1. STEAM GENERATION
1.1
BOILING STEAM GENERATION
Boiling & consequent steam generation is a quite familiar process. In brief , as we began to heat water, it goes on absorbing heat at constant pressure and is evident by rise in the temp. A stage reaches when water begins to boil and there is no rise in temp., at this stage steam is formed, which continues to be so at the same temp. unless and until pressure changes. The first stage of heat has ;due to the latent heat. Thus, thermodynamically speaking, boiling may be considered a special case of adding heat to the working substance in a constant-pressure, constant temp. process.
As the pressure rises ,latent heat decreases and a stage is reached at (221.06 bar ), when the latent heat become zero, this pressure began termed as critical pressure. (for detail see part 1 ,chapter 6).
Further , once the steam is formed & does not have any traces of water i.e. is dry & saturated , we keep the pressure constant while heat is being added, the temperature of steam will begin to rise, the heat expended being known as superheat.
As the working pressure rises, the specific weight of the water goes on decreasing while that of steam goes on rising till at critical pressure it becomes equal both for steam and water as shown in fig. No. 1.1.
Because of specific weight, differential between water & steam at a given pressure, there exists a different head, which ever a pretty long range of pressure provides a good means for circulation during steam generation. Depending upon working pressure i.e. below critical , the steam generators are designated as subcritical or super-critical.
Whereas, for a greater part of the range in subcritical pressure it is possible to have natural circulation during evaporation, but above supercritical pressure circulation is forced one. In the higher subcritical pressure range also due to the reason of economy, forced or assisted circulation may be employed.
1.2 TYPES OF BOILING
It is essential to have a temp. gradient between the products of combustion & the fluid in order to have heat transfer from the later. The temp. gradient for a given flux apart from the fluid flow & conductivity of metal will depend upon external & internal deposits on the tubes. The change from liquid to vapor take place both at the solid liquid interface , as at the inside tube surface of a water tube boiler & at the liquid vapor interface, as with the water surrounding the steam bubbles. There are two distinct viz. " nucleate" & " film type of boiling, based upon the modes of formation & release of steam bubbles. The details of the two types are given in the following sub-sections.
1.2.1
the initial stages of heating involves 'sub cooled ' water heating. i.e. below the saturation temp. when the flux is great enough with a given amount of mass flow, the water in contact with the tube begins to evaporate, but as the temp. of bulk of water has not reached saturation temp. , the bubble collapse & give latent heat to surrounding water. This is the 'sub cooled nuclear' phase. Further, when the bulk of the liquid reaches saturation temp. the bubbles will not collapse & 'nucleate boiling' will start. nucleate boiling is characterized by the formation and release of steam bubbles at the interface with the water still wetting the surface. This is a dual phenomenon where both the flow of the fluid through the tube & the flow of bubbles through the fluid effect the heat transfer. The other points with respect to nucleate boiling are as under :-
1
a) Except for the once-through type boiler designs for sub-critical pressure must stay within nucleate boiling conditions.
b) The incremental increases in temperature gradients produce uniform increase in Conductance. i.e. kcal / cm2 ,if the boiling is kept within the nucleate zone.
c) The required fluid velocities in the tubes in order to retain the conditions of nucleate boiling are :
- for vertical tubes …………0.3 to 3 mps
- for horizontal / inclined tube ………..1.5 to 3 mps
1.2.2. Beyond nucleate boiling region (i.e. higher heat fluxes ) the bubbles collapse to from a film of superheated steam over part or all the heating surfaces. This condition is known as 'film boiling'. This conditions also tends to arise when the proportion of steam in the fluid increases to a point
where complete film of steam is formed at solid, liquid interface resulting in decreased heat transfer rates. therefore, certain minimum flow velocities as discussed are essential. The point, beyond which film boiling begins is known as 'departure from nucleate boiling-(DNB)'.some of the important points associated with the phenomenon are :-
a) the metal temp. till the occurrence of (D.N.B.) is slightly higher above the water temp., when water start boiling , the metal temp. is slightly above the saturation temp., but at D.N.B. the difference between the two temperature is much higher.
b) For higher heat flux, the D.N.B. point is reached at lower steam quality & peak metal temperature is higher.
c) Break down of mode of boiling heat transfer i.e. D.N.B. leads to ' burn -out' of the tube by gross overheating.
1.3 CIRCULATION SYSTEMS
1.3.1 GENERAL
Modern boilers may be with or without the drum. The boilers working below the sub-critical pressure are generally provided with drums, or a small separating vessel (high pressure nearing critical ) in its place irrespective of the type of the type of circulation employed. The drum-acts as reservoir for water & saturated steam and also provides means and arrangements for separation and purification of steam. The term circulation generally with drum type of boilers applies to the movement of fluid from the drum to the combustion zone and back to the drum. The feed water to the drum in any case reaches the drum from the boiler feed pump via the economizer, the drum - less boilers work above critical pressures & there is straight transformation to steam without passing through the boiling stage involving latent heat of evaporation. Some designs in this type of boilers provide a small mixing or separating vessel which deals with the water drops particularly during starting.
It is essential to provide an adequate flow of water and /or of water steam mixture for an efficient transfer of heat from furnace to the working fluid and to prevent 'burn outs'. This is irrespective of the mode of circulation being used.
1.3.2 TYPES OF BOILER CIRCULATION SYSTEMS
The boiler circulation systems have been designated in different ways by the various manufacturers. Of course they are similar as for as basic principles are concerned and differ in minor details and nomenclature. The various classification in vogue are given below.
1.3.2.1 SOME DESIGNERS HAVE THE FOLLOWING CLASSIFICATIONS FOR TYPES OF CIRCULATIONS
a) Natural circulation:- In this type, no external pumping device is used for the movement of the fluid. The difference in densities in contents of fluids in downcomers from the drum and risers in the furnace is used to effect the movement of fluids. This type of circulation is employed in most of the utility boilers and in fact in India excepting one unit at Trombay power station all boilers work on natural circulation. The details of this system appear in the subsequent part of this chapter Fig. 1.2.
b) Positive circulation :- In this type , use of pumps is made for movement of fluid through the combustion zone or complete heat transfer circuits. This type of circulation is further divided into.
i) Assisted circulation:- The term is generally used in case of boilers having drums and working below critical pressures. Circulating water pumps are used in between the bottom headers of downcomers and risers to overcome frictional losses and the consequent movement of water and water steam mixture. The 500 MW units of NTPC will use this system Fig.1.3
ii) Forced circulation:- This term is generally used for movement of fluid in boilers working above critical pressures. These boilers are of 'Once-through' type. The pump forces movement of fluid through all the heating zones viz. Economizers , tubes in combustion zones and superheaters. A small separating or mixing vessel may be provided for removal of moisture from steam and pumping back to the circuit. Some designers tend to use this term even for 'Assisted circulation' described above.Fig.1.4 and 1.5.
