for preheating of the charge (stock, load), preheating of combustion air or for other processes as described below. a) Charge pre-heating When raw materials are preheated by exhaust gases before being placed in a heating furnace, the amount of fuel necessary to heat them in the furnace is reduced. Since raw materials are usually at room temperature, they can be heated sufficiently using high-temperature flue gases to noticeably reduce the fuel consumption rate. b) Preheating of combustion air For a long time, fuel gases were only use for preheating of combustion air for large boilers, metal-heating furnaces and high-temperature kilns. But preheating using heat from flue gases is now also applied to compact boilers and compact industrial furnaces.

31


A variety of equipment is available to recover waste heat. External recuperators are most common, but other techniques are also used, such as self-recuperative burners. For example, a modern recuperator use furnace exhaust gas of 1000°C can preheat the combustion air to over 500 oC, which results in energy savings of up to 30 percent compared with using cold combustion air entering the furnace. Since the volume of combustion air increases when it is preheated, it is necessary to consider this when modifying air-duct diameters and blowers. It should be noted that preheating of combustion gases from high-density oils with a high sulphur content, could cause clogging with dust or sulphides, corrosion or increases in nitrogen oxides. c) Utilizing waste heat as a heat source for other processes Other process (to generate steam or hot water by a waste heat boiler) The temperature of furnace exhaust gas can be as high as 400- 600 °C, even after heat has been recovered from it for preheating the charge or combustion air. One possibility is to install a waste heat boiler to produce steam or hot water from this heat, especially when large quantities steam or hot water are needed in a plant. Sometimes exhaust gas heat can be used for heating purposes in other equipment, but only if the heat quantity, temperature range, operation time etc are suitable for this. Fuel consumption can be greatly reduced. One existing example is the use of exhaust gas from a quenching furnace as a heat source in a tempering furnace. 4.8. Minimizing furnace skin losses About 30 to 40 percent of the fuel used in intermittent or continuous furnaces is used to make up for heat lost through the furnace skin/surface or walls. The extent of wall losses depend on: Emissivity of wall Thermal conductivity of refractories Wall thickness Whether the furnace is operated continuously or intermittently

There are several ways to minimize heat loss through the furnace skin: Choosing the appropriate refractory materials Increasing the wall thickness Installing insulating bricks. Outside wall temperatures and heat losses of a composite wall

are much lower for a wall of firebrick and insulation brick compared to a wall of the same thickness that consists only of refractory bricks. The reason is that insulating bricks have a much lower conductivity.

Planning operating times of furnaces. For most small furnaces, the operating periods alternate with the idle periods. When the furnaces are turn off, heat that was absorbed by the refractories during operation gradually dissipates through radiation and convection from the cold face and through air flowing through the furnace. When the furnace is turned on again, additional fuel is needed to heat up the refractories again. If a furnace is operated continuously for 24 hours in three days, practically all the heat stored in the refractories is lost. But if the furnace is operated 8 hours per day all the heat stored in the refractories is not dissipated. For a furnace with a firebrick wall of 350 mm thickness, it is estimated that during

32


the 16 hours that the furnace is turned off, only 55 percent of the heat stored in the refractories is dissipated from the cold surface. Careful planning of the furnace operation schedule can therefore reduce heat loss and save fuel.

The quantity (Q) of heat loss from the furnace skin is the sum of natural convection and thermal radiation. In addition to the method explain in section 3.3.3, the following equation can also be used:

Where,

Q = Quantity of heat released (kCal/hr) a = factor regarding direction of the surface of natural convection ceiling = 2.8, side walls = 2.2, hearth = 1.5 tl = temperature of external wall surface of the furnace (°C), based on the average of as

many measurements as possible to reduce the error margin t2 = temperature of air around the furnace (°C) E = emissivity of external wall surface of the furnace

The first part of the equation gives the heat loss though natural convection, and the second part the heat loss through radiation. Figure 14 shows the relation between the temperature of external wall surface and the quantity of heat release calculated with this formula. An example calculation of the heat loss from a furnace’s surface is as follows: A reheating furnace has a ceiling, sidewalls and hearth with a 20 m2, 50 m2 and 20 m2 surface area respectively. Their average measured surface temperatures 80°C, 90°C and 100°C respectively. Based on Figure 14, the quantities of heat release from ceiling, sidewalls and hearth per unit area are respectively 650 kCal/m2h, 720 kCal/m2h and 730 kCal/m2h. Therefore, the total quantity of heat release Q

= loss through ceiling + loss through sidewalls + loss through hearth = (650 x 20) + (720 x 50) + (730 x 20) = 13000 + 36000 +14600= 63,600 kCal/hr

33


Figure 15. Relationship between Surface Temperature and Quantity of Heat Loss (BEE, 2005)

4.9 Use of ceramic coatings (high emissivity coatings) Ceramic coatings in the furnace chamber promote rapid and efficient transfer of heat, uniform heating and extended life of refractories. The emissivity of conventional refractories decreases with increase in temperature whereas for ceramic coatings it increases slightly. This outstanding property has been exploited by using ceramic coatings in hot face insulation. Ceramic coatings are high emissivity coatings and a have a long life at temperatures up to 1350oC. There are two types of ceramic coatings: those used for coating metal substrates, and those used for coating refractory substrates. The coatings are non-toxic, non-flammable and water based. Applied at room temperatures, they are sprayed and air-dried in less than five minutes. The coatings allow the substrate to maintain its designed metallurgical properties and mechanical strength. Installation is quick and can be completed during shut down. Energy savings of the order of 8-20 percent have been reported depending on the type of furnace and operating conditions. High emissivity coatings are further described in section 2.3.3. 4.10 Selection of refractories The selection of refractories aims to maximize the performance of the furnace, kiln or boiler. Furnace manufacturers or users should consider the following points in the selection of a refractory: Type of furnace

34


Type of metal charge Presence of slag Area of application Working temperatures Extent of abrasion and impact Structural load of the furnace Stress due to temperature gradient in the structures and temperature fluctuations Chemical compatibility to the furnace environment Heat transfer and fuel conservation Cost considerations

5. OPTIONS CHECKLIST It is difficult to make a checklist of general options for furnaces, because options to improve energy efficiency vary between furnaces. But the main options that are applicable to most furnaces are:

Check against infiltration of air: use doors or air curtains

Monitor O2 /CO2/CO and control excess air to the optimum level

Improve burner design, combustion control and instrumentation

Ensure that the furnace combustion chamber is under slight positive pressure

Use ceramic fibers in the case of batch operations

Match the load to the furnace capacity

Retrofit with heat recovery device

Investigate cycle times and reduce

Provide temperature controllers

Ensure that flame does not touch the stock

35


Fuels and Combustion

1

FUELS & COMBUSTION

1. INTRODUCTION..........................................................................................................1

2 TYPE OF FUELS............................................................................................................1

3. PERFORMANCE EVALUATION OF FUELS ...............................................11

4. ENERGY EFFICIENCY OPPORTUNITIES...................................................17

5. OPTION CHECKLIST..............................................................................................20

6. WORKSHEETS ...........................................................................................................23

7. REFERENCES..............................................................................................................24

1. INTRODUCTION This section briefly describes the main features of fuels. Energy from the Sun is converted into chemical energy by photosynthesis. But, as we know, when we burn dried plants or wood, producing energy in the form of heat and light, we are releasing the Sun’s energy originally stored in that plant or in that wood through photosynthesis. We know that, in most of the world today, wood is not the main source of fuel. We generally use natural gas or oil in our homes, and we use mainly oil and coal to heat the water to produce the steam to drive the turbines for our huge power generation systems. These fuels - coal, oil, and natural gas - are often referred to as fossil fuels. The various types of fuels (like liquid, solid and gaseous fuels) that are available depend on various factors such as costs, availability, storage, handling, pollution and landed boilers, furnaces and other combustion equipments. The knowledge of the fuel properties helps in selecting the right fuel for the right purpose and for the efficient use of the fuel. Laboratory tests are generally used for assessing the nature and quality of fuels. 2 TYPE OF FUELS This section describes types of fuels: solid, liquid, and gaseous. 2.1 Liquid Fuels Liquid fuels like furnace oil and LSHS (low sulphur heavy stock) are predominantly used in industrial applications. The various properties of liquid fuels are given below.

Chapter - 2


2

Density is defined as the ratio of the mass of the fuel to the volume of the fuel at a reference temperature of 15°C. Density is measured by an instrument called a hydrometer. The knowledge of density is useful for quantitative calculations and assessing ignition qualities. The unit of density is kg/m3. 2.1.2 Specific gravity This is defined as the ratio of the weight of a given volume of oil to the weight of the same volume of water at a given temperature. The density of fuel, relative to water, is called specific gravity. The specific gravity of water is defined as 1. Since specific gravity is a ratio, it has no units. The measurement of specific gravity is generally made by a hydrometer. Specific gravity is used in calculations involving weights and volumes. The specific gravity of various fuel oils are given in Table below: Table 1. Specific gravity of various fuel oils (adapted from Thermax India Ltd.) Fuel Oil L.D.O

(Light Diesel Oil) Furnace oil L.S.H.S

(Low Sulphur Heavy Stock)

Specific Gravity 0.85 - 0.87 0.89 - 0.95 0.88 - 0.98 2.1.3 Viscosity The viscosity of a fluid is a measure of its internal resistance to flow. Viscosity depends on the temperature and decreases as the temperature increases. Any numerical value for viscosity has no meaning unless the temperature is also specified. Viscosity is measured in Stokes / Centistokes. Sometimes viscosity is also quoted in Engler, Saybolt or Redwood. Each type of oil has its own temperature - viscosity relationship. The measurement of viscosity is made with an instrument called a Viscometer. Viscosity is the most important characteristic in the storage and use of fuel oil. It influences the degree of pre-heating required for handling, storage and satisfactory atomization. If the oil is too viscous, it may become difficult to pump, hard to light the burner, and difficult to handle. Poor atomization may result in the formation of carbon deposits on the burner tips or on the walls. Therefore pre-heating is necessary for proper atomization. 2.1.4 Flash Point The flash point of a fuel is the lowest temperature at which the fuel can be heated so that the vapour gives off flashes momentarily when an open flame is passed over it. The flash point for furnace oil is 66 0C. 2.1.5 Pour Point The pour point of a fuel is the lowest temperature at which it will pour or flow when cooled under prescribed conditions. It is a very rough indication of the lowest temperature at which fuel oil is ready to be pumped.

2.1.1 Density


– 3

Specific heat is the amount of kCals needed to raise the temperature of 1 kg of oil by 10C. The unit of specific heat is kcal/kg0C. It varies from 0.22 to 0.28 depending on the oil specific gravity. The specific heat determines how much steam or electrical energy it takes to heat oil to a desired temperature. Light oils have a low specific heat, whereas heavier oils have a higher specific heat. 2.1.7 Calorific Value The calorific value is the measurement of heat or energy produced, and is measured either as gross calorific value or net calorific value. The difference is determined by the latent heat of condensation of the water vapour produced during the combustion process. Gross calorific value (GCV) assumes all vapour produced dur ing the combustion process is fully condensed. Net calorific value (NCV) assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value. The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal while calorific value of fuel oils are much more consistent. The typical GCVs of some of the commonly used liquid fuels are given below: Table 2. Gross calorific values for different fuel oils (adapted from Thermax India Ltd.)

Fuel Oil Gross Calorific Value (kCal/kg) Kerosene - 11,100

Diesel Oil - 10,800 L.D.O - 10,700 Furnace Oil - 10,500 LSHS - 10,600

2.1.8 Sulphur The amount of sulphur in the fuel oil depends mainly on the source of the crude oil and to a lesser extent on the refining process. The normal sulfur content for the residual fuel oil (furnace oil) is in the order of 2 - 4 %. Typical figures for different fuel oils are shown in Table 3. Table 3. Percentages of sulphur for different fuel oils (adapted from Thermax India Ltd.)

Fuel oil Percentage of Sulphur Kerosene 0.05 - 0.2 Diesel Oil 0.05 - 0.25 L.D.O 0.5 - 1.8 Furnace Oil 2.0 - 4.0 LSHS < 0.5

The main disadvantage of sulphur is the risk of corrosion by sulphuric acid formed during and after combustion, and condensation in cool parts of the chimney or stack, air pre-heater and economizer.

2.1.6 Specific Heat

4

The ash value is related to the inorganic material or salts in the fuel oil. The ash levels in distillate fuels are negligible. Residual fuels have higher ash levels. These salts may be compounds of sodium, vanadium, calcium, magnesium, silicon, iron, aluminum, nickel, etc. Typically, the ash value is in the range 0.03 - 0.07 %. Excessive ash in liquid fuels can cause fouling deposits in the combustion equipment. Ash has an erosive effect on the burner tips, causes damage to the refractories at high temperatures and gives rise to high temperature corrosion and fouling of equipments. 2.1.10 Carbon Residue Carbon residue indicates the tendency of oil to deposit a carbonaceous solid residue on a hot surface, such as a burner or injection nozzle, when its vaporizable constituents evaporate. Residual oil contains carbon residue of 1 percent or more. 2.1.11 Water Content The water content of furnace oil when it is supplied is normally very low because the product at refinery site is handled hot. An upper limit of 1% is specified as a standard. Water may be present in free or emulsified form and can cause damage to the inside surfaces of the furnace during combustion especially if it contains dissolved salts. It can also cause spluttering of the flame at the burner tip, possibly extinguishing the flame, reducing the flame temperature or lengthening the flame. Typical specifications of fuel oils are summarized in the Table below. Table 4. Typical specifications of fuel oils (adapted from Thermax India Ltd.)

Fuel Oils Properties Furnace Oil L.S.H.S L.D.O

Density (Approx. g/cc at 150C)

0.89 - 0.95 0.88 - 0.98 0.85 - 0.87

Flash Point (0C) 66 93 66 Pour Point (0C) 20 72 18 G.C.V. (kCal/kg) 10500 10600 10700 Sediment, % Wt. Max.

0.25 0.25 0.1

Sulphur Total, % Wt. Max.

Up to 4.0 Up to 0.5 Up to 1.8

Water Content, % Vol. Max.

1.0 1.0 0.25

Ash % Wt. Max. 0.1 0.1 0.02 2.1.12 Storage of Fuel oil It can be potentially hazardous to store furnace oil in barrels. A better practice is to store it in cylindrical tanks, either above or below the ground. Furnace oil that is delivered may contain dust, water and other contaminants.

2.1.9 Ash Content


5

The sizing of the storage tank facility is very important. A recommended storage size estimate is to provide for at least 10 days of normal consumption. Industrial heating fuel storage tanks are generally vertical mild steel tanks mounted above the ground. It is prudent for safety and environmental reasons to build bund walls around tanks to contain accidental spillages. As a certain amount of settlement of solids and sludge will occur in tanks over time, tanks should be cleaned at regular intervals: annually for heavy fuels and every two years for light fuels. Care should be taken when oil is decanted from the tanker to the storage tank. All leaks from joints, flanges and pipelines must be attended to at the earliest. Fuel oil should be free from possible contaminants such as dirt, sludge and water before it is fed to the combustion system. 2.2 Solid Fuel (Coal)


Subject not detailed

9

2.3 Gaseous Fuel

Gas fuels are the most convenient because they require the least amount of handling and are used in the simplest and most maintenance-free burner systems. Gas is delivered "on tap" via a distribution network and so is suited for areas with a high population or industrial density. However, large individual consumers do have gasholders and some produce their own gas.

2.3.1 Types of gaseous fuel

The following is a list of the types of gaseous fuel: § Fuels naturally found in nature:

− Natural gas − Methane from coal mines

§ Fuel gases made from solid fuel − Gases derived from coal − Gases derived from waste and biomass − From other industrial processes (blast furnace gas)

§ Gases made from petroleum − Liquefied Petroleum gas (LPG) − Refinery gases − Gases from oil gasification

§ Gases from some fermentation process

Gaseous fuels in common use are liquefied petroleum gases (LPG), Natural gas, producer gas, blast furnace gas, coke oven gas etc. The calorific value of gaseous fuel is expressed in Kilocalories per normal cubic meter (kCal/Nm3) i.e. at normal temperature (20 0C) and pressure (760 mm Hg).

2.3.2 Properties of gaseous fuels Since most gas combustion appliances cannot utilize the heat content of the water vapour, gross calorific value is of little interest. Fuel should be compared based on the net calorific value. This is especially true for natural gas, since increased hydrogen content results in high water formation during combustion. Typical physical and chemical properties of various gaseous fuels are given in Table 9.


10

Table 9. Typical physical and chemical properties of various gaseous fuels Fuel Gas Relative

Density Higher Heating Value kcal/Nm3

Air/Fuel ratio- m3

of air to m3

of Fuel

Flame

Temp. oC

Flame Speed m/s

Natural Gas

0.6 9350 10 1954 0.290

Propane 1.52 22200 25 1967 0.460 Butane 1.96 28500 32 1973 0.870

2.3.3 LPG

LPG is a predominant mixture of propane and Butane with a small percentage of unsaturates (Propylene and Butylene) and some lighter C

2 as well as heavier C

5 fractions. Included in the

LPG range are propane (C3H

8), Propylene(C

3H

6), normal and iso-butane (C

4H

10)and

Butylene(C4H

8). LPG may be defined as those hydrocarbons, which are gaseous at normal

atmospheric pressure, but may be condensed to the liquid state at normal temperature, by the application of moderate pressures. Although they are normally used as gases, they are stored and transported as liquids under pressure for convenience and ease of handling. Liquid LPG evaporates to produce about 250 times volume of gas. LPG vapour is denser than air: butane is about twice as heavy as air and propane about one and a half times as heavy as air. Consequently, the vapour may flow along the ground and into drains sinking to the lowest level of the surroundings and be ignited at a considerable distance from the source of leakage. In still air vapour will disperse slowly. Escape of even small quantities of the liquefied gas can give rise to large volumes of vapour / air mixture and thus cause considerable hazard. To aid in the detection of atmospheric leaks, all LPG’s are required to be odorized. There should be adequate ground level ventilation where LPG is stored. For this very reason LPG cylinders should not be stored in cellars or basements, which have no ventilation at ground level. 2.3.4 Natural gas Methane is the main constituent of natural gas and accounting for about 95% of the total volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen, Carbon Dioxide, and traces of other gases. Very small amounts of sulphur compounds are also present. Since methane is the largest component of natural gas, generally properties of methane are used when comparing the properties of natural gas to other fuels. Natural gas is a high calorific value fuel requiring no storage facilities. It mixes with air readily and does not produce smoke or soot. It contains no sulphur. It is lighter than air and disperses into air easily in case of leak. A typical comparison of carbon contents in oil, coal and gas is given in the table below.


11

Table 10. Comparison of chemical composition of various fuels Fuel Oil Coal Natural Gas Carbon 84 41.11 74 Hydrogen 12 2.76 25 Sulphur 3 0.41 - Oxygen 1 9.89 Trace Nitrogen Trace 1.22 0.75 Ash Trace 38.63 - Water Trace 5.98 - 3. PERFORMANCE EVALUATION OF FUELS This section explains the principles of combustion, how fuel performance can be evaluated using the stochiometric calculation of air requirement, the concept of excess air, and the draft system of exhaust gases. 3.1 Principles of Combustion 3.1.1 Combustion process Combustion refers to the rapid oxidation of fuel accompanied by the production of heat, or heat and light. Complete combustion of a fuel is possible only in the presence of an adequate supply of oxygen. Oxygen (O2) is one of the most common elements on earth making up 20.9% of our air. Rapid fuel oxidation results in large amounts of heat. Solid or liquid fuels must be changed to a gas before they will burn. Usually heat is required to change liquids or solids into gases. Fuel gases will burn in their normal state if enough air is present. Most of the 79% of air (that is not oxygen) is nitrogen, with traces of other elements. Nitrogen is considered to be a temperature reducing diluter that must be present to obtain the oxygen required for combustion. Nitrogen reduces combustion efficiency by absorbing heat from the combustion of fuels and diluting the flue gases. This reduces the heat available for transfer through the heat exchange surfaces. It also increases the volume of combustion by-products, which then have to travel through the heat exchanger and up the stack faster to allow the introduction of additional fuel-air mixture. This nitrogen also can combine with oxygen (particularly at high flame temperatures) to produce oxides of nitrogen (NOx), which are toxic pollutants. Carbon, hydrogen and sulphur in the fuel combine with oxygen in the air to form carbon dioxide, water vapour and sulphur dioxide, releasing 8,084 kcals, 28,922 kcals and 2,224 kcals of heat respectively. Under certain conditions, carbon may also combine with oxygen to form carbon monoxide, which results in the release of a smaller quantity of heat (2,430 kcals/kg of carbon). Carbon burned to CO2 will produce more heat per unit of fuel than when CO or smoke are produced.


12

C + O2 → CO 2 + 8,084 kcals/kg of Carbon 2C + O2 → 2 CO + 2,430 kcals/kg of Carbon 2H 2 + O2 → 2H2O + 28,922 kcals/kg of Hydrogen S + O2 → SO2 + 2,224 kcals/kg of Sulphur Each kilogram of CO formed means a loss of 5654 kCal of heat (8084 – 2430). 3.1.2 Three T’s of combustion The objective of good combustion is to release all of the heat in the fuel. This is accomplished by controlling the "three T's" of combustion which are (1) Temperature high enough to ignite and maintain ignition of the fuel, (2) Turbulence or intimate mixing of the fuel and oxygen, and (3) Time, sufficient for complete combustion. Commonly used fuels like natural gas and propane generally consist of carbon and hydrogen. Water vapor is a by-product of burning hydrogen. This removes heat from the flue gases, which would otherwise be available for more heat transfer. Natural gas contains more hydrogen and less carbon per kg than fuel oils and as such produces more water vapor. Consequently, more heat will be carried away by exhaust while firing natural gas. Too much, or too little fuel with the available combustion air may potentially result in unburned fuel and carbon monoxide generation. A very specific amount of O2 is needed for perfect combustion and some additional (excess) air is required for ensuring complete combustion. However, too much excess air will result in heat and efficiency losses. Not all of the fuel is converted to heat and absorbed by the steam generation equipment. Usually all of the hydrogen in the fuel is burned and most boiler fuels, allowable with today's air pollution standards, contain little or no sulfur. So the main challenge in combustion efficiency is directed toward unburned carbon (in the ash or incompletely burned gas), which forms CO instead of CO2.

Figure 1. Perfect, good and incomplete combustion

(Bureau of Energy Efficiency, 2004)


13

3.2 Stochiometric Calculation of Air Requirement 3.2.1 Calculation of stochiometric air needed for combustion of furnace oil For combustion air is needed. The amount of air needed can be calculated using the method given below. The first step is to determine the composition of the furnace oil. Typical specifications of furnace oil from lab analysis is given below:

Constituents % By weight Carbon 85.9 Hydrogen 12 Oxygen 0.7 Nitrogen 0.5 Sulphur 0.5 H2O 0.35 Ash 0.05 GCV of fuel 10880 kcal/kg If we take these analysis data, and considering a sample of 100 kg of furnace oil, then the chemical reactions are as follows: Element Molecular Weight (kg / kg mole) C 12 O2 32 H2 2 S 32 N2 28 CO2 44 SO2 64 H2O 18 C + O2 → CO2 H2 + 1/2O2 → H2O S + O2 → SO2 Constituents of fuel C + O2 → CO2 12 + 32 → 44 12 kg of carbon requires 32 kg of oxygen to form 44 kg of carbon dioxide therefore 1 kg of carbon requires 32/12 kg i.e 2.67 kg of oxygen (85.9) C + (85.9 x 2.67) O2 → 315.25 CO2 2H2 + O2 → 2H2O 4 + 32 → 36 4 kg of hydrogen requires 32 kg of oxygen to form 36 kg of water, therefore 1 kg of hydrogen requires 32/4 kg i.e. 8 kg of oxygen.


14

(12) H2 + (12 x 8) O2 → (12 x 9 ) H2O S + O2 → SO2 32 + 32 → 64 32 kg of sulphur requires 32 kg of oxygen to form 64 kg of sulphur dioxide, therefore 1 kg of sulphur requires 32/32 kg i.e. 1 kg of oxygen (0.5) S + (0.5 x 1) O2 → 1.0 SO2 Total oxygen required = 325.57 kg (229.07+96+0.5) Oxygen already present in 100 kg fuel (given) = 0.7 kg Additional oxygen required = 325.57 – 0.7

= 324.87 kg

Therefore quantity of dry air needed = (324.87) / 0.23 (air contains 23% oxygen by weight)

= 1412.45 kg of air

Theoretical air required = (1412.45) / 100 = 14.12 kg of air / kg of fuel

Therefore, in this example, for each kg of furnace oil burnt, 14.12 kg of air is required. 3.2.2 Calculation of theoretical CO2 content in the flue gases It is necessary to also calculate the CO2 content in the flue gases, which then can be used to calculate the excess air in the flue gases. A certain amount of excess air is needed for complete combustion of furnace oils. However, too much excess air points to heat losses and too little excess air points to incomplete combustion. The CO2 in flue gases can be calculated as follows: Nitrogen in flue gas = 1412.45 – 324.87

= 1087.58 kg

Theoretical CO2% in dry flue gas by volume is calculated as below: Moles of CO2 in flue gas = (314.97) / 44 = 7.16 Moles of N2 in flue gas = (1087.58) / 28 = 38.84 Moles of SO2 in flue gas = 1/64 = 0.016 Theoretical CO2 % by Volume = (Moles of CO2 x 100) / Total Moles (Dry) = (7.16 x 100) / (7.16 + 38.84 + 0.016) = 15.5%


15

3.2.3 Calculation of constituents of flue gas with excess air Now we know the theoretical air requirements and the theoretical CO2 content of flue gases. The next step is to measure the actual CO2 percentage in the flue gases. In the calculation below it is assumed that the measured %CO2 in the flue gas is 10%. % Excess air = [(Theoretical CO2%/Actual CO2) – 1] x 100

= [(15.5/10 – 1)] x 100 = 55% Theoretical air required for 100kg of fuel burnt = 1412.45 kg Total quantity of air supply required with 55% excess air = 1412.45 x 1.55 = 2189.30 kg Excess air quantity (actual – theoretical excess air) = 2189.30 – 1412.45 = 776.85

O2 (23%) = 776.85 x 0.23 = 178.68 kg N2 (77%) = 776.85 – 178.68 = 598.17 kg

The final constituents of flue gas with 55% excess air for every 100 kg fuel is as follows:

CO2 = 314.97 kg H2O = 108.00 kg SO2 = 1 kg O2 = 178.68 kg N2 = 1685.75 kg (= 1087.58 in air + 598.17 in excess air)

3.2.4 Calculation of theoretical CO2% in dry flue gas by volume Now that we have the constituents by weight, we can calculate the constituents on a volume basis as follows: Moles of CO2 in flue gas = 314.97 / 44 = 7.16 Moles of SO2 in flue gas = 1/64 = 0.016 Moles of O2 in flue gas = 178.68 / 32 = 5.58 Moles of N2 in flue gas = 1685.75 / 28 = 60.20 Theoretical CO2% by volume = (Moles of CO2 x 100) / Total moles (dry) = (7.16 x 100) / (7.16 + 0.016 + 5.58 + 60.20) = 10% Theoretical O2% by volume = (5.58 x 100) / 72.956

= 7.5%


16

3.3 Concept of Excess Air For optimum combustion, the real amount of combustion air must be greater than that required theoretically. Part of the stack gas consists of pure air, i.e. air that is simply heated to stack gas temperature and leaves the boiler through the stack. Chemical analysis of the gases is an objective method that helps to achieve finer air control. By measuring CO2 or O2 in flue gases (by continuous recording instruments or Orsat apparatus or some cheaper portable instruments) the excess air level and stack losses can be estimated. The excess air to be supplied depends on the type of fuel and the firing system. A faster way to calculate the excess air is by using the figures 2 and 3, provided the percentage of CO2 or O2 in the flue gases have been measured.

Figure 2. Relation between CO2 & Excess Air (Bureau of Energy Efficiency, 2004)

Figure 3. Relationship between residual oxygen and excess air (Bureau of Energy Efficiency, 2004)


17

For optimum combustion of fuel oil the CO2 or O2 in flue gases should be maintained as follows:

CO2 = 14.5–15 % O2 = 2–3 %

3.4 Draft System The function of draft in a combustion system is to exhaust the products of combustion, i.e. flue gases, into the atmosphere. The draft can be classified into two types namely natural draft and mechanical draft. 3.4.1 Natural draft Natural draft is the draft produced by a chimney alone. It is caused by the difference in weight between the column of hot gas inside the chimney and column of outside air of the same height and cross section. Being much lighter than outside air, chimney flue gas tends to rise, and the heavier outside air flows in through the ash pit to take its place. Draft is usually controlled by hand-operated dampers in the chimney and breeching connecting the boiler to the chimney. Here no fans or blowers are used. The products of combustion are discharged at such a height that it will not be a nuisance to the surrounding community. 3.4.2 Mechanical draft It is draft artificially produced by fans. Three basic types of drafts that are applied are: § Balanced draft: Forced-draft (F-D) fan (blower) pushes air into the furnace and an

induced draft (I-D) fan draws gases into the chimney thereby providing draft to remove the gases from the boiler. Here the pressure is maintained between 0.05 to 0.10 in. of water gauge below atmospheric pressure in the case of boilers and slightly positive for reheating and heat treatment furnaces.

§ Induced draft: An induced-draft fan draws enough draft for flow into the furnace, causing the products of combustion to discharge to atmosphere. Here the furnace is kept at a slight negative pressure below the atmospheric pressure so that combustion air flows through the system.

§ Forced draft: The Forced draft system uses a fan to deliver the air to the furnace, forcing combustion products to flow through the unit and up the stack.

4. ENERGY EFFICIENCY OPPORTUNITIES This section includes energy efficiency opportunities in Fuel Combustion 4.1 Pre-heating of the Combustion Oil The viscosity of furnace oil and LSHS (Low Sulphur Heavy Stock) increases with decreasing temperature, which makes it difficult to pump the oil. At low ambient temperatures (below 25 0C), furnace oil cannot be pumped easily. To circumvent this, preheating of oil is accomplished in two ways: § The entire tank may be preheated. In this form of bulk heating, steam coils are placed at

the bottom of the tank, which is fully insulated;


18

§ The oil can be heated as it flows out with an outflow heater. To reduce steam requirements, it is advisable to insulate tanks where bulk heating is used.

Bulk heating may be necessary if flow rates are high enough to make outflow heaters of adequate capacity impractical, or when a fuel such as LSHS is used. In the case of outflow heating, only the oil, which leaves the tank, is heated to the pumping temperature. The outflow heater is essentially a heat exchanger with steam or electricity as the heating medium. 4.2 Temperature control of Combustion Oil Thermostatic temperature control of the oil is necessary to prevent overheating, especially when oil flow is reduced or stopped. This is particularly important for electric heaters, since oil may get carbonized when there is no flow and the heater is on. Thermostats should be provided at a region where the oil flows freely into the suction pipe. The temperature at which oil can readily be pumped depends on the grade of oil being handled. Oil should never be stored at a temperature above that necessary for pumping as this leads to higher energy consumption.


4.3 Combustion Controls

Combustion controls assist the burner in regulation of fuel supply, air supply, (fuel to air ratio), and removal of gases of combustion to achieve optimum boiler efficiency. The amount of fuel supplied to the burner must be in proportion to the steam pressure and the quantity of steam required. The combustion controls are also necessary as safety device to ensure that the boiler operates safely.

Various types of combustion controls in use are: § On/Off control: The simplest control, ON/OFF control means that either the burner is

firing at full rate or it is OFF. This type of control is limited to small boilers. § High/low/off control: Slightly more complex is HIGH/LOW/OFF system where the

burner has two firing rates. The burner operates at slower firing rate and then switches to

full firing as needed. Burners can also revert to the low firing position at reduced load. This control is fitted to medium sized boilers.

§ Modulating control: The modulating control operates on the principle of matching the steam pressure demand by altering the firing rate over the entire operating range of the boiler. Modulating motors use conventional mechanical linkage or electric valves to regulate the primary air, secondary air, and fuel supplied to the burner. Full modulation means that boiler keeps firing, and fuel and air are carefully matched over the whole firing range to maximize thermal efficiency.

