Air Preheaters Superheaters Economizers

8/13/2019 Air Preheaters Superheaters Economizers

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Superheater

Superheater is a device consists of tubes which get the heat fromthe product of combustion to add additional heat into steam, so the

temperature of the steam passing through the tubes raise higherthan its saturation temperature. The super heater can help steam

boiler increase its capacity because help turbine generator get itsrequired steam both in terms pressure and temperature.

Advantages and disadvantages

The main advantages of using a superheater are reduced fuel andwater consumption but there is a price to pay in increasedmaintenance costs

Classification of Super heater in Steam Boiler

Classification of superheater can be classified based on designof flow:

1. Parallel Flow Superheater A superheater which has direction of steam flow in superheatersame with the direction of flue gas flow

. !ounter Flow Super heater A superheater which direction of steam flow in super heateropposite with the direction of flue gas flow

". #i$ Flow Super heater A super heater which direction of steam flow in super heater sameand opposite with the direction of flue gas flow

Classification of super heater can be classified based on heat

transfer:1. !onvection Super heater

The convection super heater is located somewhere in the flue gasflow, where it absorbs heat transfer by convection.

. %adiant Super heater
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%adiant super heater is located in or near furnace of steam boilerand receives heat transfer from combustion process by radiation..Classification of super heater can be classified based onarrangement:

1. &ertical Super heater The super heaters are arranged vertically. This type has is easy tosuspend and has free moving to e$pand. 'ut the disadvantage, it isnot drainable.

. (ori)ontal Super heater The super heaters are arranged hori)ontally. This type has is moredifficult to suspend and should give special support to give moving

for e$pand. 'ut the advantage, it is drainable.What about platen & pendant super heater? What are they?

The super heater could be set up as pendant surface area *veryclear bac+space among tubes or maybe platen surface area *closeto-tangent bac+space among tubes . &arious heat transferapproaches are applied if determining platen and pendant

placements. The pendant design provides better efficient areacaused by a higher number of the tube circumference obtainable tota+e in heat out of the flue gases. This particular brings aboutlesser number of superheater rows within depth pertaining to thesimilar steam temperature increase.


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Air Pre heaterThe function of an air pre-heater is similar to that of aneconomi)er. t recovers some portionof the waste heat of hot flue gases going to chimney, and transferssame to the fresh air

before it enters the combustion chamber./ue to preheating of air, the furnace temperature increases. tresults in rapid combustion of fuel with less soot, smo+e and ash. The high furnace temperaturecan permit low grade fuel

with less atmospheric pollution. The air pre-heater is placed between economi)er andchimney.!eed Water Pump

t is used to feed the water at a high pressure against the high pressure of steam alreadye$isting inside the boiler.Steam "n#ector

A steam in0ector lifts and forces the feed water into the boiler. t isusually used for verticaland locomotive boilers and can be accommodated in small space.

t is less costly. t does nothave any moving parts thus operation is salient.

Air preheater is auxiliary equipment for steam generators. It is a heattransfer surface (heat exchanger) in which air temperature is raised bytransferring heat from exit flue gas in boiler. Since air heater can besuccessfully employed to reclaim heat from flue gas at low temperature

levels than is possible with economizer, the heat re ected to chimney can bereduced to higher extent thus increasing the efficiency of the boiler. !orevery "# #$ drop in flue gas exist temperature, the boiler efficiency increasedby about % &. In utility and process boilers it is used to heat the air requiredfor combustion purpose and to dry and transport coal.

SELECTION PARAMETERS:1 . 'umber of heaters.


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". ype of fuel. *as flow entering + -. +ir flow leaving + -

/. *as inlet temperature0. 1esired gas leaving temperature.2. +ir inlet temperature3. +llowed pressure drop air4gas or total5. rimary air flow and temperature.%#. -ot end pressure differential.

Advantage of air heaters : In addition to increase in boiler efficiency theother advantages that may result are listed below 6

%. Stability of combustion is improved by use of hot air.". Intensified and improved combustion.

. ermitting to burn poor quality coal.

. -igh heat transfer rate in the furnace and hence lesser heattransfer area requirement.

