19307501-Boilers

BOILERS AND ITS CLASSIFICATION

SALMAN ALI SYEDPROJECT ENGINEER

GAS GENERATION PROJECT EXECUTION, ABHA.

INTRODUCTION: Steam generators, or boilers as they are often called, form an essential part of any power plant

or cogeneration system. The steam-based Rankine cycle has been synonymous with power generation

for centuries. Though steam parameters such as pressure and temperature have been steadily

ncreasing during the last several decades, the function of the boiler remains the same, namely, 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

are used for generating power via steam turbines. Steam is used in a variety of ways in process

industries, so boilers form an important part of the plant utilities. In addition to 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 world.

Though pulverized coal–fired boilers form the backbone of utility plants, fluidized bed boilers

are finding increasing application when it comes to handling solid fuels with varying moisture, ash, and

heating values; they also generate lower emissions of NOx and SOx. Oil- and gas-fired fire tube boilers

are widely used in small process plants for generating low pressure saturated steam.

Main uses of these systems are found in:

Generation of power

Process industry like paper, textile etc.

Centrally heating offices/homes.

i. BOILER CLASSIFICATION: The terms boiler and steam generator are often used in the same context. Boilers may be

classified into several categories as follows:

A) Application:

Utility

Marine or

Industrial boiler.

Utility boilers are the large steam generators used in power plants generating 500–1000MW 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 2400 psig, 1000 F. A few utility boilers generate supercritical steam at pressures

in excess of 3500 psig, 1100 F. Double reheat cycles are also in operation. Industrial boilers used in

cogeneration plants generate low pressure steam at 150 psig to superheated steam at 1500 psig at

temperatures ranging from 700 to 1000 F.

B) Pressure:

Low to medium pressure

High pressure, and

Supercritical pressure.

Engr. Salman Ali SyedSEC-SOA, Abha

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Process plants need low to medium pressure steam in the range of 150–1500 psig, which is

generated by field-erected or packaged boilers, whereas large utility boilers generate high pressure

(above 2000psig) and supercritical pressure steam.

C) Circulation method:

Natural,

Controlled,

Once-through, or

Combined circulation

Natural circulation is widely used for up to 2400psig 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–water mixture

through the evaporator tubes. Supercritical boilers are of the once-through type. It may be noted that

once-through designs can be employed at any pressure, whereas supercritical pressure boilers must be

of a once-through design.

D) Firing Methods:

Stoker

cyclone furnace

fluidized bed

register burner

fixed or moving grate

E) Construction:

Field-erected

Shop-assembled


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Large industrial and utility boilers are field-erected, whereas small packaged fire tube boilers up

to 90,000 lb/h capacity and water tube boilers up to 250,000 lb/h are generally assembled in the shop.

Depending on shipping dimensions, these capacities could vary slightly.

F) Heat Source and Fuel:

Solid gaseous or liquid fuels

Waste fuel

Waste heat

The type of fuel used has a significant impact 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 as shown:

G) Steam is Generated Inside or Outside the Boiler Tubes:

Fire tube boilers, in which steam is generated outside the tubes, are used in small plants

up to a capacity of about 60,000 lb/h of saturated steam at 300 psig or less; they typically

fire oil or gaseous fuels.

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

generally economical above 50,000 lb/h capacity

Fire-Tube Boilers: The firetube boiler requires a “shell” to enclose the

water and steam to complete the pressure vessel portion of the

boiler and that shell is the principal limit on the size of a firetube

boiler.

To understand why the shell is the limiting factor we

have to understand some basics about strength of materials and

how we determine the required thickness of the shell, tubes,


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and other parts of a boiler. We know that the required thickness of the shell of a boiler or a boiler tube

is a function of the radius. As the tubes get larger the thickness has to increase to hold the same

pressure.

Since the outer shell of a firetube boiler is very large it has to be quite thick. Thicker materials

require more elaborate construction practices in addition to more weight so the price of a boiler

increases proportional to its diameter with sudden large steps in price associated with different

construction rules depending on the thickness and temperature.

A big break point for high pressure boilers come at 1/2 inch thick and 650°F. The increasing

thickness has imposed a normal limit on firetube boilers of 250 psig MAWP (maximum allowable

working pressure). It’s possible to get a firetube boiler for a higher pressure but it’s not a common one.

