3. Boilers

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• The function of the boiler is to generate steam

at the desired conditions efficiently and with

low operating costs.

• Low pressure steam is used in cogeneration

plants for heating or process applications, and

high pressure superheated steam is used for

generating power via steam turbines.

•

Boilers form an important part of the plantutilities.

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• In addition to boiler efficiency and operating

costs, another factor that has introduced

several changes in the design of boilers and

associated systems is the stringent emission

regulations in various parts of the.

• The limits on emissions of NOx; CO; SOx,

and particulates have impacted the design and

features of steam generators and steam

plants.

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BOILER CLASSIFICATION

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By Application:

• Utility, marine, or industrial boiler. Utility

boilers are the large steam generators used inpower plants generating 500 – 1000 MW of

electricity. They are generally fired with

pulverized coal, though fluidized bed boilers are

popping up in some plants.

• Utility boilers generate high pressure, high

temperature superheated and reheat steam;

typical parameters are 185 bars, 540 oC. A few

utility boilers generate supercritical steam at

pressures in excess of 240 bars, 600 oC.

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By Pressure:

• Low to medium pressure, high pressure, and

supercritical pressure. Process plants need low tomedium pressure steam in the range of 10 – 100

bars, which is generated by field-erected or

packaged boilers.• Large utility boilers generate high pressure

(above 140 bar) and supercritical pressure steam.

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By Circulation Method:

• Natural, controlled, once-through, or combined

circulation

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Boiler circulation

methods.

(a)Natural;

(b) forced circulation;(c) once-through;

(d) once-through with

superimposed

circulation.

1, Economizer;2, furnace;

3, superheater;

4, drum;

5, orifice;

6, circulatingpumps;

7, separator.

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• Natural circulation is widely used for up to 165

bar steam pressure. There is no operating cost

incurred for ensuring circulation through the

furnace tubes, because gravity aids the circulation

process.

• Controlled and combined circulation boilers use

pumps to ensure circulation of a steam –

watermixture through the evaporator tubes.

• supercritical boilers are of the once through type.

It may be noted that once-through designs can beemployed at any pressure, whereas supercritical

pressure boilers must be of a once-through

design.

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By Firing Method:

• Stoker, cyclone furnace, fluidized bed, register

burner, fixed or moving grate.

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By Construction:

• Field-erected or shop-assembled. Large industrial

and utility boilers are field-erected, whereas smallpackaged fire tube boilers up to 40,000 kg/h

capacity and water tube boilers up to 100,000

kg/h are generally assembled in the shop.Depending on shipping dimensions, these

capacities could vary slightly.

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By Slag Removal Method:

• Dry or wet bottom, applicable to solid-fuel-fired

boilers.

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By Heat Source and Fuel:

• Solid, gaseous, or liquid fuels, waste fuel or waste

heat. The type of fuel used has a significantimpact on boiler size. For example, coal-fired

boiler furnaces are large, because a long residence

time is required for coal combustion, whereas oil-and gas-fired boilers can be smaller.

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http://slidepdf.com/reader/full/3-boilers 14/22The impact of fuel on furnace size

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According to Whether Steam is Generated Inside

or Outside the Boiler Tubes:

• Fire tube boilers in which steam is generatedoutside the tubes, are used in small plants up to a

capacity of about 27,000 kg/hr of saturated steam

at 20 bar or less; they typically fire oil or gaseousfuels.

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Fire Tube Boiler

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http://slidepdf.com/reader/full/3-boilers 17/22 A 4-pass fire tube boiler

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• Water tube boilers, in which steam is generated

inside the tubes, can burn any fuel, be of any size,

and operate at any pressure but are generallyeconomical above 20,000 kg/h capacity.

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Water tube boiler

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• Watertube boilers generate steam more quickly

than fire tube boilers because the heating surface

is arranged to provide more intimate contact of the water and gases of combustion with the

heating surface.

•This allows for faster conduction of heat into thewater. In addition, the curvature of the tubes in a

water tube boiler allows for greater flexibility

during expansion; therefore, watertube boilers

generally may be brought up to normal operating

temperature more quickly than fire tube boilers.

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• A water tube boiler can handle load

changes more quickly than a fire tube

boiler because the water circulation is

better in a water tube boiler. This

allows faster heat transfer from the

hot exhaust gases to the water, which

results in more rapid changes in the

steam generation rate.

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3. Boilers

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