/. 7ess unburned fuel particle in flue gas thus combustion andboiler efficiency is improved.

0. Intensified combustion permits faster load variation andfluctuation.

2. In the case of pulverized coal combustion, hot air can be usedfor drying the coal as well as for transporting the pulverized coalto burners.

3. his being a non pressure part will not warrant shutdown ofunits due to corrosion of heat transfer surface, which is inherentwith lowering of flue gas temperature.

3. T pe of Air heaters: +ir heaters can be classified as re!"perative andregenerative types based on their operating prin!ip#e .

1. % !2P %AT & T3P a T2'24A%

b P4ATc ST A# !5 4

. % 6 7 %AT & T3Pa 48276ST%5#

b %52T( #24

.% Re!"perative Air heaters :In 8ecuperative type, heating medium is on one side and air is on the

other side of tube or plate and heat transfer is by conduction through thematerial which separates the media. hese are of static construction andhence there is only nominal lea9age through expansion oints, access doors,casings etc.

hese types of air heaters can further be classified as follows 6


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.%.%. abular air heater 6 his usually consists of large number of steel tubesof # to 0/mm dia. :ither welded or expanded into the tube plates at theend. :ither gas or air may be designed to flow through the tube. *asthrough the tube normally requires higher size tube and vertical flow toreduce fouling. Single or more passes on the gas side and multi pass to andfro on the airside usually fits in with the overall plant design. +ir heaterportion at low temperature is designed normally with a shorter tube length soas to facilitate maintenance of surface due to corrosion and fouling. In somecases instead of using boiler flue gases, separate external firing is usedparticularly during starting.

.%." late type air heaters 6 hese comprise of parallel plats, which providealternate passage for gas and air. his type is simple and compact comparedto that of tubular type. he narrow passes between plates ma9e the cleaningtedious but with shot cleaning method it is improved. ;t replacement is ama or tas9.

.%. . Steam air pre


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he 7 ungstrom regenerative + - absorbs waste heat from flue gasand transfers this heat to incoming cold air by means of continuously rotatingheat transfer elements of specially formed metal plates.

-igh efficiency elements are spaced and compactly arranged withintwelve sectors shaped compartments for heater sizes from " ./ to "0./ andtwenty four sector shaped compartments for heater sizes from "2 to 0, of aradially divided cylindrical shell called the rotor.

he housing surrounding the rotor is provided with duct connections atboth the ends and is adequately sealed by radial and axial sealing members =forming an air passage through one half of the preheater and the gaspassage through the other.

+s the rotor slowly revolves the mass of the elements alternatively

through the air and gas passages, heat is absorbed by the element surfacespassing through the hot gas stream, then as the same surfaces are carriedthrough the air stream, they release the stored up heat thus increasing thetemperature of the incoming combustion or process air.

."." 8othemuhle ype6

his type is the same as the 7 ungstorm except that the matrixelement is stationary and the air4gas hoods rotate. +gain axis of rotationmay be horizontal or vertical.

.". . ri


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+s the rotor slowly revolves the mass of the elements alternatelythrough air and gas passages, heat is absorbed by the element surfacespassing through the hot gas stream, then, as the same surfaces are carriedthrough the air steam, they release the stored up heat thus increasing thetemperature of process air.

he heating elements of specially formed metal plates form thebas9ets, which are arranged compactly in three layers and within twelvesectors shaped compartment of the radially devided cylindrical shell calledrotor. he housing surrounding rotor is adequately sealed by radialcircumferential and axial sealing members forming passages for secondaryair, primary air and flue gases. he complete rotor is supported by thrustbearing at the bottom and guided by radial bearing at the upper end. wo +$motors and one +ir motor are provided to ensure the continuous operation of+ -.

.". .% 8otor 1rive Anit (@ain and +ux)6

ype % speed reducer ratio %%#6% with built in over running clutch,%%B?, :!$, I / enclosure, -orizontal foot mounted, squirrel cage inductionmotor, %/ C, phase, /# -z, with class ! insulation. he method of startingis direct on line. his motor is coupled with speed reducer by fluid coupling.