20

5. OPTION CHECKLIST This section includes most important options to improve energy efficiency of fuel use and in combustion processes.

Fuel Checklist § Daily check: Oil temperature at the burner and oil/steam leakages § Weekly task: Cleaning of all filters and draining of water from all tanks § Yearly task: Cleaning of all tanks

Troubleshooting for fuels

1. Oil not pumpable • Viscosity too high • Blocked lines and filters • Sludge in oil • Leak in oil suction • Vent pipe choked

2. Blocking of strainers • Sludge or wax in oil • Heavy precipitated compounds in oil • Rust or scale in tank • Carbonization of oil due to excessive heating

3. Excess water in oil • Water delivered along with oil • Leaking manhole • Seepage from underground tank • Ingress of moisture from vent pipe • Leaking heater steam coils

4. Pipeline plugged • Sludge in oil • High viscosity oil • Foreign materials such as rags, scale and wood splinters in line • Carbonization of oil


21

Combustion Checklist 1. Start up

• Check for correct sized burner/nozzle. • Establish air supply first (start blower). Ensure no vapour/gases are present before

light up. • Ensure a flame from a torch or other source is placed in front of the nozzle. • Turn ON the (preheated) oil supply (before start-up, drain off cold oil).

2. Operations

• Check for correct temperature of oil at the burner tip (consult viscosity vs. temperature chart).

• Check air pressure for LAP burners (63.5 cm to 76.2 cm w.c. air pressure is commonly adopted).

• Check for oil drips near burner. • Check for flame fading/flame pulsation. • Check positioning of burner (ensure no flame impingement on refractory walls or

charge). • Adjust flame length to suit the conditions (ensure flame does not extend beyond the

furnace).

3. Load changes • Operate both air and oil valves simultaneously (For self-proportioned burner, operate

the self-proportioning lever. Do not adjust valve only in oil line). • Adjust burners and damper for a light brown (hazy) smoke from chimney and at least

12 percent CO2. 4. Shut down

• Close oil line first. • Shut the blower after a few seconds (ensure gases are purged from combustion

chamber). • Do not expose the burner nozzle to the radiant heat of the furnace. (When oil is shut

off, remove burner/nozzle or interpose a thin refractory between nozzle and furnace). Troubleshooting for combustion The checklist in the Table below can help find the causes and solutions for typical problems found with fuel combustion.


22

TROUBLESHOOTING CHART FOR COMBUSTION

No Problems Causes & solutions 1. Starting difficult 1. No oil in the tank.

2. Excess sludge and water in storage tanks. 3. Oil not flowing due to high viscosity/low temperature. 4. Choked burner tip. 5. No air. 6. Strainers choked.

2. Flame goes out or splutters

1. Sludge or water in oil. 2. Unsteady oil and air pressures. 3. Too high a pressure for atomizing medium which tends to blow

out flame. 4. Presence of air in oil line. Look for leakages in suction line of

pump. 5. Broken burner block, or burner without block.

3. Flame flashes back 1. Oil supply left in ‘ON’ position after air supply cut off during earlier shut off.

2. Too high a positive pressure in combustion chamber. 3. Furnace too cold during starting to complete combustion (when

temperature rises, unburned oil particles burn). 4. Oil pressure too low.

4 Smoke and soot 1. Insufficient draft or blower of inadequate 2. Oil flow excessive. 3. Oil too heavy and not preheated to 4. Suction air holes in blower plugged. 5. Chimney clogged with soot/damper 6. Blower operating speed too low.

5. Clinker on refractory 1. Flame hits refractory because combustion chamber is too small or

2. is not correctly aligned. 3. Oil dripping from nozzle. 4. Oil supply not ’cut off’ before the air supply during shut-offs.

6. Cooking of fuel in burner

1. Nozzle exposed to furnace radiation after shut- 2. Burner fed with atomizing air over 300 °C. 3. Burner block too short or too wide. 4. Oil not drained from nozzle after shut off.

7. Excessive fuel oil consumption

1. Improper ratio of oil and air. 2. Burner nozzle oversized. 3. Excessive draft. 4. Improper oil/air mixing by burner. 5. Air and oil pressure not correct 6. Oil not preheated properly. 7. Oil viscosity too low for the type of burner used. 8. Oil leaks in oil pipelines/preheater. 9. Bad maintenance (too high or rising stack gas temperature).


23

6. WORKSHEETS Worksheet 1: Excess Air Calculation

No Parameters Formula Units Value 1 Carbon (C) % by Weight 2 Hydrogen (H) % by Weight 3 Oxygen (O4) % by Weight 4 Nitrogen % by Weight 5 Sulphur % by Weight 6 H2O % by Weight 7 Ash % by Weight 8 GCV of Fuel kCal/kg 9 Oxygen Required for

burning of Carbon (O1) C x (32/12) kg/100 kg of Fuel

10 Oxygen Required for burning of Hydrogen (O2)

H x (32/4) kg/100 kg of Fuel

11 Oxygen Required for burning of Sulphur (O3)

S x (32/32) kg/100 kg of Fuel

12 Total Oxygen Required (O)

O1 + O2 + O3 – O4 kg/100 kg of Fuel

13 Stochiometric Amount of Air Required (S.A)

O / 0.23 kg/100 kg of Fuel

14 Excess Air (EA) % 15 Actual Amount of Air

Required S.A x (1+ EA/100) kg/100 kg of Fuel


Electrical Energy Equipment: Fans and Blowers

1

FANS AND BLOWERS

1. INTRODUCTION................................................................................ 1 2. TYPE OF FANS AND BLOWERS..................................................... 6 3. ASSESSMENT OF FANS AND BLOWERS................................... 10 4. ENERGY EFFICIENCY OPPORTUNITIES ................................. 13 5. OPTION CHECKLIST ..................................................................... 18 6. WORKSHEETS ................................................................................. 19 7. REFERENCES ................................................................................... 21 1. INTRODUCTION This section describes the main features of fans and blowers. 1.1 What are fans and blowers? Most manufacturing plants use fans and blowers for ventilation and for industrial processes that need an air flow. Fan systems are essential to keep manufacturing processes working, and consist of a fan, an electric motor, a drive system, ducts or piping, flow control devices, and air conditioning equipment (filters, cooling coils, heat exchangers, etc.). An example system is illustrated in Figure 1. The US Department of Energy estimates that 15 percent of electricity in the US manufacturing industry is used by motors. Similarly, in the commercial sector, electricity needed to operate fan motors composes a large portion of the energy costs for space conditioning (US DOE, 1989). Fans, blowers and compressors are differentiated by the method used to move the air, and by the system pressure they must operate against. The American Society of Mechanical Engineers (ASME) uses the specific ratio, which is the ratio of the discharge pressure over the suction pressure, to define fans, blowers and compressors (see Table 1). Table 1: Difference between Fans, Blowers and Compressors (Ganasean) Equipment Specific Ratio Pressure rise (mmWg) Fans up to 1.11 1136 Blowers 1.11 to 1.20 1136 –2066 Compressors more than 1.20 -

Chapter-3


2

Figure 1: Typical Fan System Components (US DOE, 1989)

1.2 Important terms and definitions Before types of fans and blowers are described it is important to first understand terms and definitions.1 1.2.1 System characteristics The term “system resistance” is used when referring to the static pressure. The system resistance is the sum of static pressure losses in the system. The system resistance is a function of the configuration of ducts, pickups, elbows and the pressure drops across equipment, for example bag filter or cyclone. The system resistance varies with the square of the volume of air flowing through the system. For a given volume of air, the fan in a system with narrow ducts and multiple short radius elbows is going to have to work harder to overcome a greater system resistance than it would in a system with larger ducts and a minimum number of long radius turns. Long narrow ducts with many bends and twists will require more energy to pull the air through them. Consequently, for a given fan speed, the fan will be able to pull less air through this system than through a short system with no elbows. Thus, the system resistance increases substantially as the volume of air flowing through the system increases; square of air flow. Conversely, resistance decreases as flow decreases. To determine what volume the fan will produce, it is therefore necessary to know the system resistance characteristics. In existing systems, the system resistance can be measured. In systems that have been designed, but not built, the system resistance must be calculated. Typically a system resistance curve (see

.


3

Figure 2) is generated with for various flow rates on the x-axis and the associated resistance on the y-axis.

Figure 2. System Curve of a Fan and Effect of System Resistance (US DOE, 1989)

1.2.2 Fan characteristics Fan characteristics can be represented in form of fan curve(s). The fan curve is a performance curve for the particular fan under a specific set of conditions. The fan curve is a graphical representation of a number of inter-related parameters. Typically a curve will be developed for a given set of conditions usually including: fan volume, system static pressure, fan speed, and brake horsepower required to drive the fan under the stated conditions. Some fan curves will also include an efficiency curve so that a system designer will know where on that curve the fan will be operating under the chosen conditions (see Figure 3). Of the many curves shown in the figure, the curve static pressure (SP) versus flow is especially important. The intersection of the system curve and the static pressure curve defines the operating point. When the system resistance changes, the operating point also changes. Once the operating point is fixed, the power required can be determined by following a vertical line that passes through the operating point to an intersection with the power (BHP) curve. A horizontal line drawn through the intersection with the power curve will lead to the required power on the right vertical axis. In the depicted curves, the fan efficiency curve is also presented.


4

Figure 3. Typical Fan Efficiency Curve (BEE India, 2004) 1.2.3 System characteristics and fan curves In any fan system, the resistance to air flow (pressure) increases when the flow of air is increased. As mentioned before, it varies as the square of the flow. The pressure required by a system over a range of flows can be determined and a "system performance curve" can be developed (shown as SC) (see Figure 4). This system curve can then be plotted on the fan curve to show the fan's actual operating point at "A" where the two curves (N1 and SC1) intersect. This operating point is at air flow Q1

delivered against pressure P1. A fan operates at a performance given by the manufacturer for a particular fan speed. (The fan performance chart shows performance curves for a series of fan speeds.) At fan speed N1, the fan will operate along the N1

performance curve as shown in Figure 4. The fan's actual operating point on this curve will depend on the system resistance; fan’s operating point at “A” is flow (Q1) against pressure (P1). Two methods can be used to reduce air flow from Q1 to Q2: The first method is to restrict the air flow by partially closing a damper in the system.

This action causes a new system performance curve (SC2) where the required pressure is greater for any given air flow. The fan will now operate at "B" to provide the reduced air flow Q2

against higher pressure P2. The second method to reduce air flow is by reducing the speed from N1 to N2, keeping

the damper fully open. The fan would operate at "C" to provide the same Q2 air flow, but

at a lower pressure P3. Thus, reducing the fan speed is a much more efficient method to decrease airflow since less power is required and less energy is consumed.


5

Figure 4. Fan performance curve (BEE India, 2004) 1.2.4 Fan laws The fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (revolutions per minute or RPM) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM. This is shown in Figure 5.

Figure 5. Speed, pressure and power of fans (BEE India, 2004)


6

2. Type of fans and blowers This section briefly describes different types of fans and blowers. 2.1 Types of fans There exist two main fan types. Centrifugal fans used a rotating impeller to move the air stream. Axial fans move the air stream along the axis of the fan. 2.1.1 Centrifugal fans Centrifugal fans (Figure 6) increase the speed of an air stream with a rotating impeller. The speed increases as the reaches the ends of the blades and is then converted to pressure. These fans are able to produce high pressures, which makes them suitable for harsh operating conditions, such as systems with high temperatures, moist or dirty air streams, and material handling. Centrifugal fans are categorized by their blade shapes as summarized in Table 2. Table 2. Characteristics of Different Centrifugal Fans (adapted from US DOE, 1989) Type of fan and blade

Advantages Disadvantages

Radial fans, with flat blades (Figure 7)

Suitable for high static pressures (up to 1400 mmWC) and high temperatures

Simple design allows custom build units for special applications

Can operate at low air flows without vibration problems

High durability Efficiencies up to 75% Have large running clearances, which is

useful for airborne-solids (dust, wood chips and metal scraps) handling services

Only suitable for low-medium airflow rates

Forward curved fans, with forward curved blades (Figure 8)

Can move large air volumes against relatively low pressure

Relative small size Low noise level (due to low speed) and

well suited for residential heating, ventilation, and air conditioning (HVAC) applications

Only suitable for clean service applications but not for high pressure and harsh services

Fan output is difficult to adjust accurately

Driver must be selected carefully to avoid motor overload because power curve increases steadily with airflow

Relatively low energy efficiency (55-65%)

Backward inclined fan, with blades that tilt away from the direction of rotation: flat, curved, and airfoil (Figure 9)

Can operate with changing static pressure (as this does not overload the motor)

Suitable when system behavior at high air flow is uncertain

Suitable for forced-draft services Flat bladed fans are more robust Curved blades fans are more efficient

(exceeding 85%) Thin air-foil blades fans are most

efficient

Not suitable for dirty air streams (as fan shape promotes accumulation of dust)

Airfoil blades fans are less stable because of staff as they rely on the lift created by each blade

Thin airfoil blades fans subject to erosion


7

Figure 6. Centrifugal Fan

(FanAir Company)

Figure 7. Radia

l Blade

Centrifugal Fan (Canadian Blower)

Figure 8. Forward-Curved Fan

(Canadian Blower)

Figure 9. Backward Inclined Fan (Canadian Blower)

2.1.2 Axial fans Axial fans (Figure 10) move an air stream along the axis of the fan. The way these fans work can be compared to a propeller on an airplane: the fan blades generate an aerodynamic lift that pressurizes the air. They are popular with industry because they are inexpensive, compact and light. The main types of axial flow fans (propeller, tube-axial and vane-axial) are summarized in Table 3. Table 3. Characteristics of Different Axial Fans (adapted from US DOE, 1989) Type of fan Advantages Disadvantages Propeller fan (Figure 11)

Generate high airflow rates at low pressures Not combined with extensive ductwork (because the

generate little pressure) Inexpensive because of their simple construction Achieve maximum efficiency, near-free delivery, and are

often used in rooftop ventilation applications Can generate flow in reverse direction, which is helpful in

ventilation applications

Relative low energy efficiency

Comparatively noisy

Tube-axial fan, essentially a propeller fan

Higher pressures and better operating efficiencies than propeller fans

Suited for medium-pressure, high airflow rate

Relatively expensive

Moderate


8

Type of fan Advantages Disadvantages placed inside a cylinder (Figure 12)

applications, e.g. ducted HVAC installations Can quickly accelerate to rated speed (because of their

low rotating mass) and generate flow in reverse direction, which is useful in many ventilation applications

Create sufficient pressure to overcome duct losses and are relatively space efficient, which is useful for exhaust applications

airflow noise Relatively low

energy efficiency (65%)

Vane-axial fan (Figure 13)

Suited for medium- to high-pressure applications (up to 500 mmWC), such as induced draft service for a boiler exhaust

Can quickly accelerate to rated speech (because of their low rotating mass) and generate flow in reverse directions, which is useful in many ventilation applications

Suited for direct connection to motor shafts Most energy efficient (up to 85% if equipped with airfoil

fans and small clearances)

Relatively expensive compared to propeller fans

Figure 10. Axial Fan (NISCO)

Figure 11. Propeller Fans (FanAir Company)

Figure 12. Tube Axial Fan

(NISCO)

Figure 13. Vane-axial Fan

(NISCO)


9

2.2 Types of blowers Blowers can achieve much higher pressures than fans, as high as 1.20 kg/cm2. They are also used to produce negative pressures for industrial vacuum systems. The centrifugal blower and the positive displacement blower are two main types of blowers, which are described below.2 2.2.1 Centrifugal blowers Centrifugal blowers look more like centrifugal pumps than fans. The impeller is typically gear-driven and rotates as fast as 15,000 rpm. In multi-stage blowers, air is accelerated as it passes through each impeller. In single-stage blower, air does not take many turns, and hence it is more efficient. Centrifugal blowers typically operate against pressures of 0.35 to 0.70 kg/cm2, but can achieve higher pressures. One characteristic is that airflow tends to drop drastically as system pressure increases, which can be a disadvantage in material conveying systems that depend on a steady air volume. Because of this, they are most often used in applications that are not prone to clogging.

Figure 14. Centrifugal Blower (FanAir Company)

2.2.2 Positive-displacement blowers Positive displacement blowers have rotors, which "trap" air and push it through housing. These blowers provide a constant volume of air even if the system pressure varies. They are especially suitable for applications prone to clogging, since they can produce enough pressure (typically up to 1.25 kg/cm2) to blow clogged materials free. They turn much slower than centrifugal blowers (e.g. 3,600 rpm) and are often belt driven to facilitate speed changes.


10

3. ASSESSMENT OF FANS AND BLOWERS This section describes how to evaluate the performance of fans, but it is also applicable to blowers.3 3.1 What is fan efficiency / performance? Fan efficiency is the ratio between the power transferred to the air stream and the power delivered by the motor to the fan. The power of the airflow is the product of the pressure and the flow, corrected for unit consistency. Another term for efficiency that is often used with fans is static efficiency, which uses static pressure instead of total pressure in estimating the efficiency. When evaluating fan performance, it is important to know which efficiency term is being used. The fan efficiency depends on the type of fan and impeller. As the flow rate increases, the efficiency increases to certain height (“peak efficiency”) and then decreases with further increasing flow rate (see Figure 15). The peak efficiency ranges for different types of centrifugal and axial fans are given in Table 2.

Figure 14. Efficiency versus Flowrate (BEE India, 2004)

Table 4. Efficiency of Various Fans (BEE India, 2004)

Fan performance is typically estimated by using a graph that shows the different pressures developed by the fan and the corresponding required power. The manufacturers normally provide these fan performance curves. Understanding this relationship is essential to designing, sourcing, and operating a fan system and is the key to optimum fan selection.


11

3.2 Methodology of fan performance assessment Before the fan efficiency can be calculated, a number of operating parameters must be measured, including air velocity, pressure head, temperature of air stream on the fan side and electrical motor kW input. In order to obtain correct operating figures it should be ensured that: Fan and its associated components are operating properly at its rated speed Operations are at stable condition i.e. steady temperature, densities, system resistance etc.

The calculation of fan efficiency is explained in 5 steps. Step 1: calculate the gas density The first step is to calculate the air or gas density using the following equation: Gas density (y) = 273 X 1.293 273 + t oC

Where, t oC = Temperature of air or gas at site condition Step 2: measure the air velocity and calculate average air velocity The air velocity can be measured with a pitot tube and a manometer, or a flow sensor (differential pressure instrument), or an accurate anemometer. Figure 15 shows how the velocity pressure is measured using a pitot tube and a manometer. The total pressure is measured using the inner tube of pitot tube and static pressure is measured using the outer tube of pitot tube. When the inner and outer tube ends are connected to a manometer, we get the velocity pressure (i.e. the difference between total pressure and static pressure). For measuring low velocities, it is preferable to use an inclined tube manometer instead of U-tube manometer. See the chapter on Monitoring Equipment for a explanation of manometers.

Figure 16. Velocity Pressure Measurement using Pilot Tube

(BEE India, 2004)


12

Calculate the average air velocity by taking number of velocity pressure readings across the cross-section of the duct using the following equation (note: do not average the velocity pressure, but average the velocities!):

Where:

Cp = Pitot tube constant, 0.85 (or) as given by the manufacturer

∆p = Average differential pressure measured by pitot tube by taking measurement at number of points over the entire cross section of the duct.

γ = Density of air or gas at test condition

Step 3: calculate the volumetric flow The third step is to calculate the volumetric flow as follows: Take the duct diameter (or the circumference from which the diameter can be estimated). Calculate the volume of air/gas in the duct by following relation

Step 4: measure the power of the drive motor The power of the drive motor (kW) can be measured by a load analyzer. This kW multiplied by motor efficiency gives the shaft power to the fan.

Step 5: calculate the fan efficiency Now the fan’s mechanical and static efficiencies can be calculated as follows: a). Mechanical efficiency:

b) Static efficiency, which is the same except that the outlet velocity pressure is not added to the fan static pressure:


13

3.3 Difficulties in assessing the performance of fans and blowers In practice certain difficulties have to be faced when assessing the fan and blower performance, some of which are explained below: Non-availability of fan specification data: Fan specification data (see Worksheet 1) are

essential to assess the fan performance. Most of the industries do not keep these data systematically or have none of these data available at all. In these cases, the percentage of fan loading with respect to flow or pressure can not be estimated satisfactorily. Fan specification data should be collected from the original equipment manufacturer (OEM) and kept on record.

Difficulty in velocity measurement: Actual velocity measurement becomes a difficult task in fan performance assessment. In most cases the location of duct makes it difficult to take measurements and in other cases it becomes impossible to traverse the duct in both directions. If this is the case, then the velocity pressure can be measured in the center of the duct and corrected by multiplying it with a factor 0.9.

Improper calibration of the pitot tube, manometer, anemometer & measuring instruments: All instruments and other power measuring instruments should be calibrated correctly to avoid an incorrect assessment of fans and blowers. Assessments should not be carried out by applying correction factors to compensate for this.

Variation of process parameters during tests: If there is a large variation of process parameters measured during test periods, then the performance assessment becomes unreliable.

4. ENERGY EFFICIENCY OPPORTUNITIES This section describes the most important energy efficiency opportunities for fans and blowers. 4.1 Choose the right fan Important considerations when selecting a fan are (US DOE, 1989): Noise Rotational speed Air stream characteristics Temperature range Variations in operating conditions Space constraints and system layout Purchase costs, operating costs (determined by efficiency and maintenance), and

operating life But as a general rule it is important to know that to effectively improve the performance of fan systems, designers and operators must understand how other system components function as well. The “systems approach” requires knowing the interaction between fans, the equipment that supports fan operation, and the components that are served by fans. The use of a “systems approach” in the fan selection process will result in a quieter, more efficient, and more reliable system.


14

A common problem is that companies purchase oversized fans for their service requirements. They will not operate at their best efficiency point (BEP) and in extreme cases these fans may operate in an unstable manner because of the point of operation on the fan airflow-pressure curve. Oversized fans generate excess flow energy, resulting in high airflow noise and increased stress on the fan and the system. Consequently, oversized fans not only cost more to purchase and to operate, they create avoidable system performance problems. Possible solutions include, amongst other replacing the fan, replacing the motor, or introducing a variable speed drive motor. 4.2 Reduce the system resistance The system resistance curve and the fan curve were explained in section 1.2. The fan operates at a point where the system resistance curve and the fan curve intersects. The system resistance has a major role in determining the performance and efficiency of a fan. The system resistance also changes depending on the process. For example, the formation of the coatings / erosion of the lining in the ducts, changes the system resistance marginally. In some cases, the change of equipment, duct modifications, drastically shift the operating point, resulting in lower efficiency (See Figure 2). In such cases, to maintain the efficiency as before, the fan has to be changed. Hence, the system resistance has to be periodically checked, more so when modifications are introduced and action taken accordingly, for efficient operation of the fan. 4.3 Operate close to BEP It is earlier described that the fan efficiency increases as the flow increases to certain point and thereafter it decreases. The point at which maximum efficiency is obtained is called the peak efficiency or “Best Efficiency Point” (BEP). Normally it is closer to the rated capacity of the fan at a particular designed speed and system resistance. Deviation from the BEP will result in increased loss and inefficiency. 4.4 Maintain fans regularly Regular maintenance of fans is important to maintain their performance levels. Maintenance activities include (US DOE, 1989): Periodic inspection of all system components Bearing lubrication and replacement Belt tightening and replacement Motor repair or replacement Fan cleaning

4.5 Control the fan air flow Normally, an installed fan operates at a constant speed. But some situations may require a speed change, for example more airflow may be needed from the fan when a new run of duct is added, or less air flow may be needed if the fan is oversized. There are several ways to


15

reduce or control the airflow of fans. These are summarized in Table 5 and a comparison of full load power against percentage full flow by different flow control is given in Figure 17.

Figure 17. Relative Power Consumption among Flow Control Options (US DOE, 1989)

Figure 18. Pulley Dimension Change (BEE India, 2004)


16

Table 5. Comparison of Different Ways to Control Fan Flow (adapted from US DOE, 1989, and BEE, 2004) Type of flow control Advantages Disadvantages Pulley change: reduces the motor / drive pulley size

Permanent speed decrease Real energy reduction (see Figure 18:

a 2 inch reduction in pulley results in 12 kW savings)

Fan must be able to handle capacity change

Fan must be driven by V-belt system or motor

Dampers: reduce the amount of flow and increases the upstream pressure, which reduces fan output

Inexpensive Easy to install

Provide a limited amount of adjustment

Reduce the flow but not the energy consumption

Higher operating and maintenance costs

Inlet guide vanes: create swirls in the fan direction thereby lessening the angle between incoming air and fan blades, and thus lowering fan load, pressure and airflow

Improve fan efficiency because both fan load and delivered airflow are reduced

Cost effective at airflows between 80-100% of full flow

Less efficient at airflows lower than 80% of full flow

Variable pitch fans: change the angle between incoming airflow and the blade by tilting the fan blades, thereby reducing both the motor load and airflow

Can keep fan efficiency high over a range of operating conditions.

Avoid resonance problems as normal operating speed is maintained

Can operate from a no-flow to a full-flow condition without stall problems

Applicable to some axial fan types only

Fouling problems if contaminants accumulate in the mechanical actuator that controls the blades

Operating at low loads for long periods reduces the power factor and motor efficiency, thus loosing efficiency advantages and risking low power factor charge from the utility

Variable Speed Drive (VSD): reducing the speed of motor of the fan to meet reduced flow requirements Mechanical VSDs:

hydraulic clutches, fluid couplings, and adjustable belts and pulleys

Electrical VSDs: eddy current clutches, wound-rotor motor controllers, and variable frequency drives (VFDs: change motor’s rotational speed by adjusting electrical frequency of power supplied)

Most improved and efficient flow control

Allow fan speed adjustments over a continuous range

For VFDs specifically: Effective and easy flow control Improve fan operating efficiency over

a wide range of operating conditions Can be retrofitted to existing motors

Compactness No fouling problems Reduce energy losses and costs by

lowering overall system flow

Mechanical VSDs have fouling problems

Investment costs can be a barrier


17

Type of flow control Advantages Disadvantages Multiple speed pump Efficient control of flow

Suitable if only two fixed speeds are required

Need to jump from speed to speed

Investment costs can be a barrier

Disc throttle: a sliding throttle that changes the width of the impeller that is exposed to the air stream

Simple design Feasible in some applications only

Operate fans in parallel: two or more fans in parallel instead of one large one

High efficiencies across wide variations in system demand

Redundancy to mitigate the risk of downtime because of failure or unexpected maintenance

Two smaller fans are less expensive and offer better performance than one relatively large one

Can be equipped with other flow controls to increase flexibility and reliability

Should only be used when the fans can operate in a low resistance almost in a free delivery condition (see Figure 19)

Operate fans in series: using multiple fans in a push-pull arrangement

Lower average duct pressure Lower noise generation Lower structural and electrical

support requirements Suited for systems with long ducts,

large pressure drops across system components, or high resistances

Not suited for low resistance systems (see Figure 19)

Figure 19. Fans Operating in Series and in Parallel (BEE India, 2004)


18

5. OPTION CHECKLIST This section lists the most important energy efficiency options. Use smooth, well-rounded air inlet cones for fan air intake Avoid poor flow distribution at the fan inlet Minimize fan inlet and outlet obstructions Clean screens, filters and fan blades regularly Minimize fan speed Use low slip or flat belts for power transmission Check belt tension regularly Eliminate variable pitch pulleys Use variable speed drives for large variable fan loads Use energy-efficient motors for continuous or near continuous operation Eliminate leaks in duct works Minimize bends in duct works Turn fans and blowers off when not needed Reduce the fan speed by pulley diameter modifications incase of oversized motors Adopt inlet guide vanes in place of discharge damper control Change metallic / Glass reinforced plastic (GRP) impeller by more energy efficient

hollow FRP impeller with aerofoil design Try to operate the fan near its best operating point (BEP) Reduce transmission losses by using energy efficient flat belts or cogged raw-edged V-

belts instead of conventional V-belt systems Minimizing system resistance and pressure drops by improving the duct system Ensure proper alignment between drive and driven system Ensure proper power supply quality to the motor drive Regularly check for vibration trend to predict any incipient failures like bearing damage,

misalignments, unbalance, foundation looseness etc.


19

6. WORKSHEETS This section includes following worksheets: Fans and Blowers Specification Data Fans and Blowers Efficiency Calculation

Worksheet #1: FANS AND BLOWERS SPECIFICATION DATA

No. Parameter Units Fan/Blower number 1 2 3 1 Make 2 Type

(Axial/Centrifugal)

3 Discharge Flow m3/hr 4 Head Developed mmWC 5 Fluid Handlled 6 Density of Fluid kg/m3 7 Dust Concentration kg/m3 8 Temperature of Fluid 0C 9 Flow Control Type 10 Flow Control Range % 11 Fan Input Power kW 12 Fan Speed RPM 13 Fan Rated Efficiency % 14 Specific Power

Consumption kW/(m3/hr)

15 Fan Motor Rated Power kW Full Load Current Amp Rated Speed RPM Supply Voltage Volts Rated Efficiency % Rated Power Factor Supply Frequency Hz 16 Bearing Type Fan (Driving End) Fan (Non-Driving End) Motor (Driving End) Motor (Non-Driving

End)

17 Lubricant Grade


20

Worksheet 2: FANS AND BLOWERS EFFICIENCY CALCULATION

No. Parameter Units Fan/Blower reference 1 2 3 1 Fluid (medium) flow (Q)

(measured using pitot tube at fan discharge)

m3/sec

2 For suction pressure (measured at fan inlet using U-tube manometer)

mmWC

3 For discharge pressure (measured at fan discharge using U-tube manometer)

mmWC

4 Total Static Pressure (∆P) [3–4] mmWC 5 Total Differential Pressure (dP)

(measured by pitot tube by taking measurement at number of points over the duct cross section)

mmWC

6 Pitot tube constant (Cp) 7 Duct Cross sectional Area (A) m2 8 Temperature of fluid medium

(measured at fan inlet using a thermometer)

0C

9 Density of fluid medium handled (r) (taken from standard data and corrected to operating temperature/pressure conditions)

kg/m3

10 Motor input power (P) (measured at motor terminals or switchgear using panel or portable energy meter/power analyzer)

kW

11 Power input to shaft (P1) (P x motor efficiency X transmission efficiency)

%

12 Supply frequency Hz 13 Pump input power kW 14 Air/Gas velocity (V)

[= (Cpx√(2x9.81xdPxr)]/r m/sec

15 Flow rate (Q) (= V x A) m3/sec 16 Fan mechanical efficiency (ηF)

(Qx∆P)/(102xP1) x 100 %

17 Specific Power Consumption (P/Q) kW/(m3/sec) 18 % Motor loading with respect to

power %

19 % Fan loading with respect to flow % 20 % Fan loading with respect to total

static pressure %

1

WASTE HEAT RECOVERY

1. INTRODUCTION.................................................................................... 1

2. TYPES OF WASTE HEAT RECOVERY EQUIPMENT....................... 1

3. ASSESSMENT OF WASTE HEAT RECOVERY................................ 12

4. ENERGY EFFICIENCY OPPORTUNITIES ....................................... 15

5. OPTION CHECKLIST.......................................................................... 15

6. WORKSHEETS..................................................................................... 16

7. REFERENCES ...................................................................................... 18 1. INTRODUCTION This section briefly describes the main features of waste heat recovery. Waste heat is heat generated in a process by way of fuel combustion or chemical reaction, which is then “dumped” into the environment and not reused for useful and economic purposes. The essential fact is not the amount of heat, but rather its “value”. The mechanism to recover the unused heat depends on the temperature of the waste heat gases and the economics involved. Large quantities of hot flue gases are generated from boilers, kilns, ovens and furnaces. If some of the waste heat could be recovered then a considerable amount of primary fuel could be saved. The energy lost in waste gases cannot be fully recovered. However, much of the heat could be recovered and adopting the following measures as outlined in this chapter can minimize losses. 2. TYPES OF WASTE HEAT RECOVERY EQUIPMENT . This section describes the various commercial equipment that can be used to recover waste heat and for other applications and uses. 2.1 Recuperators In a recuperator, heat exchange takes place between the flue gases and the air through metallic or ceramic walls. Ducts or tubes carry the air for combustion to be preheated, the other side contains the waste heat stream. A recuperator for recovering waste heat from flue gases is shown in Figure 1.

