BOILERS Nitesh Dattaram Kamerkar

INTRODUCTION

WORKING PRINCIPLE

KEY COMPONENTS OF BOILERS

CLASSIFICATIONS

APPLICATIONS

CHALLENGES

Overview of

Boilers

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INTRODUCTION TO BOILERS :-

Boilers are pressure vessels designed to heat water or produce steam, which can then be

used to provide space heating and/or service water heating to a building. In most

commercial building heating applications, the heating source in the boiler is a natural gas

fired burner. Oil fired burners and electric resistance heaters can be used as well. Steam is

preferred over hot water in some applications, including absorption cooling, kitchens,

laundries, sterilizers, and steam driven equipment.

Boilers have several strengths that have made them a common feature of buildings. They

have a long life, can achieve efficiencies up to 95% or greater, provide an effective method

of heating a building, and in the case of steam systems, require little or no pumping energy.

However, fuel costs can be considerable, regular maintenance is required, and if

maintenance is delayed, repair can be costly.

The pressure vessel of a boiler is usually made of steel (or alloy steel), or historically of wrought

Iron. Stainless steel, especially of the austenitic types, is not used in wetted parts of boilers

due to corrosion and stress corrosion cracking. However, ferritic stainless steel is often used in

superheater sections that will not be exposed to boiling water, and electrically-heated

stainless steel shell boilers are allowed under the European "Pressure Equipment Directive" for

production of steam for sterilizers and disinfectors.

In live steam models, copper or brass is often used because it is more easily fabricated in

smaller size boilers. Historically, copper was often used for fireboxes (particularly for steam

locomotives), because of its better formability and higher thermal conductivity; however, in

more recent times, the high price of copper often makes this an uneconomic choice and

cheaper substitutes (such as steel) are used instead.

For much of the Victorian "age of steam", the only material used for boiler making was the

highest grade of wrought iron, with assembly by riveting. This iron was often obtained from

specialist iron works, such as at Cleator Moor (UK), noted for the high quality of their rolled

plate and its suitability for high-reliability use in critical applications, such as high-pressure

boilers. In the 20th century, design practice instead moved towards the use of steel, which is

stronger and cheaper, with welded construction, which is quicker and requires less labor. It

should be noted, however, that wrought iron boilers corrode far slower than their modern-

day steel counterparts, and are less susceptible to localized pitting and stress-corrosion. This

makes the longevity of older wrought-iron boilers far superior to those of welded steel boilers.

Cast iron may be used for the heating vessel of domestic water heaters. Although such

heaters are usually termed "boilers" in some countries, their purpose is usually to produce hot

water, not steam, and so they run at low pressure and try to avoid actual boiling. The

brittleness of cast iron makes it impractical for high-pressure steam boilers.

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WORKING PRINCIPLE OF BOILERS :-

Both gas and oil fired boilers use controlled combustion of the fuel to heat water. The key

boiler components involved in this process are the burner, combustion chamber, heat

exchanger, and controls.

The burner mixes the fuel and oxygen together and, with the assistance of an ignition

device, provides a platform for combustion. This combustion takes place in the combustion

chamber, and the heat that it generates is transferred to the water through the heat

exchanger. Controls regulate the ignition, burner firing rate, fuel supply, air supply, exhaust

draft, water temperature, steam pressure, and boiler pressure.

Hot water produced by a boiler is pumped through pipes and delivered to equipment

throughout the building, which can include hot water coils in air handling units, service hot

water heating equipment, and terminal units. Steam boilers produce steam that flows

through pipes from areas of high pressure to areas of low pressure, unaided by an external

energy source such as a pump. Steam utilized for heating can be directly utilized by steam

using equipment or can provide heat through a heat exchanger that supplies hot water to

the equipment.