1.3.2.2 THE C.E. (U.S.A.) & ALSO B.H.E.L. (INDIA) WHO HAVE TECHNICAL COLLABORATION FOR MANUFACTURE OF BOILER IN INDIA, CLASSIFY CIRCULATION SYSTEM AS UNDER :
a) Natural circulation ; as described earlier.
b) Controlled circulation ; This system is akin to "ASSISTED CIRCULATION" system as described above and is generally adopted beyond 180 kg/cm2.
c) Combined Circulation ; The system is similar to "Forced circulation" system described earlier. It is applicable beyond the critical pressure where phase transformation is absent. The C.E. design in this range provide arrangements for recalculation of water through the furnace tubes at low loads. This protects tubes and simplifies the start up procedures. A typical operating pressure for such a system is 260 Kg/cm2.
1.4 NATURAL CIRCULATION
1.4.1 A steam generation may be basically classified according to the method employed to establish flow through its evaporator, since for all types flows through the economizer upto the drum are established by feed pump. Natural circulation is the movement of the circulating fluid in conformance with the available differential head and in a boiler, this is due to the difference in densities of the contents of down comers and upriser. The circulation in this case is said to be taking place on Thermo-syphon' principles. As the pressure rises the difference is densities between water & steam (Fig.1.1) and consequently the driving force reduces. Thus the head available will not be able to overcome the frictional resistance required for the flow. Therefore, the natural circulation is limited to boilers with drum operating around 175 Kg/cm2.
In any given natural circulation system, the movement of the steam and water will increase with increased heat input to a maximum value or so called end point, after which further increase in heat absorption will result in a decrease in flow. The general form of the curve is shown in Fig. 1.6. In the process of circulation, following two opposing forces are present:-
(I) The increase in flow results from the increase in the difference of the densities of the respective fluids in down comers and risers caused by the increase in heat absorption.
(II) However, at the same time, the friction and impact losses in the system increase, mainly due to increase in specific volume in the riser circuits. Therefore, when the losses increase as compared to gain due to pressure differential the flow rate will begin to drop.
1.4.2 In natural circulation boilers the objective is to design the circuits in the region of rising part of the curve. In this region, the boiler tends to be self-compensating for the usual heat absorption variations such as:
(i) Sudden over loads
(ii) Soot and other deposits on heating surface. (iii)Non uniform fuel bed or burner conditions.
(iv) Inability to forecast actual conditions over the operating life time.
Typical flow curves for steam output with increasing circulating flow are shown in Fig.1.7 for a 186 bar natural circulation boiler working in the rising region of operative curves.
The heat absorption rates may be somewhat higher than the predicted once because of miscalculation of friction loss, localized hot spots, or other unforeseen circumstances. These reduce the average density of the circuit and thus the flow to the circuit may be more than that calculated such as assisted or controlled self compensation is not available.
One of the characteristics of natural circulation is its tendency to provide the highest flow in the tubes with the greatest heat absorption.
Heat transfer rates between the inside surface of steam generating tube and the boiling water in it are extremely high, and the tube inner wall is normally only a few degrees above the saturation temperature corresponding to the operating pressure.
Tube overheating and failures are almost invariably due to internal deposits that insulate the tube from the cooling effects of the water flowing in the tube. Those failures are mostly in high heat flux zones.
1.5 CIRCULATION RATIO
1.5.1 It is essential to maintain a certain amount of flow of water to the steam generating circuits in commensurate with the amount of steam generated from them, in order to prevent `Burn-outs' and `On-load Corrosion'. The ratio by weight of the water fed to the steam-generating circuits to the steam actually generated (Kg water : Kg steam ) is called `Circulation Ratio'. Taking circuit shown in Fig. 1.8 as an example for a unit period of time 5 kg water is admitted to risers (which will get converted into mixture of water and steam during its passage through the furnace) and 1 kg of steam is taken out of the drum, the value of circulation ratio will be five. The remaining 4 Kg of water will be recirculated in the system. To compensate for 1 Kg. Of steam taken out, 1 Kg. Of water will have to be added to the drum, which will enter the risers alongwith the water under recirculation.
1.5.2 Circulation ratio to be adopted for a particular design will be influenced by the operating pressure and the available head. The values of these parameters will decide the circulation head available to produce the driving force available. The curves in graph of Fig. 1.8 show:
(i) Typical design values of circulation ratio as a function of operating pressure.
(ii) With given circulation ratio and uniform heat absorption over height `h' the maximum available circulation head for overcoming flow resistance.
1.5.3 The circulation ratio for various types of boilers are:- (i) Utility Boilers …. 6 to 9
(ii) Industrial Boiler….8 to 30
Higher circulation provides higher thermal inertia and faster response essential for industrial boilers.
1.6
DESIGN CRITERIA OF A CIRCULATION SYSTEM
.1.6.1 The primary requisite of a circulation system design is to ensure prevention against burn-out and on-load corrosion. In order to prevent burn-out it is essential to maintain nucleate boiling conditions, i.e., the flow commensurate with the heat flux. The prevention of on-load corrosion in addition to requirement of a certain minimum flow also needs a stricter control on boiler water quality. The natural circulation within the boiler is a quite complex phenomenon and most of the deductions have been based upto experience and experimental date rather than the theoretical computations. The broad principles of natural boiler circulation are:-
a) Nucleate boiling conditions shall be maintained for all operating conditions ( This is applicable to assisted circulation also).
b) An acceptable percentage of steam by volume (%SBV) or corresponding percentage of steam by weight (%SBW) shall be maintained. This is shown by curves of Fig. 1.9. The inverse of SBW is the circulation ratio which is discussed before. The limit SBV or SBW is a function of many variables such as pressure, heat flux and mass flow. For each pressure and heat flux, there is a maximum permissible quality which is dependent on mass velocity.
c) Minimum velocity of water entering the heated portion of tubes A circulation rate of 1 to 5 fps (0.3 to 1.5 mps) is recommended.
d) Separation of steam and water in Drum to give steam free water to down comers. e) Segregation of circuits having different absorption rates.
1.7 FORCED CIRCULATION
1.7.1 This is also termed as `Positive Circulation'. In this system circulation pumps are employed to force movement of water through different circuits.
1.7.2 Under certain conditions, forced circulation can be usefully employed for steam generation, particularly when the pressure are very high. This is due to the reason that circulation head caused by density differential is too low to cause effective circulation. Generally positive circulation is resorted above 2650 psi ( 182.7 bars), but certain boilers at lower pressure 1800 PSI (124.1 bares) are also designed on this system to take advantage of the small thin tubes and higher velocities made possible by pumping. The boiler at lower pressures will have the conventional drum. The circulation in such a case may be classified as `Assisted Circulation' or as per the criteria adopted by C.E/BHEL it may be termed controlled Circulation. The forced circulation also makes it possible to have most optimum utilization of available space.