Figure 1. Waste Heat Recovery using Recuperator (SEAV, 2004)

Waste Heat RecoveryChapter-4

2

2.1.1 Metallic radiation recuperator The simplest configuration for a recuperator is the metallic radiation recuperator, which consists of two concentric lengths of metal tubing as shown in Figure 2.

The inner tube carries the hot exhaust gases while the external annulus carries the combustion air from the atmosphere to the air inlets of the furnace burners. The hot gases are cooled by the incoming combustion air, which now carries additional energy into the combustion chamber. This is the energy, which does not have to be supplied by the fuel; consequently, less fuel is burned for a given furnace loading. The saving in fuel also means a decrease in combustion air and therefore, stack losses are decreased not only by lowering the stack gas temperatures but also by discharging smaller quantities of exhaust gas. The radiation recuperator gets its name from the fact that a substantial portion of the heat transfer from the hot gases to the surface of the inner tube takes place by radiative heat transfer. The cold air in the annuals, however, is almost transparent to infrared radiation so that only convection heat transfer takes place to the incoming air. As shown in the diagram, the two gas flows are usually parallel, although the configuration would be simpler and the heat transfer would be more efficient if the flows were opposed in direction (or counterflow). The reason for the use of parallel flow is that recuperators frequently serve the additional function of cooling the duct carrying away the exhaust gases and consequently extending its service life.

2.1.2 Convective recuperator A second common configuration for recuperators is called the tube type or convective recuperator. As seen in the figure below, the hot gases are carried through a number of parallel small diameter tubes, while the incoming air to be heated enters a shell surrounding the tubes and passes over the hot tubes one or more times in the direction normal to their axes. If the tubes are baffled to allow the gas to pass over them twice, the heat exchanger is termed a two-pass recuperator; if two baffles are used, a three-pass recuperator, etc. Although baffling increases both the cost of

Figure 2. Metallic Radiation Recuperator (Hardtech Group)

Figure 3. Convective Recuperator (Reay, D.A., 1996)

Waste Heat Recovery

3

the exchanger and the pressure drop in the combustion air path, it increases the effectiveness of heat exchange. Shell and tube type recuperators are generally more compact and have a higher effectiveness than radiation recuperators, because of the larger heat transfer area made possible through the use of multiple tubes and multiple passes of the gases. 2.1.3 Hybrid recuperator

For maximum effectiveness of heat transfer, hybrid recuperators are used. These are combinations of radiation and convective designs, with a high-temperature radiation section followed a by convective section (see Figure 4). These are more expensive than simple metallic radiation recuperators, but are less bulky. 2.1.4 Ceramic recuperator The principal limitation on the heat recovery of metal recuperators is the reduced life of the liner at inlet temperatures exceeding 11000C. In order to overcome the temperature limitations of metal

recuperators, ceramic tube recuperators have been developed whose materials allow operation on the gas side to be at 1550 0C and on the preheated air side to be 815 0C on a

more or less practical basis. Early ceramic recuperators were built of tile and joined with furnace cement, and thermal cycling caused cracking of joints and rapid deterioration of the tubes. Later developments introduced various kinds of short silicon carbide tubes, which can be joined by flexible seals located in the air headers. Earlier designs had experienced leakage rates from 8 to 60 per cent. The new designs are reported to last two years with air preheat temperatures as high as 7000C, with much lower leakage rates. 2.2 Regenerators Regenerators are suitable for large capacities and have been widely used in glass and steel melting furnaces. Important relations exist

Figure 4. Hybrid Recuperator (Reay, D.A., 1996)

Figure 5. Regenerator (Department of Coal, India, 1985)

Waste Heat Recovery

4

between the sizes of the regenerator, time between reversals, thickness of brick, conductivity of brick and heat storage ratio of the brick. In a regenerator, the time between the reversals is an important aspect. Long periods would mean higher thermal storage and hence higher cost. Also long periods of reversal result in lower average temperature of preheat and consequently reduction in the fuel economy. Accumulation of dust and slagging on the surfaces reduce efficiency of the heat transfer as the furnace becomes old. Heat losses from the walls of the regenerator and air in- leaks during the gas period and out- leaks during the air period also reduces the heat transfer. 2.3 Heat Wheels A heat wheel is finding increasing applications in low to medium temperature waste heat recovery systems.

It is a sizable porous disk, fabricated with material having a fairly high heat capacity, which rotates between two side-by-side ducts: one is a cold gas duct, the other a hot gas duct. The axis of the disk is located parallel and on the partition between the two ducts. As the disk slowly rotates, sensible heat (moisture that contains latent heat) is transferred to the disk by the hot air and, as the disk rotates, from the disk to the cold air. The overall efficiency of sensible heat transfer for this kind of regenerator can be as high as 85 per cent. Heat wheels have been built as large as 21 meters in diameter with air capacities up to 1130 m3 / min.

Figure 6. Heat Wheel (SADC, 1999)

Waste Heat Recovery

5

A variation of the heat wheel is the rotary regenerator where the matrix is in a cylinder rotating across the waste gas and air streams. The heat or energy recovery wheel is a rotary gas heat regenerator, which can transfer heat from exhaust to incoming gases. Its main area of application is where heat is exchanged between large masses of air having small temperature differences. Heating and ventilation systems and recovery of heat from dryer exhaust air are typical applications. 2.4 Heat Pipe 2.4.1 Description A heat pipe can transfer up to 100 times more thermal energy than copper, the best-known conductor. In other words, heat pipe is a thermal energy absorbing and transferring system having no moving parts and hence requires minimal maintenance. The heat pipe comprises of three elements – a sealed container, a capillary wick structure and a working fluid. The capillary wick structure is integrally fabricated into the interior surface of the container tube and sealed under vacuum. Thermal energy applied to the external surface of the heat pipe is in equilibrium with its own vapour as the container tube is sealed under vacuum. Thermal energy applied to the external surface of the heat pipe causes the working fluid near the surface to evaporate instantaneously. Vapour thus formed absorbs the latent heat of vaporization and this part of the heat pipe becomes an evaporator region. The vapour then travels to the other end the pipe where the thermal energy is removed causing the vapour to condense into liquid again, thereby giving up the latent heat of the condensation. This part of the heat pipe works as the condenser region. The condensed liquid then flows back to the evaporated region. Figure 7 shows the heat pipe.

Figure 7. Heat Pipe (SADC, 1999)

Waste Heat Recovery

6

2.4.2 Performance and advantages The heat pipe exchanger (HPHE) is a lightweight compact heat recovery system. It virtually does not need mechanical maintenance, as there are no moving parts that wear out. It does not need input power for its operation and is free from cooling water and lubrication systems. It also lowers the fan horsepower requirement and increases the overall thermal efficiency of the system. The heat pipe heat recovery systems are capable of operating at 315oC with 60% to 80% heat recovery capability. 2.4.3 Typical applications Heat pipes are used in the following industrial applications: § Process of Space Heating: The heat pipe heat exchanger transfers the thermal energy from

process exhaust for building heating. The preheated air can be blended if required. The requirement of additional heating equipment to deliver heated make up air is drastically reduced or eliminated.

§ Process to Process: The heat pipe heat exchangers recover thermal energy waste from the exhaust process and transfer this energy to the incoming process air. The incoming air thus becomes warm and can be used either for the same process/other processes and hence, reduce process energy consumption.

§ HVAC Applications: − Cooling: Heat pipe heat exchangers pre-cools the building make up air in

summer and thus reduces the total tones of refrigeration, apart from the operational saving of the cooling system. Thermal energy is supply recovered from the cool exhaust and transferred to the hot supply make up air.

− Heating: The above process is reversed during winter to preheat the make up air.

§ Other applications in industries are: − Preheating of boiler combustion air − Recovery of Waste heat from furnaces − Reheating of fresh air for hot air driers − Recovery of waste heat from catalytic deodorizing equipment − Reuse of Furnace waste heat as heat source for other oven − Cooling of closed rooms with outside air − Preheating of boiler feed water with waste heat recovery from flue gases in the

heat pipe economizers. − Drying, curing and baking ovens − Waste steam reclamation − Brick kilns (secondary recovery) − Reverberatory furnaces (secondary recovery) − Heating, ventilating and air-conditioning systems

Waste Heat Recovery

7

2.5 Economizers In the case of boiler systems, an economizer can be provided to utilize the flue gas heat for pre-heating the boiler feed water. On the other hand, in an air pre-heater, the waste heat is used to heat combustion air. In both the cases, there is a corresponding reduction in the fuel requirements of the boiler. For every 220 0C reduction in flue gas temperature by passing through an economizer or a pre-heater, there is 1% saving of fuel in the boiler. In other words, for every 60 0C rise in feed water temperature through an economizer, or 200 0C rise in combustion air temperature through an air pre-heater, there is 1% saving of fuel in the boiler. 2.6 Shell and Tube Heat Exchangers When the medium containing waste heat is a liquid or a vapor which heats another liquid, then the shell and tube heat exchanger must be used since both paths must be sealed to contain the pressures of their respective fluids. The shell contains the tube bundle, and usually internal baffles, to direct the fluid in the shell over the tubes in multiple passes. The shell is inherently weaker than the tube, so that the higher-pressure fluid is circulated in the tubes while the lower pressure fluid flows through the shell. When a vapor contains the waste heat, it usually condenses, giving up its latent heat to the liquid being heated. In this application, the vapor is almost invariably contained within the shell. If the reverse is attempted, the condensation of vapors within small diameter parallel tubes causes flow instabilities. Tube and shell heat exchangers are available in a wide range of standard sizes with many combinations of materials for the tubes and shells. A shell and tube heat exchanger is illustrated in Figure 9 below.

Figure 8. Economizer (Bureau of Energy Efficiency, 2004)

Figure 9. Shell & Tube Heat Exchanger (King Fahad University of Petroleum & Minerals, 2003)

Waste Heat Recovery

8

Typical applications of shell and tube heat exchangers include heating liquids with the heat contained by condensates from refrigeration and air-conditioning systems; condensate from process steam; coolants from furnace doors, grates, and pipe supports; coolants from engines, air compressors, bearings, and lubricants; and the condensates from distillation processes. 2.7 Plate Heat Exchanger The cost of a heat exchange surface is a major cost factor when the temperature differences are not large. One way of meeting this problem is the plate type heat exchanger, which consists of a series of separate parallel plates forming a thin flow pass. Each plate is separated from the next by gaskets and the hot stream passes in parallel through alternative plates whilst the liquid to be heated passes in parallel between the hot plates. To improve heat transfer the plates are corrugated. Hot liquid passing through a bottom port in the head is permitted to pass upwards between every second plate while cold liquid at the top of the head is permitted to pass downwards between the odd plates. When the directions of hot & cold fluids are opposite, the arrangement is described as counter current. A plate heat exchanger is shown in Figure 10. Typical industrial applications are: § Pasteurization section in a milk

packaging plant. § Evaporation plants in the food

industry. 2.8 Run Around Coil Exchangers Run Around Coil Exchangers are quite similar in principle to the heat pipe exchanger. The heat from hot fluid is transferred to the colder fluid via an intermediate fluid known as the Heat Transfer Fluid. One coil of this closed loop is installed in the hot stream, while the other is in the cold stream. Circulation of this fluid is maintained by means of a circulating pump. It is more useful when the hot land cold fluids are located far away from each other and are not easily accessible.

Figure 10. Plate Heat Exchanger (Canada Agriculture and Agri-Food)

Figure 11. Run Around Coil Exchanger SADC , 1999

Waste Heat Recovery

9

Typical industrial applications are heat recovery from ventilation, air conditioning and low temperature heat recovery. 2.9 Waste Heat Recovery Boilers Waste heat boilers are ordinarily water tube boilers in which the hot exhaust gases from gas turbines, incinerators, etc., pass over a number of parallel tubes containing water. The water is vaporized in the tubes and collected in a steam drum from which it is drawn out for use as heating or processing steam. Because the exhaust gases are usually in the medium temperature range and in order to conserve space, a more compact boiler can be produced if the water tubes are finned in order to increase the effective heat transfer area on the gas side. Figure 12 shows a mud drum, a set of tubes over which the hot gases make a double pass, and a steam drum which collects the steam generated above the water surface. The pressure at which the steam is generated and the rate at which steam is produced depend on the temperature of waste heat. The pressure of a pure vapor in the presence of its liquid is a function of the temperature of the liquid from which it is evaporated. The steam tables tabulate this relationship between saturation pressure and temperature. If the waste heat in the exhaust gases is insufficient for generating the required amount of process steam, auxiliary burners, which burn fuel in the waste heat boiler or an after-burner in which the exhaust gases flue are added. Waste heat boilers are built in capacities from 25 m3 almost 30,000 m3 /min. of exhaust gas. 2.10 Heat Pumps In the various commercial options previously discussed, we find waste heat being transferred from a hot fluid to a fluid at a lower temperature. Heat must flow spontaneously “downhill”, that is from a system at high temperature to one at a lower temperature. When energy is repeatedly transferred or transformed, it becomes less and less available for use. Eventually, energy has such low intensity (resides in a medium at such low temperature) that it is no longer available to function.

Figure 12. Two-Pass Water Tube Waste Heat Recovery Boiler

(Canada Agriculture and Agri-Food)

Waste Heat Recovery

10

It has been a general rule of thumb in industrial operations that fluids with temperatures less than 120oC (or, better, 150oC to provide a safe margin), are set as the limit for waste heat recovery because of the risk of condensation of corrosive liquids. However, as fuel costs continue to rise, even such waste heat can be used economically for space heating and other low temperature applications. It is possible to reverse the direction of spontaneous energy flow by the use of a thermodynamic system known as heat pump. The majority of heat pumps work on the principle of the vapour compression cycle. In this cycle, the circulating substance is physically separated from the source (waste heat, with a temperature of Tin) and user (heat to be used in the process, Tout) streams, and is re-used in a cyclical fashion, therefore being called 'closed cycle'. In the heat pump, the following processes take place: § In the evaporator, the heat is extracted from the heat source to boil the circulating

substance; § The compressor compresses the circulating substance, thereby raising its pressure and

temperature. The low temperature vapor is compressed by a compressor, which requires external work. The work done on the vapor raises its pressure and temperature to a level where its energy becomes available for use.

§ The heat is delivered to the condenser; § The pressure of the circulating substance (working fluid) is reduced back to the

evaporator condition in the throttling valve, where the cycle repeats. The heat pump was developed as a space heating system where low temperature energy from the ambient air, water, or earth is raised to heating system temperatures by doing compression work with an electric motor-driven compressor. The arrangement of a heat pump is shown in figure 13.

Figure 13. Heat Pump Arrangement (SADC, 1999)

Waste Heat Recovery

11

The heat pumps have the ability to upgrade heat to a value more than twice the energy consumed by the device. The potential for application of heat pumps is growing and a growing number of industries have been benefited by recovering low grade waste heat by upgrading it and using it in the main process stream. Heat pump applications are most promising when both the heating and cooling capabilities can be used in combination. One such example of this is a plastics factory where chilled water from a heat is used to cool injection-moulding machines, whilst the heat output from the heat pump is used to provide factory or office heating. Other examples of heat pump installation include product drying, maintaining dry atmosphere for storage and drying compressed air. 2.11 Thermo-compressor In many cases, very low-pressure steam is reused as water after condensation for lack of any better option of reuse. In many cases it becomes feasible to compress this low-pressure steam by very high-pressure steam and reuse it as a medium pressure steam. The major energy in steam is in its latent heat value, and thus thermo compressing would give a big improvement in waste heat recovery.

The thermo-compressor is a simple equipment with a nozzle where HP steam is accelerated into a high velocity fluid. This entrains the LP steam by momentum transfer and then recompresses in a divergent venturi. A figure of thermo compressor is shown in Figure 14. It is typically used in evaporators where the boiling steam is recompressed and used as heating steam.

Figure 14. Thermo-compressor

Waste Heat Recovery

12

3. ASSESSMENT OF WASTE HEAT RECOVERY This section explains how to assess the potential for waste heat recovery and gives examples. 3.1 Determining the Waste Heat Quality When recovering waste heat, the quality of waste heat must be considered first. Depending upon the type of process, waste heat can be discarded at virtually any temperature from that of chilled cooling water to high temperature waste gases in an industrial furnace or kiln. Usually, higher temperatures equate to higher quality of heat recovery and greater cost effectiveness. In any study of waste heat recovery, it is absolutely necessary that there is some use for the recovered heat. Typical examples of use would be preheating of combustion air, space heating, or pre-heating boiler feed water or process water. With high temperature heat recovery, a cascade system of waste heat recovery may be practiced to ensure that the maximum amount of heat is recovered with the highest potential. An example of this technique of waste heat recovery is where the high temperature stage is used for air pre-heating and the low temperature stage is used for process feed water heating or steam generation. 3.1.1 Quality and potential uses In considering the potential to recover heat, it is useful to note all the possible sources of waste and their quality and possible uses (see Table 1) Table 1. Waste Heat Source and Quality No Source of waste heat Quality of waste heat and possible use 1 Heat in flue gases The higher the temperature, the greater the potential

value for heat recovery 2 Heat in vapor streams As for heat in flue gases, but when condensed,

latent heat is also recoverable 3 Convective & radiant heat lost from

exterior of equipment Low grade – if collected, may be used for space heating or air preheats

4 Heat losses in cooling water Low grade – useful gains if heat is exchanged with incoming fresh water

5 Heat losses in providing chilled water or in the disposal of chilled water

1. High grade if it can be utilized to reduce demand for refrigeration

2. Low grade if refrigeration unit used as a form of Heat pump

6 Heat stored in products leaving the process

Quality depends upon temperature

7 Heat in gaseous & liquid effluents leaving process

Poor, if heavily contaminated & thus require alloy heat exchanger

3.1.2 Recovery potential for different industrial processes Waste heat can be recovered from various industrial processes. A distinction is made between high, medium and low temperatures of waste heat.

Waste Heat Recovery

13

Table 2 gives the temperatures of waste gases from industrial process equipment in the high temperature range. All of these results are from direct fuel fired processes. Table 2. Typical Waste Heat Temperature at High Temperature Range from Various Sources Types of Devices Temperature (0C) Nickel refining furnace 1370 – 1650 Aluminium refining furnace 650 –760 Zinc refining furnace 760 – 1100 Copper refining furnace 760 – 815 Steel heating furnace 925 – 1050 Copper reverberatory furnace 900 – 1100 Open hearth furnace 650 – 700 Cement kiln (Dry process) 620 – 730 Glass melting furnace 1000 – 1550 Hydrogen plants 650 – 1000 Solid waste incinerators 650 – 1000 Fume incinerators 650 – 1450 Table 3 gives the temperatures of waste gases from process equipment in the medium temperature range. Most of the waste heat in this temperature range comes from the exhaust of directly fired process units. Table 3. Typical Waste Heat Temperature at Medium Temperature Range from Various Sources Types of Devices Temperature (0C) Steam boiler exhaust 230 – 480 Gas turbine exhaust 370 – 540 Reciprocating engine exhaust 315 – 600 Reciprocating engine exhaust (turbo charged) 230 – 370 Heat treatment furnace 425 – 650 Drying & baking ovens 230 – 600 Catalytic crackers 425 – 650 Annealing furnace cooling systems 425 – 650 Table 4 lists some heat sources in the low temperature range. In this range, it is usually not practical to extract work from the source, though steam production may not be completely excluded if there is a need for low-pressure steam. Low temperature waste heat may be useful in a supplementary way for preheating purposes. Table 4. Typical Waste Heat Temperature at Low Temperature Range from Various Sources Source Temperature 0C Process steam condensate 55-88 Cooling water from: Furnace doors

32-55

Bearings 32-88 Welding machines 32-88 Injection molding machines 32-88 Annealing furnaces 66-230

Waste Heat Recovery

14

Source Temperature 0C Forming dies 27-88 Air compressors 27-50 Pumps 27-88 Internal combustion engines 66-120 Air conditioning and refrigeration condensers 32–43 Liquid still condensers 32-88 Drying, baking and curing ovens 93-230 Hot processed liquids 32-232 Hot processed solids 93-232 3.2 Determining the Waste Heat Quantity In any heat recovery situation it is essential to know the amount of heat recoverable and also its usage. The total heat that could potentially be recovered can be calculated using this formula:

Q = V x ρ x Cp x ∆T Where,

Q is the heat content in kcal V is the flow rate of the substance in m3/hr ρ is density of the flue gas in kg/m3 Cp is the specific heat of the substance in kCal/kg oC ∆T is the temperature difference in oC

Example A large paper manufacturing company identifies an opportunity to save money by recovering heat from hot wastewater. The discharge of the wastewater from the operation range is 10000 kg/hr at 750C. Rather than discharging this water to drain, it was decided to preheat the 10000 kg/hr of cold inlet water having a yearly average temperature of 200C, by passing it through a counterflow heat exchanger with automatic back flushing to reduce fouling. Based on a heat recovery factor of 58% and an operation of 5000 hours per year, the annual heat saving (Q) is:

Q = m x η x Cp x ∆T Where, Q is the heat content in kcal m is the mass flow rate

Cp is the specific heat of the substance in kcal/kg oC, in the case water ∆T is the temperature difference in oC η is the recovery factor

Therefore, for this example m = 1000 kg/hr = 10000 x 5000 kg/yr = 50000000 kg/year Cp = 1 kCal/kg0C

Waste Heat Recovery

15

∆T = (75 – 20) 0C = 55 0C η = Heat Recovery Factor = 58% or 0.58 The calculation of Q is as follows: Q = 50000000 x 1 x 55 x 0.58

= 1595000000 kCal/year Gross calorific value (GCV of oil) = 10,200 kCal/kg Equivalent oil savings = 159500000 / 10200 = 156372 liters Cost of oil = 0.35 US$/liter Financial savings = 54730 US$/year 4. ENERGY EFFICIENCY OPPORTUNITIES Areas for potential waste recovery are dependent on the type of industrial process, and are therefore covered in other energy equipment modules. 5. OPTION CHECKLIST The most important options to maximize energy efficiency when applying waste heat recovery are § Recover heat from flue gas, engine cooling water, engine exhaust, low pressure waste

steam, drying oven exhaust, boiler blowdown, etc. § Recover heat from incinerator off-gas. § Use waste heat for fuel oil heating, boiler feedwater heating, outside air heating, etc. § Use chiller waste heat to preheat hot water. § Use heat pumps. § Use absorption refrigeration. § Use thermal wheels, run-around systems, heat pipe systems, and air-to-air exchangers. Options to recover waste heat are covered in other energy equipment modules.

Waste Heat Recovery

16

6. WORKSHEETS This section includes the following worksheets: § Heat Recovery Questionnaire § Matrix of Waste Heat Recovery Devices & Applications Worksheet 1. Heat Recovery Questionnaire

1. From which equipment do you want to recover heat? Oven, furnace, etc.

• Oven • Flue Gas • Dryer • Bake Oven • Furnace • Paint Dryer

• Kiln • Melting Furnace • Boiler • Die Cast Machine • Cupola • Exhaust Air • Other (Please specify)

2. Hot Side Flows:

a. At what temperature does hot exhaust leave this equipment? b. What is the quantity of this hot exhaust?

3. Is this hot exhaust gas clean (natural gas, propane, #2 fuel oil) or does it contain contaminates or corrodents such as sulphur, chlorides, etc?

Clean: Dirty: Exhaust is from: Exhaust is from and/or contains: _________ Air _________ Fuel Oil _________ Natural Gas ______________ Coal _________ Propane ______________ Sulphur ______% _________ Fuel Oil ______________ Chloride ______% _________ Electricity _________ Paint Vapours ______% _________ Other _________ Other ______%

4. Cold Side Flows:

Entering Fluid Temperature

0C

Entering Fluid Volume 0C Leaving Fluid Temperature Desired

0C

Energy to be recovered kJ/hr Available Flow L/s

5. Fuel Cost: (USD/kg) 6. Operating Hours

Waste Heat Recovery

17

Worksheet 2. Matrix of Waste Heat Recovery Devices And Applications Heat Recovery Device

Temp. Range

Typical Sources Typical Uses

Radiation Recuperator

H Incinerator or boiler exhaust Combustion air preheat

Convective Recuperator

M-H Soaking or annealing ovens, melting furnaces, afterburners, gas incinerators, radiant tube burners, reheat furnaces

Combustion air preheat

Furnace Regenerator

H Glass and Steel melting furnaces Combustion air preheat

Metallic Heat Wheel

L-M Curing and drying ovens, boiler exhaust

Combustion air preheat, Space preheat

Ceramic Heat Wheel

M-H Large boiler or Incinerator exhaust

Combustion air preheat

Finned tube Regenerator

L-M Boiler Exhaust Boiler makeup water preheat

Shell & tube Regenerator

L Refrigeration condensates, waste steam, distillation condensates, coolants from engines, air compressors, bearings and lubricants

Liquid flows requiring heating

Heat Pipes L-M Drying, curing and baking ovens, Waste steam, air dryers, Kilns and Reverberatory furnaces

Combustion air preheat, boiler makeup water preheat, Steam generation, domestic hot water, space heat

Waste heat boiler M-H Exhaust from gas turbines, reciprocating engines, incinerators and furnaces

Hot water or steam generation

Waste Heat Recovery

OF HEAT EXCHANGERS

Heat exchangers are equipment that transfer heat from one medium to another. The properdesign, operation and maintenance of heat exchangers will make the process energy efficientand minimize energy losses. Heat exchanger performance can deteriorate with time, offdesign operations and other interferences such as fouling, scaling etc. It is necessary toassess periodically the heat exchanger performance in order to maintain them at a high effi-ciency level. This section comprises certain proven techniques of monitoring the perfor-mance of heat exchangers, coolers and condensers from observed operating data of theequipment.

To determine the overall heat transfer coefficient for assessing the performance of the heatexchanger. Any deviation from the design heat transfer coefficient will indicate occurrence offouling.

Overall heat transfer coefficient, U

Heat exchanger performance is normally evaluated by the overall heat transfer coefficient Uthat is defined by the equation

When the hot and cold stream flows and inlet temperatures are constant, the heat transfercoefficient may be evaluated using the above formula. It may be observed that the heat pick upby the cold fluid starts reducing with time.

ENERGY PERFORMANCE ASSESSMENT

1 Introduction

2 Purpose of the Performance Test

3 Performance Terms and Definitions

Chapter-5Chapter-5Chapter-5

Chapter-5

1

Nomenclature

A typical heat exchanger is shown in figure 4.1 with nomenclature.

Heat duty of the exchanger can be calculated either on the hot side fluid or cold side fluidas given below.Heat Duty for Hot fluid, Qh = Wx Cph x (Ti–To) ………..Eqn–1, Heat Duty for Cold fluid, Qc = wx Cpc x ( to–ti) ………...Eqn–2 If the operating heat duty is less than design heat duty, it may be due to heat losses, fouling

in tubes, reduced flow rate (hot or cold) etc. Hence, for simple performance monitoring ofexchanger, efficiency may be considered as factor of performance irrespective of other para-meter. However, in industrial practice, fouling factor method is more predominantly used.

4.4.1 Procedure for determination of Overall heat transfer Coefficient, U at field

This is a fairly rigorous method of monitoring the heat exchanger performance by calculatingthe overall heat transfer coefficient periodically. Technical records are to be maintained for allthe exchangers, so that problems associated with reduced efficiency and heat transfer can beidentified easily. The record should basically contain historical heat transfer coefficient dataversus time / date of observation. A plot of heat transfer coefficient versus time permits ratio-nal planning of an exchanger-cleaning program.

The heat transfer coefficient is calculated by the equation

U = Q / (A x LMTD)

Where Q is the heat duty, A is the heat transfer area of the exchanger and LMTD is tem-perature driving force.

The step by step procedure for determination of Overall heat transfer Coefficient aredescribed below

Energy Performance Assessment Of Heat Exchangers

4 Methodology of Heat Exchanger Performance Assessment

2

Density and viscosity can be determined by analysis of the samples taken from the flowstream at the recorded temperature in the plant laboratory. Thermal conductivity and specificheat capacity if not determined from the samples can be collected from handbooks.


3


4

a. Liquid - Liquid Exchanger

A shell and tube exchanger of following configuration is considered being used for oil cool-er with oil at the shell side and cooling water at the tube side.

Tube Side

• 460 Nos x 25.4mmOD x 2.11mm thick x 7211mm long• Pitch - 31.75mm 30° triangular• 2 Pass

Shell Side

• 787 mm ID • Baffle space - 787 mm• 1 Pass


4.2 Examples

5


6


7

Heat Duty: Actual duty differences will be practically negligible as these duty differencescould be because of the specific heat capacity deviation with the temperature. Also, there couldbe some heat loss due to radiation from the hot shell side.

Pressure drop: Also, the pressure drop in the shell side of the hot fluid is reported normal(only slightly less than the design figure). This is attributed with the increased average bulktemperature of the hot side due to decreased performance of the exchanger.

Temperature range: As seen from the data the deviation in the temperature ranges could bedue to the increased fouling in the tubes (cold stream), since a higher pressure drop is noticed.

Heat Transfer coefficient: The estimated value has decreased due to increased fouling that hasresulted in minimized active area of heat transfer.

Physical properties: If available from the data or Lab analysis can be used for verificationwith the design data sheet as a cross check towards design considerations.

Troubleshooting: Fouled exchanger needs cleaning.

b. Surface Condenser

A shell and tube exchanger of following configuration is considered being used for Condensingturbine exhaust steam with cooling water at the tube side.

Tube Side

20648 Nos x 25.4mmOD x 1.22mm thk x 18300mm longPitch - 31.75mm 60° triangular1 PassThe monitored parameters are as below:

Parameters Units Inlet Outlet

Hot fluid flow, W kg/h 939888 939888

Cold fluid flow, w kg/h 55584000 55584000

Hot fluid Temp, T °C No data 34.9

Cold fluid Temp, t °C 18 27

Hot fluid Pressure, P m Bar g 52.3 mbar 48.3

Cold fluid Pressure, p Bar g 4 3.6

Calculation of Thermal data:

Area = 27871 m2

1. Duty:Q = qS + qL

Hot fluid, Q = 576990 kWCold Fluid, Q = 581825.5 kW


8

2. Hot Fluid Pressure DropPressure Drop = Pi – Po = 52.3 – 48.3 = 4.0 mbar.

3. Cold Fluid Pressure DropPressure Drop = pi – po = 4 – 3.6 = 0.4 bar.

4. Temperature range hot fluidTemperature Range ∆T = Ti– To = No data

5. Temperature Range Cold FluidTemperature Range ∆t = ti – to = 27 – 18 = 9 °C.

6. Capacity RatioCapacity ratio, R = Not significant in evaluation here.

7. EffectivenessEffectiveness, S = (to – ti) / (Ti – ti) = Not significant in evaluation here.

8. LMTD

Calculated considering condensing part only

a). LMTD, Counter Flow = ((34.9 – 18)–(34.9–27))/ ln ((34.9–18)/(34.9–27)) = 11.8 deg C.

b). Correction Factor to account for Cross flow

F = 1.0.

9. Corrected LMTDMTD = F x LMTD = 1.0 x 11.8 = 11.8 deg C.

10. Heat Transfer Co-efficientOverall HTC, U = Q/ A ∆T = 576990/ (27871 x 11.8) = 1.75 kW/m2. K

Comparison of Calculated data with Design Data

Parameters Units Test Data Design Data

Duty, Q kW 576990 588430

Hot fluid side pressure drop, ∆Ph mBar 4 mbar 3.7 mbar

Cold fluid side pressure drop, ∆Pc Bar 0.4

Temperature Range hot fluid, ∆T °C

Temperature Range cold fluid, ∆t °C (27–18) = 9 (28–19) = 9

Capacity ratio, R -----

Effectiveness, S -----

Corrected LMTD, MTD °C 11.8 8.9

Heat Transfer Coefficient, U kW/(m2. K) 1.75 2.37


9

Heat Duty: Actual duty differences will be practically negligible as these duty differencescould be because of the specific heat capacity deviation with the temperature. Also, there couldbe some heat loss due to radiation from the hot shell side.

