Lecture7-SteamGenerators

8/9/2019 Lecture7-SteamGenerators

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Page 1of 7Lecture 7

Lecture 7 - Steam GeneratorsA steam generator is a complex combination of econo9mizer, boiler, superheater, reheater, and air

preheater. In addition, it has various auxiliaries, such as stokers, pulverisers, burners, fans, emission

control equipment, stack, and ash handling equipment. A boiler is that portion of the steam generatorwhere saturated liquid is converted into saturated steam. A steam boiler is a vessel that contains water

and a heat source powered by oil, coal or gas. he boiler transfers heat from the heat source to the

water vessel, thereby turning the water into steam. his steam exits the vessel through a pipe and is

transported to another location where it can be used for cleaning, to power equipment, to provide heat

or for a number of other functions. A closed boiler system is one in which !"" percent of the steam is

returned to the vessel in the form of water and reused. #losed boiler systems are very efficient and

less costly to operate. $owever, some industrial processes contaminate the steam and prevent it from

being reused. A boiler that does not return the used steam to the water vessel is called an open system.

Mechanics of Steam Boilers

he heat source in a boiler system is located in a separate compartment within the water vessel. he

water vessel is attached to the heat source by metal rods, which heat the water directly and convert it

to steam. he steam initially collects in an area above the water vessel known as the dome before

exiting the boiler. he dome forces the steam to become highly condensed so that it will exit the boiler

with a significant amount of pressure. %ressurized steam is particularly important for industrial

applications such as powering turbines and other heavy equipment. All boilers have a safety valve,

which allows excess steam to be released to prevent explosions. A boiler also contains a drain, which

removes contaminants and sediment from the water vessel, and a chimney, which allows heat to

escape once it has passed through the water vessel.

&sually boilers are coal or oil fired. A boiler should fulfil the following requirements'

!. Safety. he boiler should be safe under operating conditions.

(. Accessibility. he various parts of the boiler should be accessible for repair and maintenance.

). Capacity. he boiler should be capable of supplying steam according to the requirements.

*. Efficiency. The boiler should be able to absorb a maximum amount of heat produced due to

burning of fuel in the furnace.

+. It should be simple in construction and its maintenance cost should be low. he boiler should

be capable of quic startin! and loa"in!.

#. he performance of a boiler may be measured in terms of its e$aporati$e capacity also

called po%er of a boiler. It is defined as the amount of water evaporated or steam produced in

! per hour.

Types of boilers

here are two main types of boilers' fire tubes and water tubes. In a fire tube boiler, heat is directed

through metal rods that pass through the middle of the water vessel. he rods are arranged in banks so

that the heat can pass through the vessel many times before escaping through the chimney. he fire

tube system exposes the water to the maximum amount of heated surface and also has the maximum

amount of water storage space. In a water tube boiler, the heat is directed through metal rods near the

outside of the water vessel. -ater tube boilers are vertical and have the ability to generate more units

of steam per hour but also provide less water storage than fire tube configurations.

$owever, the boilers can be classified according to the following criteria'


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Page 2of 7Lecture 7

Accor"in! to flo% of %ater an" hot !ases.

ire tube

-ater tube

Accor"in! to the circulatin! %ater.

/atural circulation

orced circulationAccor"in! to steam pressure.

0ow pressure

1edium pressure

$igher pressure

&ire Tube Boilers

In fire tube boiler the hot products of combustion pass through the tubes, which are surrounded, by

water.

&ire tube boilers are classifie" as'

2xternal furnace

Internal furnace

(arious a"$anta!es of fire tube boilers are'

0ow cost

luctuations of steam demand can be met easily

It is compact in size.

)

&ire Tube Boilers' C*C+,A B*LE,

#ylindrical shell

Its crown having a spherical shape.

$emispherical furnace

3pherical shape requires least material for the volume

3pherical crown gives maximum strength to withstand

the pressure of the steam

#oal or oil can be used as fuel in this boiler

#oal is fed into the grate through the fire door

ormed ash is collected in the ash4pit and is removed

manually.

5il burners are fitted below the fire door

$eat transfer to the water by convection

*ther &ire Tube Boilers


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Page 3of 7Lecture 7

he 3traight4ube boiler

he 6ent4ube boiler more detail al4wakil p47*8

/ater Tube Boilers

In water tube boilers, water circulates through thetubes and hot products of combustion flow over these

tubes.

