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Includes bibliographical references pages and index "Want to specify, operate, or troubleshoot a boiler system fast? Whether you're an operator, inspector, maintenance engineer, or technician, this guide is your direct route to the answers you need in day-to-day boiler and pressure vessel operations. Pulverized coal firing can also be incor- porated. The water then circulates down through downtakes to the rear header and finally to the tubes to complete the cycle.
To ensure better circulation. Briefly describe a bent tube boiler. The drums are protected from the radiant heat of fire. The mode of steam-water circufation is as shown in Fig. The boilers lend themsc! They can be fired with oil, gas, coal, bagasse or wood. Bent tube boilers ave high steam generation rate and are quick to respond to fluctuating loads.
What are the aavantages of bent tube boilers over straight tube boilers? Deliver more drier steam 3. More responsive to load fluctuations: 4, Lend greater economy in fabrication and operation 5. Afford greater accessibility for inspection, cleaning and ma:ntenance 6. Greater design flexibility as the tubes enter the «drum radially 7. The bent tubes aliow free expansion and con- traction 8, Boiler capacity can be increased without in- creasing the drum diameters, Steam outlet Blowdown Fig.
Ans, They have very good evaporating flexibility because of the relatively small water volume for the. When are vertical tubes preferred 10 inclined tubes in designing bent tube boilers? This is given primary desig consideration when bent tube boilers are fired with coal having Jow-melting ash or particularly abrasive ash.
What isa waste heat boiler? Q How may waste heat boilers be classified? They can be classified as: 1. Watertube boiler 2. Gastube boiler 3. Bent tube boiler 4, Positive circulation boiler 5. Supercharged boiler 6. Three-drum-low-head boiler 7.
Waterwall bidrum type boiler, Q. Where are these generally employed? Gastube For relatively clean flue gases : 3. Bent tube Handles heavily dust laden gases 4, Positive circulation For clean, low temperature gases, Gas turbine exhaust Suitable for light dust loadings For gases with suspended sticky particles, S. Supercharged 6, Three-drum-low-head boil 7, Waterwall bidrum type Q.
What should be the design considerations in the selection of a waste heat boiler? Ans, 1, Heat load and temperature of the gases avail- able for waste heat recovery for steam genera- tion 2. Chemical nature and corrosiveness of the gases 3. Available draught draft Boilers 13, 4, Dust load and its nature in the gases 5. Whether the gases are under pressure or suc- tion Available space 7.
Requirement for a start-up furnace, gas preheating emergency use or added capacity, etc, 8. Location for the outlet in the case of flue gases, Incase of lower gas temperature what madifica- tions should be made in designing a waste neat boiler? Insome cases a large quanium of dust particles is found laden in the waste gases available for steam generation.
Whar problems are associated with dust load? Ans, Dust laden waste gases, if they contain abrasive dust particles, may cause severe damage 10 boiler tubes by erosion. Therefore, waste gases loaded with such particles require low gas velocities and that decreases the heat rasfer rate in the WHB.
If the gases contain sticky or tacky particles they will deposit on heat transfer tubes, impair the heat exchange process and reduce the efficiency of the WH, Q.
By a installing dust collectors 8. What is a gasube WHB? Such boilers are shell- and -tube type boilers with hot gases flowing in the tube-side and BFW in.
They have a high weight-to-heat output ratio. They are usually found in applications for gas pressure atm. The boiler tubes are of smaller diameter and more closely spaced than direct-fired waste heat boilers. Soot blowers with nozzles directed towards the tube ends are used to clean the deposits, Gastube boilers are used in the case of gases with light dust loadings, for obvious reasons, Q.
How many types of gastube WHBs are there? Ans, Two types : Horizontal and Vertical. Ans, It is preferred when it is a prime necessity to save floor space. Avertical gastube WHB with shell diameter 2. What is the cause of such a limita- tion? This is due to limitations inherent in tanspor- tation and drum attachments.
They can work successfully at higher pres- sures 2. Since water is circulated in the tube-side which can be readily cleaned, watertube WHB3 are not so susceptible to Gamage from poor feedwater quality 3.
The slagging and erosion problems can be minimized by varying the tube size and spac- ing 6. Dust particles may be recovered 7. Lends itself toa more economic arrangement, Q. What are the primary considerations in design- ing a watertube WHB? What are the reasons that led 10 the application of bent tube boilers for waste heat recovery?