1.7.3 In the forced circulation system, we have following types of boilers:
a) Once-Through Type…. Water from the feed supply is pumped to inlet ends of the heat absorbing circuits. Evaporation or change of stage gradually takes along the length of the circuit and when the evaporation is complete, further progress through the heated circuits results in superheating the vapor. No steam and water drum is required in this system and is generally applicable above supercritical pressures. Fig. No.1.4 shows the scheme of forced circulation boilers at supercritical pressures.
b) A modification of once through type boiler is that, evaporation is up to partial dryness (9o%) and the water is removed in a separator and the dry steam passed further through the circuit for superheating. The system is used at sub-critical pressures in the higher pressure zone as shown in Fig. No. 1.5
c) Even near on beyond the critical pressures it has been found advantageous to recirculate the water through the furnace tube at low loads. This protects the furnace tubes and simplifies the start-up procedure. A typical operating pressure for such a system is 260 Kg/cm2. This system is called `Combined Circulation System' and has been adopted in C.E. Boiler designs.
1.8 ASSISTED CIRCULATION, `RECIRCULATING' FORCED CIRCULATION.
In this type provision of steam-water drum is made as in the case of natural circulation boilers and circulating pumps for circulation in various circuits like the forced circulation boilers. This corresponds to `Controlled Circulation System' as per the CE/BHEL classification. In this system of circulation, there is net thermal loss for the boiler unit because of the separate circulating pump. See Fig. 1.10
The flow to individual tubes is controlled by orifice plates to compensate for different positions along the feed headers and different heat absorption.
Though it is possible to adopt natural circulation for boilers working up to the range of 170 to 180 Kg/cm2 but some designers feel it necessary to have assisted circulation even in this or lower ranges. The advantages are:
(i) It is possible to have tubes of smaller diameter for driving up the steam water mixture by making the pumps to do a little more work.
(ii) The provisions of orifices helps to have a more uniform temperature giving another slight saving in tube wall thickness over and above that already obtained by using smaller tubes. (iii)There is overall reduction in furnace size:
1.9 STEAM SEPARATION & PURITY
1.9.1
Steam separation and purity involve the complete elimination of water and detraining the salts from steam before it leaves the drums. It will be seen from the discussions which follow that for steam generation below critical pressures, drum is an important functionary. The feed input, separation of steam, boiler water treatment and below down etc. are all carried through the drum. The priming carry (carry over the water in gulps) and foaming (carry over the water due to formation of foams) problems are also dealt within the drum. The process is carried out in following three distinct stages:
Steam separation …. This is the process of removing bulk masses of water from steam. This may be carried out buy gravity, centrifugal force or by change of direction. It is termed as Primary Separation. The steam may still contaminants which must be removed on reduced in amount before the
steam is sufficiently pure for use in HP turbines. This step is called `Secondary Separation' or `Steam Scrubbing' All these steps are usually accomplished in the boiler drum.
Steam Washing…. This action takes place after primary separation and is the process of rinsing the steam with relatively clean feed water or steam condensate resulting from cooling with feed water. Its purpose is mainly to obtain contact with low silica content water, wash out impurities and condense vaporous silica. Boilers used in India for power generation do not have this system. In order to limit silica carry over care is taken that silica content of drum water always remain within specified limit.
Blow Down…. The salts from the above process will fall into the drum and thus in its water salt concentration will increase. The removal of a part of drum water preferably that which contains high concentration of salts is known as blow down.
1.9.1.2 THE STEAM CONTAMINATION MAY ALSO ARISE BECAUSE OF CARRY
OVER DUE TO PRIMING & FOAMING PHENOMENON DESCRIBED BELOW:
Priming….This is the carry over of water in pulps to the steam during higher water level periods i.e. when the separators (drum internals) get in-effective. There will be entertainment of water in steam due to this. Foaming is primarily the result of the chemical conditions of the boiler water caused by concentration of oil, soap, organic matter, suspended particles or other foreign matter. Excess foaming may result in foam over i.e. carry over of the foam with the steam.
1.9.2 Factors affecting steam separation….. Both the design features and operating factors, affect the separation of steam from steam water mixture reaching the drum through the risers. These factors influence the separation as discussed below:
1.9.2.1 DESIGN FACTORS
Operating Pressure…. Due to increase in sp. wt of steam with increased pressure and greater decrease in sp. wt. of water, the net available head for causing flow of the steam water mixture in the riser decreases as the pressure increases as shown in Fig. 1.8. This also affects the natural tendency of steam and water to separate, as the limiting velocity of a water particle conveyed din steam and force to gravity both vary directly with the differential in the specific weights of the water and the steam, which will affect steam flow per unit of flow area, the steam velocity and the force of gravity as shown in Fig. 1.11.
Rate of steam generation and water circulation…. For a low rate of steam generation (velocity of steam leaving water-upto 1 mps), the steam bubbles have enough time to separate from the mixture by gravity without being drawn into the down comers and without carrying entrained water drops into the steam outlet. At higher rate of generation, not only this advantage is denied, but also the dense upward traffic of steam bubbles causes swelling of level and false level rise is indicated.
Arrangement of Down Comers and Risers… The effect of the location of the riser circuits in relation to the water level is shown in Fig. 1.12 (a & b). Neither of the arrangement is capable of producing desired results if only gravity separation is to be depended upon.
Size of the drum… The lower rate of steam generation per unit area is required if separation is to be carried effectively . Further to permit separation of moisture droplets, a certain minimum distance between the swelled water level and the steam outlet is required, which has to be increased alongwith rise in pressure, at 300 psia (20.68 bars) this distance is approximately 24' (61cm). Thus, it will appear that size of the drum has marked affect on steam separation.
1.9.2.2
OPERATING FACTORS
Boiler water analysis…. The increase in boiler water concentration will lead to higher contamination of steam. Also if the water has higher concent4ation of dissolved solids, increased foaming will take place, which will make ineffective the drum internals which have been fitted for effective steam separation and reduce available space where separation is by gravity only.
Type of steam load…. With rapid increase in boiler load, steam pressure will reduce causing thereby increase in steam volume through out. The resulting `Swell' will increase the level which effects separation adversely and results in water carry over. Thus to prevent carry over larger drum will be required or proper control over boiler load.
Water level carried… The rapid fluctuation in water level are not desirable as these affect the separation adversely and on increased level moisture carry over will result because of the reduced available space in the drum in case of steam separation by gravity and the ineffectiveness of drum internals by flooding. The modern boilers are ,therefore, equipped with proper and effective water level regulation system.
1.10
METHODS OF STEAM SEPARATION
By gravity separation…. The preceding discussion clearly reveals that gravity separation can be only
employed in case of boilers having low generation rates, constant loads and low operating pressures. In case otherwise size of the drum has to be increased which will not be economical. Therefore, other methods for effective primary separation are employed.
By use of Baffles… These are the simplest type of primary separation devices and are in the form of
obstacles in the direct path of steam to outlet. At lower rate of generation single V type baffle may be suitable, but for higher rates other designs such as compartment baffle be used. Diagrammatically baffles are shown in Figure 1.13 ( a & b)
By use of Multiple drums and baffle combination…. The arrangement is shown in Fig. 1.14. The
steam outlet is in dry drum where provision of baffles is also made.