Fire tube Boiler (image source: www.hurstboiler.com)

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A boiler is an enclosed vessel that provides a means for combustion heat to be transferred to

water until it becomes heated water or steam. The hot water or steam under pressure is then

usable for transferring the heat to a process. Water is a useful and inexpensive medium for

transferring heat to a process. When water at atmospheric pressure is boiled into steam its

volume increases about 1,600 times, producing a force that is almost as explosive as gun

powder. This causes the boiler to be an equipment that must be treated with utmost care.

The boiler system comprises of: a feed water system, steam system and fuel system. The feed

water system provides water to the boiler and regulates it automatically to meet the steam

demand. Various valves provide access for maintenance and repair. The steam system

collects and controls the steam produced in the boiler. Steam is directed through a piping

system to the point of use. Throughout the system, steam pressure is regulated using valves

and checked with steam pressure gauges. The fuel system includes all equipment used to

provide fuel to generate the necessary heat. The equipment required in the fuel system

depends on the type of fuel used in the system.

The water supplied to the boiler that is converted into steam is called feed water.

The two sources of feed water are:

1. Condensate or condensed steam returned from the processes and

2. Makeup water (treated raw water) which must come from outside the boiler room

and plant processes. For higher boiler efficiencies, an economizer preheats the feed

water using the waste heat in the flue gas.

Schematic Diagram of a Boiler Room

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KEY COMPONENTS OF BOILERS :-

Boilers are part of a hydronic heating system. Hydronic systems use water to transfer heat to

a distribution source, like a radiator, to heat a home. Hydronic systems can heat via hot

water or steam, depending on the type of boiler used. The boiler is the part of the system

that heats the water to be distributed. The key elements of a boiler include the burner,

combustion chamber, heat exchanger, exhaust stack, and controls. Boiler accessories

including the flue gas economizer are also commonly used as an effective method to

recover heat from a boiler.

Key Components of Boilers are:-

Burner – The burner is the component of boiler that provides the heat that heats the

water of system. The fuels used can be natural gas or oil.

Heat exchanger – The heat exchanger of boiler allows the heat from the burner to

heat the water in system. The job of the heat exchanger is to carry the heat from the

burner to the water without having direct contact with the water. It’s a similar idea to

boiling water in a pot.

Supply lines – Hydronic heating systems use piping to deliver the heated water or

steam to the distribution points, and the supply lines are the pipes that distribute the

hot water or steam to distributor.

Return lines – When the water cools, or the steam cools and changes states back to

water, the return lines bring this water back to the boiler for re-heating.

Firebox – The firebox is where the fuel of system meets the air, creating a flame.

Refractory – Refractory actually refers to refractory materials that are used for filling

any gaps and/or openings that may be around the fire box – this helps ensure the fire

stays in the fire box.

Circulator pumps – circulator pumps push the hot water or steam from system to the

heat distributors in our homes.

Deaerators/Condenser - Deaerator and condenser tanks are only used in steam

boiler systems and not in hot water and hot oil boil because here the fluid always is on

liquid form. The construction of these two types of tanks is almost identical, but as their

names suggest, they are used for different purposes.

Two primary principles are used with this form of tank design: thermal and vacuum.

This depends on which type of boiler being used. Each principle also has different

pump construction requirements.

Thermo principle

A tank using the thermal principle is connected to the atmosphere. This design is

normally used in smaller plants. Here, steam is used to maintain tank water

temperature at around 105°C, which removes air from the water.

Vacuum principle

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Here, an ejector pump is used to create a vacuum in the tank. This causes the tank

water to start boiling because of the low temperature, which in turn removes air from

the water. This principle is normally used for steam turbine applications.

Economizer

Historically, economizers have only been used in large-scale power plants. However,

the demand for more efficient boilers within industry and marine means that

economizers are now far more commonplace. An economizer is a heat exchanger

that is placed in the exhaust from a boiler or in the exhaust funnel of the main engine

of a ship. Pump requirements differ greatly, depending on where the economizer is

installed.