-ater ube 6oiler is most common because

of its ability to deliver large quantities of

steam.

-ater ube 6oiler looks very complicated.

housands of tubes are placed in strategic location to optimize the exchange of energy from

the heat to the water in the tubes.

he large tube like structure at the top of the boiler is called the steam drum.

he hundreds of tube start and eventually end up at the steam drum.

(arious a"$anta!es of %ater tube boilers are as follo%s'

!. $igh pressure can be obtained.

(. 0arge heating surface can be obtained by use of large number of tubes. herefore steam can

be generated easily.

6ecause of high movement of water in the tubes the rate of heat transfer becomes large resulting into

a greater efficiency.

The ,a"iant boiler

It is the boiler that receives most of its heat by

radiation and is designed for electric4generating

stations that use coal or lignite for pulverized orcyclone furnace applications, oil or natural gas.

hey are limited to subcritical pressure !(+4!"8

bar.

*nce throu!h Boiler

It is also called the forced circulation, 6enson or universal pressure boiler. It is applicable to all

temperatures and pressures, but economically is suited to large sizes and pressure in the high

subcritical and supercritical range. In this type the feedwater goes through the economizer, furnace

walls and super heater sections in one continuous pass, so no drum is required and no water

recirculation takes place. It demands very high purity water because of the one through process and it

is the only type that can be used by supercritical pressure operation.

/ater Tube Boilers $s. &ire Tube Boilers

/ater Tube Boilers

are less liable to explosion,

produce higher pressure,

Are accessible and can response quickly to change in steam demand.

ubes and drums of water4tube boilers are smaller than that of fire4tube boilers and due to

smaller size of drum higher pressure can be used easily.

-ater4tube boilers require lesser floor space. he efficiency of water4tube boilers is more.


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Page 4of 7Lecture 7

-ater tube boilers require less weight of metal for a given size

&ire tube boilers

ire tube boilers have low initial cost,

3imple design.

-ater volume is large and due to poor circulation they cannot

meet quickly the change in steam demand.

Steam Theory -ithin the boiler, fuel and air are force into the furnace by the

burner.

here, it burns to produce heat.

rom there, the heat flue gases8 travels throughout the boiler.

he water absorbs the heat, and eventually absorbs enough to

change into a gaseous state : steam.

The %ater tube boiler

-ater enters the boiler, preheated, at the top.

he water naturally circulates through the tubes down to the lower area.

he water heats up and flows back to the steam drum where the steam collects.

/ot all the water gets turn to steam, so the process starts again.

-ater keeps on circulating until it becomes steam.

1eanwhile, the control system is taking the temperature of the steam drum, along with

numerous other readings, to determine if it should keep the burner burning, or shut it down.

As well, sensors control the amount of water entering the boiler, this water is known as

feedwater.

eedwater is treated with chemicals to neutralize various minerals in the water, which

untreated, would cling to the tubes clogging or worst, rusting them.

his would make the boiler expensive to operate because it would not be very efficient.

5n the fire side of the boiler, carbon deposit resulting from improper combustion or

impurities in the fuel can accumulate on the outer surface of the water tube.

his creates an insulation which quickly decreases the heat transfer from the flue gas to the

water.

o remedy this problem the engineer will carry out soot blowing. At a specified time using a

long device, which has a tip at the end which ;blows; steam.

his blowing action of the steam ;scrubs; the outside of the water tubes, cleaning the carbon

build up.

-ater tube boilers can have pressures from 7bar to as high as 012bar.

he steam temperature


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Page 5of 7Lecture 7

he hot gases from the combustion process move from the bottom left to the top and then down the

right hand side of the boiler structure. As the hot gas moves through the boiler it loses energy to the

water and the gases becomes cooler as a result. here are various heat exchangers in the boiler which

extract energy from the hot gases and transfer it to water.

he following diagram shows the inner workings of a boiler. he numbers in the diagram are

described below.

". #oal powder and air are blown into the boiler and combust to form a fireball.

!. #ool feedwater enters the boiler at the bottom right.

(. he feedwater is heated in the economizer by the warm exhaust gases exiting the boiler.

). he warm feedwater is sent to the steam drum >8 where liquid water is separated from steam

formed in the waterwall. In a super critical coal plant steam is not created. 0iquid water becomes a

supercritical fluid in the superheaters at a pressure of greater than ((! bar and a temperature of greater

than *""o#. herefore there is no steam drum. In a supercritical boiler warmed feedwater passes

directly into the downcomers.

