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Compressed Air System by SWAPNIL S MAHADIK THIRD YEAR CHEMICAL ROLL NO 3431

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AIR COMPRESSOR

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Page 1: AIR COMPRESSOR

Compressed Air System

bySWAPNIL S MAHADIK

THIRD YEAR CHEMICALROLL NO 3431

Page 2: AIR COMPRESSOR

contents1. Introduction

2. Types of Air Compressor

3. System Components

4. Energy conservation opportunities

5. Energy performance assessment

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Air Is Free !!!

Compressed Air Is Free !!!Not

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Air compressors account for significant amount of electricity used in Indian industries.

Air compressors are used in a variety of industries to supply process requirements, to operate pneumatic tools and equipment, and to meet instrumentation needs.

Only 10-30% of energy reaches the point of end-use, and balance 70-90% of energy of the power of the prime mover being converted to unusable heat energy and to a lesser extent lost in form of friction, misuse and noise.

Introduction

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Types of Air Compressors

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Types of Air Compressors

There are three basic types of air compressors:Reciprocating (Recip)Rotary Screw (Screw) Rotary Centrifugal (Centrifugal)

These types are further defined by:The number of compression stages Method of cooling (air, water, oil) Drive method (motor, engine, steam) How they are lubricated (oil, oil-free)

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Positive-Displacement Compressors – Reciprocating

A piston, driven through a crankshaft and connecting rod by an electric motor reduces the volume in the cylinder occupied by the air or gas, compressing it to a higher pressure. Single-acting compressors have a compression stroke in only one direction, while double-acting units provide a compression stroke as the piston moves in each direction. Large industrial reciprocating air compressors are double-acting and water-cooled. Multi-stage double-acting compressors are the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units. Reciprocating compressors are available in sizes from less than 1 HP to more than 600 HP.

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Reciprocating compressor

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Positive-Displacement Compressors -

Rotary compressors

Most commonly used sizes from about 30-200 hp. Most common type of rotary compressor is the helical twin screw-type (also known as rotary screw or helical lobe). Male and female screw-rotors mesh, trapping air, and reducing the volume of the air along the rotors to the air discharge point. Rotary screw compressors have low initial cost, compact size, low weight, and are easy to maintain. Less common rotary compressors include sliding-vane, liquid-ring, and scroll-type.

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Screw compressor

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Dynamic Compressors -centrifugal

These compressors raise the pressure of air or gas by imparting velocity energy and converting it to pressure energy. The centrifugal-type is the most common and is widely used for industrial compressed air. Each impeller, rotating at high speed, imparts primarily radial flow to the air or gas which then passes through a volute or diffuser to convert the residual velocity energy to pressure energy. Some large manufacturing plants use centrifugal compressors for general plant air, and, in some cases, plants use other compressor types to accommodate demand load swings while the centrifugal compressors handle the base load. 

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Centrifugal Compressor

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A typical compressed air system

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SYSTEM COMPONENTS Intake Air Filters : Prevent dust and atmospheric impurities from entering compressor. Dust causes sticking valves, scored cylinders, excessive wear etc.

Inter-stage Coolers : Reduce the temperature of the air (gas) before it enters the next stage to reduce the work of compression and increase efficiency. They can be water-or air-cooled.

After Coolers : Reduce the temperature of the discharge air, and thereby reduce the moisture carrying capacity of air.

Air-dryers : Air dryers are used to remove moisture, as air for instrument and pneumatic equipment needs to be relatively free of any moisture. The moisture is removed by suing adsorbents or refrigerant dryers, or state of the art heatless dryers.

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SYSTEM COMPONENTSMoisture Traps : Air traps are used for removal of moisture in the compressed air distribution lines. They resemble steam traps wherein the air is trapped and moisture is removed.

Receivers : Depending on the system requirements, one or more air receivers are generally provided to reduce output pulsations and pressure variations.

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Air Distribution Systems

The air distribution system links the various components of the compressed air system to deliver air to the points of use with minimal pressure loss. The specific configuration of a distribution system depends on the needs of the individual plant, but frequently consists of an extended network of main lines, branch lines, valves, and air hoses. The length of the network should be kept to a minimum to reduce pressure drop. Air distribution piping should be large enough in diameter to minimize pressure drop. A loop system is generally recommended, with all piping sloped to accessible drop legs and drain points.When designing an air distribution system layout, it is best to place the air compressor and its related accessories where temperature inside the plant is the lowest. A projection of future demands and tie-ins to the existing distribution system should also be considered.

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Heat RecoveryAs noted earlier, compressing air generates heat. In fact, industrial-sized air compressors generate a substantial amount of heat that can be recovered and put to useful work. More than 80% of the electrical energy going to a compressor converts to heat. Much of this heat can be recovered and used for producing hot water or hot air

Typical uses for recovered heat include supplemental space heating, industrial process heating, water heating, makeup air heating, and boiler makeup water preheating. Recoverable heat from a compressed air system is not, however, normally hot enough to be used to produce steam directly. 