Superheater

It is integral part of boiler and is placed in the path of hot flue gases from the furnace.

The heat recovered from the flue gases is used in superheating the steam before

entering into the turbine (i.e. prime mover).Its main purpose is to increase the

temperature of saturated steam without raising its pressure.

Deaerator :-

A deaerator is used to reduce

oxygen (O2) and carbonic acid

(CO2) levels in boiler feed water to

protect a boiler against corrosion. It

is possible to reduce oxygen and

carbonic acid levels to about <

0.02 mg/l of O2 and 0 mg/l of CO2,

depending on deaerator

construction.

Condenser :-

A condenser ensures that all steam is

condensed before being pumped

back into the deaerator and on into

the boiler. New treated water is

normally fed into the condenser.

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Natural gas boilers employ one of two types of burners, atmospheric burners, also called

natural draft burners and forced draft burners, also called power burners. Due to more

stringent air quality regulations, low NOx burners and pre-mix burners are becoming more

commonly used and even required in some areas. By ensuring efficient mixing of air and fuel

as it enters the burner, these types of burners can ensure that NOx emissions are reduced.

The combustion chamber, usually made of cast iron or steel, houses the burners and

combustion process. Temperatures inside the combustion chamber can reach several

hundred degrees very quickly.

Heat exchangers may be made from cast iron, steel tube bundles, or, in the case of some

smaller boilers, copper or copper-clad steel.

The exhaust stack or flue is the piping that conveys the hot combustion gasses away from

the boiler to the outside. Typically this piping is made of steel, but in the case of condensing

boilers it needs to be constructed of stainless steel to handle the corrosive condensate.

Another consideration is whether the exhaust stack will be under a positive or negative

pressure. This can determine how the joints of the exhaust stack must be sealed.

Boiler controls help produce hot water or steam in a regulated, efficient, and safe manner.

Combustion and operating controls regulate the rate of fuel use to meet the demand. The

main operating control monitors hot water temperature or steam pressure and sends a signal

to control the firing rate, the rate at which fuel and air enters the burner. Common burner

firing sequences include on/off, high/low/off and modulating.

Boiler safety controls include high pressure and temperature, high and low gas/oil pressure,

and high and low water level and flame safeguard controls. These controls are considered

safeties or limits that break the electrical circuit to prevent firing of the boiler. For example, if

the event pressure in the boiler exceeds the pressure limit setting, the fuel valve is closed to

prevent an unsafe, high pressure condition. The safety circuit of a flame safeguard control

system typically includes switch contacts for low water cutoff, high limits, air providing

switches, redundant safety and operating controls, and flame detectors. Flame detectors

often consist of flame rods, and ultraviolet or infrared scanners to monitor the flame

condition and deactivate the burner in the event of a non-ignition or other unsafe condition.

Flame safeguard controls are programmed to operate the burner and cycle it through the

stages of operation.

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CLASSIFICATIONS OF BOILERS :-

Boilers are classified into different types based on their working pressure and temperature,

fuel type, draft method, size and capacity, and whether they condense the water vapor in

the combustion gases. Boilers are also sometimes described by their key components, such

as heat exchanger materials or tube design.

Two primary types of boilers include Fire- tube and Water- tube boilers. In a Fire-tube boiler,

hot gases of combustion flow through a series of tubes surrounded by water. Alternatively, in

a water - tube boiler, water flows in the inside of the tubes and the hot gases from

combustion flow around the outside of the tubes.

Fire tube boilers are more commonly available for low pressure steam or hot water

applications, and are available in sizes ranging from 500,000 to 75,000,000 BTU input. Water-

tube boilers are primarily used in higher pressure steam applications and are used extensively

for comfort heating applications. They typically range in size from 500,000 to more than

20,000,000 BTU input.