*. 0iquid water flows down the downcomers on the

outside of the boiler to the bottom of the boiler.+. -ater flows up the tubes in the waterwalls of the

boiler and surround the fireball of burning coal. he

water in the tubes is heated by radiation from the

fireball. 3ome of the water in these tubes turns into

steam.

>. he steam and water in the waterwall tubes is sent

to the steam drum, where steam is separated from

liquid water. 0iquid water in the steam drum flows

down the downcomers on the outside of the boiler.

. 3team from the steam drum is sent to the primary

superheater.

7. 3team is super heated to a high temperature andpressure in the primary superheaters and then moves

to the secondary superheaters.

9. In the secondary superheaters steam meets the very

hot gases exiting the top of the boiler. he steam is

heated to its final temperature and pressure before

leaving the boiler as main steam. he main steam is

sent to the high pressure steam turbine. he steam turbines drive a generator which produces

electricity.

!". 3team exiting the high pressure steam turbine is called cold reheat. It is reheated in the reheater

and sent back to the second steam turbine.

6y the time the flue gas exits the boiler most of the energy in the coal will be transferred to the waterin the boiler. he hot gases are cleaned up before they are sent to a stack. eedwater is generally

supplied by condensed steam from the low pressure steam turbine. 0arge amounts of cooling water

are used to condense this steam into liquid water in the condenser.

Boiler Calculations

$eat transfer required to form steam

he steam in the boiler is formed at constant pressure, so

$eat transfer required to form ! kg of steam in the boiler (h2h1 )kJ

Ener!y recei$e" from the fuel


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Page 6of 7Lecture 7

his is obtained from knowledge of the mass of fuel used and its calorific value.

If the mass of the fuel used is m kg, and the calorific value of fuel is #? k@=kg, then

Energy received from fuel=(mCV) kJ

Boiler thermal efficiencyhis is given by the ratio of the energy received by the steam to the energy supplied by the fuel to

produce the steam, thus

Boiler thermal efficiency=Energy to steam

Energy from fuel=

ms(h2h1)

( mCV) 100

-here,

ms=mass of steam raised in a given time

m=mass of fuel used in the same time

Equi$alent e$aporation of a boiler3ome boilers can be operated under many different running conditions for these boilers it is

necessary to have some standard upon which to base, and compare, their respective evaporative

capacities. he standard commonly adopted is the equivalent evaporation of a boiler from and at

!""o#.

Energy received by steam=ms(h2h1 )kJ

rom this it determined the amount of water at !""o# which could be evaporated into dry saturated

steam at !""o# if supplied with this same amount of energy. his is then called the equivalent

evaporation of the boiler, from and at !""o#.

At !""o# it will be enthalpy of evaporation B ((+>.9 k@=kg8 which is supplied,

hus, the equivalent evaporation of boiler, from and a !""o# is

ms (h2h1 )2256.9

kg

in the given time or per kilogram of fuel

The Steam 5rum

3team drum is where feedwater from economizer is fed, saturated steam is separated from the boiling

water and the remaining water is circulated above. he drum is also used for chemical water treatment

and blow down to reduce solids in the water. 3team drums are provided in all modern generators

except the once4through types.

he drum should stand the flow rate changes and prevent the carryover of water towards the supper4

heater which may lead to distortion or burnout.

3team is separated in the drum by two methods'

a. Cravity separation b. 1echanical separation


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c.

". Gra$ity Separatione. actors that affect the gravity separation are'

o 3team velocity

o %ositions of the down comer and riser nozzle with respect to the steam outlet

o 5perating pressuref. Cravity separation is economical only for low4steam4capacity, low pressure service.

g.

h. Mechanical Separationi. It has three steps'

o %rimary separation' removes most of the water from the steam and prevents the carry under of

steam with the recirculating water to down4comer and risers. 6affle plates and the bent or

corrugated plates are used for primary separation.

o 3econdary separation, also called steam scrubbing or drying. It removes mist or fine droplets

and solids from steam. 3creens, bent or corrugated plates and centrifugal separators are

considered types of the secondary separation.

o #entrifugal separation is used at high pressures and it is called cyclone or turbo separators.

D.

k. ypical utility steam drums range in length to more than !"" ft in diameter to more than !+ ft

long and flow rate in terms of hundreds tons per hour.

l.

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Lecture7-SteamGenerators