These boilers can tolerate beavy dus! The vertical tubes collect less dust 2. Greater flexibility in tube size, spacing and arrangement 3. Tube damage problem is minimized by more positive circulation 4, Allows pendant superheater installation 5.
Less space is required for tube removal. Whar is te chiefadvamage of aposiuive circuta- tion waste heat boiler? QName one such boiler. What are the applications of waste heat boilers? Waste heal boiler finds its application in: 1, Steel ealits — use all types of smali boilers: horizontal ang vertical gasmbe WHE QO borizomal straight tubulas WHE do Bent waterube type boilers, Waste heat boilers are fitted to openstearth, forge and continuous heating furnaces.
Bollers 15 5. Paper making-uses « WHB 1 a generate process steam by burning the waste liquor b recover the salt cake 40 efiminate syeam pollution. The waste liquor is debydrated to produce char which is bumt in 2 large heap in a reducing atmos- phere in the recovery funace.
The furnace tempesa- ture is as high as " K. Gas velocities are kept tow to avoid fouling of the heat absorbing sur- faces which comprise waterwall tubes taid af the reftactory lined boiler furnace, The Aue gases after passing theough the economizer go dirough ibe evaporator to concentrate the black liquor. How are marine botlers classified? Ans, 1. Sectionat header boiler—either horizontat aight tube type or the sectional express type 2, Drum type—these.
Positive circulation boiler 4, Thermonuclcar steam generator Q. How are marine botlers typified? Ans, Depending upon the specific usuage of steam they produce, the marine voilers are divided into: 1. Main boilers — used for ship propulsion 2, Auxiliary boilers —used for running the ausiiary systems abroad the ship. QL Whar are these aitxiliary systems? Space heating 5.
Cooking 6. The boiler must be compact it size and mini- MUM ity weigh! Fuel and its efficient utifization. Pulverized coal is seldom used because of space limitation. Itis, fitted with a separately fired superheater and extended surface economizer. Close control over superheater temperature ensures greater efficiency Generation of steam at almost any desired temperature and pressure condition can be effected at any load condition by varying the rate of combustion in the twin furnaces Desuperheater is not required Requires less maintenauce Ensures good fuel economy at bigh efficiency under norma conditions Compact.
Less floor space is required. Q What are its main drawbacks? Since the saturated auxiliary steam is taken directly from the steam dmum, under condi- tions of low evaporation rate the superheater may receive insufficient steam, causing su- perhater starvation. Ans, J pees More compact and light for a given steam output More reliable in operation Higher steam generation efficiency More economical at normal load Operation is more simplified.
Ans, 1, Requires greater care during cold start-up. Efficiency goes down at higher steam genera- tion rate. Naval boilers have been found to insialiall tubes straight, What are its advaniages? Straight tubes: a lend themselves to greater ease of inspection, cleaning and replacement if necessary b require a small number of spares. What are the disadvantages associated with all- tubes- straight installation? It weakens the drum structure 2.
It delimits the number of tubes that can be installed for a specified drum diameter and length. Modern high pressure navai steam boiters are designed with all tubes bent to.
Maximum number of tubes can be provided for a given drum length and diameter 2. Dritkng hates for tube fitting on the drum is easy 3. Easqof filling tubes to the drum, 4, Permits operation at higher boiler pressure. The bent iube design has cerrain disadvantages. Tube cleaning becomes more difficult and time consuming 2. Tube replacement is troublesome 3. Because of variation in length, size and cur- vature, a large number of spare tubes must always be kept ready.
Bolles 17 What are the usual methods of installing tubes in marine boilers? Straight ube design, known as Yarrow design. All tubes are straight 2. Foster design, in which all tubes are bent to an. Bent tube design, in which all tubes are bent at right angles to the drum surface at their entry to the drum.
Whar types of superheaters are generally used in marine boilers? Convective type superheate Q. Why not radiant or radiant-convective super- heaters? Marine boiless have a low steam generation rate. Ata low steaming rate, the radiant and radiant-convee- tive types of superheaters will not operate satisfactorily. How does this problem arise, if almost all marine boilers are oil fired?
Ans, As the fuef oil tanks, during the operation of the marine boiler, get gradually depleted, seawater is pumped into the tanks for batlast.
As a result of this, the seawater salinity is imparted to the fuel oil whose. Q What constitutes the furnace wall of a marine boiter? Itis constructed of a refractory wall cooled by waterwall tubes facing the furnace core. The refrac- tory wall is backed up with insulating material and then a metaf casing.