Cast iron sectional boilers are another type of boiler commonly used in commercial space

heating applications. These types of boilers don’t use tubes. Instead, they’re built up from

cast iron sections that have water and combustion gas passages. The iron castings are

bolted together, similar to an old steam radiator. The sections are sealed together by

gaskets. They’re available for producing steam or hot water, and are available in sizes

ranging from 35,000 to 14,000,000 BTU input.

Cast iron sectional boilers are advantageous because they can be assembled on site,

allowing them to be transported through doors and smaller openings. Their main

disadvantage is that because the sections are sealed together with gaskets, they are prone

to leakage as the gaskets age and are attacked by boiler treatment chemicals.

Some of the well- known Boilers are:–

Babcock & Wilcox boiler

Cochran boiler - A vertical boiler with horizontal fire-tubes.

Cornish boiler - A large horizontal stationary boiler with a single flue.

La Mont boiler - A forced-water-circulation boiler. They are often used as

marine heat-recovery boilers. It was also used, unsuccessfully, for an

experimental steam locomotive in East Germany in the 1950s.

Lancashire boiler - A development of the Cornish boiler, with two flues.

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BOILERS

(According to Firing Method)

Grate Firing

Traveling grate stoker firing

Front fired Burner

Tangential fired

Opposed jet firing

Down- jet firing

Cyclone firing

BOILERS

(According to Energy Source)

Coal

Liquid Fuel

Gas

Solid waste

Biomass

Recovery Fuel (RDF)

Waste heat

Nuclear Fuel

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BOILERS

(According to Use of Steam)

Utility ( Electricity)

Industrial

Cogeneration

Naval

Marine

Domestic heating

Cooking

BOILERS

(According to Type of construction)

Packaged shell(Fire Tube)

Packaged water tube

Field erected

BOILERS

(According to Type of water circulation)

Natural

Forced

Combined

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APPLICATIONS OF BOILERS :-

Heat is basis of our life. If heat came only from the sun’s rays, large areas of the earth would

be uninhabitable for man. Artificial heating (thermal heat) is therefore necessary, depending

on the geographic position and season of the year. In addition to this, there are also a great

number of technical processes that are only made possible through heat, for example,

cooking, boiling and cleaning processes in the food and drink industry. But in many other

branches, too, such as the paper, building, chemical or textile industry, many processes

function only with heat (process heat).

Industrial hot water boiler systems for generating thermal heat are very similar to the

household heating boilers in our cellars. The main difference is that industrial boilers are

dimensioned significantly larger, so their heating capacity is not only sufficient for a family

home but also for hotels, hospitals, skyscrapers, industrial buildings or entire districts. When

using process heat generated by steam boiler systems the individual applications are far

more versatile. They are used in many industry sectors.

Laundries and cleaning firms

This is an obvious example of what steam is used for. It is just easier to get rid of spots and dirt

when the washing water is heated. Our washing machine at home does the same, however

with electrical heating. In large laundries this would be inefficient as electrical energy is too

expensive. Steam can also be perfectly used for downstream processes like pressing, using

the mangle, ironing or finishing. We know this process from steam-ironing at home; steam

simply removes all creases.

Food Industry

Food must often be heated or boiled during processing. Thus this industrial sector obviously

needs plenty of thermal energy. However, some steam applications are still stunning; a good

example is potato processing.

Breweries

Most people know that a good and tasty beer consists of hops, malt and water. However,

before enjoying the beer there is a complex production process. Malt has to be ground

Cold potatoes are filled in a big pressure vessel and hot

steam is abruptly injected. The abrupt heat supply causes

the potato peels to fall off so they do not have to be

peeled by hand any more. Now, isn’t it tempting to fetch

the old steam cleaner from the cellar and use it as a

cooking aid?

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coarsely and mixed with water. The brewer calls this mashing. The mash must be heated to

various temperatures in two to four hours. Steam generated with the steam boiler is normally

used as a heat carrier. Subsequently, hops are added and the mixture has to cool down.