.". . . +ir @otor 1rive Anit 6

neumatic air motor type I:$, I @ ##, +ir pressure 0. 9g4cm "

consumption 3 '@ 4min at approx. #./ rpm, :nergized to close, de


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he impeller, runner and casing are high tensile +luminum alloycastings, and the impeller and the runner both have a large no. of straightradial vanes. he runner shaft is carried in ball and roller bearings in thecasing and impeller. here is no mechanical connection between the impellerand runner. he amount of oil in the coupling is varied over a wide range togive ad ustment of acceleration and over load torques. + gland seal is fittedbetween the casing and runner shaft.

he impeller, driven by the motor, and the runner, coupled to thedriven machine both has a large no. of straight radial vanes. he impellerbehaves li9e a centrifugal pump, creating up outwardly flowing steam of oil,which crosses the gap to the runner, which acts as a turbine. he oil steamgives up power as it flows inwards between the vanes of the runner and as itreturns to the impeller again, the cycle is repeated.

.". ./ Sealing +rrangement 6

Seals are provided at both the ends of the + - to minimize lea9agebetween the airside and the gas side. he hot and cold end radial seals areattached to each diaphragm of the rotor and are set at a specified clearancefrom the sector plates which separate the gas and air steam. Seals providedat the rotor post are set to operate with minimum clearance with respect tothe horizontal sealing surface of the sector plate. he circumferential sealsprovide sealing between the periphery of the rotor and the connectingplate4housing.

3.$.3.% S!oot o'ing Arrange(ent:

he cleaning device is provided at the cold end (gas side) to removesoot deposits on the heating elements. he hot end elements are cleaneddue to erosive action of the fly ash and do not need cleaning.

he cleaning device consists of an electric motor coupled to a feardriven cran9 mechanism, which oscillates the swivel header carrying thenozzle pipe. he cleaning medium i.e. superheated steam is conveyedthrough the swivel header and respective nozzle pipe, to the nozzle at theend. + rotary oint in the supply line permits free motion of the swivel headerwhile connected to the source of supply. he arc travel by the nozzle and therotation of the rotor sub ects the entire area of the rotor to the action of thecleaning et.

1rain connections are provided in the steam piping layout for removingcondensate from the piping system while the device is idle and ust before itis placed in operation.

!or normal operation the recommended steam pressure is % 9g4cm "

at %/# #$ superheat, with the inlet valve wide open. :ffective cleaning actionwill not be obtained if the steam pressure is too low. +lso steam pressureshould not exceed %2 9g4cm " as excessive pressure for an extended periodmay damage rotor seals and heating surface.


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;efore admitting the steam to the cleaning device nozzle, drain thesupply lines thoroughly to assure complete removal of wet steam andcondensate ahead of the cleaning device.

3.$.3.) *ater 'ashing of Air pre+heaters :

In case when residual deposit accumulations can not be readilyremoved by soot blowing, it some times become necessary to water wash theheating surface to maintain acceptable draft losses through air preil circulating system is provided to supply support bearingwith the bath of continuously clean oil at proper viscosity. he bearing oilsupply is circulated by means of a motor driven screw pump through an


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external filtering system. *uide bearing is a double row cylindrical rollertype.

,. Air heater fires :

+ir heater fires may occur during cold start up following hot stand bybecause of poor combustion of the oil fuel. he improper combustion resultsin un burnt or partially burnt oil condensing and depositing on the +ir heaterelement surface.

+s the temperature entering the +ir pre " , -alon and other extinguishing agents areineffective under these circumstances. ?ater washing and deluge systemshould be used to extinguish the above said fires by flooding the water in theair heaters.

Air preheater


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Schematic diagram of typical coal-fired power plant steam generator highlighting the air preheater *AP( location. *For simplicity, any radiant section tubing is not shown.

An air preheater *AP( is a device designed to heat air before another process *fore$ample, combustion in a boiler with the primary ob0ective of increasing the thermalefficiency of the process. They may be used alone or to replace a recuperative heatsystem or to replace a steam coil.