By centrifugal action…. In all the modern high pressure and high capacity boilers separation of steam
is obtained buy employing cyclones which work on centrifugal action. The three basic effects which a good separation system shall promote are:
(i) Centrifugal action to produce separation forces many time greater than those produced by gravity.
(ii) Alter direction of steam water mixture so that the upward velocity vector is zero. (iii)Provision of drainable wetted surface into which fine spray can coalesce.
An arrangement of steam drum internals for effecting steam separation incorporating the three requirements mentioned above the shown in Fig. 1.15. This is used in natural circulation type boilers of CE/BHEL designs. The steam water mixture from the furnace passes through the centrifugal separators where a spin by the spinner blades is imparted to it. This forces the water to the outer edge
of the centrifugal separator where it is separated from the steam. The water flows downward through the annuals space in the cylinder and this is free from steam bubbles. The partially deride steam passes at a low velocity through corrugated plates (secondary separators) where additional mixture is removed by wetting action on the plates. A similar process occurs in the final screen dryers.
1.11 BLOW DOWN
The removal of a portion of water containing impurities from the Boiler Drum is termed as blow down. This is resorted in order to remove the impurities is which increase in the drum after the separation and purification of steam. The blow down water is first passed through a flash tank, where the flash steam is admitted to the deaerator and then water is led through a heat exchanger, where it gives its heat to D. M. water. These steps help to recover heat from a part of blow down water, which finally goes to waste channel. The blow down may be continuous and or periodic. Continuous blow down tapings are from the drum and periodic blow down is from low point drains fitted to lower headers of water walls. Periodic blow down is resorted to when concentrations increase and are not cleared by continuous blow down, and also to remove those impurities which reach the lower headers and get accumulated there. The amount of continuous blow down may be determined as given (Ref.Fig.1.17)
Let T tonnes of steam taken out per hr. P tonnes of steam blow down per hr. Then water to be admitted = T+P tonnes/hr.
(This ignores a slight wastage of water which may be taking place due to leaks etc.) Solids allowed in steam per tonne = Ks. mg.
Solids going down in blow down =Kp. mg. Solids coming along with water = Kf. mg.
Kf assumed to be equal for make up water and boiler water in the circuit. T x Ks + P x Kp = (T + P)Kf.
P = T (Kf - Ks) (P x Kf is neglected as quantity is quite small) ___________
Kp
% blow down = (Kf - Ks) x 100 based upon steam out-put. ---
2.
BOILERS PLANT DESCRIPTION DEVELOPMENT ETC.
2.1 INTRODUCTION
During the last thirty years there has been a phenomenal growth in the size of the generating plant. The increase in size has been necessitated due to many economical & technical factors such as the demand for increased power, desire to have it at low cost despite the steady increase in cost of materials & labour, and general deterioration in fuel quality. The last factor is of grate importance to India where large reserves of low grade coals can be utilized for power generation.
The present technological advancements have placed practically no restrictions on unit size provided the grid has the capacity to bear it during tripping etc. In India the units of 500 MW sizes are being operated at present.
The increase in size has led to increase in over all dimensions. These mammoth Boilers may be as high as a 12 to 15 story building. Fig. No. 2.1 compares the sizes of a 200 MW and 500 MW unit with Kutub Minar.
The increase in boiler size also helps in reduction of vol. And area per KW capacity. This helps to utilize the power station to their utmost advantage. Typical figures are given below:
Capacity Rel Vol.
30MW 1.0
120MW 0.61
200 MW 0.58
350 MW 0.47
A 30 MW, Boiler will require around 6.5 m3 of furnace space.
2.2 STEAM PRESSURE
2.2.1
The economic gains due to increased pressure are not great, but still high pressures are required as it is necessary to reduce the specific volume of steam to the lowest possible level on units having large steaming rates.
2.2.2
To obtain the maximum technical benefit from high pressure if would be necessary to operate near the supercritical regime. The choice of pressure is guided by the load factor, fuel costs and type of circulation used. Natural circulation boilers have been proved for operation at superheater outlet pressure up to 2500 p.s.i. or 177 Kg/cm2 and few units have been tested at even higher pressure. Taking into account economic considerations 2000 psi or 140 Kg/cm2 pressure at super-heater exit may be considered adequate. The higher pressure ranges may be adopted for assisted circulation or once - through type boilers.
2.3 STEAM TEMPERATURE
2.3.1
The maximum efficiency between two limits of the working fluid as give by the cCarnot cycle is represented by the following equation :
T1 - T2 Efficiency = --- T1
T1 & T2 are max. & min. absolute temperatures. As T2 is limited by ambient conditions so T1 must be as high as possible.
Thus from the efficiency point of view the largest gain can be obtained by the use of higher temperatures, but against this must be set the increasing cost of pressure part materials as temperature is increased.
2.3.2
The relative gains due to increase in pressure and due to temperature can be studied with the help of T -S diagram. The area between water and saturated steam lines go on reducing as the pressure rises and so the incremental work done, but gain to superheating steam is significant.
2.3.3
When operating at steam temps, between 538 oC and 593 oC serious consideration must also be given to be possibilities of gas-side corrosion. In India unfortunately no data on this aspect by taking into consideration characteristics of our coals, has been built up. However 200/210 Mw units with 540
o
C temperatures as super-heater and reheater exists are now in operation. The investigations carried out by Central Electricity. Generating Board (United Kingdom) with respect to boilers using local coals show an operating steam temperature range of 538 oC to 593 oC without serious risk of gas side corrosion.
The increase in operating temps. will involve the use of high cost an static material. The C.E.G.B (U.K) taking into consideration the load factor of the system has established 566 oC as the most economical steam temperature for coal fired units. If the load factor goes below 20%, temp. can be reduced. The corresponding steam temp. limit for the oil fired boiler is 538oC
2.4 BASIC BOILER TYPES
See previous chapter on 'steam generation, separation and purification.
2.5
IMPACT OF FUEL
The fuel or combination of fuels influence the design of modern high pressure & temperature boilers considerably. The cost of the fuel and the effect of constituents on corrosion also influence the choice of steam cycle. The main component which are likely to be effected and required consideration during design stage are :-
2.5.1
The furnace size……The choice of fuel will considerably influences the size of the furnace. The coal burning steam generators, because of the ash produced, have larger furnaces. The approximate comparison is as given below :-
FUEL WIDTH DEPTH HEIGHT
Gas W D H Oil 1.06 W 1.05 D 1.2 H
Coal 1.12 W 1.10 D 1.5 H
2.5.2 FUEL BURNING & PREPARATION EQUIPMENT:
2.5.2.1
The medium and high volatile coals can be fired through horizontal circular burners in the front or and rear wall or through corners. These fuels are normally direct fired through medium speed ball or roller mills. Low volatile coals require the application of the U-flame and direct firing of these fuels is favored in using medium speed ball mills or tube mills.