The other practical limit on the size of a firetube boiler is its diameter. Anything larger than 8 feet 6

inches in diameter will require special permits for transporting it.

Types of Fire-Tube Boilers:

Firetube boilers come in several configurations and arrangements. Basically they are cylindrical

in shape and are further defined by position and modifications to the general form. Most commonly

used fire tube boilers are:

Horizontal Return Tubular (HRT) boiler

Locomotive boiler

Firebox boiler

Scotch Marine boiler

1. Horizontal Return Tubular (HRT) boiler:

HRT boiler is an early design of boiler that has survived to modern times. Return in the label indicates the flue gasses flow down some of the boiler tubes from one end to the other then return through the remaining tubes.


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A cross section is shown in the middle of the figure that shows the tubes, how they’re arranged to permit the baffle at the rear and location of an access door for scraping off the bottom. Typically the shell of the boiler is extended at the end where the gas makes the turn to form a “turning box” which is closed by large cast iron doors. The doors could be at the front or rear of the boiler depending on how it’s constructed relative to the furnace.

2. Locomotive boiler: A locomotive boiler is a good example of a firetube boiler modified to provide some water

cooling of the furnace. The increased cost of the boiler to create a water jacket around the furnace was justified for locomotive service because the steel and water were considerably lighter than the refractory that would be required while providing more heating surface to make the locomotive more powerful. Stay bolts are used to hold the flat surfaces against the internal pressure and their failure was one reason many of these boilers are no longer around.

3. Firebox Boiler:

The firebox boiler was the first potential “package” boiler because it only required construction of

an insulated base in the field with all other parts assembled in the factory. A partial form of the boiler was also built to provide comparable performance at lower construction and shipping costs by requiring construction of part of the furnace as a brickwork base then setting the boiler on top of that base

4. Scotch Marine Boiler: This type of boiler incorporates the insertion of a large furnace tube in the boiler (Figure 9-10)

eliminating the requirements for an external furnace and providing a furnace that is almost completely water cooled.

Many of the original boilers of this design, the ones that were used on ships, were coal fired and required multiple furnaces to provide enough furnace volume and grate surface. The furnace tube diameters range from two feet to four feet and are welded to the tube sheets. The tube sheet to shell joint is also welded. The scotch marine design comes in two general arrangements;

Dry-back design

Wet-back design the most common is a dry back design where the turning chambers at either end of the boiler are formed by an extension of the shell and/or a door that forms the turning chambers. In either case both ends of the

boiler are fitted with doors to gain access to the tube ends. The doors can be full size, covering the entire end of the boiler or they can be multiple with separate doors providing access to various portions of the


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tube ends and furnace. In almost every case the door covering the end of the boiler and furnace tube is refractory lined because the temperatures of flue gas leaving the furnace can be over 1200°F. Some doors contain integral baffles to divert the flow of flue gas back into other tubes in the boilers. The baffle arrangement varies with the boiler design principally to separate the passes.

The wet back arrangement is a more efficient boiler with less refractory to maintain but the higher cost and limited tube removal (front only) has resulted in a decline of its use.

The locomotive boiler is a basic single pass design. The flue gases enter the boiler proper and flow through all the tubes to the outlet of the boiler. The HRT design provided improved heat transfer by providing two passes, the flue gases are turned and return down a portion of the tubes on their way to the stack.

Note that a pass consists of a path for flue gas to travel from one extreme end of the flue gas containing parts of the boiler to another. Neither of these designs required a baffle to direct the flow of flue gas. Scotch marine designs can have two, three, or four passes. A two pass scotch marine boiler requires no baffles other than means to separate the burner from the returning flue gas. Three pass scotch marine construction requires one baffle in the rear of the boiler to separate the first and second pass turning box from the third pass outlet while four pass boilers require a baffle there plus one at the front to separate the second and third pass turning box from the fourth pass outlet


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Sometimes furnace tubes are called Morrison tubes, and it’s done without distinction. Some

furnace tubes are not Morrison tubes; they’re the ones that are basically a simple cylinder. Morrison is

the guy that realized the furnace tube could be made thinner and still withstands the external pressure

without collapsing if it was corrugated. If the tube is corrugated it’s a Morrison tube and if it’s not it’s

just a furnace tube.