Then yeast is added and triggers the fermentation so that the beer gets the desired effect.

Again after bottles or barrels have been emptied they are normally returned to the brewery.

Of course the breweries first have to clean them before they can be refilled. For this process

steam is again required to heat up the water needed for cleaning.

Building materials industry

Large amounts of steam are also necessary for the production of molded bricks. The basic

materials like sand, lime, water, etc. are mixed and pressed to relatively loose stone

compounds. Subsequently, the stones are transported to a huge pressure vessel (autoclave)

which is then closed and steam is injected. The stones have to harden at a temperature of

approximately 200 °C and a pressure of about 16 bar for a certain period of time and can

then be withdrawn as finished stones.

Sewer pipe rehabilitation

What to do if there is a drain leakage? This problem can either be solved by means of

excavation works at the underground pipes and renewal of the sewage pipe systems or with

rehabilitation tubes. These tubes are overdimensioned hoses that are inserted in the pipes

without excavation work and then inflated with steam. The plastic hose attaches itself to the

sewage pipe under application of pressure and temperature and the pipe can continue to

be used for many years.

…… and many other Industries

Electrical industry

Food packaging

industry

Glass fibre production

Hospitals

Metal-working industry

Paper industry

Pharmaceutical

industry

Plastics

manufacturing

Primary industry

Print offices

Refineries

Steel works

Tank farms

Tobacco industry

Tyre manufacturing

Vulcanisation

Waste disposal

Waste incineration

Slaughterhouses

Agriculture

Animal food industry

Automotive industry

Bakeries

Ceramic industry

Cheese dairies

Dairies

Distilleries

Fruit processing

Dyeing factories

Chemical industry

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CHALLENGES IN BOILERS :-

Some of the major Challenges in Boilers are :-

o To minimize size and weight.

o To meet required life.

o To be resistant to fouling and corrosion.

o To minimize cost.

Poor Maintenance

Statistics indicates that about two-thirds of all boiler failures and nearly all unscheduled

shutdowns are caused by poor maintenance and operation.

Scaling

Scale formations in boilers are responsible for lost efficiency, increased maintenance

and operating costs not to mention lost revenue due to outages and downtime. Most

scale formations in boilers occur due to the presence of hardness in the make-up water.

This hardness reacts in the high temperatures environment within the boiler to form and

insoluble scale. This insoluble scale coats the heat transfer surfaces, acting as an

insulator to impede heat transfer. Hardness isn't the only cause of scale formation in boilers, other impurities such as iron,

silica, copper, oil, etc. are often found in samples of boiler scale. In fact, it is rare to find

scale which isn't the result of several of these impurities.

Slagging and Fouling

Slagging occurs in boiler furnaces where ash deposits are exposed to the radiant heat

of the coal flames, while fouling occurs in the boiler’s convective passes. Slagging &

fouling are mainly observed in coal- fired boilers.

Excessive ash deposits on a coal fired boiler’s heat transfer surfaces will reduce its

efficiency, and in extreme cases a boiler can be shut down by ash related problems. It

Slag on platen superheater

tubes

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has been estimated that slagging incidents cost the global utility industry several billion

dollars annually in reduced power generation and equipment maintenance.

Corrosion

Corrosion occurs in boilers due to two major reasons. The most common cause

is dissolved oxygen entering the system via the feed-water. Another common cause of

corrosion in boiler systems is low pH within the boiler. This reduced pH may result from

carbon dioxide infiltration or form contamination by other chemicals. The oxygen causes

very localized corrosion to occur in the form of pitting. The pits are small but deep

pinpoint holes which eventually can penetrate tube walls and cause their failure. Oxygen corrosion is normally controlled by driving the oxygen from the feed-water in a

deaerating heater/deaerator or by chemically removing it with an oxygen scavenger

such as sodium sulfite. There are many contaminates which can infiltrate a boiler system

and cause low pH levels to develop.