The purpose of the air preheater is to recover the heat from the boiler flue gas whichincreases the thermal efficiency of the boiler by reducing the useful heat lost in the fluegas. As a consequence, the flue gases are also sent to the flue gas stac+ *or chimney at alower temperature, allowing simplified design of the ducting and the flue gas stac+. talso allows control over the temperature of gases leaving the stac+ *to meet emissionsregulations, for e$ample .

$ypes of Air preheaters:
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There are two types of air preheaters for use in steam generators in thermal powerstations 9 5ne is a tubular type built into the boiler flue gas ducting, and the other is aregenerative air preheater. These may be arranged so the gas flows hori)ontally orvertically across the a$is of rotation.

Another type of air preheater is the regenerator used in iron or glass manufacture.

$ubular type

Construction features

Tubular preheaters consist of straight tube bundles which pass through the outlet ductingof the boiler and open at each end outside of the ducting. nside the ducting, the hotfurnace gases pass around the preheater tubes, transferring heat from the e$haust gas tothe air inside the preheater. Ambient air is forced by a fan through ducting at one end ofthe preheater tubes and at other end the heated air from inside of the tubes emerges into

another set of ducting, which carries it to the boiler furnace for combustion.

Problems

The tubular preheater ductings for cold and hot air require more space and structuralsupports than a rotating preheater design. Further, due to dust-laden abrasive flue gases,the tubes outside the ducting wear out faster on the side facing the gas current. #anyadvances have been made to eliminate this problem such as the use of ceramic andhardened steel.

#any new circulating fluidi)ed bed *!F' and bubbling fluidi)ed bed *'F' steam

generators are currently incorporating tubular air heaters offering an advantage withregards to the moving parts of a rotary type.

Dew point corrosion

/ew point corrosion occurs for a variety of reasons .:;?@F *==@! and "?@F *11?@! , but sometimes at temperatures as high as?@F *1 ;@! , then the ris+ of dew point corrosion damage becomes considerable.

%egenerative air preheaters

There are two types of regenerative air preheaters9 the rotating-plate regenerative air preheaters *%AP( and the stationary-plate regenerative air preheaters *%othemuhle .

%otating plate regenerative air preheater
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Typical %otating-plate %egenerative Air Preheater *'i-sector type

Principle function for the 40ungstrom regenerative preheater.

The rotating-plate design *%AP( consists of a central rotating-plate element installedwithin a casing that is divided into two * bi-sector type , three *tri-sector type or four*quad-sector type sectors containing seals around the element. The seals allow theelement to rotate through all the sectors, but +eep gas lea+age between sectors to aminimum while providing separate gas air and flue gas paths through each sector.

Tri-sector types are the most common in modern power generation facilities. n the tri-sector design, the largest sector *usually spanning about half the cross-section of thecasing is connected to the boiler hot gas outlet. The hot e$haust gas flows over thecentral element, transferring some of its heat to the element, and is then ducted away forfurther treatment in dust collectors and other equipment before being e$pelled from the
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flue gas stac+ . The second, smaller sector, is fed with ambient air by a fan , which passesover the heated element as it rotates into the sector, and is heated before being carried tothe boiler furnace for combustion. The third sector is the smallest one and it heats airwhich is routed into the pulveri)ers and used to carry the coal-air mi$ture to coal boiler

burners. Thus, the total air heated in the %AP( provides9 heating air to remove the

moisture from the pulverised coal dust, carrier air for transporting the pulverised coal tothe boiler burners and the primary air for combustion.

The rotor itself is the medium of heat transfer in this system, and is usually composed ofsome form of steel andBor ceramic structure. t rotates quite slowly *around "-C %P# toallow optimum heat transfer first from the hot e$haust gases to the element, then as itrotates, from the element to the cooler air in the other sectors.


n this design the whole air preheater casing is supported on the boiler supporting

structure itself with necessary e$pansion 0oints in the ducting.

The vertical rotor is supported on thrust bearings at the lower end and has an oil bathlubrication, cooled by water circulating in coils inside the oil bath. This arrangement isfor cooling the lower end of the shaft, as this end of the vertical rotor is on the hot end ofthe ducting. The top end of the rotor has a simple roller bearing to hold the shaft in avertical position.