The section through a firetube boiler in figure also

reveals another important element of their construction,

staybolts. The tube sheet isn’t supported by the boiler

tubes in the top of the boiler (what we call the steam

space) so staybolts are required to keep that portion of

the tube sheet from buckling out. Part of a boiler

internal inspection is checking the fillet welds attaching

the staybolts to the top of the boiler shell, and the

staybolts themselves, for corrosion. The staybolts

normally penetrate the tube sheet and their welds

should be checked on the outside as well as the inside.

There’s another classification of firetube boiler

that you may encounter. They’re called “oil field boilers”

and they’re designed for that application. Boilers used in

oil fields get little care, normally run on raw water with little condensate return and don’t get the quality

treatment provided by a wise boiler operator so they’re designed for the abuse. They have thicker hells,

thicker tubes, and lower heat transfer rates.

There are many advantages to a scotch marine firetube boiler which includes simplicity in

design. They’re relatively easy to clean completely on the fire side, once you get those heavy doors off.

They can be packaged in most of the sizes, they contain minimal refractory. Tube replacement is less

expensive because all the tubes are straight. They also hold a larger volume of water compared to a

watertube boiler so they absorb load swings a little better.

Water-Tube Boilers: Water tube boilers just like firetube boilers need a shell to contain the water and steam most

watertube boilers require drums or headers to close off the ends of the tubes, provide a path for the

water and steam to flow into and out of the tubes, and provide a place for steam and water to separate.

The difference between the header and drum is that drums are big and headers are small. That rule

doesn’t always work when it comes to what we call a mud drum which is the lowest drum in a boiler and

has connecting piping for blowoff so the mud can be removed from the boiler.

Waterwalls consist of tubes that may be bent to connect to a steam or mud drum or connect to

a header that is connected to one of the drums with more tubes.

Types of water tube boilers:


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University of engineering and technology, Lahore, Pakistan Most common designs of water tube boilers are:

Cross-drum boiler

Bent tube boiler

1. Cross Drum Boiler:

This is a three pass boiler. The flue gases traverse the furnace from the burners to the rear but that’s not counted as a pass. The gases turn up at the back of the boiler and pass up through the superheater and boiler tubes until they reach the top (first pass) then drop down through the middle of the tubes (second pass) and finally up through the tubes at the front of the boiler and out the stack. The baffles are made out of refractory and include tile laid on top of the screen tubes to form the bottom of the second and third passes.

The bottom two rows of tubes are called screen tubes because they form a screen that blocks the radiant energy from the superheater. They also protect the baffle. The sectional header part of this boiler involved the forged square headers shown in the detail which were connected to the steam drum and bottom header by tube nipples (short lengths of tube) and contained hand holes on the side to gain access to the tube ends so they could be rolled. The headers were forged in a semi-square shape to provide a uniform surface for rolling the tubes. Drums are normally of sufficient diameter that there is no problem rolling a tube in them. To gain access to the tube ends to roll them and for other parts the drums have manholes, usually a 12-inch by 16-inch oval opening.

Water separated from the steam and boiler feedwater mixes in the steam drum (a common

arrangement) then drops down the front header s (which are exposed to the coolest flue gas) and rises up the sloped tubes going from the front of the boiler to the rear. In those tubes the water is heated to the point of saturation and starts boiling, changing from water to steam. The steam forms small bubbles in the water, displacing the heavier water and reducing the density of the steam and water mixture as it travels along the tube.


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By the time the mixture reaches the rear headers it is significantly lighter than the water so the weight of the water in the front header is just like a piston pushing down to force the water and steam mixture up the rear headers and back the return tubes to the steam drum. There’s only a little difference in pressure between the water in the front header and the mixture at the rear header, perhaps half the height of the boiler (inches water column) but that’s enough to force the water and steam to flow around with the flow rate of the steam and water mixture through the top tubes at least five times the rate of the steam going out the nozzle, perhaps more. In the case of this boiler all tubes are risers, the front headers are downcomers.

2. Bent tube boilers: Bent tube boilers come in various designs, the most common are the A, D, and O-designs. These

designs provide the current optimum in cost and performance, some better than others, and represent the heart of the packaged watertube boiler industry.