Yes, Both forred and induced draught draft find their way into all marine boilers. Either F. Ans, Forced draught draft. Why is LD. Fans occupy more vertical space, which is ata premium in marine installations. In which cases are both forced and induced draughts drafis used in marine boilers? In cases of coal-fired marine boilers, Q.
Why is the use of a desuperheater in marine boiters not favoured? Is any reheater installed in marine boiler units? Q Why? Itadds to the weight and size of the boiler unit 2. Itimparts operational complexities. What are the primary advantage of steam reheating?
When the reheat cycle is incorporated, the overall plant efficiency increases. The superheated steam may become wet after being expanded through turbine stages which tells upon the body of the turbine blades by eroding its surface and thereby reducing its power output. To avoid this problem, steam, after its Partial expansion in the turbine, is reheated to super- heated steam and is again delivered to the turbine.
When is it more economic 10 adopt a reheat cycle? Why is H. Ans, HP steam possesses higher enthalpy content than MP or saturated steam and hence a higher quan- tum of heat drop, during expansion over turbine blades, is available from HP steam before it becomes wel. What do you mean by wility boilers? Ans, There are three main factors—efficiency, reliability and cost.
These smaller sized units require less floor space. They can be fitted comfortably to the available space of existing utility central sta- tions, 2. They require less erection time. Economies of scale do not favour large units. Q Why are supercritical once-through units preferred to subcritical drum-type units for large units — MW and above? For very large sizes, the supercritical once- through units are more efficient i.
Hence supercritical boilers are favoured in those countries viz. Monobloc units with drum-type boiler for large capacity MW power plants need a high-capacity, high-pressure boiler drum. These giant boiler drums boiler drum of a MW boiler unit may weigh as much as on are considerably larger in dimension and tlave much thicker wall cross-sections, The operating experience of these high-capacity, high-pressure drum-type boilers reveals that at vary- ing operating modes load conditions in which the unit is shutdown and started up too frequently, non- uniform temperature fields appear in the walf cross- section, producing high thermal stresses that bring about cracks of corrosion fatigue in the boiler drum.
However, in once-through boilers there are 20 such heavy metal elements as the drum. They are much lighter in weight and can be shutdown and started up more rapilly, For these reasons, once- through boilers are recommended for units with the duty of supplying semi-peak and peak loads.
Ans, No, Recirculation of combustion products drawn at relatively fow temperature, te. Therefore, the average heating intensity of waterwalls in the flame core zone is reduced, whereupon the possibility of high- temperature corrosion is minimized, Q. Of tate, steam boilers with gas-tight enclosures are guining ground. Wextends the reliability of the boiler units 3. Heat losses to waste gases are reduced as there is noairin-leakage in the furnace and the ducts 4, Low auxiliary power consumption in the supply of air and discharge of combustion products 5.
Combustion uader optima conditions with least excess air ratio is possible. This eliminates low-temperature corrosion and fouling of heat-transfer surfaces 6.
Fasier removal of slag and soot from the fur- nace by water without the tisk of damaging the lining. Q Why is she load-carrying boiler structure predominanily combined with the building structure in modern high-capacity boiter design?
Ans, To achieve aconsiderable saving in mel. For example, such a desigh of an MW monobloc unit results in the saving of ton of metal. Briefly describe the boiter of a superthermal power station.
Its a gas and fuel oil fired supercritical pressure where the burvers are laid out in two fronts and two tiers sce Fig. Fuelflexibility—A variety of solid fuels right from low cost solid fuels to conventional fos- sil fuels can be bumed.
Fuels burned include many types of coals, biomass, petroleum cokes and other wastes, viz. This means flexible, reactive capabilities of fucl market fluctuation. High turndown ratio. Combined cycle—Pressurized Fluidized Bed PFB combustion technology affords highly efficient gas-turbine-based combine cycle 8, Minimum bailding volume 9.
Advanced control system—Allows accurate control of temperature of combustion products in the fumace, minimizing NO, xe Me 12, Hoduction, particulate formation and com- plete combustion of fuels. Their flexibility permits simultaneous combustion of solids, sludges, slurries and gases in the same unit. Coftring—With refuse derived fuel RDF as well as conventional pulverized coal, fuel oils and gas is possible.
This is an added bonanza for fluidized combustion systems using low- Cost fuels to meet higher load demands. Low operating cost Short construction time Low cost of electricity Better ash quality—Ash produced is non- toxic Q. What are the disadvaniages of fluidized-bed boilers as compared to pulverized coal fired boilers?