Pressure drop: The condensing side operating pressure raised due to the backpressurecaused by the non-condensable. This has resulted in increased pressure drop across the steamside

Temperature range: With reference to cooling waterside there is no difference in the rangehowever, the terminal temperature differences has increased indicating lack of proper heattransfer.

Heat Transfer coefficient: Heat transfer coefficient has decreased due to increased amount ofnon-condensable with the steam.

Trouble shooting: Operations may be checked for tightness of the circuit and ensure proper venting of the system. The vacuum source might be verified for proper functioning.

C. Vaporizer

A shell and tube exchanger of following configuration is considered being used for vaporizingchlorine with steam at the shell side.

Tube Side

200 Nos x 25.4mmOD x 1.22mm thick x 6000mm longPitch - 31.75mm 30° triangular2 PassArea = 95.7.m2


10

Calculation of Thermal data:

1. Duty:Q = qS + qL

Hot fluid, Q = 3130 kW

Cold Fluid, Q = qS + qL = 180.3 kW + 2948 kW = 3128.3 kW

2. Hot Fluid Pressure DropPressure Drop = Pi – Po = 0.4 – 0.3 = 0.1 bar

3. Cold Fluid Pressure DropPressure Drop = pi – po = 9 – 8.8 = 0.2 bar.

4. Temperature range hot fluidTemperature Range ∆T = Ti – To = 0 °C




8. LMTDCalculated considering condensing part only

a). LMTD, Counter Flow =((108 – 30)–(108–34))/ ln ((108–30)/(108–34)) = 76 °C.

b). Correction Factor to account for Cross flow

F = 1.0.

9. Corrected LMTDMTD = F x LMTD = 1.0 x 76 = 76 °C.

10. Heat Transfer Co-efficientOverall HTC, U = Q/ A ∆T = 3130/ (95.7 x 76) = 0.43 kW/m2. K




Hot fluid Temp, T °C 108 108


Hot fluid Pressure, P Bar g 0.4 0.3

Cold fluid Pressure, p Bar g 9 8.8

The monitored parameters are as below:


11


Duty, Q kW 3130 3130

Hot fluid side pressure drop, ∆Ph Bar 0.1 Neg

Cold fluid side pressure drop, ∆Pc Bar 0.2


Temperature Range cold fluid, ∆t °C 4 4



Corrected LMTD, MTD °C 76



Heat Duty: There is no difference inferred from the duty as the exchanger is performing as perthe requirement

Pressure drop: The steam side pressure drop has increased in spite of condensation at thesteam side. Indication of non-condensable presence in steam side

Temperature range: No deviations

Heat Transfer coefficient: Even at no deviation in the temperature profile at the chlorine side,heat transfer coefficient has decreased with an indication of overpressure at the shell side. Thisindicates disturbances to the condensation of steam at the shell side. Non-condensable suspect-ed at steam side.

Trouble shooting: Operations may be checked for presence of chlorine at the shell sidethrough tube leakages. Observing the steam side vent could do this. Alternately condensate pHcould be tested for presence of acidity.


12

d. Air heater

A finned tube exchanger of following configuration is considered being used for heating airwith steam in the tube side.

The monitored parameters are as below:




Hot fluid Temp, T °C 150 150


Hot fluid Pressure, P Bar g

Cold fluid Pressure, p mBar g 200 mbar 180 mbar

Calculation of Thermal data:Bare tube Area = 42.8 m2; Fined tube area = 856 m2

1.Duty:Hot fluid, Q = 1748 kW

Cold Fluid, Q = 1726 kW

2. Hot Fluid Pressure DropPressure Drop = Pi – Po = Neg

3. Cold Fluid Pressure DropPressure Drop = pi – po = 200–180 = 20 mbar.

4. Temperature range hot fluidTemperature Range ∆T = Ti – To = Not required.




8. LMTDCalculated considering condensing part only

a). LMTD, Counter Flow =((150 – 30)–(150–95)/ ln ((150–30)/(150–95)) = 83.3 °C.

b). Correction Factor to account for cross flow

F = 0.95

9. Corrected LMTD

MTD = F x LMTD = 0.95 x 83.3 = 79 °C.

10. Overall Heat Transfer Co-efficient (HTC)U = Q/ A ∆T = 1748/ (856 x 79) = 0.026 kW/m22. K


13



Duty, Q kW 1748 1800

Hot fluid side pressure drop, ∆Ph Bar Neg Neg

Cold fluid side pressure drop, ∆Pc Bar 20 15


Temperature Range cold fluid, ∆t °C 65 65



Corrected LMTD, MTD °C 79 79


Heat Duty: The difference inferred from the duty as the exchanger is under performing thanrequired

Pressure drop: The airside pressure drop has increased in spite of condensation at the steamside. Indication of choking and dirt blocking at the airside.

Temperature range: No deviations

Heat Transfer coefficient: Decreased because of decreased fin efficiency due to choking onair side.

Trouble shooting: Operations may be checked to perform pulsejet cleaning with steam / blowair jet on air side if the facility is available. Mechanical cleaning may have to be planned dur-ing any down time in the immediate future.


14

The test and evaluation of the performance of the heat exchanger equipment is carried out bymeasurement of operating parameters upstream and downstream of the exchanger. Due careneeds to be taken to ensure the accuracy and correctness of the measured parameter. The instru-ments used for measurements require calibration and verification prior to measurement.

Parameters Units Instruments used

Fluid flow kg/h Flow can be measured with instruments like Orifice flow meter, Vortex flow meter, Venturi meters, Coriollis flow meters, Magnetic flowmeter as applicable to the fluid service and flow ranges

Temperature °C Thermo gauge for low ranges, RTD, etc.

Pressure Bar g Liquid manometers, Draft gauge, Pressure gauges Bourdon and diaphragm type, Absolute pressure transmitters, etc.

Density kg/m3 Measured in the Laboratory as per ASTM standards, hydrometer, etc

Viscosity MpaS Measured in the Laboratory as per ASTM standards, viscometer, etc.

Specific heat capacity J/(kg.K) Measured in the Laboratory as per ASTM standards

Thermal conductivity W/(m.K) Measured in the Laboratory as per ASTM standards

Composition+ %wt (or) % Vol Measured in the Laboratory as per ASTM standards using Chemical analysis, HPLC, GC, Spectrophotometer, etc.

Terminology Definition Unit

Capacity ratio Ratio of the products of mass flow rate and specific heat capacity of the cold fluid to that of the hot fluid. Also computed by the ratio of temperature range of the hot fluid to that of the cold fluid. Higher the ratio greater will be size of the exchanger

Co current flow An exchanger wherein the fluid flow direction of the exchanger cold and hot fluids are same

Counter flow Exchangers wherein the fluid flow direction of the cold and exchanger hot fluids are opposite. Normally preferred

Cross flow An exchanger wherein the fluid flow direction of the cold and hot fluids are in cross


4.3 Instruments for monitoring:

4.4 Terminology used in Heat Exchangers

15

Density It is the mass per unit volume of a material kg/m3

Effectiveness Ratio of the cold fluid temperature range to that of the inlet temperature difference of the hot and cold fluid. Higher the ratio lesser will be requirement of heat transfer surface

Fouling The phenomenon of formation and development of scales and deposits over the heat transfer surface diminishing the heat flux. The process of fouling will get indicated by the increase in pressure drop

Fouling Factor The reciprocal of heat transfer coefficient of the dirt formed in the heat exchange process. Higher the factor lesser will be the overall heat transfer coefficient. (m2.K)/W

Heat Duty The capacity of the heat exchanger equipment expressed in terms of heat transfer rate, viz. magnitude of energy or heat transferred per time. It means the exchanger is capable of performing at this capacity in the given system W

Heat exchanger Refers to the nomenclature of equipment designed and constructed to transmit heat content (enthalpy or energy) of a comparatively high temperature hot fluid to a lower temperature cold fluid wherein the temperature of the hot fluid decreases (or remain constant in case of losing latent heat of condensation) and the temperature of the cold fluid increases (or remain constant in case of gaining latent heat of vaporisation). A heat exchanger will normally provide indirect contact heating. E.g. Acooling tower cannot be called a heat exchanger where water is cooled by direct contact with air

Heat Flux The rate of heat transfer per unit surface of a heat exchanger W/ m2

Heat transfer The process of transport of heat energy from a hot source to the comparatively cold surrounding

Heat transfer Refers to the surface area of the heat exchanger that surface or heat provides the indirect contact between the hot and cold Transfer area fluid in effecting the heat transfer. Thus the heat transfer

area is defined as the surface having both sides wetted with one side by the hot fluid and the other side by the cold fluid providing indirect contact for heat transfer m2

Individual The heat flux per unit temperature difference across Heat transfer boundary layer of the hot / cold fluid film formed Coefficient at the heat transfer surface. The magnitude of heat

transfer coefficient indicates the ability of heat conductivity of the given fluid. It increases with increase in density, velocity, specific heat, geometry of the film forming surface W/( m2.K)


16

LMTD Calculated considering the Capacity and effectiveness Correction of a heat exchanging process. When multiplied with factor LMTD gives the corrected LMTD thus accounting

for the temperature driving force for the cross flow pattern as applicable inside the exchanger

Logarithmic The logarithmic average of the terminal temperature Mean approaches across a heat exchangerTemperature difference, LMTD °C

Overall Heat The ratio of heat flux per unit difference in approach transfer across a heat exchange equipment considering the Coefficient individual coefficient and heat exchanger metal surface

conductivity. The magnitude indicates the ability of heat transfer for a given surface. Higher the coefficient lesser will be the heat transfer surface requirement W/(m2.K)

Pressure drop The difference in pressure between the inlet and outlet of a heat exchanger Bar

Specific The heat content per unit weight of any material per heat capacity degree raise/fall in temperature J/(kg.K)

Temperature The difference in the temperature between the hot and Approach cold fluids at the inlet / outlet of the heat exchanger.

The greater the difference greater will be heat transfer flux °C

Temperature The difference in the temperature between the inlet Range and outlet of a hot/cold fluid in a heat exchanger °C

Terminal The temperatures at the inlet / outlet of the hot / cold temperature fluid steams across a heat exchanger °C

Thermal The rate of heat transfer by conduction though any Conductivity substance across a distance per unit

temperature difference W/(m2.K)

Viscosity The force on unit volume of any material that will cause per velocity Pa


17

Cooling Towers

1

COOLING TOWERS

1. INTRODUCTION...........................................................................................1 2. TYPES OF COOLING TOWERS ................................................................4 3. ASSESSMENT OF COOLING TOWERS ..................................................7 4. ENERGY EFFICIENCY OPPORTUNITIES .............................................9 5. OPTION CHECKLIST ................................................................................14 6. WORKSHEETS............................................................................................15 7. REFERENCES..............................................................................................17 1. INTRODUCTION This section briefly describes the main features of cooling towers. 1.1 What is a cooling tower? Cooled water is needed for, for example, air conditioners, manufacturing processes or power generation. A cooling tower is an equipment used to reduce the temperature of a water stream by extracting heat from water and emitting it to the atmosphere. Cooling towers make use of evaporation whereby some of the water is evaporated into a moving air stream and subsequently discharged into the atmosphere. As a result, the remainder of the water is cooled down significantly (Figure 1). Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and are therefore more cost-effective and energy efficient.

Figure 1. Schematic diagram of a cooling water system (Pacific Northwest National Laboratory, 2001)

Chapter-6

Electrical Energy Equipment: Cooling Towers

2

1.2 Components of a cooling tower The basic components of a cooling tower include the frame and casing, fill, cold-water basin, drift eliminators, air inlet, louvers, nozzles and fans. These are described below.1 Frame and casing. Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame. Fill. Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. There are two types of fill: Splash fill: water falls over successive layers of horizontal splash bars, continuously

breaking into smaller droplets, while also wetting the fill surface. Plastic splash fills promote better heat transfer than wood splash fills.

Film fill: consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.

Cold-water basin. The cold-water basin is located at or near the bottom of the tower, and it receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold-water discharge connection. In many tower designs, the cold-water basin is beneath the entire fill. In some forced draft counter flow design, however, the water at the bottom of the fill is channeled to a perimeter trough that functions as the cold-water basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors. Drift eliminators. These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere. Air inlet. This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower (cross-flow design) or be located low on the side or the bottom of the tower (counter-flow design). Louvers. Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers. Nozzles. These spray water to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed and spray in a round or square patterns, or they can be part of a rotating assembly as found in some circular cross-section towers. Fans. Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, the type of propeller fans used is either fixed 1


3

or variable pitch. A fan with non-automatic adjustable pitch blades can be used over a wide kW range because the fan can be adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions. 1.3 Tower materials Originally, cooling towers were constructed primarily with wood, including the frame, casing, louvers, fill and cold-water basin. Sometimes the cold-water basin was made of concrete. Today, manufacturers use a variety of materials to construct cooling towers. Materials are chosen to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fiber, and concrete are widely used in tower construction, as well as aluminum and plastics for some components.2 Frame and casing. Wooden towers are still available, but many components are made of different materials, such as the casing around the wooden framework of glass fiber, the inlet air louvers of glass fiber, the fill of plastic and the cold-water basin of steel. Many towers (casings and basins) are constructed of galvanized steel or, where a corrosive atmosphere is a problem, the tower and/or the basis are made of stainless steel. Larger towers sometimes are made of concrete. Glass fiber is also widely used for cooling tower casings and basins, because they extend the life of the cooling tower and provide protection against harmful chemicals. Fill. Plastics are widely used for fill, including PVC, polypropylene, and other polymers. When water conditions require the use of splash fill, treated wood splash fill is still used in wooden towers, but plastic splash fill is also widely used. Because of greater heat transfer efficiency, film fill is chosen for applications where the circulating water is generally free of debris that could block the fill passageways. Nozzles. Plastics are also widely used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon. Fans. Aluminum, glass fiber and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are made from galvanized steel, aluminum, or molded glass fiber reinforced plastic.


4

2. TYPES OF COOLING TOWERS This section describes the two main types of cooling towers: the natural draft and mechanical draft cooling towers. 2.1 Natural draft cooling tower The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. As hot air moves upwards through the tower (because hot air rises), fresh cool air is drawn into the tower through an air inlet at the bottom. Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive.

There are two main types of natural draft towers: Cross flow tower (Figure 2): air is drawn across the falling water and the fill is located

outside the tower Counter flow tower (Figure 3): air is drawn up through the falling water and the fill is

therefore located inside the tower, although design depends on specific site conditions 2.2 Mechanical draft cooling tower Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc.

Figure 2. Cross flow natural draft cooling tower Figure 3. Counter flow natural draft cooling tower

(Gulf Coast Chemical Commercial Inc, 1995)


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Mechanical draft towers are available in a large range of capacities. Towers can be either factory built or field erected – for example concrete towers are only field erected. Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., a eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells. The three types of mechanical draft towers are summarized in Table 1. Table 1. Main features of different types of draft cooling towers (based on AIRAH) Type of cooling tower Advantages Disadvantages Forced draft cooling tower (Figure 4): air is blown through the tower by a fan located in the air inlet

Suited for high air resistance due to centrifugal blower fans

Fans are relatively quiet

Recirculation due to high air-entry and low air-exit velocities, which can be solved by locating towers in plant rooms combined with discharge ducts

Induced draft cross flow cooling tower (Figure 5): water enters at top and passes over fill air enters on one side (single-flow tower)

or opposite sides (double-flow tower) an induced draft fan draws air across fill

towards exit at top of tower Induced draft counter flow cooling tower (Figure 6): hot water enters at the top air enters bottom and exits at the top uses forced and induced draft fans

Less recirculation than forced draft towers because the speed of exit air is 3-4 times higher than entering air

Fans and the motor drive mechanism require weather-proofing against moisture and corrosion because they are in the path of humid exit air

Figure 4. Forced Draft Cooling Tower ((GEO4VA))


6

Figure 5. Induced draft counter flow cooling tower (GEO4VA)

Figure 6. Induced draft cross flow cooling tower (GEO4VA)


7

3. ASSESSMENT OF COOLING TOWERS This section describes how the performance of cooling powers can be assessed.3 The performance of cooling towers is evaluated to assess present levels of approach and range against their design values, identify areas of energy wastage and to suggest improvements. During the performance evaluation, portable monitoring instruments are used to measure the following parameters: Wet bulb temperature of air Dry bulb temperature of air Cooling tower inlet water temperature Cooling tower outlet water temperature Exhaust air temperature Electrical readings of pump and fan motors Water flow rate Air flow rate

Figure 7. Range and approach of cooling towers These measured parameters and then used to determine the cooling tower performance in several ways. (Note: CT = cooling tower; CW = cooling water). These are: a) Range (see Figure 7). This is the difference between the cooling tower water inlet and

outlet temperature. A high CT Range means that the cooling tower has been able to reduce the water temperature effectively, and is thus performing well. The formula is:

CT Range (°C) = [CW inlet temp (°C) – CW outlet temp (°C)]

Range

Approach

Hot Water Temperature (In)

Cold-water Temperature (Out)

Wet Bulb Temperature (Ambient)

(In) to the Tower (Out) from the Tower


8

b) Approach (see Figure 7). This is the difference between the cooling tower outlet cold-

water temperature and ambient wet bulb temperature. The lower the approach the better the cooling tower performance. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance.

CT Approach (°C) = [CW outlet temp (°C) – Wet bulb temp (°C)]

c) Effectiveness. This is the ratio between the range and the ideal range (in percentage), i.e.

difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach). The higher this ratio, the higher the cooling tower effectiveness.

CT Effectiveness (%) = 100 x (CW temp – CW out temp) / (CW in temp – WB temp)

d) Cooling capacity. This is the heat rejected in kCal/hr or TR, given as product of mass

flow rate of water, specific heat and temperature difference. e) Evaporation loss. This is the water quantity evaporated for cooling duty. Theoretically

the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected. The following formula can be used (Perry):

Evaporation loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2) T1 - T2 = temperature difference between inlet and outlet water

f) Cycles of concentration (C.O.C). This is the ratio of dissolved solids in circulating water

to the dissolved solids in make up water. g) Blow down losses depend upon cycles of concentration and the evaporation losses and is

given by formula:

Blow down = Evaporation loss / (C.O.C. – 1)

h) Liquid/Gas (L/G) ratio. The L/G ratio of a cooling tower is the ratio between the water and the air mass flow rates. Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynamic rules also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used:

L(T1 – T2) = G(h2 – h1)

L/G = (h2 – h1) / (T1 – T2)

Where: L/G = liquid to gas mass flow ratio (kg/kg) T1 = hot water temperature (0C) T2 = cold-water temperature (0C)


9

h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature (same units as above) h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature (same units as above)

4. ENERGY EFFICIENCY OPPORTUNITIES This section includes main areas for improving energy efficiency of cooling towers. The main areas for energy conservation include: 4 Selecting the right cooling tower (because the structural aspects of the cooling tower

cannot be changed after it is installed) Fills Pumps and water distribution system Fans and motors

4.1 Selecting the right cooling towers Once a cooling tower is in place it is very difficult to significantly improve its energy performance. A number of factors are of influence on the cooling tower’s performance and should be considered when choosing a cooling tower: capacity, range, approach, heat load, wet bulb temperature, and the relationship between these factors. This is described below. 4.1.1 Capacity Heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are an indication of the capacity of cooling towers. However, these design parameters are not sufficient to understand the cooling tower performance. For example, a cooling tower sized to cool 4540 m3/hr through a 13.9 0C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5 0C range. Therefore other design parameters are also needed. 4.1.2 Range Range is determined not by the cooling tower, but by the process it is serving. The range at the exchanger is determined entirely by the heat load and the water circulation rate through the exchanger and going to the cooling water. The range is a function of the heat load and the flow circulated through the system:

Range 0C = Heat load (in kCal/hour) / Water circulation rate (l/hour) Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 4540 m3/hr from 48.9oC to 32.2oC at 26.7oC wet bulb temperature. 4.1.3 Approach As a general rule, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.8oC approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. When the size of the


10

tower has to be chosen, then the approach is most important, closely followed by the flow rate, and the range and wet bulb would be of lesser importance.

Approach (5.50C) = Cold-water temperature 32.2 0C – Wet bulb temperature (26.7 0C)

4.1.4 Heat load The heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired operating temperature of the process. In most cases, a low operating temperature is desirable to increase process efficiency or to improve the quality or quantity of the product. However, in some applications (e.g. internal combustion engines) high operating temperatures are desirable. The size and cost of the cooling tower is increases with increasing heat load. Purchasing undersized equipment (if the calculated heat load is too low) and oversized equipment (if the calculated heat load is too high) is something to be aware of. Process heat loads may vary considerably depending upon the process involved and are therefore difficult to determine accurately. On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy. Information is available for the heat rejection requirements of various types of power equipment. A sample list is as follows (BEE, 2004): Air Compressor

- Single-stage - 129 kCal/kW/hr - Single-stage with after cooler - 862 kCal/kW/hr - Two-stage with intercooler - 518 kCal/kW/hr - Two-stage with intercooler and after cooler - 862 kCal/kW/hr

Refrigeration, Compression - 63 kCal/min/TR Refrigeration, Absorption - 127 kCal/min/TR Steam Turbine Condenser - 555 kCal/kg of steam Diesel Engine, Four-Cycle, Supercharged - 880 kCal/kW/hr Natural Gas Engine, Four-cycle - 1523 kCal/kW/hr (= 18 kg/cm2 compression)

4.1.5 Wet bulb temperature Wet bulb temperature is an important factor in performance of evaporative water cooling equipment, because it is the lowest temperature to which water can be cooled. For this reason, the wet bulb temperature of the air entering the cooling tower determines the minimum operating temperature level throughout the plant, process, or system. The following should be considered when pre-selecting a cooling tower based on the wet bulb temperature: Theoretically, a cooling tower will cool water to the entering wet bulb temperature. In

practice, however, water is cooled to a temperature higher than the wet bulb temperature because heat needs to be rejected from the cooling tower.

A pre-selection of towers based on the design wet bulb temperature must consider conditions at the tower site. The design wet bulb temperature also should not be exceeded for more than 5 percent of the time. In general, the design temperature selected is close to the average maximum wet bulb temperature in summer.

Confirm whether the wet bulb temperature is specified as ambient (the temperature in the cooling tower area) or inlet (the temperature of the air entering the tower, which is often affected by discharge vapors recirculated into the tower). As the impact of


11

recirculation cannot be known in advance, the ambient wet bulb temperature is preferred.

Confirm with the supplier if the cooling tower is able to deal with the effects of increased wet bulb temperatures.

The cold-water temperature must be low enough to exchange heat or to condense vapors at the optimum temperature level. The quantity and temperature of heat exchanged can be considered when choosing the right size cooling tower and heat exchangers at the lowest costs.

4.1.6 Relationship between range, flow and heat load The range increases when the quantity of circulated water and heat load increase. This means that increasing the range as a result of added heat load requires a larger tower. There are two possible causes for the increased range: The inlet water temperature is increased (and the cold-water temperature at the exit

remains the same). In this case it is economical to invest in removing the additional heat. The exit water temperature is decreased (and the hot water temperature at the inlet

remains the same). In this case the tower size would have to be increased considerably because the approach is also reduced, and this is not always economical.

4.1.7 Relationship between approach and wet bulb temperature The design wet bulb temperature is determined by the geographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, the smaller the tower required. For example, a 4540 m3/hr cooling tower selected for a 16.67oC range and a 4.45oC approach to 21.11oC wet bulb would be larger than the same tower to a 26.67oC wet bulb. The reason is that air at the higher wet bulb temperature is capable of picking up more heat. This is explained for the two different wet bulb temperatures: Each kg of air entering the tower at a wet bulb temperature of 21.1oC contains 18.86

kCal. If the air leaves the tower at 32.2oC wet bulb temperature, each kg of air contains 24.17 kCal. At an increase of 11.1oC, the air picks up 12.1 kCal per kg of air.

Each kg of air entering the tower at a wet bulb temperature of 26.67oC contains 24.17 kCals. If the air leaves at 37.8oC wet bulb temperature, each kg of air contains 39.67 kCal. At an increase of 11.1oC, the air picks up 15.5 kCal per kg of air, which is much more than the first scenario.

4.2 Fill media effects In a cooling tower, hot water is distributed above fill media and is cooled down through evaporation as it flows down the tower and gets in contact with air. The fill media impacts energy consumption in two ways: Electricity is used for pumping above the fill and for fans that create the air draft. An

efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore lead to lower electricity consumption.

Heat exchange between air and water is influenced by surface area of heat exchange, duration of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. The fill media determines all of these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower becomes.

There are three types of fills:


12

Splash fill media. Splash fill media generates the required heat exchange area by splashing water over the fill media into smaller water droplets. The surface area of the water droplets is the surface area for heat exchange with the air.

Film fill media. In a film fill, water forms a thin film on either side of fill sheets. The surface area of the fill sheets is the area for heat exchange with the surrounding air. Film fill can result in significant electricity savings due to fewer air and pumping head requirements.

Low-clog film fills. Low-clog film fills with higher flute sizes were recently developed to handle high turbid waters. Low clog film fills are considered as the best choice for sea water in terms of power savings and performance compared to conventional splash type fills.

Table 1: Design Values of Different Types of Fill

(BEE India, 2004; Ramarao; and Shivaraman) Splash fill Film fill Low clog film fill

Possible L/G ratio 1.1 – 1.5 1.5 – 2.0 1.4 – 1.8 Effective heat exchange area 30 – 45 m2/m3 150 m2/m3 85 - 100 m2/m3 Fill height required 5 – 10 m 1.2 – 1.5 m 1.5 – 1.8 m Pumping head required 9 – 12 m 5 – 8 m 6 – 9 m Quantity of air required High Lowest Low 4.3 Pumps and water distribution 4.3.1 Pumps Areas for energy efficiency improvements are discussed in details in the Pumps and Pumping Systems chapter. 4.3.2 Optimize cooling water treatment Cooling water treatment (e.g. to control suspended solids, algae growth) is mandatory for any cooling tower independent of what fill media is used. With increasing costs of water, efforts to increase Cycles of Concentration (COC), by cooling water treatment would help to reduce make up water requirements significantly. In large industries and power plants improving the COC is often considered a key area for water conservation. 4.3.3 Install drift eliminators It is very difficult to ignore drift problems in cooling towers. Nowadays most of the end user specifications assume a 0.02% drift loss. But thanks to technological developments and the production of PVC, manufacturers have improved drift eliminator designs. As a result drift losses can now be as low as 0.003 – 0.001%. 4.4 Cooling tower fans The purpose of a cooling tower fan is to move a specified quantity of air through the system. The fan has to overcome the system resistance, which is defined as the pressure loss, to move the air. The fan output or work done by the fan is the product of air flow and the pressure loss. The fan output and kW input determines the fan efficiency.


13

The fan efficiency in turn is greatly dependent on the profile of the blade. Blades include: Metallic blades, which are manufactured by extrusion or casting processes and therefore

it is difficult to produce ideal aerodynamic profiles Fiber reinforced plastic (FRP) blades are normally hand molded which makes it easier to

produce an optimum aerodynamic profile tailored to specific duty conditions. Because FRP fans are light, they need a low starting torque requiring a lower HP motor, the lives of the gear box, motor and bearing is increased, and maintenance is easier.

A 85-92% efficiency can be achieved with blades with an aerodynamic profile, optimum twist, taper and a high coefficient of lift to coefficient of drop ratio. However, this efficiency is drastically affected by factors such as tip clearance, obstacles to airflow and inlet shape, etc. Cases reported where metallic or glass fiber reinforced plastic fan blades have been replaced by efficient hollow FRP blades. The resulting fan energy savings were in the order of 20-30% and with simple pay back period of 6 to 7 months (NPC). The chapter Fans and Blowers gives more information about fans.


14

5. OPTION CHECKLIST This section lists the most important options to improve energy efficiency of cooling towers.

Follow manufacturer’s recommended clearances around cooling towers and relocate or modify structures that interfere with the air intake or exhaust

Optimize cooling tower fan blade angle on a seasonal and/or load basis

Correct excessive and/or uneven fan blade tip clearance and poor fan balance

In old counter-flow cooling towers, replace old spray type nozzles with new square spray nozzles that do not clog

Replace splash bars with self-extinguishing PVC cellular film fill

Install nozzles that spray in a more uniform water pattern

Clean plugged cooling tower distribution nozzles regularly

Balance flow to cooling tower hot water basins

Cover hot water basins to minimize algae growth that contributes to fouling

Optimize the blow down flow rate, taking into account the cycles of concentration (COC) limit

Replace slat type drift eliminators with low-pressure drop, self-extinguishing PVC cellular units

Restrict flows through large loads to design values

Keep the cooling water temperature to a minimum level by (a) segregating high heat loads like furnaces, air compressors, DG sets and (b) isolating cooling towers from sensitive applications like A/C plants, condensers of captive power plant etc. Note: A 1oC cooling water temperature increase may increase the A/C compressor electricity consumption by 2.7%. A 1oC drop in cooling water temperature can give a heat rate saving of 5 kCal/kWh in a thermal power plant

Monitor approach, effectiveness and cooling capacity to continuously optimize the cooling tower performance, but consider seasonal variations and side variations

Monitor liquid to gas ratio and cooling water flow rates and amend these depending on the design values and seasonal variations. For example: increase water loads during summer and times when approach is high and increase air flow during monsoon times and when approach is low.

Consider COC improvement measures for water savings

Consider energy efficient fibre reinforced plastic blade adoption for fan energy savings

Control cooling tower fans based on exit water temperatures especially in small units

Check cooling water pumps regularly to maximize their efficiency


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6. WORKSHEETS This section includes following worksheets: 1. Key Technical Specifications 2. Cooling Tower Performance

Worksheet 1 : KEY TECHNICAL SPECIFICATION

No. Parameter Units Cooling tower reference

CT 1 CT 2

1. Type of cooling tower

2. Number of tower

3. Number of cells per tower

4. Area per cell

5. Water flow m3/hr

6. Pumping power kW

7. Pumping head m

8. Fan power kW

9. Design hot water temperature 0C

10. Design cold-water temperature 0C

11. Design wet bulb temperature 0C


16

Worksheet 2: COOLING TOWER PERFORMANCE

No. Parameter reference Units Cooling tower (CT)

CT 1 CT 2

1. Dry bulb temperature °C

2. Wet bulb temperature °C

3. CT inlet temperature °C

4. CT outlet temperature °C

5. Range °C

6. Approach °C

7. CT effectiveness %

8. Average water flow kg/hr

9. Average air quantity kg/hr

10. Liquid/gas (L/G) ratio kg water/kg air

11. Evaporation loss m3/hr

12. CT heat loading kCal/hr

Monitoring Equipment

1

MONITORING EQUIPMENT

1. ELECTRICAL MEASURING INSTRUMENTS .........................................2

2. COMBUSTION ANALYZERS.....................................................................7

3. MANOMETERS...........................................................................................9

4. THERMOMETERS....................................................................................12

5. WATER FLOW METERS.........................................................................15

6. TACHOMETERS / STROBOSCOPES......................................................19

7. LEAK DETECTORS ..................................................................................22

8. LUX METERS............................................................................................24

9. REFERENCES ...........................................................................................26 Monitoring equipment can be useful to measure the actual operating parameters of various energy equipment and compare them with the design parameters to determine if energy efficiency can be improved. Or monitoring equipment can be used to identify measure steam or compressed air leaks. Parameters that are often monitored during an energy assessment are: § Basic electrical parameters in AC & DC systems: voltage (V), current (I), power factor,

active power (kW), maximum demand (kVA), reactive power (kVAr), energy consumption (kWh), frequency (Hz), harmonics, etc.

§ Other non-electrical parameters: temperature and heat flow, radiation, air and gas flow, liquid flow, revolutions per minute (RPM), air velocity, noise and vibration, dust concentration, total dissolved solids (TDS), pH, moisture content, relative humidity, flue gas analysis (CO2, O2, CO, SOx, NOx), combustion efficiency, etc.