A Type boiler: The A shape is attributed to the single steam drum at the center top and the two mud drums,

commonly called headers, at the bottom. They require a second blow down line and more soot blowers but provided features like a water cooled furnace from one end to the other and balanced construction which makes them easy to transport as package boilers.

The tubes inside that form the furnace have alternating

shapes. One will drop from the steam drum around the furnace and down into the bottom header while the next tube turns above the bottom header and crosses the bottom of the furnace to enter the side of the opposite bottom header. Shifting the tube arrangement by one sets up the crossing pattern with a tangent tube wall construction (Figure 9-19) in most of the roof and sides of the furnace. The furnace floor (the tubes at the bottom) has a maximum spacing of one tube width.

O Type boiler: The O type boiler (Figure 9-20) is similar to the A while

eliminating one header by providing a drum in the bottom center just like the top. The headers required many handholes for rolling the tubes in an A type boiler so the single drum eliminated that expense but produced a boiler with a smaller furnace cross section.

D Type boiler: The predominant design is the D type (Figure 9-21) which has only one drawback and that ’s the

problem with transporting and supporting something with most of the weight on one side. The D tubes extend out of the drum to form the roof of the furnace, drop to form the furnace side wall, and return under the furnace to the mud drum. It has one convection bank of tubes centered between the drums to limit sootblower requirements. This construction makes it possible for the flue gas to leave the boiler via the front or side.


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Muhammad Uzair Barry +92-333-4425262

A more

detailed diagram shows some of the standard features of this construction:


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WATER TUBE VERSUS FIRE TUBE BOILERS:

Generally water tube boilers are suitable for large gas flows exceeding millions of pounds per

hour and can handle high steam pressures and temperatures. Fire tubeboilers are suitable for low steam

pressures, generally below 500psig. Table shows the effect of pressure on tube thickness in both types

of boilers, and one can see why fire tube boilers are not suggested for high steam pressure applications.

In water tube boilers, extended surfaces can be used to make them compact if the gas stream is

clean. Flue gas pressure drop will also be lower than for an equivalent fire tube boiler owing to the

compactness of the design. Water tube boilers can be smaller and weigh less, particularly if the gas flow

is large, exceeding 100,000 lb/h. Superheaters can be used in both types. In a water tube boiler they can

be located in an optimum gas temperature zone. A shield screen section or a large convection section

precedes the superheater. In a fire tube boiler, the superheater has to be located at either the gas inlet

or exit, making the design less flexible and vulnerable to slagging or corrosion. If the waste gas is

slagging in nature, a water tube boiler is desired because the surfaces can be cleaned by using

retractable soot blowers.

ii. Drum: The purpose of the drum is to separate water from steam. Its lower part is full of water. It comes

from the economizer through tubes external to the boiler or if it is missing, directly from the feed pumps. The upper part is filled with steam instead taken from the main valve. If the superheater is included, the steam passes through the entry header of the latter instead.

In big radiation units, downcomers are inserted into the lower part of the drum feeding the steam-generating tubes of the bank (if there is one), as well as the screens of the furnace. Return tubes coming from the upper headers are inserted laterally into the drum.


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In small units, the downcomers

can also be coupled to the lower part of the drum while the return tubes are connected to the sides. Otherwise, the tubes making up the screens of the furnace and those making up the steam-generating tubes of the bank can be coupled directly to the drum.

iii. Superheaters: Superheaters may be of

convection type superheater or

radiation type superheater

1. Radiation-type superheaters:

Radiation-type superheaters are those placed along the walls of the furnace. In small

generators, this solution is used at times for superheaters that push up the steam temperature by a few

dozen degrees above the temperature of the saturated steam. The goal is to dry the steam taken from

the drum and to bring it to such a temperature that it reaches the machines it is meant for, still

saturated and dry, regardless of the heat loss occurring in the external piping between generator and

usage. These superheaters are done by substituting some of the steam-generating tubes on the walls of

the furnace with the tubes of the superheater. The location is in the terminal part of the furnace. This

way, the tubes will not see the flame from the front.

Superheaters placed along the walls of the furnace are used even in very large units. In that

case, they represent the first stage of the superheater (primary superheater). This prevents the

temperature of the fluid inside the tubes to increase too much.