2.5.2.2
For coal preparation equipment is wide and varied. This may comprise apart from the equipment in storage yards; the crushers, conveyors, raw coal bunkers, pulverizes and classifiers etc.
For fuel oils the burning equipment consists of burners of various types. The main problem is to atomize the oil which may be done by raising pressure or in combination with steam or in combination with air.
For fuel oil the preparation equipment will consist of pumps, storage tanks, pressure regulator, and heating arrangement right from the unloading stage to final burning into the furnace.
2.5.3
Heat surface Area & Placement …. Low calorific value coals will require low furnace plan area heat release. The fuel properties also influence the spacing of the superheater and reheater banks to prevent serious fouling of the heating surfaces and the velocity of flues to minimize erosion. Particular care is taken to reduce speeds wherever local dust concentration can occur, ash content is high and it is of abrasive nature.
Apart from the shape and dimensions of the furnace the fuel specifications also influence the arrangement of economizers and air heaters.
2.5.4 Choice of pollution control equipment including ash precipitators also depends upon the fuel specifications.
2.6
EFFECT OF STEAM CYCLE
The increase in unit size along with corresponding increase in pressure and superheat and reheat steam temperatures has brought considerable changes in boiler design. The distribution of heat within the boiler varies as tem. And pressure are increase. With the increase in superheat temp. the proportional heat required in heating the water and bringing it up to saturation temperature progressively decrease and the effect is more marked with the reheat cycle. The furnace must be able to complete the combustion of the fuel and reduce the flue temp. entering convection surfaces to a level at which fouling and corrosion can be controlled. With modern boilers this may not be possible and radiant superheaters surfaces may have to be added or increase.
2.7
FURNACES
2.7.1
Furnace is the primary part of boiler where the chemical energy available in the fuel is converted to thermal energy by combustion. Furnace is designed for efficient and complete combustion. Major factors that assist for efficient combustion are time of residence (fuel) inside the furnace, temperature inside the furnace and turbulence which causes rapid mixing between fuel and air. In modern boilers water cooled furnaces are used which has the following advantages :
(i) In furnace not only combustion but also heat transfer is taking place simultaneously.
(ii) The maintenance work involved in repairing the fire bricks (which is otherwise necessary) is practically eliminated.
(iii)Due to heat transfer in the furnace, temperature of the flue gas leaving the furnace is reduced to the acceptable level of the superheating surfaces.
(iv) Higher heat loading in the furnace is possible as heat is being simultaneously removed by heat transfer, and hence economy in surfacing.
For the reasons explained in 8.6 the furnace rating of coal fired boilers are low. It will be seen from Fig. No. 2.3 that inspite of the large increase in boiler sizes, furnace rating have not increased. This is equally true , both for the heat release and the heat absorption rates which have remained reasonably constant at 15,000 tu /Cu. Ft/hr. (558263KJ/m/hr) and 35,000 to 40,000 Btu/ Sq.ft. / hr. (397081 - 453806 Kjm2/hr) respectively.
2.7.2
Front wall opposed and corner firing are the various systems used in furnace designs. The firing system will effect the shape and dimensions of the furnace. The horizontal circular turbulent burners for coal or other fuels can be designed upto 170 x 106 kj / hr. capacity. Opposed firing is very suitable for burning low volatile coals. In case of coal burners the furnace depth should be 8m and 11m for front wall and opposed firing. Corner firing requires extra fan power to maintain the high primary and secondary air velocities which are necessary to maintain the flame vortex. Furnace depth may be increased or multi furnace design may be adopted for large boilers.
Note : All ratings are estimated on all projected radiant heating surface (P.R.H.S.) Inside the Furnace upto the top of the furnace arch.[a1]
Fig. 2.3
The adequate distance between wing burners & side walls should also be decided. The number of burners & widths can be increased in proportion to the heat input. Furnace heating surface can be increased either by increased height or by the use of division walls.
A limiting factor with coal firing is the need to ensure that the residence time of fuel particles is sufficient to ensure complete combustion and transfer of heat and as per the present experience it should be 2 seconds, though only a fraction of a second is required for combustion of pulverized coal particles. The improvements in burner, air distribution & pulverizes are aimed towards reduction in furnace residence time of fuel particle.
The other important feature of the furnace construction is to reduce the air infiltration. This is to ensure sufficient air through the burners for efficient combustion, prevention of heat losses and proper steam temperature control. The tangent skin type of casing and membrane wall methods of construction for furnace are in use. The later two methods are now being used widely.
2.7.3.1
In the tangent tube construction generally a tube of smaller dia. is placed in between two larger tubes which touch each other. The wall is backed by fire bricks or moldable refractory Typical construction is shown in Fig. No. 2.4.
2.7.3.2
The skin type of casing is shown in fig No.2.5 and has been mostly used on 120 to 500 MW boilers. The casing plate is carried by mild steel channels welded to tubes which are stiffened by beams attached to the channels by some form o spigot or clip to allow for differential expansion between them.
2.7.3.3
A recent development in gas tight casings is the membrane was shown if Fig. No. 2.6 . The tightness is accomplished by either welding tubes together by means of flats or bars. This actually eliminates the casing and many of its accompanying problems. Insulation is directly applied to tubes and metal lagging is attached to give the outer surface durability and good appearance. The use of the membrane wall method of construction facilitates and construction of large surface panels (20m - 2.5m) in the shops and transported to site as such. Thus lot of assembly and welding work can be avoided. The method can be used in large boilers as well, (500, 660 MW). The two factors which have delayed the introduction of this form of construction have been fear of excessive stresses, particularly during starting and the high cost of plant needed for manufacture.
2.7.4 ASH HOPPER SEAL OR THROUGH SEAL
On the older type boiler downward expansion of the boiler is absorbed by large metal bellows type joints, between the boiler base and ash hopper. On modern boilers, a simple, very effective water seal is employed, fig. 2.7 shows the layout of such a seal.
Downward expansion of the furnace walls is absorbed by the ash hopper water seal. The seal also prevents air ingress into the combustion chamber which is normally maintained at a controlled suction by the induced draught fans.
i) Ash hopper staggered throats: Ashing has often proved a difficult and costly operation. This is often due to the radiant heat from the furnace fusing the coarse ash in the hopper into large, extremely hard masses.
The introduction of the staggered throat did much to alleviate this problem. In the staggered throat design, the configuration of the ash slopes reduces the penetration of the radiant heat to the ash hopper.
ii) Ash quench sprays : (Fig 2.7) These sprays in a considerable number are arranged around the top of the ash hopper. Supplied from a ring main, produce a finely divided spray of cooling water across the mouth of the furnace throat.