This module provides information for various monitoring equipment that are often used during energy assessments in industry: 1. Electrical measuring instruments 2. Combustion analyzer 3. Thermometers 4. Manometers 5. Water flow meters 6. Tachometers / Stroboscopes 7. Leak detectors 8. Lux meters For each type of monitoring equipment the following information is given: § What the monitoring equipment does § Where the monitoring equipment is used § How to operate the monitoring equipment § Precautions and safety measures necessary for the monitoring equipment

Chapter-7


2

1. ELECTRICAL MEASURING INSTRUMENTS 1.1 What electrical measuring instruments do Electrical measuring instruments include clamp-on or power analyzers and are used to measure main electrical parameters such as KVA, kW, PF, Hertz, KVAr, Amps and Volts. Some of these instruments also measure harmonics. Instant measurements can be taken with hand-held meters, while more advanced ones facilitates cumulative readings with print outs at specified intervals. There are several models available in the market from different companies. One suc h instrument is the HIOKI 3286-20 Clamp-on Power Hitester (Figure 1). It measures the following parameters: § Voltage: 150 V to 600 V, 3 ranges § Current: 200 A or 1000 A, 2 ranges § Voltage/current peak § Effective/reactive/apparent power (single-phase

or 3-phase): 30 kW to 1200 kW, 14 combination patterns

§ Power factor § Reactivity § Phase angle § Frequency, § Phase detection (3-phase) § Voltage/current harmonic levels (up to 20th) Figure 1. Hioki 3286-20 Clamp-on Power Hitester (Hioko Ltd.) 1.2 Where electrical measuring instruments are used These instruments are applied on-line to measure various electrical parameters of motors, transformers, and electrical heaters. There is no need to stop the equipment while taking the measurements. 1.3 How to operate electrical measuring instruments The instrument has three leads (wires), which are connected to the crocodile clips at the end. The three leads are yellow, black and red. Figures 2 to 8 give illustrate the measurement method for various conditions. However, operating procedures may vary for different types of clamp-on or power analyzers. For the correct operation procedure the operator should always check the instruction manual supplied with the instrument.


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Figure 2. Power Measurement on Single-phase Two-wire Circuit (Hioki Ltd)

Figure 3. Power and Power Factor Measurement on Single-phase Three-wire Circuit

(Hioki Ltd) The power and power factor of a single-phase three-wire circuit are measured similarly to a single -phase two-wire circuit. Connect the black lead to the neutral wire as shown, and then switch the red lead and clamp sensor to the respective wires. Now the power and power factor between the wires can be measured.


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Figure 4. Power and Power Factor Measurement on Three-phase Three-wire Circuit

(Hioki Ltd)

Figure 5. Alternative Method of Power and Power Factor Measurement on Three-phase

Three-wire Circuit (Hioki Ltd)


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Figure 6. Power and Power Factor Measurement on Three-phase Four-wire Circuit

(Hioki Ltd)

Figure 7. Current Measurement (Hioki Ltd)

Figure 8. Voltage Measurement (Hioki Ltd)


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1.4 Precautions and safety measures Some precautions and safety measures to be taken while using clamp-on and power analyzers are: § To avoid short circuits and potentially life-threatening hazards, never attach the clamp to a

circuit that operates at more than the maximum rated voltage, or over bare conductors. § The clamp-on probe should be connected to the secondary side of a breaker, so the breaker

can prevent an accident if a short circuit occurs. § While using the instrument, use rubber hand gloves, boots, and a safety helmet, to avoid

electrical shocks, and do not use the instrument when hands are wet. § Check the operating manual of the monitoring equipment for more detailed instructions on

safety and precautions before using the equipment.


Energy Efficiency Guide for Industry in Asia – www.energyefficiencyasia.org ©UNEP 7

2. COMBUSTION ANALYZERS 2.1 What a combustion analyzer does A combustion analyzer is used to measure the composition of the flue gases after combustion has taken place. Different combustion analyzers can be ordered to match the requirements at a plant. Basically all combustion analyzers measure the percentage oxygen (O2) or carbon dioxide (CO2) in the exit flue gases and then use an inbuilt programme to calculate the combustion efficiency if required. The various types of combustion analyzers are given below:

Fuel Efficiency Monitor This measures oxygen and temperature of the flue gas. Calorific values of common fuels are fed into the microprocessor which calculates the combustion efficiency

Fyrite A hand bellow pump draws the flue gas sample into a solution inside the fyrite. A chemical reaction changes the liquid volume revealing the amount of gas. The percentage oxygen or CO2 can be read from the scale.

Gas Analyzer

This instrument has in-built chemical cells which measure various gases such as CO2, CO, NO X, SOX etc.


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2.2 Where a combustion analyzer is used Combustion analyzers are used to determine the composition of the flue gases in the duct. The duct is the large piping arrangement of rectangular configuration and is used to flush out the combusted gases to the chimney. The values for the different components of the flue gases are volume-based. Mostly these instruments measure the percentage oxygen and carbon dioxide and the temperature of the flue gas. During energy audits it is desirable to know the composition of the flue gases in order to assess the combustion conditions and efficiency and leakages of atmospheric air into the system. 2.3 How to operate Different types of the combustion analyzers are operated differently. For all types of combustion analyzers the probe is inserted into the duct through a small hole made in the duct for monitoring purposes. In case of a fyrite combustion analyzer, which is manually operated, the flue gas from the duct is sucked out using a manual pumping device. In most of the other analyzers the flue gases are pumped out of the ducts using a suction pump. The collected gases react with the chemical/cells and give readings of % oxygen or carbon dioxide. 2.4 Precautions and safety measures Some precautions and safety measures to be taken while using combustion analyzers are: § Always calibrate the instrument in open fresh air before taking a set of measurements. § Check for the clogging of the air filters of the instrument. § During measurements, ensure that the rubber tubing carrying the gases from the duct to the

instrument is not bent. § After insertion of the probe into the duct, care should be taken to wrap the left opening space

with cotton rags to ensure that there is no filtration of air into the system or air escaping the system.

§ Thick cotton hand gloves, goggles, a safety helmet and other safety gear should be worn before taking the readings. Remember that the gases you are handling are very hot!

§ Check the operating manual of the monitoring equipment for more detailed instructions on safety and precautions before using the equipment.


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3. MANOMETERS 3.1 What a manometer does Manometers are widely used instrument in energy audits for measuring the differential pressure across two points. The oldest type is the liquid -column manometer. A very simple version of a liquid-column manometer is a U-shaped tube (see Figure 9) that is half- full of liquid (usually oil, water or mercury) where the measured pressure is applied to one side of the tube whilst the reference pressure (which might be that of the atmosphere) is applied to the other. The difference in liquid level represents the applied pressure.

a b c

Figure 9. Schematic illustration of a liquid-column manometer

(Dwyer Instruments Inc.)

The principles of how a manometer works are as follows: § Figure 9a. In its simplest form the manometer is a U-tube about half filled with liquid. With

both ends of the tube open, the liquid is at the same height in each leg. § Figure 9b. When positive pressure is applied to one leg, the liquid is forced down in that leg

and up in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the pressure.

§ Figure 9c. When a vacuum is applied to one leg, the liquid rises in that leg and falls in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the amount of vacuum.

There are three main types of manometers: § Single- limb liquid-column manometer has a larger reservoir instead of one side of the U-tube

and has a scale beside the narrower column. This column can further amplify the liquid movement. Liquid-column manometers can be used to measure small differences between high pressures.

§ Flexible Membrane Type: This type uses the deflection of a flexible membrane that seals off a volume with a fixed reference-pressure. The degree of deflection of the membrane corresponds with a specific pressure. Reference tables exist to determine the pressure for different deflections.


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§ Coiled Tube Type: A third variant uses a coiled tube which will expand with increasing pressure. This causes a rotation of an arm connected to the tube.

3.2 Where the manometer is used During conducting of energy audit studies manometers are used to determine the differential pressure between two points in a duct carrying exhaust gases or air. The pressure differential is then used to calculate the velocity of flow in the duct using the Bernoulli’s equation. ( Pressure Differential = v2/2g). A more detailed account of use of manometer is given in section on hoe to operate a manometer. However the same can be used for measuring the differential pressure across two points in a pipe carrying any fluid. In this case the precaution to be taken is that the manometer should be compatible to be used for fluid flows. The velocity of flow of fluid is given by Differential pressure = f LV2/2gD where f is the friction factor of the pipe material, L is distance between two points across which pressure differential is taken, D is the diameter of the pipe and g is the gravitational constant. 3.3 How to operate a manometer It is not easy to explain one way of operating manometers. The reason is that there are many different kinds of manometers that require different ways of handling. However, some operating steps are the same. During energy audits, air velocity in ducts can be measured using a pitot tube and flows are calculated using a manometer. A sampling hole is made in the duct (pipe carrying exhaust gases) and the pitot tube is inserted into the duct. The two openings at the end of the pitot tube are connected to the two openings of the manometer. The difference in level of the manometer gives the total velocity pressure. For example, in case of a digital manometer the readings are displayed as mm of the water column.

Pitot tube Manometer

Figure 10. Measurement using Pitot Tube and Manometer (Dwyer Instruments Inc.)


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3.4 Precautions and safety measures § Manometers should not be exposed to very high pressures. In case of high pressures inclined

tune manometers should be used. § Check the operating manual of the monitoring equipment for more detailed instructions on



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4. THERMOMETERS 4.1 What a thermometer does Thermometers are instruments used to measure the temperature of fluids, surfaces or gases, for example of the flue gases after combustion has taken place. Thermometers are classified as contact thermometers or non-contact or infrared thermometers and are described below. Contact thermometer There are many types of contact thermometers. A simple clinical thermometer is the best known example of a contact thermometer. However, for the purpose of energy audits in an industrial plant we generally use thermocouples for measuring temperatures with a high accuracy. It consists of two dissimilar metals, joined together at one end. The thermocouple metal alloys are commonly available as wire. A thermocouple is available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. There are high temperature calibrations R, S, C and GB. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. Although the thermocouple calibration dictates the temperature range, the maximum range is also limited by the diameter of the thermocouple wire. Non-contact or infrared thermometer A non-contact or infrared thermometer allows the measurement of temperatures without physical contact between the thermometer and the object of which the temperature is determined. The thermometer is directed at the surface and immediately gives a temperature reading. This instrument is useful for measuring hot spots in furnaces, surface temperatures etc. Infrared thermometer allows users to measure temperature in applications where conventional sensors cannot be used or cannot produce accurate temperature readings, such as:

Figure 11. Thermocouple Thermometer (Reliability Direct, Inc)


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§ When a fast response or measurement is required, such as moving objects (i.e. rollers, moving machinery, or a conveyor belt)

§ Where non-contact measurements are required because of contamination or hazardous reasons (such as high voltage)

§ Too large distances or heights § Too high temperatures for thermocouples or other contact sensors § The object is contained in a vacuum or other controlled atmosphere § The object is surrounded by an electromagnetic field (such as induction heating) The basic principle of the infrared thermometer is that all objects emit infrared energy. The hotter an object, the more active its molecules are, and the more infrared energy it emits. An infrared thermometer contains a lens that focuses the collected infrared energy from the object onto a detector. The detector converts the energy into an electrical signal, which is amplified and displayed in units of temperature after corrections for ambient temperature variations.

4.2 Where the thermometer is used In energy audits, the temperature is one of the most important parameters to be measured in order to determine the thermal energy loss or to make a thermal energy balance. Temperature measurements are taken for the audit of air conditioning units, boilers, furnaces, steam systems, waste heat recovery systems, heat exchangers, etc. During the audits, the temperature can be measured of the: § Ambient air § Chilled water in refrigeration plants § Inlet air into the Air Handling unit of AC plant § Cooling water inlet and out let at the Cooling Tower

Figure 12. Non-contact or Infrared Thermometer (Nitonuk Ltd. 2003)


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§ Surfaces of steam pipelines, boilers, kilns § Input water to the boiler § Exhaust gases § Condensate returned § Pre heated air supply for combustion § Temperature of the fuel oil 4.3 How to operate a thermometer The thermocouple (contact thermometer) consists of two dissimilar metals, joined together at one end. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature. A probe is inserted into a liquid or gaseous stream to measure the temperature of, for example, flue gas, hot air, or water. A leaf type probe is used to measure surface temperatures. In most of the cases the thermocouple directly gives the reading in the desired units( Centigrade or Fahrenheit) on a digital panel. The operation of a non-contact or infrared thermometer is simple. The infrared thermometer (gun) is pointed towards the surface where the temperature must be measured. The measurement result is read directly from the panel. 4.4 Precautions and safety measures The following precautions and safety measures apply when using a thermometer: § The probe must be immersed in the fluid and the measurement must be taken after 1-2

minutes, i.e. after the stabilization of the readings. § Before using the thermocouple, the temperature range for which the thermocouple is

designed for should be checked. § The probe of the thermocouple should never touch the bare flame. § Before using a non-contact thermometer the emissivity should be set in accordance with the

surface where the temperature is to be measured. § Check the operating manual of the monitoring equipment for more detailed instructions on



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5. WATER FLOW METERS 5.1 What a water flow meter does A flow meter is an instrument used to measure the linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. This section deals specifically with water flow meters. The choice of method or type of water flow meter depends on the site conditions and the required measurement accuracy. Apart from water flow meters, there are several methods available to measure water flow during an audit. Two common methods to obtain reasonably accurate estimates of the water flow are: § Time of fill method: Water is allowed to fill a known volume of a vessel or tank (m3 ). The

time taken to fill up this volume is recorded using a stop watch (seconds). The volume divided by the time gives the average flow in m3/sec.

§ Float method: This method is generally used to measure the flow in an open channel. A specific distance (for example 25 meters or 50 meters) is marked on the side of the channel. A ping-pong ball is placed on the water and the time it takes for the ball to float to the marked distance is recorded. Multiple readings are taken to get a more accurate timing. The velocity of the water is calculated by Distance traveled by the ball / Average time taken. Depending on the flow conditions and the site characteristics the calculated velocity is further divided by a factor 0.8 to 0.9 to obtain the peak velocity in an open channel, since the velocity at the surface is reduced due to drag forces of the wind etc.

Some of the most common types of flow meters are given below: Rotameter or variable area flow meter for gases and liquids. The rotameter consists of a tapered tube and a float. It is the most widely used variable-area flow meter because of its low cost, simplicity, low pressure drop, relatively wide range ability, and linear output.

Figure 13. Rotameter (Omega Engineering Ltd)


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Variable flow meters - spring and piston flow meters for gases and liquids. Piston-type flow meters use an annular orifice formed by a piston and a tapered cone. The piston is held in place at the base of the cone (in the "no flow position") by a calibrated spring. Scales are based on specific gravities of 0.84 for oil meters and 1.0 for water meters. Their simple design and the ease with which they can be equipped to transmit electrical signals has made them an economical alternative to rotameters for flow rate indication and control. Ultrasonic flow meters (Non-Intrusive or Doppler) for Liquids Ultrasonic doppler flow meters are commonly used in dirty applications such as wastewater and other dirty fluids and slurries which ordinarily cause damage to conventional sensors. The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion.

Figure 14. Spring and Piston Flow Meters (Omega Engineering Ltd)

Figure 15. Ultrasonic Flow Meter (Dynasonics Ltd)


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Turbine flow meters The turbine meter is a very accurate meter (0.5% of the reading) and can be used for clean liquids and viscous liquids up to 100 centistokes. A minimum of 10 pipe diameters of straight pipe is required on the inlet. The most common outputs are a sine wave or square wave frequencies, but signal conditioners can be placed on the top of the meter for analog outputs and explosion proof classifications. The meters consist of a multi-bladed rotor mounted at right angles to the flow and suspended in the fluid stream on a free-running bearing. Paddlewheel sensors Paddlewheel sensors are one of the most popular cost effective flow meters for water or water-like fluids. Many are offered with flow fittings or insertions styles. These meters, like the turbine meter, require a minimum of 10 pipe diameters of straight pipe on the inlet and 5 on the outlet. Chemical compatibility should be verified when not using water. Sine wave and square wave pulse outputs are typical but transmitters are available for integral or panel mounting. The rotor of the paddlewheel sensor is perpendicular to the flow and is in contact with only a limited cross section of the flow. Positive displacement flow meters These meters are used for water applications when no straight pipe is available and when turbine meters and paddlewheel sensor would cause too much turbulence. The positive displacement flow meters are also used for measuring the flow of viscous liquids. Vortex meters The main advantages of vortex meters are their low sensitivity to variations in process conditions and low wear relative to orifices or turbine meters. Also, initial and maintenance costs are low. For these reasons, they have are widely accepted by users. Vortex meters do require sizing. Magnetic flow meters for conductive liquids These flow meters are available in in- line or insertion style. The magnetic flow meters do not have any moving parts and are ideal for measuring the flow of wastewater or any dirty liquid that

Figure 16. How an Ultrasonic Flow Meter works (Eesiflow International Pty Ltd.)


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is conductive. Displays are integral or an analog output can be used for remote monitoring or data logging. 5.2 Where the water flow meter is used During energy audits, water flow measurements are of significant importance. Generally the measurements are taken to quantify the amo unt of liquid/water flowing in a pipe. If there is no flow measurement device attached to the pipe line, then the flow can be quantified using an ultrasonic flow meter. Typical cases where the measurement of the water flow is absolutely essential are as pa rt of determining the efficiency of pumps, the efficiency of cooling towers, chillers and air conditioning plants, heat exchangers, and condensers. 5.3 How to operate water flow meters There are many varieties of the ultrasonic flow meters available on the market. The functioning of each model differs from the other. However the basic principle of all remains same. The 2 probes/sensors of the ultrasonic flow meters are placed on the surface of the pipe at some distance apart along a straight line. The diameter of the pipe determines the distance between the probes. When the meter is turned on, it generates sonic waves which are transmitted via one of the probes/sensors and are received by the other. The meter is calibrated to display the velocity or volume of the flow of liquid inside the pipe, based on the time required by the sonic waves to travel the distance from one sensor to the other. 5.4 Precautions and safety measures The following precautions should be taken when using water flow meters: § The probes/sensors should be placed on the surface of the pipes after thorough cleaning of

the spot. Care should be taken that there is no speck of paint etc. Ideally the spot where the sensors are placed should be filed by a emery paper.

§ The meter would not give the measurements if the internal condition of the pipe is corroded or has any alga growth.

§ The measurement should be taken where the pipe flow is expected to be laminar and the pipe must be flowing full.

§ Check the operating manual of the monitoring equipment for more detailed instructions on safety and precautions before using the equipment.


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6. TACHOMETERS / STROBOSCOPES 6.1 What tachometers and stroboscopes do In any audit exercise speed measurements of for example motors are critical as they may change with frequency, belt slip and loading. There are two main types of speed measurement instruments: the tachometer and the stroboscope.

Tachometer A simple tachometer is a contact type instrument, which can be used to measure speeds where direct access is possible. Stroboscope More sophisticated and safer instruments to measure speed are non-contact instruments, such as stroboscopes. A stroboscope is a source of flashing light that can be synchronized with any fast, repetitive motion so that a rapidly moving device seems to stand still, or to move slowly. To illustrate this principle, consider the following example: Assume a white disk with a single black dot mounted on the shaft of an 1800-rpm motor. When the disk is rotating at 1800 rpm, it is impossible for the human eye to distinguish a single image and the dot will appear to be a blurred continuous circle. When illuminated by the flashing stroboscope light, synchronized to flash once every revolution of the disk (when the dot is at 3 o’clock, for example), the dot will be seen at this position - and only at this position - at a rate of 1800 times each minute. Thus, the dot will appear to “freeze” or stand still. If the flash rate of the stroboscope is slowed to 1799 flashes per minute, the dot will be illuminated at a slightly different position each time the disc revolves, and the dot will appear to

Figure 17. A tachometer (left) and a stroboscope (right) (Reliability Direct, Inc)


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move slowly in the direction of rotation through 360° and arrive at its original position 1 minute later. A similar movement, but in a direction opposite the rotation of the dot, will be observed if the flash rate of the stroboscope is increased to 1801 fpm. If desired, the rate of apparent movement can be speeded up by further increases or decreases in the strobe flash rate. When the image is stopped, the flash rate of the strobe equals the speed of the moving object. And since the flash rate is known the speed of the object is also known. Thus the stroboscope has a dual purpose of measuring speed and of apparently slowing down or stopping rapid motion for observation. The practical significance of the slow-motion effect is that, since it is the true copy of the high speed motion, all irregularities (vibration, torsion, chattering, whip) present in the high speed motion can be studied. .

For audit studies we generally use contact type of tachometers since they are readily available. 6.2 Where tachometers and stroboscopes are used Tachometers and stroboscopes are used measure the speed of rotation of motors, fans, pulleys etc. 6.3 How to operate a tachometer and stroboscope In a contact-type tachometer, the wheel of the tachometer is brought in contact with the rotating body. Due to friction between the two, after few seconds the speed of the wheel of the tachometer is the same as the speed of the rotating body. This speed is displayed on the panel as rpm. The digital stroboscope is a versatile flashing light source that is used to measure the speed of fast-moving objects or to produce the optical effect of stopping or slowing down high-speed motion for purposes of observation, analysis, or high-speed photography. The stroboscope emits a high- intensity, short-duration flash of light. The instrument features an electronic pulse

Figure 18. Principle of the Working of a Stroboscope (NPC, 2006)


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generator that controls the flash rate, a line-operated power supply, and a light-emitting diode (LED) readout in flashes per minute. The light can be aimed at most moving objects, including those in otherwise inaccessible areas. When measuring the rotational speed of an object, set the flash rate initially to a higher setting than the estimated speed of the object. Then, slowly reduce the flash rate until the first single image appears. At this point, the strobe flash rate is equal to the rotational speed of the object, and the speed can be read directly from the digital display. 6.4 Precautions and safety measures The following precautions should be taken when using a tachometer and stroboscopes: § Care should be taken while bringing the wheel of the tachometer in contact with the rotating

body. § From a safety point of view never wear loose clothing when taking measurements with

tachometers. § Avoid being alone when taking measurements. § Check the operating manual of the monitoring equipment for more detailed instructions on



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7. LEAK DETECTORS 7.1 What leak detectors do As the name implies, ultrasonic leak detectors detect the ultrasonic sound of a leak. You are probably familiar with the hissing sound a large leak makes. Smaller leaks also emit a sound, however the frequency is too high for our ears to detect it. An ultrasonic leak detector transforms the ultrasonic hissing sound to a sound that humans can hear, and thus leads to the source of the leak.

Some other information about ultrasonic leak detectors includes: § Distance and access. Some leaks can be heard from a several meters away, therefore access

to the leak is not always necessary. As long as the leak is turbulent, there will be enough sound that can be detected ultrasonically.

§ Pressure. High pressure of leaks is not necessary. Ultrasonic can detect pinhole leaks with a pressure as low as 1 psi. However, the more pressure behind the leak, the easier it will be to locate.

§ Sensitivity to sound. Ultrasonic leak detectors are very sensitive to sound. A good ultrasonic leak detector can actually let you hear the blink of the human eye. A leak test can be done in an enclosed area which is saturated with refrigerant, and the only indication an ultrasonic will give you is the sound of the leak. A good ultrasonic detector uses an electronic process called “heterodyning” to convert this high frequency leak sound to a lower range where the hissing of the leak can be heard through a set of headphones, and traced to its source. Any turbulent gas will generate ultrasound when it leaks, therefore it does not matter what refrigerant you are leak testing. Ultrasonic detectors will even detect air as it rushes into a system under vacuum.

§ Background noises. Because the ultrasonic detector is focused on a specific band/frequency of sound, it will not detect wind, voices, traffic, and most normal operational sounds. However, larger systems with a multitude of pressure regulating valves and high velocity flow may produce hissing sounds at frequencies where ultrasonic detectors are most

Figure 19. Leak Detectors (Reliability Direct, Inc)


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sensitive. In this case it would be necessary to shut the system down, or use another method of detecting leaks.

§ Choice of leak detector. Always consider the capabilities and the limitations of the leak detection method used. It is thereby important to consider more than just the sensitivity under laboratory / testing conditions when choosing a leak detector. For example, a highly sensitive “sniffer” type was able to detect a leak of 0.25oz. of refrigerant per year in a controlled laboratory setting. But the leak detector would give different results when used on the windy and dirty rooftop where you might be trying to find refrigerant leaks.

7.2 Where leak detectors are used Ultrasonic leak detectors are used to detect leaks of compressed air and other gases which are normally not possible to detect with the human ear. No leak detector will find every leak, every time. It is often a combination of several available methods which will insure the highest rate of success. 7.3 How to operate a leak detectors It is not easy to generalize the operating method of a leak detector. The reason is that there are many different kinds of leak detectors that require different ways of handling. However, some steps are the same: § The probe of the ultrasonic leak detector instrument is placed near the gas/steam pipe line

where a leak is suspected § The headphone attached to the instrument is placed on the ears § The probe is moved slowly until the person can hear a hissing sound through headphone,

indicating a leak § The position is marked to identify the location of the leakage 7.4 Precautions and safety measures The following measures should be taken when using ultrasonic leak detectors: § Dust or smoke should not be coming out of the pipe, otherwise dust/smoke will choke the

probe and leads to the instrument failure § Avoid measurement at the places where sound levels are high § Check the operating manual of the monitoring equipment for more detailed instructions on



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8. LUX METERS 8.1 What lux meters do Lux meters are used to measure illumination (light) levels. Most lux meters consist of a body, a sensor with a photo cell, and a display panel. The sensor is placed under the light source. The light that falls on the photo cell has energy, which is transferred by the photo cell into electric current. The more light is absorbed by the cell, the higher the generated current. The meter reads the electrical current and calculates the appropriate value of either Lux or Foot candles. This value is shown on the display panel. A key thing to remember about light is that it is usually made up of many different types (colors) of light at different wavelengths. The reading, therefore, is a result of the combined effects of all the wavelengths. A standard color can be referred to as colo r temperature and is expressed in degrees Kelvin. The standard color temperature for calibration of most light meters is 2856 degrees Kelvin, which is more yellow than pure white. Different types of light bulbs burn at different color temperatures. Lux meter readings will, therefore, vary with different light sources of the same intensity. This is why some lights seem "harsher" or “softer” than others.

8.2 Where lux meters are used Lux meters are used to measure illumination levels in offices, factories etc. 8.3 How to operate a lux meters This instrument is very simple to operate. The sensor is to be placed at the work station or at the place where intensity of the light is to be measured, and the instrument will directly give the reading on the display panel.

Figure 20. Lux Meters (Reliability Direct, Inc)


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8.4 Precautions and safety measures The following measures should be taken when working with lux meters: § The sensor is to be properly placed on the work station to obtain an accurate reading § Due to the high sensitivity of sensor it should be stored safely § Check the operating manual of the monitoring equipment for more detailed instructions on


Energy is the ability to do work and work is the transfer of energy from one form to another. Inpractical terms, energy is what we use to manipulate the world around us, whether by excitingour muscles, by using electricity, or by using mechanical devices such as automobiles. Energycomes in different forms - heat (thermal), light (radiant), mechanical, electrical, chemical, andnuclear energy.

There are two types of energy - stored (potential) energy and working (kinetic) energy. Forexample, the food we eat contains chemical energy, and our body stores this energy until werelease it when we work or play.

Potential energy is stored energy and the energy of position (gravitational). It exists in variousforms.

Chemical EnergyChemical energy is the energy stored in the bonds of atoms and molecules. Biomass, petrole-um, natural gas, propane and coal are examples of stored chemical energy.

Nuclear EnergyNuclear energy is the energy stored in the nucleus of an atom - the energy that holds the nucle-us together. The nucleus of a uranium atom is an example of nuclear energy.

Stored Mechanical EnergyStored mechanical energy is energy stored in objects by the application of a force. Compressedsprings and stretched rubber bands are examples of stored mechanical energy.

BASICS OF ENERGY AND ITS VARIOUS FORMS

1 Definition

2 Various Forms of Energy

2.1 Potential Energy

Chapter- 8

1

Gravitational EnergyGravitational energy is the energy of place or position. Water in a reservoir behind a hydropow-er dam is an example of gravitational energy. When the water is released to spin the turbines, itbecomes motion energy.

2.2.2 Kinetic Energy

Kinetic energy is energy in motion- the motion of waves, electrons, atoms, molecules and sub-stances. It exists in various forms.

Radiant EnergyRadiant energy is electromagnetic energy that travels in transverse waves. Radiant energyincludes visible light, x-rays, gamma rays and radio waves. Solar energy is an example of radi-ant energy.

Thermal EnergyThermal energy (or heat) is the internal energy in substances- the vibration and movement ofatoms and molecules within substances. Geothermal energy is an example of thermal energy.

MotionThe movement of objects or substances from one place to another is motion. Wind andhydropower are examples of motion.

SoundSound is the movement of energy through substances in longitudinal (compression/rarefaction)waves.

Electrical EnergyElectrical energy is the movement of electrons. Lightning and electricity are examples of elec-trical energy.

2.2.3 Energy Conversion

Energy is defined as "the ability to do work." In this sense, examples of work include movingsomething, lifting something, warming something, or lighting something. The following is anexample of the transformation of different types of energy into heat and power.

It is difficult to imagine spending an entire day without using energy. We use energy to light ourcities and homes, to power machinery in factories, cook our food, play music, and operate ourTV.

More the number of conversion stages, lesserthe overall energy efficiency

Oil burns to generate heat --> Heat boils water -->

Water turns to steam -->Steam pressure turns a turbine -->

Turbine turns an electric generator -->Generator produces electricity -->

Electricity powers light bulbs -->Light bulbs give off light and heat

Basics of Energy and its Various Forms

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2.2.4 Grades of Energy

High-Grade Energy

Electrical and chemical energy are high-grade energy, because the energy is concentrated in asmall space. Even a small amount of electrical and chemical energy can do a great amount ofwork. The molecules or particles that store these forms of energy are highly ordered and com-pact and thus considered as high grade energy. High-grade energy like electricity is better usedfor high grade applications like melting of metals rather than simply heating of water.

Low-Grade Energy

Heat is low-grade energy. Heat can still be used to do work (example of a heater boiling water),but it rapidly dissipates. The molecules, in which this kind of energy is stored (air and watermolecules), are more randomly distributed than the molecules of carbon in a coal. This disor-dered state of the molecules and the dissipated energy are classified as low-grade energy.

Electric current is divided into two types: Directional Current (DC) and Alternating Current(AC).

Directional (Direct) Current

A non-varying, unidirectional electric current (Example: Current produced by batteries)

Characteristics: • Direction of the flow of positive and negative charges does not change with time• Direction of current (direction of flow for positive charges) is constant with time• Potential difference (voltage) between two points of the circuit does not change polarity

with time

Alternating Current

A current which reverses in regularly recurring intervals of time and which has alternately pos-itive and negative values, and occurring a specified number of times per second. (Example:Household electricity produced by generators, Electricity supplied by utilities.)

Characteristics: · Direction of the current reverses periodically with time· Voltage (tension) between two points of the circuit changes polarity with time.· In 50 cycle AC, current reverses direction 100 times a second (two times during onecycle)

Ampere (A)

Current is the rate of flow of charge. The ampere is the basic unit of electric current. It is thatcurrent which produces a specified force between two parallel wires, which are 1 metre apartin a vacuum.

Voltage (V)

The volt is the International System of Units (SI) measure of electric potential or electromo-


3 Electrical Energy Basics

3

kVAr (Reactive Power)

kVAr is the reactive power. Reactive power is the portion of apparent power that does no work.This type of power must be supplied to all types of magnetic equipment, such as motors, trans-formers etc. Larger the magnetizing requirement, larger the kVAr.

Kilowatt (kW) (Active Power)

kW is the active power or the work-producing part of apparent power.

tive force. A potential of one volt appears across a resistance of one ohm when a current of oneampere flows through that resistance.

1000 V = 1 kiloVolts (kV)

Resistance

VoltageResistance =

_______

Current

The unit of resistance is ohm (Ω)

Ohm' Law

Ohm's law states that the current through a conductor is directly proportional to the potentialdifference across it, provided the temperature and other external conditions remain constant.

Frequency

The supply frequency tells us the cycles at which alternating current changes. The unit of fre-quency is hertz (Hz :cycles per second).

Kilovolt Ampere (kVA)

It is the product of kilovolts and amperes. This measures the electrical load on a circuit or sys-tem. It is also called the apparent power.