Hanging superheaters of radiation-type are called SH platen, if they are placed at the exit of the

furnace and consist of far apart coils (see Fig. 3.35). In fact, in this case, the heat transferred directly

from the flame, or through radiation by the flue gas at high temperature, is greater than the heat

transferred by convection, given the low velocity of the gas and the value of the so-called mean beam

length.

2. Convection type superheater:


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Convection superheaters are those consisting of a coil bank with the coils placed very close to

one another in an area of the generator not radiated by the flame. In this case, there is still a quota of

heat transferred by radiation by the gas (due to its high temperature), but the heat transferred by

convection is greater.

The functional characteristics of superheaters by convection or radiation with regard to load

variations are completely different. In fact, in convection superheaters, the temperature of the steam at

the exit increases with the load, whereas it decreases in radiation superheaters. This behavior depends

on the following causes. In convection superheaters, a load increase (it will increase the burned fuel) is

matched by an increase in temperature of the flue gas that hit it. In addition, the velocity of flue gas and,

by default, the overall heat transfer coefficient goes up. Therefore, the transferred heat increases

considerably, prevailing on the simultaneous increase in steam to be superheated. Thus, the

temperature goes up.

In radiation superheaters, the load increase will increase the temperature of both flame and flue

gas. Consequently, the radiated heat goes up, but this increase does not compensate for the increase in

steam to be superheated. For this reason, the temperature decreases.

By combining a convection superheater with a radiation superheater, it is possible to achieve a

temperature of the superheated steam that will remain constant, regardless of load variations or that

will at least reduce temperature variations. Note that radiation superheaters carry smaller costs. In fact,

given equal heat absorption, the radiation superheater has a much smaller surface and is much easier to

build compared to the convection superheater.


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Steam Temperature Control:

The steam temperature in packaged boilers is often controlled from 60%to 100% load

by using a two-stage superheater design with interstage attemperation as shown in Fig. 3.18. Steam

temperature can also be maintained from 10% to 100%; however, this calls for a much larger

superheater surface area. Demineralized water should be used for attemperation, because it does not

add solids to the steam. The solids in the feedwater used for attemperation should be in the 30–100

ppb. If solids are deposited inside the superheater, the tubes can become overheated, particularly if

operated at high loads and high heat flux conditions. The convective superheaters are generally

oversized at 100% load as explained earlier. The quantity of water spray is larger at higher load. In the

radiant design, the steam temperature remains nearly flat over the load range because the radiant

component of energy increases at lower loads and decreases at higher loads. Thus many radiant

superheaters do not use a two-stage design.


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The steam temperature in packaged boilers is often controlled from 60%to 100% load by using a

two-stage superheater design with interstage attemperation as shown in Fig. 3.18. Steam temperature

can also be maintained from 10% to 100%; however, this calls for a much larger superheater surface

area. Demineralized water should be used for attemperation, because it does not add solids to the

steam. The solids in the feedwater used for attemperation should be in the 30–100 ppb. If solids are

deposited inside the superheater, the tubes can become overheated, particularly if operated at high

loads and high heat flux conditions. The convective superheaters are generally oversized at 100% load as

explained earlier. The quantity of water spray is larger at higher load. In the radiant design, the steam

temperature remains nearly flat over the load range because the radiant component of energy increases

at lower loads and decreases at higher loads. Thus many radiant superheaters do not use a two-stage

design.

However, reviewing other concerns such as possible overheating of tubes and higher tube wall

temperatures, the choice is left to the user. When demineralized water is not available, a portion of the

saturated steam from the drum is taken and cooled in a heat exchanger, preheating the feedwater as

shown in Fig. 3.18. The condensed water is then sprayed into the attemperator between the two stages

of the superheater. Often, in order to balance the pressure drops in the two parallel paths, a resistance

is introduced into each path or the exchanger is located vertically up, say 30–40 ft above the boiler, to

provide additional head for the spray water control valve operation.

Spraying downstream of the superheater for steam temperature control is not recommended,

because the steam temperature at the superheater exit increases with load, thus increasing the

superheater tube wall temperature, which can lead to tube failures. For example, if 800_F is the final

steam temperature desired, the steam temperature at the superheater exit may run as high as 875–

925_F, which will diminish the life of the tubes over a period of time. Also, the water droplets may not

evaporate completely in the piping and the steam turbine could end up with water droplets and the

solids present in the water, leading to deposits on turbine blades.