2.7.5 TYPE OF FURNACE :
The furnaces are classified according to the mode of bottom ash collection in the following three ways:
Dry bottom furnace Oil fired furnace
Slag type or wet bottom type.
i)
Dry bottom furnace
: Selected for coal of non-slagging type i.e. fusion temperature of the ash produced by combustion will be more than the temperature encountered in the furnace. Normally a maximum of 20% total ash may be collected as slag from bottom of furnace. The rest of the ash is carried away along with flue gas. If slagging type coal is used in dry bottom furnace slag will fuse and deposit in the heat transfer surfaces of furnace, superheater and reheater where removal may pose problem.Hopper at the bottom is formed by slopping the front and rear water walls, thus the amount of brick work is reduced and hence maintenance. By this arrangement loss of efficiency due to evaporation of water from hopper is also effectively reduced. Most of the Indian coals contain high amount of silica in the ash and hence ash fusion temperatures are high. Hence dry bottom types are best suited for Indian coals. In addition, loss of efficiency due to sensible heat in the molten ash of wet bottom furnace for high ash content coals.
ii)
Slag type
: Furnace of this type normally has two furnace parts. Primary furnace is used for very high rate of combustion from where the molten slag passes to ash hopper and the flue gases into the secondary furnace which is very similar to dry type furnace. Provision is made to chill the molten slag and crush to granular form for easy disposal be used. To obtain high temperature inside the primary furnace which will facilitate the easy flow of ash, very small but highly rated design is needed for primary furnace. High temp. refractory material is used inside the primary furnace and hence maintenance is needed.iii)
Oil fired boiler furnace
: Oil fired furnace is generally closed at the bottom as there is no need to remove slag as in the case of PF fired boiler. Bottom will have small amount of slope to prevent film boiling in the bottom tubes.If the boiler has to be designed for both PF as well as oil, the furnace has to be designed for coal as otherwise higher heat loading with PF will cause slagging and high furnace exit gas temperature. 2.8
SUPERHEATER
2.8.1
Superheaters(SH) are meant for raising the steam temperature above the saturation temperature. Present trend is to limit the superheated and reheated steam temperature around 540 oC. The introduction of advanced steam cycle in modern boilers has placed in greater burden on superheaters and reheaters. The percentage of heat to superheater and reheater for the 165 bar boiler is approx. 50%.
i) SH(Reheater also can be classified into convection and radiation type according to heat transfer process.
The superheaters and reheaters placed above the furnace which can view the flame is called radiant type. The other surfaces are called as convection types. This is most practical way of classification.
ii) Superheater may be classified also according to the shape of the tube banks and the position of the heaters, such as pendant SH, paten SH, horizontal SH, celling SH, wall SH etc.
iii) They may be classified according to their stages of superheating they perform, like primary SH, Secondary SH, Final SH etc.
2.8.3 Arrangements
Generally heating surfaces can be arranged either in line or staggered. Staggered arrangement requires less surface for same duty but draft losses will be more and on load cleaning of surfaces will not be as effective as in-line arrangement. So, selection of nay one of them depends on the fuel fouling characteristics, operating cost of draft loss, cost of tube materials etc.
The surfaces can be designed to place in such a way that the flow direction of flue gas and steam is parallel or opposite. Counter flow arrangement has the advantage of minimum surface but the metal temperature as the leaving selection is high compared to parallel flow. Hence the counter flow is used in most of the cases except in final section where the metal temperature limitation calls for parallel flow.
The superheaters are placed in the flue gas path to transfer heat by radiation and convection in some proportion such that the outlet steam temperature can be maintained fairly constant at all loads. Fig. 2.8 shows how steam temperature changes with load for radiant SH, convection SH and combination of both. It is evident from the fig. That combination of radiant and convection type helps to keep the superheated steam temperature nearly constant during variation of load. Any number of stages of superheater can be designed but the present trend is to limit to 3 stages so that the cost on headers, connecting pipings desuperheater can be kept minimum.
In a typical arrangement shown in Fig. 2.9 the saturated steam from drum enter the low temperature superheaters (LTSH) via ceiling superheaters. After LTSH the steam enters platen superheater through inter stage desuperheater and finally through a convection superheater.
2.9
REHEATERS
Reheaters (RH) are provided to raise the temperature of the steam from which part of energy has already been extracted by HP turbine.
The arrangement and construction of a reheater is similar to that of a superheater and in modern boilers the reheat sections are mixed with superheat section (see fig. 2.9)
Like super heater in reheater also, any number of stages can be designed. Normally most of the reheater surfaces are placed in hotter zone so that the surface requirement is kept minimum to reduce the pressure drop in steam to keep the cycle efficiency maximum.
Though superheaters are designed in such a way that heat absorbed by radiant and convection superheaters always try to maintain the steam temperature constant during variation of load, in practice the necessary control is achieved by using a de-superheater.
All modern boiler have contact type de-superheaters (Fig.2.10) by which feed water are sprayed directly into the steam for required cooling. Amount of feed water to be sprayed is controlled by automatic control system which is designed to maintain a set final steam temperature. Provision of manual control is also there for emergency or other wise.
2.11 DRAFT SYSTEM
2.11.1
The combustion process in a furnace can take place only when it receives a steady flow of air and has the combustion gases continuously removed.
The steam generator draft system includes air and flue gas flow. 2.11.2
When only a chimney (stack) is used to create the draft necessary for steady flow of air and flue gas, the system is called `natural draft'.
2.11.3
For large boilers the draft created by stack is too little to ensure steady flow. For example, a 30m stack with gas at 250 oC will develop a draft of approximately 10.12 mm of water column (WC) where as gas flow resistance of 100-120 mm of WC may be encountered. Thus it becomes essential to provide fans for developing the draft and then the draft system is called 'mechanical draft'.
2.11.4
All modern large utility boilers are fired under 'balanced draft' condition i.e. where draft is zero. This condition is created by the combination of 'forced draft' and 'induced draft'.
Forced draft represents flow of air or products of combustion at a pressure above atmosphere. The air for combustion is carried under forced draft conditions and the fan used for this purpose is called Forced Draft(FD) fan.
Induced draft represents the system where air or products of combustion are caused to flow to or through a unit by maintaining them at a progressively increasing sub-atmospheric pressure. (This, when, attained with a chimney is called natural draft). This is achieved with the help of stack and fans. These fans are called Induced Draft(ID) fans.
2.11.5
Fig.2.11 shows the principle parts of balanced draft system. The draft steadily drops from FD fan outlet to ID fan inlet.
Though theoretically balanced draft means keeping furnace pressure equal to atmospheric pressure. In practice the furnace is kept slightly below atmospheric pressure(-2.. to -5mm of W.C.). It ensures that there is no egress of air or hot gas and ash into boiler house.
2.11.6 Draft System Flow Resistance
The flow pressure pattern in the draft system may be presented as : DF+DS = DA + DG = DV
Where,
DF = Total fan - effective pressure
DS = Net stack effect (Chimney is vertical passage) DA= Draft pressure loss on air side
DG= Draft pressure loss on gas side DV= Gas exit velocity pressure
In this equation DA is the sum of friction losses in air ducts, bends, air heater, secondary air pressure at fuel burner etc.
And, DG is the sum of friction losses in gas ducts, bends, economizer, air heater, super heaters and reheaters, chimney etc.