The rotor is built up on the vertical shaft with radial supports and cages for holding the bas+ets in position. %adial and circumferential seal plates are also provided to avoidlea+ages of gases or air between the sectors or between the duct and the casing while in

rotation.

For on line cleaning of the deposits from the bas+ets steam 0ets are provided such that the blown out dust and ash are collected at the bottom ash hopper of the air preheater. Thisdust hopper is connected for emptying along with the main dust hoppers of the dustcollectors.

The rotor is turned by an air driven motor and gearing, and is required to be started beforestarting the boiler and also to be +ept in rotation for some time after the boiler is stopped,to avoid uneven e$pansion and contraction resulting in warping or crac+ing of the rotor.The station air is generally totally dry *dry air is required for the instrumentation , so the

air used to drive the rotor is in0ected with oil to lubricate the air motor.

Safety protected inspection windows are provided for viewing the preheaterDs internaloperation under all operating conditions.

The bas+ets are in the sector housings provided on the rotor and are renewable. The lifeof the bas+ets depend on the ash abrasiveness and corrosiveness of the boiler outlet gases.
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Problems

The boiler flue gas contains many dust particles *due to high ash content not contributingtowards combustion, such as silica, which cause abrasive wear of the bas+ets, and mayalso contain corrosive gases depending on the composition of the fuel. For e$ample,

ndian coals generally result in high levels of ash , sulfur and silica in the flue gas. Thewear of the bas+ets therefore is generally more than other, cleaner-burning fuels.

n this %AP(, the dust laden, corrosive boiler gases have to pass between the elements ofair preheater bas+ets. The elements are made up of )ig )ag corrugated plates pressed intoa steel bas+et giving sufficient annular space in between for the gas to pass through.These plates are corrugated to give more surface area for the heat to be absorbed and alsoto give it the rigidity for stac+ing them into the bas+ets. (ence frequent replacements arecalled for and new bas+ets are always +ept ready. n the early days, !or-ten steel was

being used for the elements. Today due to technological advance many manufacturersmay use their own patents. Some manufacturers supply different materials for the use of

the elements to lengthen the life of the bas+ets.

n certain cases the unburnt deposits may occur on the air preheater elements causing it tocatch fire during normal operations of the boiler, giving rise to e$plosions inside the air

preheater. Sometimes mild e$plosions may be detected in the control room by variationsin the inlet and outlet temperatures of the combustion air.

Schematic of typical stationary-plate regenerative air preheater

Stationary plate regenerative air preheater
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The heating plate elements in this type of regenerative air preheater are also installed in acasing, but the heating plate elements are stationary rather than rotating. nstead the airducts in the preheater are rotated so as to alternatively e$pose sections of the heating

plate elements to the upflowing cool air .:1


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'oilers have forced draft *F/ fans to supply air for the combustion of fuel. n addition tothis fan, there can be many other fans such as induced draft * / , primary air fans *PA ,seal air fans, scanner air fans, etc. n these the F/, /, and PA fans are large capacity fans,while the others are smaller fans. n the case of a pressuri)ed boiler, only an F/ fan is

present. (ere the F/ fan handles the full pressure drop of the whole boiler air and flue gassystem . n the case of a balanced draft system, the / fan evacuates the flue gas from thefurnace and handles the pressure drop in the flue gas section. The PA fan is used in thecase of solid fuels to carry the fuel to the furnace and give the primary air requirement tothe fuel.

$rip in a running boiler

Any trip of ma0or equipment in the boiler causes the boiler parameters to vary widely

before it stabili)es depending upon the action ta+en by the operator. 'oiler tripping canalso be caused if some vital equipment trips or if some unsafe condition appears. Theimportant factors to be understood in a trip of equipment in a boiler are mainly four.

1. The specific cause of trip. The plant response to the trip

". The immediate operator action required. The immediate local operator action requirement
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"n the case of both ! fans tripping' the boiler will go for trip as the air supply to thefuel is cut off in full or e$cluding the primary air depending upon the system design. ThePA fan also trip as soon as all the F/ fans trip. /epending upon whether the boiler is fora process unit or power station, the action on the steam consumer end will vary.