To prevent problems with water depositing in them many superheaters are designed to drain

completely by installing the headers at the bottom with the tubes extending up from the headers. We

call them “drainable” superheaters. Boilers in most utility plants are of a construction that doesn’t drain,

the tubes hang down from the headers into the furnace or flue gas passages and they’re called

“pendant” type superheaters.


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iv. Economizer: The function of the economizer is to heat the feed water before it enters the drum. Meanwhile

the flue gas is cooled, thus positively impacting generator efficiency, given the reduced sensible heat

loss of the flue gas.

Economizers are used as heat recovery equipment in packaged boilers instead of air heaters

because of NOx concerns. They are also less expensive and have lower gas pressure drops across them.

Economizers for gas firing typically use serrated fins at four to five fins per inch. For distillate fuel, about

4 fins/in, solid fins are preferred. For heavy oil, bare tubes or a maximum of 2–3 fins/in. are used,

depending upon the dirtiness of the flue gas and the ash content of the fuel.

Economizers are generally of vertical gas flow and counterflow configuration with horizontal

tubes as shown in Fig. 3.23. The water-side velocity ranges from 3 to 7 ft/s. Small packaged boilers,

below 40,000 lb=h capacity, use circular economizers that can be fitted into the stack. Another variation

is the horizontal gas flow configuration with vertical headers and horizontal tubes. Generally, steaming

in the economizer is not a concern, as discussed earlier. Feedwater temperatures of 230–320_F are

common, depending on acid dew point concerns. The feedwater is sometimes preheated in a steam–

water exchanger if the deaerator delivers a lower feedwater temperature than that desired to avoid acid

corrosion in the case of oil-fired boilers.

Economizer with steel finned tubes

v. Air Heater: Air heaters are used in a few waste heat boilers for preheating combustion air. Incineration

plants and reformer furnaces also use preheated air. Like the economizer, the air heater is installed to

reduce the temperature of the flue gas entering the chimney and improve the generator efficiency. Note

that a reduction of 20◦C of this temperature roughly corresponds to an increase in efficiency of 1%. The

air heater cools the flue gas, it also heats the combustion air, this way increasing the heat going into the

furnace. This strongly influences the sizing of the furnace, the amount of heat radiated by the flame, as

well as the exit temperature of the flue gas from the furnace.


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Fundamentally, there are two types of air h

eaters, the recuperative and the regenerative ones. The

recuperative air heaters are static and keep the two

fluids on both sides of the heat exchange surface. These

can be done with tubes. Generally, in that case, it is

preferable to have the flue gas flow through the tubes

and the bank hit by the air outside. This facilitates

cleaning of the surfaces licked by flue gas that can be

done with a pig.

There are two types of regenerative air eaters,

one in which the heater matrix rotates, and one in which the connecting air and flue gas duct work

rotate. The first type is called the Ljungstrom air heater. The energy from the hot flue gases is

transferred to a slowly rotating matrix made of enamel or alloy=carbon steel material, which absorbs

the heat and then transfers it to the cold air as it rotates. The elements are contained in baskets, which

makes cleaning or replacement easier. Regenerative air heaters are more compact than tubular air

heaters, which are heavy and occupy a lot of space. The gas- and air-side pressure drops are high in both

these types of air heaters, adding to the fan power consumption. Due to the low heat transfer

coefficients of air and flue gases and a low log-mean temperature difference (LMTD), surface area

requirements are large for air heaters. However, a lot of surface area can be packed into each basket of

a regenerative air heater, so they are more compact than the tubular heater.

One of the problems with regenerative air heaters is the leakage of air from the flue gas side

that affects the power consumption and efficiency of the fan. Though the leakage may be low, on the


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order of 5–10% depending upon the seal design, it is significant in large plants. In tubular air heaters,

failure of the tubes or expansion joints could result in leakage from the air side to the gas side, but this

is minimal.

vi. HRSGs AND CIRCULATION: Heat recovery steam generators are generally categorized according to the type of circulation

system used, which could be natural, forced, or once-through as illustrated in Fig. Natural circulation

units have vertical tubes and horizontal gas flow orientation, whereas the forced circulation HRSG uses

horizontal tubes and gases flow in the vertical direction. Once-through units can have either a horizontal

or vertical gas flow path. In natural circulation units, the difference in density between water and steam

drives the steam–water mixture through the evaporator tubes and risers and back to the steam drum.