As much as 80-93% of the electrical energy used by an industrial air compressor is converted into heat. In many cases, a properly designed heat recovery unit can recover anywhere from 50-90% of this available thermal energy and put it to useful work heating air or water

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ENERGY PERFORMANCE

ASSESSMENT OF COMPRESSORS

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Purpose of the Performance Test

Actual Free Air Delivery (FAD) of the compressorIsothermal power required Volumetric efficiency Specific power requirement

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Performance Terms and Definitions

Compression ratio : Absolute discharge pressure of last stage Absolute intake pressure

Isothermal Power : It is the least power required to compress the air assuming isothermal conditions.

Isothermal Efficiency : The ratio of Isothermal power to shaft power

Volumetric efficiency : The ratio of Free air delivered to compressor swept volume

Specific power requirement: The ratio of power consumption (in kW ) to the volume delivered at ambient conditions.

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IV. Volumetric efficiency = Free air delivered m3/min x 100 Compressor displacement, m3/min

Compressor Displacement = π x D2 x L x S x x n 4

D = Cylinder bore, metre L = Cylinder stroke, metre S = Compressor speed rpm = 1 for single acting and 2 for double acting cylinders n = No. of cylinders

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ExampleCalculation of Isothermal Efficiency for a Reciprocating Air Compressor. Step – 1 : Calculate Volumetric Flow Rate k : Flow coefficient (Assumed as 1) d : Nozzle diameter : 0.08 metre P2 : Receiver Pressure - 3.5 kg / cm2 (a) P1 : Inlet Pressure - 1.04 kg / cm2(a)

T1 : Inlet air temperature 30oC or 303oK

P3 : Pressure before nozzle – 1.08 kg / cm2

T3 : Temperature before the nozzle 40oC or 313oK P3 – P4 : Pressure drop across the nozzle = 0.036 kg / cm2

Ra : Gas constant : 287 Joules / kg K Free Air Delivered Qf = k x π x d2 x T1 x 2 (P3-P4) (P3 x Ra) 4 P1 T3 = 1 x π ?? x (0.08)2 x 303 x 2 x 0.036 x 1.08 x 287 4 1.04 313 = 0.391 m3/sec = 1407.6 m3 / h.

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Step – 2 : Calculate Isothermal Power Requirement Isothermal Power (kW) = P1 x Qf x loger 36.7

P1 - Absolute intake pressure = 1.04 kg / cm2 (a) Qf -Free Air Delivered = 1407.6 m3 / h.

Compression ratio r = 3.5 = 3.36 1.04 Isothermal Power = 1.04 x 1407.6 x loge3.36 = 48.34 kW 36.7

Step – 3 : Calculate Isothermal Efficiency Motor input power = 100 kW Motor and drive efficiency = 86 % Compressor input power = 86 kW Isothermal efficiency = Isothermal Power x 100 Compressor input Power = 48.34 x 100 = 56% 86.0

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Assessment of Specific Power requirement

Specific power consumption = Actual power consumed by the compressor Measured Free Air Delivery In the above example the measured flow is 1407.6 m3/hr and actual power consumption is 100 kW. Specific power requirement = 100 1407.6 = 0.071 kW/m3/hr

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Energy Conservation in compressed air systems

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• Ensure air intake to compressor is not warm and humid by locating compressor in well ventilated area or by drawing cold air from outside (Every 40C rise in inlet air temperature results in a higher energy consumption by 1 % to achieve equivalent output. Hence, cool air intake leads to a more efficient compression)

• Clean air inlet filters regularly (For every 250 mmWC pressure lost at the inlet due to choked filters, the compressor performance is reduced by about 2 percent)

• Minimize low-load compressor operation, if demand is less than 50 %, consider change over to a smaller compressor or reduce compressor speed

Energy Conservation Opportunities

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•Periodically cleaning of inter coolers must be ensured. Fouled inter coolers reduce compressor efficiency and cause more water condensation in air receivers and distribution pipe lines

•Check compressor free air delivery regularly• If more than one compressor is feeding to a

common header, compressors must be operated in such a way that only small compressor should handle the load variations whereas other compressors will operate at full load

•Reduce compressor delivery pressure wherever possible to save energy

Energy Conservation Opportunities

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• If pressure requirement for processes are widely different (e.g. 3 bar to 7 bar), it is advisable to have two separate compressed air systems

•Retrofit with variable speed drives in big compressors, say over 100 kW, to eliminate the unloaded running condition

•Keep the minimum possible range between load and unload pressure settings

•Avoid frequent drainage operation•Carry out periodic leak tests to estimate the

quantity of leakage

Energy Conservation Opportunities

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•Compressed air piping layout should be made preferably as a ring main to provide desired pressures for all users

•Avoid misuse of compressed air for body cleaning, agitation, general floor cleaning

•On account of high pressure drop, ball or plug valves are preferable over globe valves

•Reduce pressure drops by pipe size optimization

•Keep compressor valves in good condition. Worn out valves can reduce compressor efficiency by as much as 50 %

Energy Conservation Opportunities

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Thank You