1000

AmperesxVoltage(kVA)powerApparent,circuit electrical phase single aFor =

1000)(,sin

factorPowerxAmperesxVoltagekWPowerphasegleFor =

1000

732.1)(,

factorPowerxAmperesxVoltagexkWPowerphaseThreeFor =

1000

AmperesxVoltagex3(kVA)powerApparent,circuit electrical phase threeaFor =


4

Power Factor

Power Factor (PF) is the ratio between the active power (kW) and apparent power (kVA).

When current lags the voltage like in inductive loads, it is called lagging power factor and whencurrent leads the voltage like in capacitive loads, it is called leading power factor.

Inductive loads such as induction motors, transformers, discharge lamp, etc. absorb com-paratively more lagging reactive power (kVAr) and hence, their power factor is poor. Lower thepower factor; electrical network is loaded with more current. It would be advisable to havehighest power factor (close to 1) so that network carries only active power which does realwork. PF improvement is done by installing capacitors near the load centers, which improvepower factor from the point of installation back to the generating station.

Kilowatt-hour (kWh)

Kilowatt-hour is the energy consumed by 1000 Watts in one hour. If 1kW (1000 watts) of a elec-trical equipment is operated for 1 hour, it would consume 1 kWh of energy (1 unit of electrici-ty).

For a company, it is the amount of electrical units in kWh recorded in the plant over a monthfor billing purpose. The company is charged / billed based on kWh consumption.

Electricity Tariff

Calculation of electric bill for a company

Electrical utility or power supplying companies charge industrial customers not only based onthe amount of energy used (kWh) but also on the peak demand (kVA) for each month.

Contract Demand

Contract demand is the amount of electric power that a customer demands from utility in a spec-ified interval. Unit used is kVA or kW. It is the amount of electric power that the consumeragreed upon with the utility. This would mean that utility has to plan for the specified capacity.

Maximum demand

Maximum demand is the highest average kVA recorded during any one-demand interval with-in the month. The demand interval is normally 30 minutes, but may vary from utility to utilityfrom 15 minutes to 60 minutes. The demand is measured using a tri-vector meter / digital ener-gy meter.


5

41

Prediction of Load

While considering the methods of load prediction, some of the terms used in connection withpower supply must be appreciated.Connected Load - is the nameplate rating (in kW or kVA) of the apparatus installed on a con-sumer's premises.Demand Factor - is the ratio of maximum demand to the connected load.Load Factor - The ratio of average load to maximum load.

The load factor can also be defined as the ratio of the energy consumed during a given periodto the energy, which would have been used if the maximum load had been maintained through-out that period. For example, load factor for a day (24 hours) will be given by:

PF Measurement

A power analyzer can measure PF directly, or alternately kWh, kVAh or kVArh readings arerecorded from the billing meter installed at the incoming point of supply. The relation kWh /kVAh gives the power factor.

Time of Day (TOD) Tariff

Many electrical utilitieslike to have flatdemand curve toachieve high plant effi-ciency. They encourageuser to draw morepower during off-peakhours (say during nighttime) and less powerduring peak hours. Asper their plan, theyoffer TOD Tariff,which may be incen-tives or disincentives.Energy meter willrecord peak and non-peak consumption sep-arately by timer con-trol. TOD tariff givesopportunity for the user to reduce their billing, as off peak hour tariff charged are quite low incomparison to peak hour tariff.

LoadMaximum

Load Average Factor Load =

HoursxrecordedloadMaximum

hoursduringconsumedEnergyFactorLoad

24

24=


6

Three phase AC power measurement

Most of the motive drives such as pumps, compressors, machines etc. operate with 3 phase ACInduction motor. Power consumption can be determined by using the relation.

Power = √3 x V x I x CosΦPortable power analysers /instruments are available for measuring all electrical parameters.

Example:

A 3-phase AC induction motor (20 kW capacity) is used for pumping operation. Electricalparameter such as current, volt and power factor were measured with power analyzer. Findenergy consumption of motor in one hour? (line volts. = 440 V, line current = 25 amps and PF= 0.90).

Energy consumption = √ 3 x 0.440 (kV) x 25(A) x 0.90(PF) x 1(hour) = 17.15 kWh

Motor loading calculation

The nameplate details of motor, kW or HP indicate the output parameters of the motor at fullload. The voltage, amps and PF refer to the rated input parameters at full load.

Example:

A three phase,10 kW motor has the name plate details as 415 V, 18.2 amps and 0.9 PF. Actualinput measurement shows 415 V, 12 amps and 0.7 PF which was measured with power analyz-er during motor running.

Rated output at full load = 10 kWRated input at full load = 1.732 x 0.415 x 18.2 x 0.9 = 11.8 kWThe rated efficiency of motor at full load = (10 x 100) / 11.8 = 85%

Measured (Actual) input power = 1.732x 0.415 x 12x 0.7 = 6.0 kW

Which applications use single-phase power in an industry?

Single-phase power is mostly used for lighting, fractional HP motors and electric heater appli-cations.

Example :

A 400 Watt mercury vapor lamp was switched on for 10 hours per day. The supply volt is 230V. Find the power consumption per day? (Volt = 230 V, Current = 2 amps, PF = 0.8)

Electricity consumption (kWh) = V x I x Cos x No of Hours = 0.230 x 2 x 0.8 x 10 = 3.7 kWh or Units

%2.511008.11

0.6100% === xx

kWRated

kWMeasuredloadingMotor


7

Example :

An electric heater of 230 V, 5 kW rating is used for hot water generation in an industry. Findelectricity consumption per hour (a) at the rated voltage (b) at 200 V

(a) Electricity consumption (kWh) at rated voltage = 5 kW x 1 hour = 5 kWh. (b) Electricity consumption at 200 V (kWh) = (200 / 230)2 x 5 kW x 1 hour = 3.78 kWh.

Temperature and PressureTemperature and pressure are measures of the physical state of a substance. They are closelyrelated to the energy contained in the substance. As a result, measurements of temperature andpressure provide a means of determining energy content.

Temperature

It is the degree of hotness or coldness measured on a definite scale. Heat is a form of energy;temperature is a measure of its thermal effects. In other words, temperature is a means of deter-mining sensible heat content of the substanceIn the Celsius scale the freezing point of water is 0°C and the boiling point of water is 100°Cat atmospheric pressure.

To change temperature given in Fahrenheit (°F) to Celsius (°C)Start with (°F); subtract 32; multiply by 5; divide by 9; the answer is (°C)

To change temperature given in Celsius (°C) to Fahrenheit (°F)Start with (°C); multiply by 9; divide by 5; add on 32; the answer is (°F)

Pressure

It is the force per unit area applied to outside of a body. When we heat a gas in a confined space,we create more force; a pressure increase. For example, heating the air inside a balloon willcause the balloon to stretch as the pressure increases.

Pressure, therefore, is also indicative of stored energy. Steam at high pressures containsmuch more energy than at low pressures.

Heat

Heat is a form of energy, a distinct and measurable property of all matter. The quantity of heatdepends on the quantity and type of substance involved.

Unit of Heat

Calorie is the unit for measuring the quantity of heat. It is the quantity of heat, which can raisethe temperature of 1 g of water by 1°C.

Calorie is too small a unit for many purposes. Therefore, a bigger unit Kilocalorie (1 Kilocalorie

°C = (°F - 32) x 5/9


4 Thermal Energy Basics

8

= 1000 calories) is used to measure heat. 1 kilocalorie can raise the temperature of 1000g (i.e.1kg) of water by 1°C.

However, nowadays generally joule as the unit of heat energy is used. It is the internation-ally accepted unit. Its relationship with calorie is as follows:

1 Calorie = 4.187 J

Specific Heat

If the same amount of heat energy is supplied to equal quantities of water and milk, their tem-perature goes up by different amounts. This property is called the specific heat of a substanceand is defined as the quantity of heat required to raise the temperature of 1kg of a substancethrough 1°C.

The specific heat of water is very high as compared to other common substances; it takes alot of heat to raise the temperature of water. Also, when water is cooled, it gives out a largequantity of heat.

Sensible heat

It is that heat which when added or subtracted results in a change of temperature.

Quantity of Heat

The quantity of heat, Q, supplied to a substance to increase its temperature by t°C depends on

– mass of the substance (m)– increase in temperature (∆t)– specific heat of the substance (Cp)

TABLE 2.1 SPECIFIC HEAT OF SOME COMMON

SUBSTANCES

Substance Specific Heat (Joules / kg °C)

Lead 130

Mercury 140

Brass 380

Copper 390

Iron 470

Glass 670

Aluminium 910

Rubber 1890

Ice 2100

Alcohol 2400

Water 4200


9

The quantity of heat is given by:

Q = mass x specific heat x increase in temperature Q = m x Cp x ∆t

Phase Change

The change of state from the solid state to a liquid state is called fusion. The fixed temperatureat which a solid changes into a liquid is called its melting point.

The change of a state from a liquid state to a gas is called vaporization.

Latent heat of fusion

The latent heat of fusion of a substance is the quantity of heat required to convert 1kg solid toliquid state without change of temperature. It is represented by the symbol L. Its unit is Jouleper kilogram (J/Kg)

Thus, L (ice) = 336000 J/kg,

Latent Heat of Vaporization

The latent heat of vaporization of a substance is the quantity of heat required to change 1kg ofthe substance from liquid to vapour state without change of temperature. It is also denoted bythe symbol L and its unit is also J/kg. The latent heat of vaporization of water is 22,60,000 J/kg.

When 1 kg of steam at 100°C condenses to form water at 100°C, it gives out 2260 kJ (540kCals) of heat. Steam gives out more heat than an equal amount of boiling water because of itslatent heat.

Latent heat

It is the change in heat content of a substance, when its physical state is changed without achange in temperature.

Super Heat

The heating of vapour, particularly saturated steam to a temperature much higher than the boil-ing point at the existing pressure. This is done in power plants to improve efficiency and toavoid condensation in the turbine.

Humidity

The moisture content of air is referred to as humidity and may be expressed in two ways: spe-cific humidity and relative humidity.

Specific Humidity

It is the actual weight of water vapour mixed in a kg of dry air.

Humidity Factor

Humidity factor = kg of water per kg of dry air (kg/kg).


10

Relative Humidity (RH)

It is the measure of degree of saturation of the air at any dry-bulb (DB) temperature. Relativehumidity given as a percentage is the actual water content of the air divided by the moisturecontent of fully saturated air at the existing temperature.

Dew Point

It is the temperature at which condensation of water vapour from the air begins as the temper-ature of the air-water vapour mixture falls.

Dry bulb Temperature

It is an indication of the sensible heat content of air-water vapour mixtures.

Wet bulb Temperature

It is a measure of total heat content or enthalpy. It is the temperature approached by the dry bulband the dew point as saturation occurs.

Dew Point Temperature

It is a measure of the latent heat content of air-water vapour mixtures and since latent heat is afunction of moisture content, the dew point temperature is determined by the moisture content.

Fuel Density

Density is the ratio of the mass of the fuel to the volume of the fuel at a stated temperature.

Specific gravity of fuel

The density of fuel, relative to water, is called specific gravity. The specific gravity of water isdefined as 1. As it is a ratio there are no units. Higher the specific gravity, higher will be theheating values.

Viscosity

The viscosity of a fluid is a measure of its internal resistance to flow. All liquid fuels decreasein viscosity with increasing temperature

Calorific Value

Energy content in an organic matter (Calorific Value) can be measured by burning it and mea-suring the heat released. This is done by placing a sample of known mass in a bomb calorime-ter, a device that is completely sealed and insulated to prevent heat loss. A thermometer isplaced inside (but it can be read from the outside) and the increase in temperature after the sam-ple is burnt completely is measured. From this data, energy content in the organic matter can befound out.

The heating value of fuel is the measure of the heat released during the complete combus-tion of unit weight of fuel. It is expressed as Gross Calorific Value (GCV) or Net Calorific Value(NCV). The difference between GCV and NCV is the heat of vaporization of the moisture andatomic hydrogen (conversion to water vapour) in the fuel. Typical GCV and NCV for heavy fueloil are 10,500 kcal/kg and 9,800 kcal/kg.


11

Heat Transfer

Heat will always be transferred from higher temperature to lower temperature independent ofthe mode. The energy transferred is measured in Joules (kcal or Btu). The rate of energy trans-fer, more commonly called heat transfer, is measured in Joules/second (kcal/hr or Btu/hr).Heat is transferred by three primary modes: o Conduction (Energy transfer in a solid) o Convection (Energy transfer in a fluid)o Radiation (Does not need a material to travel through)

Conduction

The conduction of heat takes place, when two bodies are in contact with one another. If onebody is at a higher temperature than the other, the motion of the molecules in the hotter bodywill vibrate the molecules at the point of contact in the cooler body and consequently result inincrease in temperature.

The amount of heat transferred by conduction depends upon the temperature difference, theproperties of the material involved, the thickness of the material, the surface contact area, andthe duration of the transfer.

Good conductors of heat are typically substances that are dense as they have moleculesclose together. This allows the molecular agitation process to permeate the substance easily. So,metals are good conductors of heat, while gaseous substance, having low densities or widelyspaced molecules, are poor conductors of heat. Poor conductors of heat are usually called insu-lators.

The measure of the ability of a substance to insulate is its thermal resistance. This is com-monly referred to as the R-value (RSI in metric). The R-value is generally the inverse of thethermal conductivity, the ability to conduct heat.

Typical units of measure for conductive heat transfer are:

Per unit area (for a given thickness)Metric (SI) : Watt per square meter (W/m2 )

OverallMetric (SI) : Watt (W) or kilowatts (kW)

Convection

The transfer of heat by convection involves the movement of a fluid such as a gas or liquid fromthe hot to the cold portion. There are two types of convection: natural and forced.

In case of natural convection, the fluid in contact with or adjacent to a high temperaturebody is heated by conduction. As it is heated, it expands, becomes less dense and consequent-ly rises. This begins a fluid motion process in which a circulating current of fluid moves pastthe heated body, continuously transferring heat away from it.

In the case of forced convection, the movement of the fluid is forced by a fan, pump or otherexternal means. A centralized hot air heating system is a good example of forced convection.

Convection depends on the thermal properties of the fluid as well as surface conditions atthe body and other factors that affect the ability of the fluid to flow. With a low conductivityfluid such as air, a rough surface can trap air against the surface reducing the conductive heat


12

transfer and consequently reducing the convective currents.Units of measure for rate of convective heat transfer are:

Metric (SI) : Watt (W) or kilowatts (kW)

Thermal Radiation

Thermal radiation is a process in which energy is transferred by electromagnetic waves similarto light waves. These waves may be both visible (light) and invisible. A very common exampleof thermal radiation is a heating element on a heater. When the heater element is first switchedon, the radiation is invisible, but you can feel the warmth it radiates. As the element heats, itwill glow orange and some of the radiation is now visible. The hotter the element, the brighterit glows and the more radiant energy it emits.

The key processes in the interaction of a substance with thermal radiation are:Absorption the process by which radiation enters a body and

becomes heatTransmission the process by which radiation passes through a bodyReflection the process by which radiation is neither absorbed or transmitted

through the body; rather it bounces off

Objects receive thermal radiation when they are struck by electromagnetic waves, therebyagitating the molecules and atoms. More agitation means more energy and a higher tempera-ture. Energy is transferred to one body from another without contact or transporting mediumsuch as air or water. In fact, thermal radiation heat transfer is the only form of heat transfer pos-sible in a vacuum.

All bodies emit a certain amount of radiation. The amount depends upon the body's tem-perature and nature of its surface. Some bodies only emit a small amount of radiant energy fortheir temperature, commonly called low emissivity materials (abbreviated low-E). Low-E win-dows are used to control the heat radiation in and out of buildings. Windows can be designedto reflect, absorb and transmit different parts of the sun's radiant energy.

The condition of a body's surface will determine the amount of thermal radiation that isabsorbed, reflected or re-emitted. Surfaces that are black and rough, such as black iron, willabsorb and re-emit almost all the energy that strikes them. Polished and smooth surfaces willnot absorb, but reflect, a large part of the incoming radiant energy.

Typical units of measure for rate of radiant heat transferMetric (SI) Watt per square meter (W/m2)

Evaporation

The change by which any substance is converted from a liquid state and carried off as vapour.

Example: People are cooled by evaporation of perspiration from the skin and refrigeration isaccomplished by evaporating the liquid refrigerant. Evaporation is a cooling process.

Condensation

The change by which any substance is converted from a gaseous state to liquid state.


13

Example: Condensation on the other hand is a heating process. As molecules of vapour con-dense and become liquid, their latent heat of vapourisation evidences itself again as sensibleheat, indicated by a rise in temperature. This heating effect of condensation is what causes theconsiderable rise in atmospheric temperature often noted as fog forms and as rain or snowbegins to fall.

Steam

Steam has been a popular mode of conveying energy, since the industrial revolution. The fol-lowing characteristics of steam make it so popular and useful to the industry:• High specific heat and latent heat • High heat transfer coefficient • Easy to control and distribute • Cheap and inert

Steam is used for generating power and also used in process industries, such as, sugar,paper, fertilizer, refineries, petrochemicals, chemical, food, synthetic fibre and textiles. In theprocess industries, the high pressure steam produced in the boiler, is first expanded in a steamturbine for generating power. The extraction or bleed from the turbine, which are generally atlow pressure, are used for the process. This method of producing power, by using the steam gen-erated for process in the boiler, is called "Cogeneration."

How to read a Steam Table?

Select the pressure and temperature of the steam at which you want to find the enthalpy. Readthe intersection of pressure and temperature for enthalpy (Heat content in the steam)

First law of Thermodynamics

It states that energy may be converted from one form to another, but it is never lost from thesystem.

Second Law of Thermodynamics

• In any conversion of energy from one form to another, some amount of energy will be dissipated as heat.

• Thus no energy conversion is 100 % efficient. • This principle is used in energy equipment efficiency calculations.

Law of Conservation of Matter

• In any physical or chemical change, matter is neither created nor destroyed, but it may bechanged from one form to another.

• For example, if a sample of coal were burnt in an enclosed chamber, carbon in coal wouldend up as CO2 in the air inside the chamber; In fact, for every carbon atom there would beone carbon dioxide molecule in the combustion products (each of which has one carbonatom). So the carbon atoms would be conserved, and so would every other atom. Thus, nomatter would be lost during this conversion of the coal into heat.

• This principle is used in energy and material balance calculations


14

The energy units are wide and varied. The usage of units varies with country, industry sector,systems such as FPS, CGS, MKS and SI, and also with generations of earlier period using FPSand recent generations using MKS. Even technology/equipment suppliers adopt units that aredifferent from the one being used by the user of that technology/equipment. For example somecompressor manufacturers specify output in m3/min while some specify in cubic feet/minute oreven in litres/second. All this cause confusion and hence the need for this chapter on units andconversions.

Energy Units

1 barrel of oil = 42 U.S. gallons (gal) = 0.16 cubic meters (m3)

1 MW 1,000 kW

1 kW 1,000 Watts

1 kWh 3,412 Btu

1 kWh 1.340 Hp hours

1,000 Btu 0.293 kWh

1 Therm 100,000 Btu (British Thermal Units)

1 Million Btu 293.1 Kilowatt hours

100,000 Btu 1 Therm

1 Watt 3.412 Btu per hour

1 Horsepower 746 Watts or 0.746 Kilo Watts

1 Horsepower hr. 2,545 Btu

1 kJ 0.239005 Kilocalories

1 Calorie 4.187 Joules

1 kcal/Kg 1.8 Btu's/lb.

1 Million Btu 252 Mega calories

1 Btu 252 Calories

1 Btu 1,055 Joules

1 Btu/lb. 2.3260 kJ/kg

1 Btu/lb. 0.5559 Kilocalories/kg

Power (Energy Rate) Equivalents

1 kilowatt (kW) 1 kilo joule /second (kJ/s)

1 kilowatt (kW) 3413 BTU/hour (Btu/hr.)

1 horsepower (hp) 746 watts (0.746 kW)

1 Ton of refrigeration 12000 Btu/hr.

Pressure:

Gauge pressure is defined relative to the prevailing atmospheric pressure (101.325 kPa at sealevel), or as absolute pressure:Absolute Pressure = Gauge Pressure + Prevailing Atmospheric Pressure


5 Units and Conversions

15

Units of measure of pressure:Metric (SI) : kilopascals (kPa)1 pascal (Pa) = 1 Newton/m2 (N/m2 )

1 physical atmosphere (atm) = 101325 Pa = 760 mm of mercury (mm Hg) = 14.69 lb-force/in2 (psi)

1 technical atmosphere (ata) = 1 kilogram-force/cm2 (kg/cm2)= 9.806650 × 104 Pa

Power:1 W = 1 J/s = 0.9478×10-3 Btu/s = 3.41214 Btu/hr

Fuel to kWh (Approximate conversion)

Natural gas M3 x 10.6 kWh

Ft3 x 0.3 kWh

therms x 29.3 kWh

LPG (propane) m3 x 25 kWh

Coal kg x 8.05 kWh

Coke kg x 10.0 kWh

Gas oil litres x 12.5 kWh

Light fuel oil litres x 12.9 kWh

Medium fuel oil litres x 13.1 kWh

Heavy fuel oil litres x 13.3 kWh

Prefixes for units in the International System

Prefix Symbol Power Example USA/Other

exa E 1018 quintillion

peta P 1015 pentagram (Pg) quadrillion/billiard

tera T 1012 terawatt (TW) trillion/billion

giga G 109 gigawatt (GW) billion/milliard

mega M 106 megawatt (MW) million

kilo k 103 kilogram (kg)

hecto h 102 hectoliter (hl)

deka da 101 dekagram (dag)

deci d 10-1 decimeter (dm)

centi c 10-2 centimeter (cm)

milli m 10-3 millimeter (mm)

micro µ 10-6 micrometer (µm)

nano n 10-9 nanosecond (ns)

pico p 10-12 picofarad (pf)

femto f 10-15 femtogram (fg)

atto a 10-18


16

To: TJ Gcal Mtoe MBtu GWh

From: Multiply by:

TJ 1 238.8 2.388 x 10-5 947.8 0.2778

Gcal 4.1868 x 10-3 1 10-7 3.968 1.163 x 10-3

Mtoe 4.1868 x 104 107 1 3.968 x 107 11630

MBtu 1.0551 x 10-3 0.252 2.52 x 10-8 1 2.931 x 10-4

GWh 3.6 860 8.6 x 10-5 3412 1

Energy

To: kg t lt st lb

From: multiply by:

kilogram (kg) 1 0.001 9.84 x 10-4 1.102 x 10-3 2.2046

tonne (t) 1000 1 0.984 1.1023 2204.6

long ton (lt) 1016 1.016 1 1.120 2240.0

short ton (st) 907.2 0.9072 0.893 1 2000.0

pound (lb) 0.454 4.54 x 10-4 4.46 x 10-4 5.0 x 10-4 1

Mass

To: gal U.S. gal U.K. bbl ft3 l m3

From: multiply by:

U.S. gallon (gal) 1 0.8327 0.02381 0.1337 3.785 0.0038

U.K. gallon (gal) 1.201 1 0.02859 0.1605 4.546 0.0045

Barrel (bbl) 42.0 34.97 1 5.615 159.0 0.159

Cubic foot (ft3) 7.48 6.229 0.1781 1 28.3 0.0283

Litre (l) 0.2642 0.220 0.0063 0.0353 1 0.001

Cubic metre (m3) 264.2 220.0 6.289 35.3147 1000.0 1

Volume


17

AIR/FUEL RATIO CONTROL The chain of command for air/fuel ratio controls is usually as follows: The burner or zone input control responds to T-sensor (or steam pressure sensor in the case of boiler)The burner input control (also termed furnace input control, kiln input control, etc.) may actuate burner or zone air valve (“air primary air/fuel ratio control”) or burner or zone fuel valve (“fuel primary air/fuel ratio control”)Air primary air/fuel ratio control is more common with smaller burners. Many problems are avoided if each burner is equipped with its own ratio control. Where multiple burners are “ganged” in parallel downstream from single air/fuel ratio control, if one burner has problem with its ratio, all parallel burners of that zone will have the opposite difficulty, the intensity of which will be divided by the number of burners in the zone. AIR/FUEL RATIO CONTROL MUST BE UNDERSTOOD Furnace engineers and operators must understand the many aspects of air/fuel ratio control for safety and for equality. Mass flow control is essential if the combustion air is preheated. Changing air temperature affects the weight of air passing through control valve, affecting input rate and air/fuel ratio. Control valves are volumetric devices, but temperature changes density, which changes the weight of air delivered. The air volume delivered to furnace should be corrected for temperature changes because the chemistry of combustion really requires constant weight (or mass) ratio of air to fuel. The magnitude of the correction will vary as the square root of the absolute temperature. Most larger modern air/fuel ratio controllers have an input port for signal from an air T-sensor. This type of air/fuel ratio control is called “mass flow control. Individual ratio controls at every burner make it easy to modify the input profile pattern up and down or across furnace without having to reset the ratio of each burner afterward. Small burners without preheated air are generally controlled by cross-connected air/fuel ratio regulators (one for each burner)This arrangement is ideal because it saves the operator from constantly having to adjust the ratio—until the paint is worn off the hand dial—because of changing maldistributions of flows in either air or fuel manifold. Air and Fuel Manifolds. It is difficult to correct bad manifold designs; therefore, it is important to be generous in initial air and fuel manifold sizing, and get it right the first time. (See fig. 6.10.) Designers should think of manifolds as plenums that should be sized for low velocities. no uniform air or fuel distribution often changes its maldistribution as burners are turned up and down. An easy, safe design has the manifold cross-sectional area equal to the sum of the cross-sectional areas of all of its offtake pipes. BENEFITS OF GOOD AIR/FUEL RATIO CONTROL 1. Safety from explosions and fuel-fed fires by minimizing the chance of accumulating rich mixture in the confined space of furnace or duct. 2. Lower fuel consumption because “ff-ratio” operation leaves fuel unburned if too rich but sends too much hot gas out the stack if too lean.

Chapter-9

1

3. Better product quality, because the load surface is less likely to be oxidized when air/fuel ratio is too lean, and less likely to be carburized or have hydrogen absorption if too rich. 4. Rolled-in sticky scale is avoided by controlling air/fuel ratio to prevent reducing atmosphere in the furnace. (Rolled-in scale causes pits which generally cannot be ground out. 5. Less metal loss because less scale is formed. 6. Reduced scrap because poor air/fuel ratio control can result in the load being scrapped for fear of customer penalties. AIR/FUEL RATIO IS CRUCIAL TO SAFETY Air primary control is generally preferred over fuel primary control for safety reasons. Burners are generally more stable if they should happen to go lean than if they happen

to go rich. Having air lead the fuel (air primary) may avoid dangerous flame-out when input is rising. If burners go rich, do not try “soft shutdown” with flame-out hazard impending. Do FULL shutdown because otherwise unburned fuel may work its way back upstream into feed pipes and ducts, followed by hot furnace gases, followed by an in-duct explosion. “Soft shutdowns” that leave the air on low and do not trip the fuel safety shutoff valve (to avoid time-consuming total restart) are very likely to move the fans or blowers into the low end of their pressure curve, where surging may happen. Surging can pull unburned fuel into air-filled pipes or ducts, forming combustible mixtures, and then suck in hot furnace gas, providing source of ignition, resulting in an explosion. An explosion will be much more time consuming than proper shutdown (including fuel shutoff) than restart. If the fuel is not shut off immediately to prevent any unburned fuel accumulation or if the rich atmosphere has already accumulated considerably after loss of ignition, these situations are potential bombs. Do not open any furnace doors or other openings. Turn off air to any pilots or other sources of ignition that may still be burning, but do not change main gas or air flow. Let the furnace self-cool even though smoking “Flood” the furnace with steam or other nonreactive gas such as argon, CO2,orN2, which are better coolants than too-rich-to-burn fuel–air mixture. Figure 6.11 cites two potential hazards leading to explosions and fuel-fed fires from using constant pilots instead of interrupted pilots when single flame monitor is used to check both pilot flame and main flame. The upper time-line diagram of figure 6.11 shows burner startup situation where the air/fuel ratio control has erroneously been set too rich. The burner may have lighted as it entered the flammable zone (about 5% gas in gas–air mixture, for natural gas)but its mixture soon became too rich to burn, exceeding the upper limit of flammability (about 15% gas in natural gas–air mixture)exiting the flammable zone, with the flame going out. The pilot has its own controlled air and fuel supply set at an air/fuel ratio between the flammability limits; thus, it stays lighted even though it is surrounded by nonflammable atmosphere. The accumulated too-rich-to-burn fuel–air mixture will be ignited as an explosion when someone wonders why the burner went out after an assumed-to-be-normal startup and (a) opens the furnace door, letting in air, or (b) turns off the fuel to the main burners, allowing the continuing air supply to bring the accumulated rich mixture back to combustible (explosive) mixture. The lower diagram of figure 6.11 shows situation where burner fuel shutoff valve was not closed tightly or fuel somehow leaked into furnace or oven overnight. If pilot had been left running overnight, an explosion would occur as soon as sufficient fuel accumulated in the furnace to bring the fuel percentage up to the lower limit of

2

flammability (about 5% gas in gas–air mix, for natural gas)If there was no constant pilot or other source of ignition in the furnace while shut down, the air/fuel ratio could pass through the flammable (explosible) zone and rise above the upper limit of flammability (about 15% gas in natural gas–air mix)The asterisk marks the point at which someone trying to light burner the next morning (a) opens the furnace door, letting in air, or (b) turns on the main air, or (c) turns off the leaking gas valve. Figure 6.12 shows time line for lighting and shutting down program for one-burner furnace. The block diagram across the top shows the programmed functions designed to prevent accumulation of rich or combustible air–fuel mixtures. The bottom plot shows air flow during the programmed light-up and shutdown. This is for system with interrupted pilot or direct spark ignition with flame monitor that checks for presence of either pilot or main flame. All such programs should be designed, installed, and operated in compliance with insuring underwriter’s requirements, those of government authorities, and recommendations of the U.S. National Fire Protection Association. Ref: Industrial Furnaces. – W. TRINKS. 6th edition.

3

Excess Air and O2 and CO2 in Flue Gas

Approximate values for CO2 and O2 in the flue gas as result of excess air are estimated in the table below:

Carbon Dioxide - CO2 - in Flue Gas (% volume) Excess Air

% Natural Gas

Propane Butane Fuel Oil Bituminous

Coal Anthracite

Coal

Oxygen in Flue Gas

for all fuels (% volume)

0 12 14 15.5 18 20 0

20 10.5 12 13.5 15.5 16.5 3

40 9 10 12 13.5 14 5

60 8 9 10 12 12.5 7.5

80 7 8 9 11 11.5 9

100 6 6 8 9.5 10 10 Technical Information Propane

Characteristics Values

Specific gravity at 70oF/21oC 1.56

Calorific value (kJ/kg) 50,000

Critical temperature (oF/oC -43.6/-42

Butane




Critical temperature (oF/oC) 22.99/-5

1

Chapter-10

Typical values of excess air for some common fuels are shown in the table below:

Fuel Excess of Air-(%)

Anthracite 40

Coke oven gas 5 - 10

Natural Gas 5 - 10

Coal, pulverized 15 - 20

Coal, stoker 20 - 30

Oil (No. 2 and No. 6) 10 to 20

Semi anthracite, hand firing 70 to 100

Semi anthracite, with stoker 40 to 70

Semi anthracite, with traveling grate 30 to 60

The calorific value of a fuel is the quantity of heat produced by its combustion - at constant pressure and under a conditions known as " normal " of temperature and pressure (i.e. to 0oC and under a pressure of 1,013 mbar). The combustion of a fuel product generates water vapor. Certain techniques are used to recover the quantity of heat contained in this water vapor by condensing it.

The Higher Calorific Value (or Gross Calorific Value - GCV) supposes that the water of combustion is entirely condensed and that the heat contained in the water vapor is recovered. The Lower Calorific Value (or Net Calorific Value - NCV) supposes that the products of combustion contains the water vapor. The heat contained in the water vapor is not recovered.