In forced circulation units, a pump is used to drive the steam–water mixture through the horizontal

evaporator tubes. At the steam drum, steam separates from the steam–water mixture and dry saturated

steam flows through the superheater. In once-through designs, there is no circulation system. Water

enters at one end and leaves as steam at the other end of the tube bundle.

Once-Through Units:

A once-through HRSG (called an OTSG) does not have a steam drum like a natural or forced

circulation unit (Fig. 2.12). An OTSG is simply made up of serpentine coils like an economizer. Because

water is converted to steam inside the tubes, the water should have nearly zero solids. Otherwise

deposition of solids can occur inside the tubes to the complete evaporation process. This in turn can

lead to overheating of the tubes and consequent tube failure, particularly if the heat flux inside the

tubes is high. Like natural or forced circulation units, these units generate single- or multiple-pressure

saturated or superheated steam. A once-through unit does not have a defined economizer, evaporator,

and superheater section. The location at which boiling starts keeps moving depending upon the gas

flow, inlet gas temperature, and duty. The single-point control for the OTSG is the feedwater control

valve; valve actuation depends on predefined operating conditions that are set through the distributed

control system (DCS). The DCS is connected to a feedforward and feedback control loop, which monitors

the transients in the gas turbine load and steam conditions. If a transient in the gas turbine load is

monitored, the feedforward control sets the feedwater flow to a predicted value based on the turbine

exhaust temperature, producing steady-state superheated steam conditions.

Because there is no steam drum, the water holdup is much less than in drum-type units. Often

Alloy 800 or 825 tubes are used to ensure dry running and also to limit the sensitivity to oxygen in the

water, avoiding the need for active chemical treatment.


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Natural and Forced Circulation HRSGs:

Figures 2.12b and 2.12c show the arrangement of natural and forced circulation HRSGs. In the

natural circulation unit the differential head between the cold water in the downcomer circuit and the


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hotter, less dense mixture in the riser tubes drives the steam–water mixture through the evaporator

tubes. The circulation pumps provide the additional differential head to ensure flow through the

evaporator tubes. The following are some of the features of these types of HRSGs.

Natural circulation units do not require a pump for maintaining circulation through the

evaporator tubes. The circulation is ensured through natural gravity principles. The use of circulating

pumps inforced circulation units involves an operational and maintenance cost, and their failure for

some reason such as power outage or pump failure could shut down the HRSG.

SOOT BLOWING Soot blowing is often resorted to in coal-fired or heavy oil–fired boilers. In packaged boilers,

both steam and air have been used as the blowing media, and both have been effective with heavy oil

firing. Rotary blowers are sometimes used with distillate oil firing. Steam-blowing systems must have a

minimum blowing pressure of 170–200 psig to be effective. The steam system must be warmed up prior

to blowing to minimize condensation. The steam must be dry. Increasing the capacity of a steam system


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is easier than increasing that of an air system. With an air system, the additional capacity of the

compressor must be considered. Also, because steam has a higher heat transfer coefficient than air,

more air is required for cooling the lances in high gas temperature regions compared to steam.

Moisture droplets in steam can cause erosion of tubes, and often tube shields are required to

protect the tubes. The intensity of the retractable blower jet is more than that of the rotary blower jet,

and its blowing radius is larger, thus cleaning more surface area. However, one must be concerned

about the erosion or wear on the tubes.

Sonic cleaning has been tried on a few boilers. In this system, low frequency high energy sound

waves are produced when compressed air enters a sound generator and forces a diaphragm to flex. The

resulting sound waves cause particulate deposits to resonate and dislodge from the surfaces. Once

dislodged, they are removed by gravity or by the flowing gases. Typical frequencies range from 75 to 33

Hz. Sticky particles are difficult to clean. The nondirectional nature of the sound waves minimizes

accumulation in blind spots where soot blowers are ineffective. Piping work is minimal. Sonic blowers

operate on plant air at 40–90 psi and sound off for 10 s every 10–20 min.


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19307501-Boilers

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