$he specific cause of the trip

For both F/ fans to trip the main reasons can be as below.

. G& supply failure ?. G& supply failure !ooling water to motors failure

Plant response to the trip

The immediate plant response when both the F/ fans trip will be boiler trip. The PA fans

and mills also trip.

"mmediate operator action re(uired

The boiler will have to be prepared for light up immediately. Any delay in this actionwithin a very short period can cause lot of problems and loss for the plant. The followingwill help any boiler operator to speed up his action to bring bac+ the boiler on line.

!hec+ feed pump, maintain drum level 6et clearance from connected group to start equipment in boiler Start one / fan, as soon as possible

Start F/ fan #aintain purge conditions !omplete purge ntroduce furnace probe 4ight up boiler 5pen start-up vent valve or bring to service 4P B (P bypass depending upon the

consumer Huic+ly start PA fan B milling system and give steam to the consumer maintaining

the required steam parameters #aintain steam parameters and load the boiler as consumption of steam increases !ontrol the water level in drum #aintain furnace conditions 4oad to ma$imum load as required once all parameters stabili)e

)ocal operator action re(uirement

The local operator of the boiler will have to perform a quic+ tas+ of restoring things inthe locale and communicate to the boiler control room. This set of activity is carried out
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by the respective groups in each area li+e boiler operating floor, mill area, etc. Theactions required are9

See all fuel trip valves are closed !hec+ furnace has no flame

!hec+ if air heaters are rotating I only for regenerative air heaters !hec+ fans for starting conditions and confirm to control room #a+e ready the fuel oil system for light up of unit and inform control room !hec+ local boiler drum level and ma+e sure !hec+ and ma+e ready feeders and mills for coal firing #a+e all electrical chec+ and clear for starting the boiler

All the said actions are broad and for general understanding only. t is required to trainand educate the operators for each design of the units. There are lot of changes ta+ing

place on the controls side and equipment design side. This brings in a need to understandthe specifics of the system that is being handled.

Air preheater

Schematic diagram of typical coal-fired power plant steam

generator highlighting the air preheater *AP( location.

An air preheater *AP( is a general term to describe any devicedesigned to heat air before another process *for e$ample,combustion in a boiler with the primary ob0ective of increasing
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the thermal efficiency of the process. They may be used alone or toreplace a recuperative heat system or to replace a steam coil.

n particular, this article describes the combustion air preheaters

used in large boilers found in thermal power stations producingelectric power from e.g. fossil fuels , biomasses or waste .:1


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The tubular preheater ductings for cold and hot air require more space and structuralsupports than a rotating preheater design. Further, due to dust-laden abrasive flue gases,the tubes outside the ducting wear out faster on the side facing the gas current. #anyadvances have been made to eliminate this problem such as the use of ceramic andhardened steel.

#any new circulating fluidi)ed bed *!F' and bubbling fluidi)ed bed *'F' steamgenerators are currently incorporating tubular air heaters offering an advantage withregards to the moving parts of a rotary type.

Dew point corrosion

/ew point corrosion occurs for a variety of reasons .:;?@F *==@! and "?@F *11?@! , but sometimes at temperatures as high as?@F *1 ;@! , then the ris+ of dew point corrosion damage becomes considerable.

%egenerative air preheaters

There are two types of regenerative air preheaters9 the rotating-plate regenerative air preheaters *%AP( and the stationary-plate regenerative air preheaters *%othemuhle . :1


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Principle function for the 40ungstrom regenerative preheater.The rotating-plate design *%AP( : < consists of a central rotating-plate element installedwithin a casing that is divided into two * bi-sector type , three *tri-sector type or four*quad-sector type sectors containing seals around the element. The seals allow theelement to rotate through all the sectors, but +eep gas lea+age between sectors to aminimum while providing separate gas air and flue gas paths through each sector.