When buming low grade fuels, particularly those containing salts, care must be taken to maintain accurate bed temperature over AFT. Inbed tube erosion is higher. For utility units above MW, any economy of scale disappears for bubbling-bed boilers owing to the fact that bed plan area require- ments exceed the practical limits.
CFB is handicapped by the same problem. It is height-limited as unit output increases ddue to the large amount of solids that must be circulated through the furnace. Ans, This is mainly due to the fact that whereas a wet scrubber can operate ata calcium-to-sulphurratio Bed drain Bollers 23 close to 1.
How can fluidized-bed boilers be classified? Atmospheric Bubbling Bed boilers Fig. AA distinction can also be made on the basis of fluidiz- ing velocity of air, which is the fundamental distinguish- ing feature of fluidized-bed-combustion units.
What are the principal attributes of amas. The operating temperature range is such that the reactions of SO, with a suitable sorbent, commonly limestone, are thermally balanced 6. The fluidization mechanism feads to less volatilization of alkali compounits 7. Added turbulence minimizes the chances of hot spot on the boiler and shell surfaces 8. Less sensitivity to quantity and nature of ash 9, Smaller furnace volume.
What ts the layout of evaporator tutes in AFB units? Ans, The evaporator tubes serve as a suitable beat sink in the combustion area to control temperature They are laid out ia the traditional pattern of water- wall enclosures. There may be different designs as, for instance, where the boiter tubes are located within the bed itself Q Does this mean that only warertube design is possible?
No; both watertube and Gnetube designs are possible. Firetube boilers are more popular for larger sizes, How is steaming rate controlied in AFB units? Boilers 25 Why are the bubbling fluidized bed boilers so catied? Ans, The combustors of sach boilers are Provided with a grid upon which is supported a dense bed of fuel and limestone.
The bed is main- lained in a fluid-like state by air blown up through the grid aud the bed from the botiom of gridholes. As the air flows upwards it mixes the limestone and fuel and at the same time keeps the ed in suspension. The action of air cseates ex- Ueme turbulence in the bed material and produces the effect of a bubbling Muid. Can's ir be made amenable 10 timited oad fiuc- tation? Yes: limited toad Puctuation can be made possihte.
Q How? By lessening -the amount of air passing through the bed, a why? Ans, This lowers the bed depth and retards eomnbus- tion, Q. What is bed-slumping? Why must the velocity of air through the bed be kept always above the minimum fluidizing vetocity? To avoid incomplete combustion which results in smoking. If the velocity of air through the bed is below the minimum fluidizing vetocity.
How many types of bubbling-bed boilers are there? Two: single-stage bubbling-bed units b two-stage bubbling-bed units Q. Whar is the concept behind two-stage bubbting- bed units? What is the world's largest bubbling -bed boiter? Till date it is the largest. AFB bubbling-bed unit. What are its importan: features? Itsupponts a single-level bed divided into 12 compartments 2, Ithas under-bed feed system 3.
Its primary superhwater and evaporator are fitted in the bed 4. Ils finishing supetheater and reheater are fitted in a split convection pass 5.
Main-steam temperature control by spray water 6. Rehcat-steam temperature control by gas: Now dampers Q. Whar is its fuet? What are its important features? Ans 1, Devise-phase fue! Single combustor system fited with waters covidd walls, grate and plenum 3. What are the advantages of CFB anits over bubbling-bed systems? Greater amount of absorption of SO, and other acid gases because of longer residence time 3. What is the common disadvantage of 9 CFB unit? Ans, Higher electricity bill because of greater fan power is required to maintain the higher velocity through bed.
It couples two cycles—conventional steam turbine cycle with the gas-turbine cycle Q. How does PFBC work? Ans, Fluidized bed combustion takes place at elevated pressures. The resulting flue gas, also at high pressure, is cleaned of suspended solids and directed to drive the gas turbine.
Economizer Ans, 1. Bed height 2 Bed temperature 3. Solids inventory 4. Superficial gas velocity Q. Whar are the basic characteristics of circulating Sluidized-bed boilers? They generally do not have in-bed tubes 2, They use hot-solids separation device cyclones to minimize erosion of heat ab- sorption surfaces 3. Ans, Due ta crack or formation of patch hokes. What will happen in the case of baffle faiture?
Ans, Combustion gases will be short circuited when battles open out. A modified beat redistribution will ake place. Some parts will get more overheated than the others. Tubes may fail, Excessive flue gas temperature atthe stadk outlet will cecur and this wil bring down the overall thermat efficiency of the boiler, Q. Why isa slag sereen installed? At the entry to the convective shaft of the boiler furnace. It is required that hot fue gases and matten slag.