Fuel Higher Calorific Value-(Gross Calorific Value - GCV)

kJ/kg Btu/lb

Anthracite 32,500 - 34,000 14,000 - 14,500

Bituminous coal 17,000 - 23,250 7,300 - 10,000

Butane 49,510 20,900

2

Charcoal 29,600 12,800

Coal 15,000 - 27,000 8,000 - 14,000

Coke 28,000 - 31,000 12,000 - 13,500

Diesel 44,800 19,300

Ethanol 29,700 12,800

Lignite 16,300 7,000

Methane 55,530

Gasoline 47,300 20,400

Hydrogen 141,790 61,000

Peat 13,800 - 20,500 5,500 - 8,800

Propane 50,350

Semi anthracite 26,700 - 32,500 11,500 - 14,000

Wood (dry) 14,400 - 17,400 6,200 - 7,500

kJ/m3 Btu/ft3

Acetylene 56,000

Butane C4H10 133,000

Hydrogen 13,000

Natural gas 43,000

Methane CH4 39,820

Propane C3H8 101,000

Town gas 18,000

kJ/l Btu/gal

Acetone (kJ/kg) 29,000

3

Alcohol, 96% (kJ/kg) 30,000

Ether (kJ/kg) 43,000

Kerosene 35,000 154,000

Gas oil 38,000 164,000

Glycerin (kJ/kg) 19,000

Heavy fuel oil 41,200 177,000

Oils, vegetable (kJ/kg) 39,000 - 48,000

Petrol (kJ/kg) 48,000

Petroleum (kJ/kg) 43,000

Tar (kJ/kg) 36,000

Turpentine (kJ/kg) 44,000

1 kJ/kg = 0.4299 Btu/ lbm = 0.23884 kcal/kg

1 Btu/lbm = 2.326 kJ/kg = 1.8 kcal/kg

1 Btu/ft3 = 8.9 kcal/m3 = 3.73x104 J/m3

1 Btu/lb = 2,326.1 J/kg = 0.55556 kcal/kg

4

Heat losses

Excess air The amount of excess air used in a furnace, dryer or kiln varies according to the application; for example, a direct-fired drying oven requires large quantities of excess air to remove vapours quickly from it (see the example preceding). Excess air carries heat away from the process and up the stack, so this air should be monitored and adjusted to the minimum quantity necessary to do the job. Even small (0.16-cm, or 1/8-in.) gaps around doors, etc. quickly add up to a large open area, and substantial amounts of cold air can infiltrate. The excess air takes away from the heat required to heat the product. Savings will result when the excess air is reduced. Proper maintenance can reduce but seldom eliminate cold air infiltration (except in new equipment); instead, use furnace pressurization and burner flame management and control. Maintaining positive pressure at all times inside the furnace will prevent cold air infiltration through leaks. Technologies that regulate the chimney stack opening and a variety of pulse-fired combustion methods, together with maintaining steady heat levels (high fire is on most of the time), can also prevent cold air from entering. Combined energy savings may be as high as 60 percent along with substantial emissions reductions. Radiation and convection heat loss Heat losses due to radiation and convection from a furnace, dryer or kiln can be high if the enclosure is not properly maintained. Heat loss can occur because of deficiencies such as

• damaged or missing insulation; • missing furnace doors and covers; • damaged, warped or loose-fitting furnace doors and covers; and • openings in the furnace enclosure that allow passage of air.

Figures 2.8 and 2.9 illustrate the relationship between outside furnace wall temperature and energy loss from the furnace walls, and between furnace temperature and energy loss through openings in furnace walls. FIGURE 2.8 - Energy loss from furnace walls versus outside wall temperature

Chapter-11

1

FIGURE 2.9 - Energy loss by radiation through opening versus furnace temperature

Controls and monitoring

Without adequate controls, energy efficiency improvement efforts will fail. Monitoring equipment should be installed so that operators can determine energy consumption per unit of output. They can then identify deviations from this standard and take corrective action.

Furnace efficiency can often be improved by upgrading burner controls and their type, as mentioned above. Automating systems that include fuel and airflow meters, gas pressure control, flue damper control through pressure sensors, and tight in-furnace conditions monitoring for sloping control will permit closer energy consumption control and lower levels of excess air. Systems with oxygen trim allow for even better control of excess air.

The controlling and monitoring technologies incorporate proportional integral derivative controllers, feedback and feed forward control, process integration control, dynamic modeling and expert computer control systems.

Generally, the benefits of monitoring and controlling industrial furnaces, ovens and kilns include the following:

• reduced product losses; • improved product quality and consistency; • improved operational reliability; and • energy efficiency improvements of 50 percent or more.

2

HEAT TRANSFER 1 Principles of Heat Transfer Heat transfer is defined as energy in transit as a result of a difference in temperature. There are three main mechanisms of heat transfer: conduction, convection, and thermal radiation. In the case of conduction, heat energy is transferred on a molecular scale, with no large-scale movement of matter. Convection occurs when there is a temperature difference between a fluid and a solid boundary. It is a combination of both heat flow and fluid flow (mass transfer). Two cases of convection occur: forced convection when the flow of fluid is caused by some external means, e.g. the action of a fan or pump; natural convection when the flow is simply a result of differences in buoyancy with the fluid. Radiation does not require the existence of an intervening medium. It occurs as a result of the energy, in the form of electromagnetic waves, emitted by all matter. Quantity of energy transferred is dependent on the temperature and emissivity of the emitting body, which may be a solid, a liquid or, in case of radiative heat transfer from flames/gas. In practice, all three mechanisms operate simultaneously; however, for the purposes of heat transfer between the flame and furnace stock, convection and radiation are the main mechanisms. 2 Choice of Heat Transfer Method Radiation has two main advantages: it has high intensity or power, i.e. high heat input rates can be readily achieved to suit high-speed production processes; it can be easily spread over a wide area. Disadvantages are: it travels only in straight lines, with the result that some stock may not be adequately heated; it cannot be readily controlled. Stock may continue to be heated by the radiation from the furnace walls and ceiling, even after the flame has been turned off. Convection can be more readily directed around the interior of a furnace and can penetrate throughout the load, thus ensuring even heating of the stock. Convective heating is beneficial in treatments requiring an element of mass transfer, e.g. drying operations. The heat transfer method chosen depends largely on the application: radiation tends to be more effective in melting of metal and glass, and holding of the molten metal or glass, and for forging, galvanising and reheating processes; convection methods tend to be used for processes such as drying, normalising, stress-relieving, and heating of densely-packed loads, or for high-velocity flame heating. 3 Influence of Flame Characteristics on Heat Transfer The shape and characteristics of the flame have a strong influence on the heat transfer between the flame and the stock, and burner design and choice of fuel can be manipulated to achieve the desired effect. The design of the burner determines the relative velocities of the fuel and air streams, and hence the flame length and shape. Good mixing, as a result of high turbulence and velocity, produces a short, bushy flame, whereas poor or delayed mixing will produce a longer, slender flame. A turbulent, high-velocity flame will churn up the atmosphere within the furnace and promote high rates of convective heat transfer, whereas a long, slender flame will have a higher radiative heat transfer component. Fuel choice also has a strong bearing on the radiative heat transfer characteristics. Oil firing produces flames that are yellower and more luminous (with relatively better radiation characteristics) than a well-mixed natural-gas flame, which tends to be bluer and less luminous. A poorly mixed or delayed mix natural-gas flame may, however, give a yellower, more luminous flame due to the incandescence of carbon particles in the flame. These are produced by pyrolysis of the hydrocarbon compounds in the gas.

Chapter-12

1

The Nature of LPG Products, Their Storage, Measurement, and Delivery

Objectives Upon completion of this chapter, you should be able to: 1. Describe the basic physical properties of LPG products. 2. Describe the conditions under which these products must be stored to maintain their liquid state. 3. Describe the basic elements of an LPG liquid-measuring and delivery system. 4. Identify major types of delivery devices and installations commonly found in the marketplace. Introduction The devices used for measurement of LPG products in their liquid state are quite similar in design and operation to a number of other liquid-measuring devices such as vehicle-tank meters and loading-rack meters. Except for the special materials used for some internal components and the differential pressure valve (used to maintain system pressure at levels required to preserve the liquid state of the product) that LPG liquid-measuring systems employ, they are virtually identical to the equipment used for tank truck and bulk measurement installations for liquid petroleum products, fertilizers, chemicals, etc. What makes LPG measuring equipment different from other liquid-measuring devices and necessitates different specifications and test procedures for examining them in the field are features of the delivery systems on which they are installed. The design of the delivery system is in turn directly related to the physical and, to a lesser extent, the chemical properties of the liquid that is dispensed and the operating conditions under which it is dispensed. This chapter provides an introduction to the systems that are used for commercial measurement of liquid LPG. (Devices used for measuring LPG products by weight or by gaseous volume are not covered in this manual.) We will first look at the distinctive properties of these products, and then describe the special requirements for design and operation that arise from these properties. We will also summarize the major types of systems that are employed for deliveries in various applications. Anhydrous Ammonia Metering systems used for the measurement of anhydrous ammonia are very similar in design and operation to systems used for LPG products. In fact, when applying examination procedures, LPG and anhydrous ammonia liquid-measuring devices may be considered to be virtually identical.

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Chapter-13

Appendix C of this manual includes a discussion of the properties of anhydrous ammonia, as well as points of difference in the conduct of field examinations, especially regarding safety procedures. If your jurisdiction is involved in examining anhydrous ammonia liquid-measuring devices, your instructor will discuss this material with you. LPG Properties And Characteristics Liquefied petroleum gas (LPG) is defined as a petroleum product composed predominantly of any of the following hydrocarbons or mixtures thereof: propane, propylene, butanes (normal butane or isobutane), and butylenes. These substances are generally extracted from natural gas or produced as a byproduct of the refining of crude oil. Commercial propane and, to a lesser extent, butane are the principal LPG products that we will be concerned with. Commercial propane, however, is not a pure product but a mixture of LPG products, with the primary component being propane (commercial LPG is generally more than 90 percent propane). It may also contain up to 7 or 8 percent ethane, a neo-cryogenic hydrocarbon. A relatively rare combination of physical properties accounts for the prominent place of LPG in the marketplace. The most important of these properties is that LPG products are gases at atmospheric temperatures and pressures, but they can be liquefied and maintained in their liquid state with relative ease. For example, propane, the hydrocarbon that comprises about 90 percent of most commercial LPG, occupies approximately 270 times less space as a liquid than it does as a gas: about 270 cubic feet of propane gas are condensed into 1 cubic foot of propane liquid (at 60 °F). Butane has a slightly lower ratio, but as a liquid still occupies less than 0.5 percent of the volume it would occupy as a gas (again, at 60 °F). The main value of LPG products results from the fact that they can be stored in their liquid state and used in their gaseous state. In their condensed, liquid state they can be stored and transported much more efficiently than they could be as gases. The advantage in reduced transportation and storage costs is sufficient to offset the cost of liquefying these products and also produce a profit. For this reason, and because of the value of the products themselves to consumers, production of LPG products has grown rapidly since they were first introduced in the marketplace more than half a century ago. To use LPG in most commercial and industrial applications it must be reconverted to a gaseous state. This is easily accomplished simply by returning it to atmospheric temperature and pressure. When we say that gaseous LPG products are “easily liquefied,” this must be understood in relative terms. To be liquefied, a substance must be maintained at a temperature below its boiling point. The boiling point of a substance is the temperature at which it will change from its liquid state to its gaseous state. To bring about the transformation from a liquid to a gaseous state, a certain amount of heat must be applied to the liquid at its boiling point. This is known as the latent heat of vaporization. Propane, the principal component of most LPG, has a boiling point of -44 °F at atmospheric pressure, which is approximately 14.7 pounds per square inch (psi). The boiling point of butane (also at atmospheric pressure) is much higher, +32 °F.

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Even though such temperatures are attained under certain climatic conditions in certain parts of the country, it is obviously impracticable to transport and store the products as liquids only when and where these temperatures occur naturally. Furthermore, temperatures sufficiently low to maintain propane in its liquid state are not attainable using normal refrigeration methods. Consequently, LPG products are liquefied by refrigeration but maintained in the liquid state by pressurization. The pressure that must be applied to maintain a contained product in its liquid state at a given temperature is unique to that product and is a function of temperature. That pressure is known as the vapor pressure of the liquid. Just as the boiling point of a substance, such as water, varies with its pressure, the vapor pressure of a contained LPG product varies with temperature. Temperature and Vapor Pressure A temperature of 60 °F is more or less in the middle of the range of temperatures normal for a temperate climate. The approximate vapor pressures for the two principal LPG products at 60 °F are:

Propane - 100 psig Butane - 12 psig

For example, at a temperature of 60 °F a vapor pressure of 100 psig is needed to maintain LPG in its liquid state. If the temperature of the LPG is raised to 100 °F the amount of vapor pressure required to keep it in a liquid state would be 172 psig. Thus, it takes considerably higher vapor pressure to maintain propane in its liquid state at 100 °F than at 60 °F. At higher temperatures, higher vapor pressures will be needed to maintain the product in its liquid state. At 100 °F the vapor pressure of butane is approximately 38 psig, more than three times its vapor pressure at 60 °F, though considerably lower than the vapor pressure of propane at that temperature. Temperatures required to liquefy these products are quite easily produced by refrigeration equipment, and containers can be fabricated that will securely hold several times the vapor pressure of propane within normal temperature ranges. As a result, LPG products are stored and transported in closed containers. LPG is metered from those containers at ambient temperatures, but at pressures higher than atmospheric pressure. It is obvious that one requirement of any system used to measure and deliver these products is, therefore, that it be a closed system so that the product can be held at a pressure sufficient to maintain its liquid state. Since the product is customarily stored by the purchaser as liquid until it is used, the storage vessel must also be closed and be connected to the delivery system when product is dispensed in such a way that they function as a closed system. The properties described above also relate directly to requirements for measurement of liquid LPG products. Since they are normally metered and sold by liquid volume, it is especially important that the product be in its liquid state when it passes through the metering device. The reason for this is that, as mentioned above, a given weight of the product will occupy many times its volume in its gaseous state as in its liquid state. You may recall, for example, that gaseous propane occupies about 270 times the volume it would as a liquid. Since the buyer of the product is purchasing liquid

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product by volume, any gas in the form of vapor included in the volume measured will result in the customer paying for more product than has actually been received. Temperature and Volume Changes LPG products, either in their liquid or gaseous state, expand when heated and contract when cooled. Thus, even if it is maintained as a liquid, a given weight of propane, for example, will occupy more volume if it is warm than if it is cold. Again, since the purchaser and seller complete their commercial transaction based upon a measured volume of liquid product, the temperature at which it is sold can make a difference. In fact, for LPG products the temperature can make a very great difference, since among their physical properties is a high rate of change in volume with change in temperature. For example, commercial liquid propane expands or contracts by about 1 percent of its volume for each change of temperature of 6 °F. Because of this property, sales of LPG products are often (though not always) based upon the volume sold at a reference standard temperature of 60 °F. To avoid the necessity of heating or cooling the product to exactly this temperature, the quantity indicated by the measuring device is usually corrected for its deviation from this standard, either by an automatic device called, a temperature compensator, or by calculations based upon a volume correction table. In later chapters you will learn how such corrections are actually made. Storage Vessel Headspace There must be a certain amount of headspace above the level of liquid in the tank to allow for the expansion that will occur if the temperature of the tank and its surroundings increases, even by a small amount. In addition, as product is drained from the tank during normal operation, some liquid product will vaporize in response to the drop in pressure and will generally remain as vapor, filling the upper portion of the storage tank. We will see how the delivery system is designed to keep vapor out of the measuring elements in the next section. Design of the Delivery System Now that you are aware of the properties of LPG products that affect their storage and measurement as liquids, we can turn to the general characteristics of the design of the derepresents, in greatly simplified detail, the design of any LPG delivery system. The system has four basic components: a storage tank, a pump unit, a metering unit, and the piping (including valves and other control elements) that connects these components and leads from the metering unit to the discharge nozzle. As you can imagine, the actual design is considerably more complex and includes a variety of accessory elements. We will look at these specific details in the next chapter and concentrate for the moment on the essential functional components of the system.

livery system. Figure 1

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As mentioned above, the entire system is closed and must permit no leakage of liquid or vapor. It must also be designed and constructed to withstand high pressure. Specifications regarding operating pressures that these systems must be capable of withstanding have been developed by the American Society of Mechanical Engineers (ASME: “Pressure Vessel Code,” Section 8) and have been adopted as part of most State fire and safety codes. The system must also be equipped with pressure relief valves, which are designed to permit a controlled venting of product to the atmosphere when internal pressures exceed safe

Storage Tank The storage tank is equipped with a liquid fill inlet for supplying the system with product, and the system has a discharge line with an outlet for delivery. In addition, a vapor port is provided; this permits the use of a pressure equalization line, which is sometimes necessary for efficient delivery under certain extreme conditions (as explained below) and for volumetric testing or calibration of

Pump The pump provides the pressure needed to propel product through the delivery system and into the receiving tank. Its design and operating characteristics are determined by its application. If properly

Figure 1. Basic components of an LPG delivery system.

the system .

limits.

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selected, its capacity, in terms of discharge rate and pressure, will meet the requirements of the systems to which it delivers product. Metering Unit Liquid product is measured as it passes through the metering unit. In addition to the measuring device itself, this component includes an indicating element, which is designed to indicate, or register, the quantity of liquid that has passed through the meter. The indicating element is driven directly by the measuring element, so that measurement and registration occur simultaneously. This allows the operator of the system, and the purchaser of the product, to monitor the amount of liquid that is being delivered continuously throughout

Vapor Eliminator and Differential Pressure Valve In addition to the measuring and indicating elements, the metering unit also includes two separate devices, a vapor eliminator and a differential pressure valve. The function of these devices is to prevent vapor from entering the meter and being measured along with liquid product. The vapor eliminator separates any vapor that has been produced from the liquid flow before it reaches the meter and returns it to the vapor space of the storage tank; the differential pressure valve maintains the product in its liquid state as it passes through the meter. As liquid is drawn from the storage tank, pressure within the tank falls. When it has fallen below the vapor pressure of the product -- as will happen almost immediately -- some of the liquid will boil (that is, change from its liquid state to its vapor state). In doing so, it expands rapidly and rises from the liquid into the vapor space. This also has a cooling effect (because of the latent heat of vaporization, which must be absorbed from the product in order to bring about the transformation in state from liquid to gas). The resulting expansion in volume will restore the equilibrium condition that exists in the tank almost instantaneously. As a result, whenever liquid product is being delivered, some vapor is being produced in the storage tank at the same time. This vapor is generally of no concern to accurate measurement of liquid product since it remains in the storage tank and does not enter the pump or metering unit. However, vapor can be produced in any part of the system where even a slight pressure differential occurs. A fall in pressure is likely to occur at any point where flow is restricted for any reason. This may occur at the pump, at valves, and at points where different sizes of piping are joined. Vaporization may also occur if a temperature differential exists at any point. A temperature differential will occur if one portion of the piping is exposed to direct sunlight, heating product at that location. Liquid product is also heated by friction as it flows through the system, especially at points where it must flow against gravity, around bends, or through a restricted passage. Even though the vaporization that occurs at any one point in the system as a result of any particular factor may be very small, any accumulation of vapor may have a significant effect on measurement accuracy.

the delivery.

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The design of the system must thus be such as to minimize vaporization due to these conditions. For example, piping should be no more extensive than necessary, as straight as possible, and should avoid upward pitches. This is especially important for the piping that leads from the storage tank to the inlet of the pump. It is also generally recommended that piping leading to the pump be one size larger than the discharge piping. The number of pipe connections and valves should be kept to a minimum, and the pump and meter should be carefully selected to match the operating conditions under which the system must perform, especially in terms of capacity and pressure. As mentioned above, vaporization of product is also reduced by a differential pressure valve, which eliminates pressure differentials by restricting flow on the discharge side of the meter, thus maintaining a uniform pressure in the piping and metering element upstream that is at or above the vapor pressure of the product. This device is an important part of the system design. However, it is not always possible to eliminate all sources of vapor production in the system and deliver product efficiently at the same time. As a result, the system must incorporate effective means of eliminating the vapor that is produced by collecting it and returning it to the storage tank vapor space, where its presence is innocuous. A device called a vapor eliminator performs this

In summary, the principal design requirements of the system as a whole are that it must:

• Contain product under pressures that are within its safe operating range, and be provided with means to effect a controlled venting of product when internal pressures exceed those limits.

• Be capable of delivering product efficiently (that is, at a rate of flow and discharge pressure that are appropriate for the receiving vessels to which product is delivered).

• It must reduce to a minimum the production of vapor within the system and must be equipped to eliminate small amounts of vapor that are produced.

Receiving Vessel Before leaving this general overview, let us look briefly at the problem of moving liquid product within a closed system from the point of view offactors we have been discussing affect delivery.

function.

the receiving vessel. Figure 2 illustrates how the

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The receiving vessel, like the delivery system storage tank, will contain some vapor and some liquid at all times, although at the time of delivery its contents may be mostly vapor. The situation described above for the system storage tank works in reverse in the receiving tank during delivery of liquid product as product is pushed into the receiving tank. As liquid enters the receiving container, propelled by pressure from the delivery system’s pump, it displaces vapor. As the level of liquid rises, it acts like a slow-moving piston, compressing vapor in the space above it. This compression causes a rise in pressure and temperature. As the pressure rises, some of the vapor begins to condense and return to the liquid phase. Equilibrium is eventually, but not instantaneously, restored. In older vapor return systems, this situation was remedied by connecting a vapor line between the vapor space of the receiving tank and the vapor space of the system storage tank. Excess pressure could thus be bled from the receiving tank, and equilibrium would be maintained in both tanks. This solution, however, meant that some amount of product belonging to the purchaser was being returned to the seller in the form of vapor. A far better and more equitable solution involves an adaptation of the receiving tank. A pipe from the receiving tank inlet is extended into the vapor space. The outlet of the pipe is designed in such a way that incoming liquid product is sprayed upward toward the top of the tank. The droplets of cooler liquid spray falling through the vapor space promote condensation of the vapor, thus cooling the compressed vapor. This in turn lowers pressure and allows the system pump to deliver more efficiently. This method, called spray fill

Figure 2. Delivery of product.

, may be accomplished in several ways, as shown in Figure3.

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This design of receiving tanks has now become virtually universal and thus has made vapor-return lines unnecessary, except under the most extreme climatic conditions. As a result, most States have prohibited the use of vapor-return lines in commercial deliveries, except under very specific and limited circumstances. This prohibition is also included among the requirements of NIST Handbook

The effects of temperature expansion of the liquid product must also be considered in determining how much product should be delivered to a receiving tank. In general, the colder the liquid that is delivered, the greater the amount of head space that should be allowed for expansion. The reason for this is that if the product delivered is colder than the tank and the surrounding air temperature, it will expand as it warms. This process will be gradual, and it may take a number of hours before the product has warmed -- and expanded -- fully. For this reason, sufficient vapor space must be preserved in the top of the receiving container to permit expansion of product. Provisions of National Fire Protection Association (NFPA) 58 (“Storage and Handling of Liquefied Petroleum Gases”) and Department of Transportation (DOT) regulations (“Hazardous Materials Regulations,” 49 CFR 170-179) for filling cylinders, storage tanks, tank trucks, and tank cars allow adequate vapor space for liquid expansion as the result of a change in atmospheric temperature. Types of Delivery Systems The basic design features described above apply to all types of systems used for measurement and delivery of LPG. These are not the only design criteria, however. The design of the system must also reflect its use in a specific delivery application in the marketplace. Wholesale deliveries are generally made from bulk distribution centers or terminals to tank trucks. Because of the large capacity of truck tanks, delivery can be made safely and efficiently at relatively

Figure 3. Spray-fill method of delivery.

44 .

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high flow rates, usually in the range of 100 gallons per minute (gpm), and sometimes considerably higher. Tank trucks make deliveries either to retail stations or directly to customers. Because receiving tanks are considerably smaller, these systems generally operate at lower discharge rates, depending upon the type of service. Trucks delivering to large holding tanks at a retail installation generally are equipped for maximum discharge rates of about 50-60 gpm. Retail deliveries to smaller tanks kept by farmers and homeowners are usually still lower, about 30 gpm. In many parts of the country, retail sales of LPG products are also made from service stations. Customers may either fill small truck- or trailer-mounted tanks at these facilities or, in the case of propane products especially, refillable cylinders may be used. In some parts of the country, LPG is used extensively as a motor fuel. In recent years, filling stations for LPG-fueled vehicles have been constructed that appear quite similar to gasoline filling stations, with similar dispensers and controls familiar to motorists. However, most LPG motor-fuel refilling facilities are nothing more than a tank, pump, meter, and hose. Because receiving tanks and fill pipes on motor vehicles are relatively small, these systems typically deliver product at comparatively low flow rates, usually about 10 gpm. The tank, pump, meter, and piping of an LPG delivery system are selected and designed as a unit, depending upon the purpose of the system. Summary The appropriate design of metering and delivery systems used for LPG is determined by the physical properties of the product, especially its response to changes in temperature and pressure during delivery. To assure accurate measurement, the design of the system must incorporate means to minimize vaporization and to eliminate any vapor produced before it enters the metering element. Receiving tanks must also be designed to facilitate both efficient delivery and accurate measurement. The design of the metering system also depends upon its use and especially upon the desired maximum discharge rate. Metering systems for LPG are commonly installed in fixed locations or on tank trucks and are used in both wholesale and retail service. Terms to Know: boiling point. The temperature at which a substance will change from its liquid state to its gaseous state. The boiling point of a product is a function of pressure. latent heat of vaporization. The amount of heat that must be applied to a liquid at its boiling point to bring about the transformation from a liquid to a gaseous state. vapor pressure. The pressure that must be applied to maintain a contained product in its liquid state at a given temperature. The vapor pressure of a product is unique to that product and is a function of temperature.

Page 10

PROPERTIES OF AIR

AIR IS A GAS – 78% Nitrogen, 21% Oxygen, traces of H2O, CO2 , Ar---- Properties Dimensions Value (SLS)

Mass, Volume. Metric English Density (r) Mass / volume 1.229 Kg/m3 0.00237 slug/ft3

Specific Volume (V) Volume / mass 0.814 m3/Kg 422 ft3/slug Pressure (P) Force / area 101.0 kN/m2 14.7 lbs/in2

Temperature (T) Degrees 150C 590F Viscocity (mu) Force-time / area 1.73 x 10-5 N-s/m2 3.62 x 10-7 lb-s/ft2

SLS – Sea Level Static (Standard Day)

Air is a mixture of gases, 78% nitrogen and 21% oxygen with traces of water vapor, carbon dioxide, argon, and various other components. We usually model air as a uniform (no variation or fluctuation) gas with properties that are averaged from all the individual components. Any gas has certain properties that we can detect with our senses. The values and relations of the properties define the state of the gas.

On this slide you will find typical values of the properties of air at sea level static conditions for a standard day. We are all aware that pressure and temperature of the air depend on your location on the earth and the season of the year. And while it is hotter in some seasons than others, pressure and temperature change day to day, hour to hour, sometimes even minute to minute (during severe weather). The values presented on the slide are simply average values used by engineers to design machines. That's why they are called standard values. We also know that all of the state-of-the-gas variables will change with altitude, which is why the typical values are given at sea level, static conditions. Because the gravity of the earth holds the atmosphere to the surface, as altitude increases, air density, pressure, and temperature (for lower altitudes) decrease. In deep space, the density is almost zero. The variation of the air from the standard can be very important since it affects flow parameters like the speed of sound. You will learn that jet engines do not produce as much thrust on hot, muggy days as on cold, crisp days, and that lift, drag, and thrust vary greatly with altitude.

A gas is composed of a large number of molecules which are in constant motion. The sum of the mass of all the molecules is equal to the mass of the gas. A gas occupies some volume in three dimensional space. For a given pressure and temperature, the volume depends directly on the amount of gas. Since the mass and volume are directly related, we can express both the mass and volume by a single variable. When a gas is moving, it is convenient to use the density of a gas, which is the mass divided by the volume the gas occupies. The sea level standard value of air density r is

r = 1.229 kilograms/cubic meters = .00237 slug/cubic feet

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Chapter-14

http://www.grc.nasa.gov/WWW/K-12/airplane/thrust1.html

When working with a static (unmoving) gas, it is more convenient to use specific volume, which is the volume divided by the mass. The sea level standard value of specific volume v is

v = .814 cubic meters/kilogram = 422 cubic feet/slug

The pressure of a gas equals the perpendicular (normal) force exerted by the gas divided by the surface area on which the force is exerted. The sea level standard value of air pressure p is

p = 101.3 kilo Newton/square meter = 14.7 pounds/square inch

The temperature of a gas is a measure of the kinetic energy of the molecules of the gas. The sea level standard value of air temperature T is

T = 15 degrees C = 59 degrees Fahrenheit

A gas can exert a tangential (shearing) force on a surface, which acts like friction between solid surfaces. This "sticky" property of the gas is called the viscosity and it plays a large role in aerodynamic drag. The sea level standard value of air viscosity mu is

mu = 1.73 x 10^-5 Newton-second/square meters = 3.62 x 10^-7 pound-second/square feet

The density (specific volume), pressure, and temperature of a gas are related to each other through the equation of state. The state of a gas can be changed by external processes, and the reaction of the gas can be predicted using the laws of thermodynamics. A fundamental understanding of thermodynamics is very important in describing the operation of propulsion systems.

* * * * * * * * *

2

What is LPG or LP Gas?

LPG or LP Gas is the abbreviation of Liquefied Petroleum Gas. This group of products includes saturated Hydrocarbons - Propane (C3H8) and Butane (C4H10), which can be stored/transported separately or as a mixture. They exist as gases at normal room temperature and atmospheric pressure. Why is it called Liquefied Petroleum Gas? This is because these gases liquefy under moderate pressure. They liquefy at moderate pressures, readily vaporizing upon release of pressure. It is this property that permits transportation of and storage of LP Gas in concentrated liquid form. Where does LPG come from? LPG comes from two sources. It can be obtained from the refining of crude oil. When produced this way it is generally in pressurized form. LPG is also extracted from natural gas or crude oil streams coming from underground reservoirs. 60% of LPG in the world today is produced this way whereas 40% of LPG is extracted from refining of crude oil. What is commercial Propane & Butane? Ideally products referred to as "propane" and "butane" consist very largely of these saturated hydrocarbons; but during the process of extraction/production certain allowable unsaturated hydrocarbons like ethylene, propylene, butylenes etc. may be included in the mixture along with pure propane and butane. The presence of these in moderate amounts would not affect LPG in terms of combustion but may affect other properties slightly (such as corrosiveness or gum formation). How is LPG seen & felt?

• It is colorless and cannot be seen • It is odorless. Hence LPG is odorized by adding an odorant prior to supply to the

user, to aid the detection of any leaks. • It is slightly heavier than air and hence if there is a leak it flows to lower lying areas. • In liquid form, its density is half that of water and hence it floats initially before it is

vaporized. • It is non-toxic but can cause asphyxiation in very high concentrations in air.

LPG expands upon release and 1 liter of liquid will form approximately 250 liters of vapor What is LPG used for? LPG is used as a fuel for domestic (cooking), industrial, horticultural, agricultural, heating and drying processes. LPG can be used as an automotive fuel or as a propellant for aerosols, in addition to other specialist applications. LPG can also be used to provide lighting through the use of pressure lanterns. Why are Butane and Propane used in combination? While butane and propane are different chemical compounds, their properties are similar enough to be useful in mixtures. Butane and Propane are both saturated hydrocarbons. They do not react with other. Butane is less volatile and boils at 0.6 deg C. Propane is more volatile and boils at - 42 deg C. Both products are liquids at atmospheric pressure when cooled to temperatures lower than their boiling points. Vaporization is rapid at

1

Chapter-15

temperatures above the boiling points. The calorific (heat) values of both are almost equal. Both are thus mixed together to attain the vapor pressure that is required by the end user and depending on the ambient conditions. If the ambient temperature is very low propane is preferred to achieve higher vapor pressure at the given temperature. What are the advantages of LPG? The advantages of LPG are as follows

• Because of its relatively fewer components, it is easy to achieve the correct fuel to air mix ratio that allows the complete combustion of the product. This gives LPG its clean burning characteristics.

• Both Propane and Butane are easily liquefied and stored in pressure containers. These properties make the fuel highly portable, and hence, can be easily transported in cylinders or tanks to end-users.

• LPG is a good substitute for petrol in spark ignition engines. Its clean burning properties, in a properly tuned engine, give reduced exhaust emissions, extended lubricant and spark plug life.