Tri-sector types are the most common in modern power generation facilities. :11< n the tri-sector design, the largest sector *usually spanning about half the cross-section of thecasing is connected to the boiler hot gas outlet. The hot e$haust gas flows over thecentral element, transferring some of its heat to the element, and is then ducted away forfurther treatment in dust collectors and other equipment before being e$pelled from theflue gas stac+ . The second, smaller sector, is fed with ambient air by a fan , which passesover the heated element as it rotates into the sector, and is heated before being carried tothe boiler furnace for combustion. The third sector is the smallest one and it heats airwhich is routed into the pulveri)ers and used to carry the coal-air mi$ture to coal boiler

burners. Thus, the total air heated in the %AP( provides9 heating air to remove themoisture from the pulverised coal dust, carrier air for transporting the pulverised coal tothe boiler burners and the primary air for combustion.

The rotor itself is the medium of heat transfer in this system, and is usually composed ofsome form of steel andBor ceramic structure. t rotates quite slowly *around "-C %P# toallow optimum heat transfer first from the hot e$haust gases to the element, then as itrotates, from the element to the cooler air in the other sectors.


n this design the whole air preheater casing is supported on the boiler supportingstructure itself with necessary e$pansion 0oints in the ducting.
http://en.wikipedia.org/wiki/Air_preheater#cite_note-BW-2http://en.wikipedia.org/wiki/Flue_gashttp://en.wikipedia.org/wiki/Air_preheater#cite_note-11http://en.wikipedia.org/wiki/Dust_collectorhttp://en.wikipedia.org/wiki/Flue_gas_stackhttp://en.wikipedia.org/wiki/Flue_gas_stackhttp://en.wikipedia.org/wiki/Centrifugal_fanhttp://en.wikipedia.org/wiki/Centrifugal_fanhttp://en.wikipedia.org/wiki/Pulverizerhttp://en.wikipedia.org/wiki/Rotor_(turbine)http://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/RPMhttp://en.wikipedia.org/wiki/Expansion_jointhttp://en.wikipedia.org/wiki/Expansion_jointhttp://en.wikipedia.org/wiki/File:Ljungstr%C3%B6m_regenerative_heat_exchanger.jpghttp://en.wikipedia.org/wiki/Air_preheater#cite_note-BW-2http://en.wikipedia.org/wiki/Flue_gashttp://en.wikipedia.org/wiki/Air_preheater#cite_note-11http://en.wikipedia.org/wiki/Dust_collectorhttp://en.wikipedia.org/wiki/Flue_gas_stackhttp://en.wikipedia.org/wiki/Centrifugal_fanhttp://en.wikipedia.org/wiki/Pulverizerhttp://en.wikipedia.org/wiki/Rotor_(turbine)http://en.wikipedia.org/wiki/Heat_transferhttp://en.wikipedia.org/wiki/Ceramichttp://en.wikipedia.org/wiki/RPMhttp://en.wikipedia.org/wiki/Expansion_joint


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The vertical rotor is supported on thrust bearings at the lower end and has an oil bathlubrication, cooled by water circulating in coils inside the oil bath. This arrangement isfor cooling the lower end of the shaft, as this end of the vertical rotor is on the hot end ofthe ducting. The top end of the rotor has a simple roller bearing to hold the shaft in avertical position.

The rotor is built up on the vertical shaft with radial supports and cages for holding the bas+ets in position. %adial and circumferential seal plates are also provided to avoidlea+ages of gases or air between the sectors or between the duct and the casing while inrotation.

For on line cleaning of the deposits from the bas+ets steam 0ets are provided such that the blown out dust and ash are collected at the bottom ash hopper of the air preheater. Thisdust hopper is connected for emptying along with the main dust hoppers of the dustcollectors.

The rotor is turned by an air driven motor and gearing, and is required to be started beforestarting the boiler and also to be +ept in rotation for some time after the boiler is stopped,to avoid uneven e$pansion and contraction resulting in warping or crac+ing of the rotor.The station air is generally totally dry *dry air is required for the instrumentation , so theair used to drive the rotor is in0ected with oil to lubricate the air motor.

Safety protected inspection windows are provided for viewing the preheaterDs internaloperation under all operating conditions.

The bas+ets are in the sector housings provided on the rotor and are renewable. The lifeof the bas+ets depend on the ash abrasiveness and corrosiveness of the boiler outlet gases.