2.12 ECONOMISERS
2.12.1
The economizer absorbs heat from the flue gas and adds it mainly as sensible heat to the feed water. The temperature of feed water is kept just below the saturation in case of non-steaming economizers. The work in the evaporative section of the plant is reduced by its heat contribution as well as lowering the temperature of the flue gases prior to their entry into the air heaters.
Some of the boilers are equipped with steaming economizers. The evaporation in these economizers is limited to 20% of feed water at full load and of course less as the load decreases, because of the practical difficulties in treating a high percentage of water to a condition suitable for steaming economizers, they cannot be used good advantage in boiler units where high feed make-up is required.
2.12.2 Location, arrangement and design criteria
Earlier the economizers were introduced mainly to recover the heat available in flue gas that leaves the boiler and provision of this additional heating surface increased the efficiency of steam generation, saving in fuel consumption, thus the name 'Economizer' developed. In the modern boilers used for power generation feed water heaters were used to increase the efficiency of the unit and feed water temperature and hence the relative size of economizer is less than earlier units. This is a good proposition for pulverized fuel fired boilers.
Using of economizer or air heater or both is decided by the total economy that will result flexibility in operation, maintenance and selection of firing system and other related equipment. Modern medium and high capacity boilers used both economizer and air heater. In low capacity boilers air heater alone may be selected.
It is usual to locate economizer ahead of air heaters and following primary super-heater or reheater in the gas stream. Hence it will generally be contained in the same casing as the primary super heater or reheater . See Fig.2.12 for location and arrangement of economizer. Counter flow arrangement is normally selected so that heating surface requirement is kept minimum for the same temperature drop in the flue gas. Economizers coils are designed for horizontal placement which facilitate draining of the coils and favours the arrangement in the second pass of boiler. Water flow is from bottom to top so the steam if any formed during the heat transfer can move along with water and prevent the lock up steam which will cause over heating and failure of economizer tube.
The economizer design usually has to fit in with restrictions imposed by other aspects of boiler design. The most important items which have to be borne in mind include the following:
(a) It must reduce the gas temperature to a level which is satisfactory for the air heaters.
(b) Its surface area must be minimized and its overall dimensions must be as compact as possible. It must fit in with the design of the preceding section of the boiler, usually the reheater.
(c) Provision must be made for on-load cleaning equipment to ensure that gas-side draught loss is kept to a minimum.
(d) Water flow must be uniformly distributed between tubes, and resistance to flow must be as low as possible. A low flow through given tube or element could cause local steam formation which could result in tube failure.
(e) Economizer supports must be arranged to cater for expansions and tubes must be adequately supported to prevent sagging.
(f) A water recirculating connection may be provided from the boiler drum to give adequate circulation during periods when feed flow is absent; this prevents economizer tubes from 'boiling out' and over heating, such as during pressure raising. On once-through boilers this connection of course, is not necessary since a feed flow has to be maintained during pressure raising periods.
2.12.3 Economizer types
On modern high pressure plant only two main types of economizers are installed.. These are the plain tube type and the welded fin tube type.
Plain tube economizers are composed of several banks of tubes either in line or in staggered formation. The staggered formation induces more gas side turbulence than the in line and so results in a higher rate transfer. However, it has the disadvantage of giving a higher draught loss. In line arrangement may need about 10 to 15% more surface but effectively cleanable with the help of on load steam soot blowers. Hence selection of in-line or staggered arrangement depends on the nature of fuel (fouling) and transverse distance between tube and compactness of the assembly required.
Welded fin type economizers have the tube heat exchange surface area extended by the addition of welded fins. With earlier design of extended surface economizers, finned cast shrouds were shrunk on to a mild steel tube. This type was costly and was prone to fouling. It was claimed however, to often food resistance to gas side corrosion and erosion. With the high feed water temperatures utilized on modern plant, gas -side corrosion is unlikely to occur and consequently the air-steel welded fin tube designs have been developed; these are cheaper, lighter and more accurately constructed an so less prone to fouling.
2.13 PRESSURE PART MATERIALS
2.13.1
The selection of materials for fossil fuel fired boilers has mainly been confined to the so called 'conventional' steel, e.g. carbon manganese, the low alloys 1% Cr. 1/2% Mo. 2 1/4%Cr. 1% Mo; . 1/2%
Mo. 1/4% V, and the austenitics, ASTM, Tp321, 347 and 316( Table 1). This also shows a much
stronger austenitic material which has recently been developed. This is Esshete 1250 which has been used in recent boilers, mainly for high temperature superheater tubes and reheater tubes, and to some extent for high temperature headers and superheater integral piping. This has also been used for the supercritical boiler 600 oC main steam piping in some boilers.
A stronger material for use in boiler drums is Ducal W30 (Table 2). The advantage of this material is the high proof stress/UTS ratio and this has enabled to reduce the thickness of drum for 500 MW (CEGB) boilers from , typically, 5 3/4" thick to about 4 1/2" due to an increase in acceptable stress
from 9 tons/in2 to 11.7 tones/in2. 2.13.3
The material used in the manufacture of furnace wall tubes for coal fired boilers is ordinary carbon steel but in the 500 MW oil fires unit of CEGB the major proportion of the furnace is constructed from the 1% Cr. .5% Mo alloy. Designs submitted for 660 MW units also include this material for the whole of the furnace. The anticipated maximum heat flux in these boilers in approximately 175,000 BTU/ft2 (451395 Kcal/m2 hr) and therefore the use of this material is necessary from stress temperature considerations only.
Table - 1 Chemical Composition Austenitic Tube Steels
--- % Tp 321 Tp 347 Tp 316 Esshete 1250 --- Carbon .04.90 .04.90 .04.90 .06.15 Silicon .20.80 .20.80 .25.75 .30.75 Manganese .50-2.0 .50-2.0 1.6.20 5.50-7.0
Sulphur .030 Max .030 Max .030 Max .040 Max
Phosphorus .040 Max .040 Max .040 Max .040 Max
Nickel 9.0-13.0 11.0-13.0 12.0-14-0 9.0-11.0 Chromium 17.0-20.0 17.0-19.0 16.0-17.5 0.8-1.20 Molybdenum - - 2.0-2.75 0.8-1.20 Niobium 10 x C min - 75-1.25 1.10 Max Titanium 4 x C min - - - 0.6 Max - - - Boron - - .006 .009 Vanadium - - - .15-40 --- Table 2. Ducol W.30 Chemical composition ---
Grade 'A' Grade 'B'
Min. Max. % Min Max
---
Carbon 0.11 0.17 0.09 0.15
Silicon 0.30 0.30
Chromium 0.40 0.70 0.40 0.70 Molybdenum 0.20 0.28 0.20 0.28 Vanadium 0.04 0.12 0.04 0.12 Sulphur 0.20 0.05 0.05 Phosphorus 0.04 0.05 0.05 Nickel 0.70 0.79 1.00 Copper 0.20 0.20 Nobium 0.10 --- Tensile strength ( 3" to 6" thick) 36.44 ton f/in2
Yield stress ( 3" to 6" thick) 25 ton f/in2
0.2% proof stress ( 3" to 6" thick) at 360 oC 19.5 ton f/in2
Ducol W.30 A has been used for a large number of boiler drums including all the 500 MW units and the Drax 660 MW boilers. The grade B variant has a higher Nickel content for improved impact.