• As a replacement for aerosol propellants and refrigerants, LPG provides alternatives to fluorocarbons, which are known to cause deterioration of the earth's ozone layer.

The clean burning properties and portability of LPG provide a substitute for traditional fuels such as wood, coal, and other organic matter. This provides a solution to de-forestation and the reduction of particulate matter in the atmosphere (haze), caused by burning the traditional fuels. What are LPG properties?

Property Units CommercialPropane

Commercial Butane Mixture 50% each

Specific gravity of Liquid at 15

deg C (Water=1) 0.504 0.582 0.543

Specific gravity of Vapor at 15 deg C(Air=1)

1.5 2.01 1.75

Vapor pressure at 38 deg C

Kg/sq.cm 13.8 2.6 8.0

Boiling point at atm pressure

Deg C - 42 9 + 9 to - 42

Ignition temperature in

air

Deg C 495-605 480-535 480-605

Latent Heat of Vaporization

Btu/lb 184 167 175

2

Chapter - 16

Properties of LPG

Propane Butane

Liquid Density 0.505 0.575

Gas Density 1.5 1.95

Ratio Gas/liquid 274 230

Atm. Boiling ptc. -42 -2

Specific heat liquid 0.60 Btu/deg. 0.58 Btu/deg

Latent heat Vaporization 358 kj/kg. 372 kj/kg

Flammability limit 2.2 - 9.5% 1.8 - 8.5%

Auto temp ign 470ºC 410ºC

Mole Weight 44.10 kg/k/mole 58.12

Freezing Point -187.7ºC -138.4

Critical temp 96.7ºC 152.1ºC

Critical Press 42.5 bar 38.0 bar

Soluble in water Slight Slight

Color Colorless Colorless

1

Chapter-17 Excess Air and O2 and CO2 in Flue Gas

Approximate values for CO2 and O2 in the flue gas as result of excess air are estimated in the table below:

Carbon Dioxide - CO2 - in Flue Gas (% volume) Excess Air

% Natural Gas

Propane Butane Fuel Oil Bituminous

Coal Anthracite

Coal

Oxygen in Flue Gas

for all fuels (% volume)

0 12 14 15.5 18 20 0

20 10.5 12 13.5 15.5 16.5 3

40 9 10 12 13.5 14 5

60 8 9 10 12 12.5 7.5

80 7 8 9 11 11.5 9

100 6 6 8 9.5 10 10 Technical Information Propane




Critical temperature (oF/oC -43.6/-42

Butane




Critical temperature (oF/oC) 22.99/-5

1

Typical values of excess air for some common fuels are shown in the table below:

Fuel Excess of Air-(%)

Anthracite 40

Coke oven gas 5 - 10

Natural Gas 5 - 10

Coal, pulverized 15 - 20

Coal, stoker 20 - 30

Oil (No. 2 and No. 6) 10 to 20

Semi anthracite, hand firing 70 to 100

Semi anthracite, with stoker 40 to 70

Semi anthracite, with traveling grate 30 to 60

The calorific value of a fuel is the quantity of heat produced by its combustion - at constant pressure and under a conditions known as " normal " of temperature and pressure (i.e. to 0oC and under a pressure of 1,013 mbar). The combustion of a fuel product generates water vapor. Certain techniques are used to recover the quantity of heat contained in this water vapor by condensing it.

The Higher Calorific Value (or Gross Calorific Value - GCV) supposes that the water of combustion is entirely condensed and that the heat contained in the water vapor is recovered. The Lower Calorific Value (or Net Calorific Value - NCV) supposes that the products of combustion contains the water vapor. The heat contained in the water vapor is not recovered.

Fuel Higher Calorific Value-(Gross Calorific Value - GCV)

kJ/kg Btu/lb

Anthracite 32,500 - 34,000 14,000 - 14,500

Bituminous coal 17,000 - 23,250 7,300 - 10,000

Butane 49,510 20,900

2

Charcoal 29,600 12,800

Coal 15,000 - 27,000 8,000 - 14,000

Coke 28,000 - 31,000 12,000 - 13,500

Diesel 44,800 19,300

Ethanol 29,700 12,800

Lignite 16,300 7,000

Methane 55,530

Gasoline 47,300 20,400

Hydrogen 141,790 61,000

Peat 13,800 - 20,500 5,500 - 8,800

Propane 50,350

Semi anthracite 26,700 - 32,500 11,500 - 14,000

Wood (dry) 14,400 - 17,400 6,200 - 7,500

kJ/m3 Btu/ft3

Acetylene 56,000

Butane C4H10 133,000

Hydrogen 13,000

Natural gas 43,000

Methane CH4 39,820

Propane C3H8 101,000

Town gas 18,000

kJ/l Btu/gal

Acetone (kJ/kg) 29,000

3

Alcohol, 96% (kJ/kg) 30,000

Ether (kJ/kg) 43,000

Kerosene 35,000 154,000

Gas oil 38,000 164,000

Glycerin (kJ/kg) 19,000

Heavy fuel oil 41,200 177,000

Oils, vegetable (kJ/kg) 39,000 - 48,000

Petrol (kJ/kg) 48,000

Petroleum (kJ/kg) 43,000

Tar (kJ/kg) 36,000

Turpentine (kJ/kg) 44,000

1 kJ/kg = 0.4299 Btu/ lbm = 0.23884 kcal/kg

1 Btu/lbm = 2.326 kJ/kg = 1.8 kcal/kg

1 Btu/ft3 = 8.9 kcal/m3 = 3.73x104 J/m3

1 Btu/lb = 2,326.1 J/kg = 0.55556 kcal/kg

4

Chapter - 18 FUEL SAVING IN OIL FIRED FURNACES.

Oil fired furnaces are widely used for heating of various metals, for heat-treatment of alloys &

also for manufacturing chemical products & for controlling process in some production lines.

Practically most of the users of oil fired furnaces are running the furnaces at very low efficiency

& there is lot of scope for fuel saving upto 50%.

Thermal Efficiency of Furnaces.

The characteristics of fuel are as follows :

composition

Fuel

Sp-

Gravity

at 600F Carbon Hydrogen

Oxygen +

Sulphur

Gross

Calorific

Value

Flash

point

Kerosene o.792 86.4 13.5 0.1 11138

Kcal/kg 370C

Light Fuel

Oil 0.895 86.1 12.3 1.6

10700

Kcal/kg 790C

Heavy

Fuel Oil 0.950 86.0 11.8 2.2

10500

Kcal/kg 1000C

Carbon burned to CO2 --- 8080 Kcal/Kg of C.

Carbon burned to CO --- 2472 Kcal/Kg of C.

Hydrogen burned to H2O --- 34462 Kcal/Kg of H2

CO burned to CO2 --- 2402 Kcal/Kg of CO

Now, if we know the fuel oil consumption in a process such as melting of metals or heating

metal to a specific temp, we can calculate the heat available in oil say if fuel oil (furnace oil) for

melting 100kgs of copper to 11500C was 25kg oil, then heat available was 25 x 10,500 =

262500 Kcals.

The net heat required to heat a metal or chemical is given by –

Weight of metal x temp0C x Sp-Heat.

1

If metal is melted then latent heat of fusion (LHF) is the additional heat required.

For melting copper to 11500C – 177Kcal/kg of Cu required.

For melting aluminum to 7200C – 246 Kcal/kg of Al required.

For melting brass to 9500C – 144 Kcal/kg of Brass required.

Sp-Heat of copper – 0.095.

LHF of copper –180 KJ/kg – 43 Kcal/kg

Sp Heat of Brass – 0.094.

Sp-Heat of aluminum – 0.214.

LHF of Al – 386 KJ/kg – 92 Kcal/kg. (386/4.187=92).

Sp-Heat of steel – 0.117.

LHF of steel – 208 KJ/kg – 49.7 Kcal/kg.

Thus for melting 100kg of copper to 11500C using 25 kg of furnace oil, the thermal efficiency is –

(100 x 177 Kcal x 100) / (180 x 10500) = 6.74%.

If 180kg of oil is required to heat one ton of steel to 9500C, then the thermal efficiency will be –

(1000 x 950 x 0.117 x 100) / (180 x 10500) = 5.88%.

Thus only 6% to 7% of the heat of oil is utilized in above processes and 93% heat is wasted. If

we know the causes of the wastage of heat and correct these , we can improve efficiency &

save fuel.

The Main Causes of Heat Wastages:

A) Sensible heat in waste gases :-

The heat lost due to high temp of waste gases is called sensible heat in waste gases.

It is calculated as –

Wt of flue gases x temp0C x sp-heat.

Now theoretically, if 1 kg of oil is burned completely, it requires 14 kg of air. But in practice 25%

extra air is required for complete combustion with cold air and cold oil, i.e. 17.5 kg of air is

needed for 1 kg of oil and this will produce 18.5 kg of flue gas. The temperature of flue gas has

2

to be more than the process temperature. Thus for melting Copper to 11500C the temperature of

flue gases is 12000C and melting of Brass to 9500C or heating steel to 9500C the temperature of

flue gas will be 10000C etc.

The heat lost at various flue gas temperature and its percentage of the heat available in oil are –

Wt of the flue

gas Temp0C Specific-Heat Heat Lost - Kcal % of heat of fuel

18.5 kg 500 0.283 2167 25.0

700 0.295 3820 36.4

800 0.301 4450 42.4

1000 0.312 5772 55.0

1200 o.320 7104 67.6

Thus 50 to 70% heat of oil is wasted in flue gas due to its high temperature.

B) Latent heat in flue gas :-

We have already seen in the tables that when Carbon burns to CO2, 8080 Kcal of heat is

produced and if burns to CO, then 2472 Kcal of heat is produced.

Thus in complete combustion gives –

CO in flue gas and the (calorific value) x (weight of CO) is the heat lost as latent heat in flue

gas.

Thus if flue gas contains 4% CO, then the heat lost will be

18.5 x 0.04 x 2402 = 1777 Kcal and this will be

(1777 x 100) / 10500 = 17% heat of fuel.

This happens due to inefficient burners and faulty burner operations.

C) Unburned oil in flue gases :-

If burners are very poorly operated, then the unburned oil can shoot through the flue gas and

heat lost will be directly proportional to the percentage of oil.

D) Excess air used in burner :-

If excess air used is more than the required for combustion, the weight of the flue gas will be

more and the heat lost will be directly proportional to the excess weight of gas.

3

E) Heat lost due to convection from outer walls of the furnace :-

This is given by an approximate formulae –

Heat Lost(Kcal) = h x A x (TS - TA) Kcal / hr / m2 / 0C.

Where h = convective heat transfer co-efficient.

A = Area of walls (outside) of furnace m2.

TS = Temperature of outside walls of furnace 0C.

TA = Atmospheric temp 0C.

Where h = 4.3Kcal/hr/m2/0C for still air, and varies according to the air velocity around the

furnace outer walls.

Example –

Furnace with outer dimensions as 2mtrs x 2mtrs x 2mtrs has wall temperature of 1000C,

and uses 30kg/hr oil.

Heat lost by walls = 4.3 x (2 x 2 x 6) x 100 = 10320 Kcal/hr

Total heat lost = (10320 x 100) / (30 x 10500) = 3.25%.

Thus the % of heat lost due to outer wall emission may shoot upto 25% if the outer air velocity is

more. Higher outside temperature indicates the poor condition of the insulation inside the

furnace.

Methods to Save Fuel Consumption –

As we have seen above, the furnace efficiency is low due to wastage or inefficient use of fuel

used. Now if we control the wastages & increase the input of heat in the furnace without

increasing fuel, the efficiency will go up & we can save fuel consumption upto 30%. The various

methods of fuel savings are –

1) Recuperators :

The maximum heat losses (upto 80%) are from waste gases. If the waste heat in the flue gases

is utilized to heat the incoming air required for burner, there will be additional heat input to the

furnace while other parameters remain the same. The additional input of heat is utilized in

heating the metal. This is achieved by a heat exchanger used to heat air by the hot exhaust of

flue gas & this system is called “RECUPERATOR”.

4

The higher temperature of the combustion air has the following advantages –

a) The flame temperature will increase. In furnace the heating of metal is mainly due to radiation

and heat transfer is proportional to the fourth power of the temperature difference. Thus the

heating is faster and saves fuel.

b) Higher temperature of air improves combustion process which requires less excess air for

complete combustion. Lower the weight of the air, higher is the flame temperature which helps

in saving the fuel.

Flame temperature is calculated as –

(Kcal of oil + sensible heat in air)

----------------------------------------------

(Wt of flue gas x sp-heat of flue gas)

Thus if 1-kg of requires 17.5 kg of air & cal-value of oil is 10500Kcal/kg & sp-heat of flue gas is

0.33 at expected temperature of flame, then the flame temperature will be

10500

--------------------- = 17190C flame temperature.

(1+17.5) x 0.33

When air is heated to 1500C, the excess air may be less to 10% from 25% normally required or

14 x 0.1 + 14 = 15.4 kg of air is required & this will produce 16.4 kg of flue gas and the flame

temperature in this case will be –

10500 + (16.4 x 100 x 0.24)

------------------------------------- = 19530C.

16.4 x .34

Thus the flame temperature has shot up by 2340C with hot air.

A temperature rise of air upto 1500C, can save upto 20% fuel. Hence “Recuperator” is the best

method to save fuel.

5

c) As the input of heat is increased due to Recuperator without increase in fuel & the output in

flue gases is reduced due to reduction in weight of flue gas, more heat is used by the metal

resulting into the increased furnace efficiency.

d) As the flue gases are passed through a heat exchanger enclosed in a closed chimney the

pollution of atmosphere near the furnace is controlled. Unburned CO can be burned in

Recuperator.

e) Burner remain cleaner due to better atomisation of oil & better combustion.

2) Burner & Burner Operation :

If an inefficient burner is fitted on furnace & the burner block & blower & oil pump are not proper

& so also oil is not pre-heated, the burner will not burn oil properly & give incomplete

combustion, the complete combustion will give CO in flue gases. The CO in flue gas can be

detected by Zink test & quantitatively determined by the flue gas analyser. Selection of burner,

burner block, blower, oil pump with pre-heating arrangement & air pressure & the passage of

flue gases are very important in controlling fuel consumption & & these factors can cause upto

40% wastage of fuel oil.

3) Fuel Oil – Kerosene, High Speed Diesel Oil.

L.D.O. & Furnace oil are used as fuel & each oil has different specifications. If wrong oil is used,

the wastage of fuel can be upto 30%. Most of the furnaces are run with wrong type of fuel oil.

Simply changing the oil can save lot of fuel expenses. Fuel must be pre-heated to correct

temperature. Safety controls must be incorporated in the system to avoid excess temperature &

avoid accidents and explosions. Fuel oil measuring system to be incorporated.

4) Insulation.

If the furnaces are not properly insulated, the heat is wasted to the atmosphere through outside

walls. Proper insulation will save fuel consumption upto 25%.

6

Chapter – 19

Simple Methods to Reduce Energy Costs by Shobhan Paul

Energy conservation can significantly affect the bottom line of organizations. Soaring energy prices and expensive capital equipment are a dual challenge today.

Fig. 1. Depiction of heat-transfer process

This paper attempts to identify the “low-hanging fruit” of energy conservation that can radically save energy without huge capital outlays. The methodology and its scientific basis are discussed as simply as possible. Detailed knowledge of mathematics is not necessary. However, the mathematical basis is discussed for those who want to dig deeper. The strategy described here is based on: Simple methods that add up to energy savings of as much as 50% Quick returns on investment – sometimes a week No capital investment is required to get at the low-hanging fruit Practical Example – Stress Relieving The relief of residual stresses is a time-temperature related phenomenon parametrically correlated by the Larson-Miller equation. Thermal effect = T(log t + 20)(10-3) where T is temperature (Rankin) and t is hours For example, holding a piece at 595°C (1100°F) for six hours provides the same relief of residual stress as heating at 650°C (1200°F) for one hour. The energy savings by going to the higher temperature are in the region of 75% or more. The return on investment is as long as it takes to use a calculator – a good example of low-hanging fruit. The catch is that it takes a certain amount of finesse to do the arithmetic, and there are metallurgical concerns that may call for good technical judgment/expertise. Fundamentals Energy used = Energy produced - Energy lost The burning of natural gas is generally depicted stoichiometrically as follows: CH4 + 2O2 = CO2 + 2H2O + Heat

1

In industry, excess air is utilized to a great degree. Some furnace operators run their furnaces with as much as 300% excess air. This can result in huge energy losses as shown later. Heat Transfer Conduction, convection and radiation are the three principal modes for heat transfer. Conduction In conduction, the heat flows through the material, from a higher temperature to a lower temperature based upon the following law of conduction: Q (heat flow per unit time) = k (thermal conductivity) x A (area perpendicular to the heat flow) x Temperature Gradient (difference in temperature/distance between the high temperature of the furnace and outside wall) Convection Heat transfer in convection is governed by the following equation: Q (heat flow per unit time) = h (convective heat-transfer coefficient) x A (area of the surface) x Temperature Difference The heat-transfer coefficient (h) will vary with factors such as turbulence, rate of air/fluid flow, etc. For air, “h” may vary from 10-100 Btu/hr-ft2-°F. The equation can be used to figure out the heat lost in transporting a piece of metal for quenching. A piece of steel 1 ft2 at 1700°F will lose heat by convection, as follows. Here “h” has been assumed to be 18 Btu/hr-ft2-°F and room temperature 70°F. Q = 18 Btu/hr-ft2-°F x 1 ft2 x (1700 - 70°F) = 29,340 BTU/hr Radiation Radiation is the dominant mode of heat transfer in furnaces. The heat transferred is Q = esAtT4 where e= emissivity of the object radiating (furnace); s = Stefan-Boltzmann’s constant = 0.174 x 10-8 Btu/(hour ft2 °R4) with °R=459.67+°F; A = area of the surface radiating; t = time of the radiation; T = temperature of the object. For example, a normalizing furnace running at 1700°F with a gap between the door and furnace shell will radiate heat. Assuming it is a car-bottom furnace with a gap of 4 inches over a length of 6 feet, the area of the gap is 6 x 4/12 = 2 ft2. The emissivity can be assumed to be about 0.9. Using the law for radiation above we find out the heat lost is: Q = 0.9 x 0.174 x 10-8 Btu/(hour ft2 °R4) (1700+459.67°R)4 x 2 ft2 = 68,135 BTU/hr. Hence, it is important to shut the furnace doors properly. If a gap exists, it should be covered with a ceramic-wool blanket. If the door is left open, however, the heat lost

2

depends upon the area of the door. Assuming it is 6 x 6 = 36 ft2, the radiation equation yields Q = 1,226,433 BTU/hr. Heat Losses

Fig. 2. Furnace efficiency vs. excess air In furnaces, heat losses occur in the following ways: 1. Flues – More than 50% of the heat may be lost through the flues. This is mainly due to the use of excess air for combustion. In boilers, the flue-gas heat loss may be as much as 83%. Flue-gas heat loss is the single largest energy loss in a combustion process, and because the products of combustion are heated by the combustion itself, it is impossible to eliminate this flue-gas heat loss. Reducing the amount of excess air supplied to the burner, however, will lessen flue-gas heat loss. ....In heat-treating practice, it is assumed that excess air will contribute to furnace uniformity by causing turbulence in the furnace. At higher temperatures above 1400°F, however, more than 90% of the heating is done through radiation. Therefore, increasing turbulence does not help in achieving uniformity. It is best to go closer to stoichiometric amounts of needed air. This is best achieved by slowly fine-tuning the process. Start cutting back on excess air at temperatures above 1400°F, and use the dampers again, very gradually, to reduce the exit of gases taking into account safety of the operation first (Fig. 2). This method can reduce the heat losses by over 33%. It has to be achieved slowly, however, and one builds upon the shop-floor experience and constraints over time. 2. Conduction through the refractory walls – These are generally low, unless the refractory lining has been damaged. With proper maintenance, many of these losses can be reduced significantly.

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3. Convection and radiation – These are generally visible, and the best shop-floor remedy is to patch the area where hot gases are escaping with a ceramic-fiber blanket. Pictures can also be taken with an infrared camera to detect heat losses. Scheduling as a Tool for Energy Conservation

Another method, which does not rely upon costly new capital equipment but good shop-floor management, is scheduling. Smaller loads are very inefficient because the bulk of the heat is spent in heating the refractory lining and also exits through the flues. Bigger loads are more efficient because the losses are more or less the same, since the same amount of energy goes into heating the walls of the furnace. Flue-gas losses are similar for large and small loads because the rate at which flue gases escape depends upon the energy of the flue gas. This is primarily dependant on temperature and factors such as flues and damper position, which are generally the same for bigger and smaller loads. Therefore, for larger loads the energy consumed per unit weight is less. Table 1 indicates the increase in efficiency by increasing the load size. Loads can be combined for better economies, and furnaces should not be kept waiting for equipment and personnel. Proper scheduling can eliminate down-the-road bottlenecks and affect energy conservation. An example is an industrial facility that utilized the exhaust from its annealing furnaces to preheat boiler water. The calculated savings were about $320,000/year. Energy Audit A simple audit can be done, and this will require identifying how much natural gas/fuel is consumed every month. A breakdown of fuel consumed by each unit is then determined. These are the steps: Collect energy-consumption data from each furnace and overall facility.

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Collect other technical data needed for analysis. This may include loads heat treated/melted, exit gas temperatures, excess air use, broken insulation, condition of equipment, improper calibration of furnaces, fuel-oil leaks, steam leaks, bare hot surfaces needing insulation, burners out of adjustment, equipment idling when not needed, compressed-air leaks, gas leaks, product rejects, unnecessary handling of materials, frequent production interruption/shutdowns, unnecessary pressure-reducing stations, defective control instruments, defective steam traps, faulty installation of steam traps, plugged-up filters of blowers/compressors, dirty working environment and lack of lubrication – especially furnace wheel bearings. Fine-tuning can result in significant energy savings. Identify improved operating/maintenance procedures. Identify minor cost improvements. Identify any steps that can be minimised/simplified to reduce energy use (inter-critical heat treatments may replace two heat treatments). Identify any product rejects that can be minimized to reduce energy use. Can waste heat be recovered for preheating water/material? Are there any financial/tax incentives provided by the state or utilities? The previously reviewed calculations can be done to evaluate current efficiencies. The goals can be set to achieve these targets. The goals, methodology and results should be shared with the shop personnel. As they get involved, the contributions add up. In one heat-treating facility where this approach was adopted, the unit stopped losing $40,000/month and began making a profit of $160,000/month. The following case studies further illustrate the dramatic impact of this approach. Case Study – Forging Industry Maintaining good temperature control in hot forging can greatly reduce rework and save a lot of energy. Flow stress for a material depends upon yield stress, strain rate, etc. Strain rate is a function of the ram rate. Yield stress is a function of temperature. For a 50°F drop in temperature, the yield stress of some aerospace alloys can double. Therefore, it is very important to maintain temperature control to reduce rework and save energy. Case Study – Diffusion Bonding in the Aerospace Industry In the aerospace industry, titanium alloys are used extensively because of their high stiffness to weight ratio. These titanium alloys are difficult to machine, however. Hence, it was decided to make large parts – some over four feet in length – by joining smaller parts using diffusion bonding. Diffusion bonding can be a simple process. Parts are heated and mated, and they bond together if proper temperature and pressure is used. The problem was that in spite of tremendous capital expenditure, the process was having major problems – a reject rate of about 160%. The rejects were even getting rejected after rework. The energy costs were horrendous. Titanium diffusion bonding is done at high temperatures under pressure, and the rework required TIG welding and extensive machining. A study revealed the problem was improper tooling setup. The parts never really mated in many areas when put under pressure, resulting in a higher reject rate. Once the tooling

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was properly aligned, the process worked and energy savings were tremendous. A multi-billion dollar program stayed on schedule as a result of this simple fix. Conclusion Energy conservation can be achieved using a well-integrated approach backed with good engineering skills and shop-floor management. IH SIDEBAR: Case Study – Heat-Treating Industry The energy lost in operations such as heat treating can be as much as 93%. Hence, if a heat-treating operation has an energy bill of $100,000/month, it may be wasting up to $93,000/month. Here is a simple way of finding the approximate efficiency of a gas heat-treating furnace where the weight (W) of the steel or metal being heat treated (at 1700°F) in the furnace is 1,000 pounds. If weight is not known, make a rough estimate of the volume and multiply it by the density of the metal. References 1 and 2 are good sources for metals/materials properties for doing the calculations. The specific heat (H) of the metal = 0.116 BTU/lb°F. The energy needed to heat the metal is: W x T x H = 1,000 lbs. x 1700°F x 0.116 BTU/lb°F = 197,200 BTU Actual gas burnt = Final reading of the gas gauge – Initial reading of the gas gauge = 1,643,300 BTU. Therefore, the efficiency of this furnace operation is 197,200/1,643,300 ~ 12%, and the dollars wasted (per load) are calculated by multiplying the price of gas, $7/MMBTU (1 million BTU) or $0.000007/BTU, by the quantity of wasted gas. Money wasted = (1,643,300 BTU – 197,200 BTU) x $0.000007/BTU = $10 When there are a number of car-bottom furnaces heating huge shafts (for instance), the losses could add up to maybe $200,000/month. Prices of gas will vary as will, accordingly, the losses. Shobhan Paul Starfire Technologies LLC, Malibu, Calif. Posted: June 11, 2008

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Chapter-20

Tips for Thermal Energy Conservation General Undertake regular energy audits. Plug all oil leakage. Leakage of one drop of oil per second amounts to a loss of over 2000 liters/year. Filter oil in stages. Impurities in oil affect combustion. Pre-heat the oil. For proper combustion, oil should be at right viscosity at the burner tip. Provide adequate Pre-heat capacity. Incomplete combustion leads to wastage of fuel. Observe the colour of smoke emitted from chimney. Black smoke indicates improper combustion and fuel wastage. White smoke indicates excess air & hence loss of heat. Hazy brown smoke indicates proper combustion. Use of Low air pressure “film burners” helps save oil upto 15% in furnaces. Furnace Recover & utilize waste heat from furnace flue gas for preheating of combustion air. Every 210C rise in combustion air temperature results in 1% fuel oil savings. Control excess air in furnaces. A 10% drop in excess air amounts to 1% saving of fuel in furnaces. For an annual consumption of 3000 kl. of furnace oil. This means a saving of Rs 3 Lacs. (Cost of furnace oil-Rs. 10 per litre). Reduce heat losses through furnace openings. Observations show that a furnace operating at a temperature of 10000C having an open door (1500mm*750mm) results in a fuel loss of 10 lit/hr. For a 4000 hrs. furnace operation this translates into a loss of approx. Rs. 4 Lacs per year. Improve insulation if the surface temperature exceeds 200C above ambient. Studies have revealed that heat loss form a furnace wall 115mm thick at 6500C amounting to 2650 Kcal/m2/hr can be cut down to 850 kcal/m2/he by using 65 mm thick insulation on the 115 mm wall. Proper design of lids of melting furnaces and training of operators to close lids helps reduce losses by 10-20% in foundries. Boiler Remove soot deposits when flue gas temperature raises 400C above the normal. A coating of 3mm thick soot on the heat transfer surface can cause an increase in fuel consumption of as much as 2.5%. Recover heat from steam condense. For every 60C rise in boiler feed water temperature through condense return, there is 1% saving in fuel. Improve boiler efficiency. Boilers should be monitored for flue gas losses, radiation losses, incomplete combustion, blow down losses, excess air etc. Proper control can decrease the consumption upto 20%.

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Use only treated water in boilers. A scale formation of 1mm thickness on the waterside would increase fuel consumption by 5-8%. Stop steam leakage. Steam leakage from a 3 mm-diameter hole on a pipeline carrying steam at 7kg/cm2 would waste 32 kl of fuel oil per year amounting to a loss of Rs. 3 Lacs. Maintain steam pipe insulation. It has been estimated that a bare steam pipe, 150 mm in diameter and 100m in length, carrying saturated steam at 8kg/cm2 would waste 25 kl of furnace oil in a year amounting to an annual loss of Rs. 2.5 Lacs. DG Sets Maintain diesel engines regularly. A poorly maintained injection pump increases fuel consumption by 4Gms/KWH. A faulty nozzle increases fuel consumption by 2Gms/KWH. Blocked filters increase fuel consumption by 2Gms/KWH. A continuously running DG set can generate 0.5 Ton/Hr of steam at 10 to 12 bars from the residual heat of the engine exhaust per MW of the generator capacity. Measure fuel consumption per KWH of electricity generated regularly. Take corrective action in case this shows a rising trend. Tips for Electrical Energy Conservation General Improve power factor by installing capacitors to reduce KVA demand charges and also line losses within plant. Improvement of power factor from 0.85 to 0.96 will give 11.5% reduction of peak KVA and 21.6% reduction in peak losses. This corresponds to 14.5% reduction in average losses for a load factor of 0.8. Avoid repeated rewinding of motors. Observations show that rewound motors practically have an efficiency loss of upto 5%. This is mainly due to increase in no load losses. Hence use such rewound motors on low duty cycle applications only. Use of variable frequency drives, slip power recovery systems and fluid couplings for variable speed applications such as fans, pumps etc. helps in minimizing consumption. Illumination Use of electronic ballast in place of conventional choke saves energy upto 20%. Use of CFL lamp in place of GLS lamp can save energy upto 70%. Clean the lamps & fixtures regularly. Illumination levels fall by 20-30% due to collection of dust. Use of 36W tube-light instead of 40 W tube-light saves electricity by 8 to 10%. Use of sodium vapour lamps for area lighting in place of Mercury vapour lamps saves electricity upto 40%. Compressed Air Compressed air is very energy intensive. Only 5% of electrical energy are converted to useful energy. Use of compressed air for cleaning is rarely justified. Ensure low temperature of inlet air. Increase in inlet air temperature by 30C increases power consumption by 1%. It should be examined whether air at lower pressure can be used in the process. Reduction in discharge pressure by 10% saves energy consumption upto 5%.

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A leakage from a ½” diameter hole from a compressed air line working at a pressure of 7kg/cm2 can drain almost Rs. 2500 per day. Air output of compressors per unit of electricity input must be measured at regular intervals. Efficiency of compressors tends to deteriorate with time. Refrigeration & Air Conditioning Use of double doors, automatic door closers, air curtains, double glazed windows, polyester sun films etc. reduces heat ingress and air-conditioning load of buildings. Maintain condensers for proper heat exchange. A 50C decrease in evaporator temperature increases the specific power consumption by 15%. Utilisation of air-conditioned/refrigerated space should be examined and efforts made to reduce cooling load as far as possible. Utilise waste heat of excess steam or flue gases to change over from gas compression systems to absorption chilling systems and save energy costs in the range of 50-70%. Specific power consumption of compressors should be measured at regular intervals. The most efficient compressors to be used for continuous duty and others on standby. Cooling Towers Replacement of inefficient aluminium or fabricated steel fans by moulded FRP fans with aerofoil designs results in electricity savings in the range of 15-0%. A study on a typical 20ft. diameter fan revealed that replacing wooden blade drift eliminators with newly developed cellular PVC drift eliminators reduces the drift losses from 0.01-0.02% with a fan power energy saving of 10%. Install automatic ON-OFF switching of cooling tower fans and save upto 40% on electricity costs. Use of PVC fills in place of wooden bars results in a saving in pumping power of upto 20%. Pumps Improper selection of pumps can lead to large wastage of energy. A pump with 85% efficiency at rated flow may have only 65% efficiency at half the flow. Use of throttling valves instead of variable speed drives to change flow of fluids is a wasteful practice. Throttling can cause wastage of power to the tune of 50 to 60%. It is advisable to use a number of pumps in series and parallel to cope with variations in operating conditions by switching on or off pumps rather than running one large pump with partial load. Drive transmission between pumps & motors is very important. Loose belts can cause energy loss upto 1-20%. Modern synthetic flat belts in place of conventional V-belts can save 5% to 10% of energy. Properly organized maintenance is very important. Efficiency of worn out pumps can drop by 10-15% unless maintained properly. Ref: PCRG

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furnaces

Engineering

refractories furnaces

reheating furnaces

arc furnaces

types of furnaces

oilfired furnaces

assessment of furnaces

furnace heating materials

furnace oil