Problems

The boiler flue gas contains many dust particles *due to high ash content not contributingtowards combustion, such as silica, which cause abrasive wear of the bas+ets, and mayalso contain corrosive gases depending on the composition of the fuel. For e$ample,

ndian coals generally result in high levels of ash , sulfur and silica in the flue gas. Thewear of the bas+ets therefore is generally more than other, cleaner-burning fuels.

n this %AP(, the dust laden, corrosive boiler gases have to pass between the elements ofair preheater bas+ets. The elements are made up of )ig )ag corrugated plates pressed into

a steel bas+et giving sufficient annular space in between for the gas to pass through.These plates are corrugated to give more surface area for the heat to be absorbed and alsoto give it the rigidity for stac+ing them into the bas+ets. (ence frequent replacements arecalled for and new bas+ets are always +ept ready. n the early days, !or-ten steel was

being used for the elements. Today due to technological advance many manufacturersmay use their own patents. Some manufacturers supply different materials for the use ofthe elements to lengthen the life of the bas+ets.
http://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Fly_ashhttp://en.wikipedia.org/wiki/Fly_ashhttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Cor-tenhttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Fly_ashhttp://en.wikipedia.org/wiki/Sulfurhttp://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/wiki/Cor-ten


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n certain cases the unburnt deposits may occur on the air preheater elements causing it tocatch fire during normal operations of the boiler, giving rise to e$plosions inside the air

preheater. Sometimes mild e$plosions may be detected in the control room by variationsin the inlet and outlet temperatures of the combustion air.

Schematic of typical stationary-plate regenerative air preheater

Stationary plate regenerative air preheater

The heating plate elements in this type of regenerative air preheater are also installed in acasing, but the heating plate elements are stationary rather than rotating. nstead the airducts in the preheater are rotated so as to alternatively e$pose sections of the heating

plate elements to the upflowing cool air .:1


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give up heat to the bric+s. The airflow is then reversed, so that the hot bric+s heat up theincoming combustion air and fuel. For a glass-melting furnace, a regenerator sits oneither side of the furnace, often forming an integral whole. For a blast furnace , theregenerators *commonly called Cowper stoves sit separate to the furnace. A furnaceneeds no less than two stoves, but may have three. 5ne of the stoves is Don gasD, receiving

hot gases from the furnace top and heating the chec+erwor+ inside, whilst the other is Don blastD, receiving cold air from the blowers, heating it and passing it to the blast furnace.

See also %ecuperator conomiser %egenerator Thermal wheel

*conomi+ers and air heaters perform a +ey function in providinghigh overall boiler thermal efficiency by recovering the low level*i.e., low temperature energy from the flue gas before it ise$hausted to the atmosphere.

For each ?F * ! that the flue gas is cooled by an economi)er orair heater in a conventional boiler, the overall boiler efficiencyincreases by appro$imately 1J. conomi)ers recover the energy

by heating the boiler feed waterK air heaters heat the combustion

air.

Air heating also enhances the combustion of many fuels and iscritical for pulveri)ed coal firing to dry the fuel and ensure stableignition.

conomi)ers are basically tubular heat transfer surfaces used to preheat boiler feed water before it enters the steam drum*recirculating units or furnace surfaces *once-through units . Theterm economi)er comes from early use of such heat e$changers toreduce operating costs or economi)e on fuel by recovering e$traenergy from the flue gas.

conomi)ers also reduce the potential of thermal shoc+ and strongwater temperature fluctuations as the feed water enters the drum or
http://en.wikipedia.org/wiki/Blast_furnacehttp://en.wikipedia.org/wiki/Recuperatorhttp://en.wikipedia.org/wiki/Economiserhttp://en.wikipedia.org/wiki/Regeneratorhttp://en.wikipedia.org/wiki/Thermal_wheelhttp://en.wikipedia.org/wiki/Blast_furnacehttp://en.wikipedia.org/wiki/Recuperatorhttp://en.wikipedia.org/wiki/Economiserhttp://en.wikipedia.org/wiki/Regeneratorhttp://en.wikipedia.org/wiki/Thermal_wheel


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water walls. The economi)er is typically the last water-cooled heattransfer surface upstream of the air heater.

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