2.14 SUPPORTS
Modern high capacity boilers are top supported units. The hanger rods are designed for the direct tensile stress resulting from the weight of the unit and the bending tensile stress from the pressure part expansion that deflects the hanger rods horizontally from the vertical or cold load position. A common support elevation is normally used and the effect of temperature load distribution among them. In the walls of a top-supported unit, the tubes are carrying the lower sections and the load stress in the tubes must be added to the pressure stress to determine the total stress in these members, all the loads such as that of water, ash etc. shall be taken into account. The wind loads an earthquake effects are also to be considered. Fig. No.2.13 shows a typically top-supported boiler (500 MW)
2.15 SOOT BLOWERS
2.15.1
Because of the nature of the deposits resulting from the combustion of coal, and to a relatively smaller extent from oil, means have to be provided to prevent an accumulation of deposits from choking the boiler gas passes and to maintain the boiler heating surfaces in a suitably clean condition for effective heat transfer whilst on-load.
The most commonly used method of on-load cleaning is soot blowing, although other methods, such as shot cleaning on economizers and tubular air heaters have been used to a more limit extent on order boiler.
Steam has mainly been used as the soot blowing medium, but recently the used low-pressure air as a soot blowing medium has been introduced as this offers a number of advantages.
2.15.2 Type of Soot Blowers
a) Long Retractable Blowers
These soot blowers are normally employed for cleaning the superheater, reheater zones of the boiler.
The complete assembly of the soot blower is enclosed in the supporting case. The main parts enclosed are power packs, travelling carriage assembly, valve head and it controlling linkage. (Fig. 2.14)
The lance is attached to a travelling carriage which runs on tracks inside the blower housing. The lance gets its rotary and traverse movement from the independent gear boxes and drive chains. Control of movement is by stop and reverse limit switches fixed on extreme ends.
Flow of medium through the retractable is controlled by a valve head mounted at the rear of the blower. An adjustable bar on the travelling carriage strikes a "v" shaped lever to cause the flow of blowing medium.
Blowing pressures for each blower can be adjusted by positioning the screw attached to the valve head.
b) Half Retractable Soot Blower
The operation is same as long retractable but the lance will be extended for half boiler width. A support bearing will be provided to support the lance. This type of blower is not suitable for the regions where the gas temperature is more than 55 oC.
c) Wall Deslagger
The wall deslagger (Fig.2.15) consists of a stationary body and a gear-box for travel and another for rotation of the swivel tube. The swivel tube is supported by sleeve type bearings at each end of the body casting. The horizontal guide rods are used to assure proper alignment of the traveling gear-box. Unlike the retractable the wall blower nozzles do not follow the helical path. Since the purpose of the wall blower is to get the water walls on which it is mounted, cleaned, the swivel tube along with the nozzle first moves forward and when the nozzle cleans the water wall, the traverse movement stops and the swivel tube starts rotating.
The valve for controlling the steam flow is identical to that of long retract. This valve is opened when the swivel tube is fully extended the traverse movement is fast (~1.2m/sec.) and the rotation is very slow (n = 0.6 rpm). This gives maximum time for the cleaning effort.
The material of the nozzle and the nozzle head are made of heat resisting stainless steel containing 25% Cr. And 12% Ni.
The operation of the wall blower is controlled by 3 limit switches which from part of the control box, integral with the soot blower.
This type of blower is employed to clean the furnace walls. The travel is only 30 cms. The swivel tube will go into the wall for 4 cms. And the nozzle attached to the swivel tube cleans the surface .
2.15.3 Soot blower Piping System
The soot-blowing can be with steam or compressed air; both are equally efficient. Normally for all the boilers supplied by us we use superheated steam. The steam taping is taken from any of the intermediate superheater header. The enthalpy of superheated steam is selected such that after the steam pressure is reduced to the blowing pressure, the steam will have enough superheat and also will
be below 427 oC. This limitation is to avoid the use of alloy steel piping. About 50 oC superheat is preferred to prevent the water particle being blown through the nozzle which may lead to tube cutting and consequent tube failures. The steam taken from the intermediate header is reduced through a pressure reducing valve to approximately 25 to 30 atmosphere and his steam is directly fed to the soot blowers. A separate line from the pressure reducing station is taken to the air heater so that the air heater soot blowers can be used along with the soot-blowers in other areas normally the soot-blowers are operated one by one. Hence the piping is sized for the maximum flow required for any of the soot blowers. The lay out of the piping is carried out in such a way that the piping is self-drained and finally ending up with the electrically operated drain valve. This drain valve will have a permanent orifice in the disc so that a continuous drain can be maintained. This will keep line in the warmed up condition and will prevent condensate formation.
Modern soot blower installations are remotely operated and sequentially controlled from a separate panel in the unit control room. The operation is usually carried out with conventional switch gear, but can be by 'solid state' (electronic). Provision is made in the sequence control to skip any individual soot blower or any group of soot blower in the boiler where it is found that less cleaning is required or if one of the soot blowers is out for maintenance.
3.AIR HEATERS
3.1
The air heater is now an essential boiler auxiliary, because hot air is necessary for rapid and efficient combustion in the furnace and also for drying coal in the milling plant. This is rather different from its original purpose, which was to recover 'waste' heat from the flue gas to increase boiler efficiency. In any of the present generation of large boilers, two sets of air heaters are provided one for the normal duty of pre-heating air for combustion and the other for providing higher temperature air to the mills for drying out wet coal.
So, there are two main types of air heater in use; the static recuperative plate or tube-type and the rotary regenerative type, with its two variants (the Ljungstrom and the Rothemuhle types).
In the recuperative type, the flue gas is on one side of the tube or plate and the air is on the other side.
In the regenerative type the gas flows through a closely packed matrix or heat transfer elements giving up heat to the air heater elements and so raising the temperature of the matrix. Air is then passed through and recovers the heat. Either the matrix or the hoods may be rotated to achieve this heat transfer as a continuous process.
3.2
RECUPERATIVE AIR HEATERS
3.2.1 Tubular Air heater
This usually consist of large number of steel tubes of 40 to 65 mm dia. either welded or expanded into the tube plates at the end. Either gas or air may be allowed to flow through the tube. Gas through the tube normally requires higher size tube and vertical flow to reduce fouling. Single or more passes on the gas side and multipass cross flow on the air side usually fits in with the overall plant design. The portion of airheater at low temperature zone is designed normally with a shorter tube length so as to facilitate maintenance of surfaces due to corrosion and fouling. In some cases instead of using boiler flue gases, separate external firing is used particularly during starting.