To avoid clog. What are the drawbacks ofa stag screen? Aims, it 1. Vimits the maximum distance for effective tube cleaning by soot blowing. This imposes Tesinictions on tube hank design. Ans, Two: 1, in-line arrangement 2. Which one is bester? Ans, In-line arrangement, a Why?
Incline arrangement has wider tube spacings, Itreduces slagging, The wider tnbe spacing facilitates, manual cleaning, washing and replacement of boiler tubes. Why is the steam drum provided? Whur are drum internais? Whai sundamentat steps are invoived during the process of steam separation and purification in the Steam drum? Ans, The process involves three steps: 1. Whar is steam separasion? This can be effected by any of the following three methods: 1, gravity separation 2. The openings are Aniied holes 20 mrp dia or slots 7.
Whar are bafftes? Baffles arc steam drum intemals that break up the steam-water jets entering the drum. How many types of baffles are in use? Deflector-plate battle Fig. Offset defector-plate baffle Fig. V-baifle Fig. Pesforateu plate and V-baffe Fig. Angle-iron deflector haffle Fig. Hydraulic barrage baifle Fig, 1. Compartment baffle Fig. Why are deflectors ar compartment baffles necessary? Ans, For low steam release rates a V-baffle or single venical baffle is adequate, but for higher release rates deflectors or compartment haffles are necessary for separation.
When is a hydraulic barrage insralied? These types of baffles are installed te contro! Waar is a cyclone separator? This is an effective device to hamess steam- water separation by utilizing centrifugal for- ces fFig. How does it do so? It imparts to the steam-water mixaure, as it enters the drum, a rotary motion which develops a centrifugal force that destroys the foam bubble, climinates spray and separates out solid particles.
What is steam washing? What is its purpose? QL How much of the silica carryover by steam can be eliminased by the process of steam washing? What are the ways to reduce the silica content of steam leaving the washer?
Ans, 1, Increasing the stage efficiency of the washer 2. Adding more washer stages in series 3. Limitations of space. Even distribution of steam. Strength Method of installation, Q. Why is feedwater treatment added to the boiter rather than to the feedwater itself? This is to eliminate the problem of plugging of washer by sludge resulting from chemical teac- tions.
Qin some marine boiiers, water baffles—called swash plates—are installed in the lower haif of athwartship across-ship steam drums. ANS, These plates reduce the surging of boiler water from end to end in the drum as the ship rollsin choppy seas. FE ergy eepe Q. Why is a duat circulation boiler installed? How does it work? Since the primary section contributes to the lion's stare of overail steam output, a low con- centration of silica in the boiler wates of this section reduces the carryover of silica by steam.
Moteover, the generated stearn ia the secondary section being scrubbed by feedwater, a part of it gets condensed and the silica contamination in steam leaving this section is greatly reduced. Whar are she distinct advantages of a dual cir culation steam generation unit? Marked reduction in carryover of silica as well as total dissolved solids in steam 2 Permits higher impurity level in we intake foed 3.
Low blowdown as high concentrations are maintained in the secondary section, This also minimizes heat loss and feedwater make-up 4, Lower concentration of silica and TDS in BFW in the high duty fumace section greatly reduces the danger of scaling 5. Permits higher heat absorption rate over prolonged period 6. Outage time and maintenance cost are mini- mal 7. What are the upper pressure limitasions of boilers? Ans, Steel Firetube Boiler atm. Stee] Watertube Boiler Na effective limitations.
Boilers for operation at atm have been built Casv-lron Boiler atm steam. What are the effects of expansion and contrac- von? When heated the boilers expand, When cooled they contract. The heating and cooling is not uniform and may give rise to Kot spots within the boiler. Hence, 3 boiler designer should give necessary consideration to expansion. Whai about the expansion problem in steel boiter?
Properly designed steef boilers present litte expansion difficulty and is relatively free from stres- sesdne 19 expaasion and contraction. The connecting pipelines should be adequately provided for expan sion and moverneat.
Whar is the effect of expansion ar contraction stresses on tubes? Leakage develops where the tubes are bent into the drum or tubesheet, Q. What about the expansion and contractionprob- lem ina cast-iron boiler? Ans, Thermal stresses induced due to expansion or contraction wilt wanp sections of the cast-iron boiler.
And deformed sections take a permanent shape. Re- placement may be difficult, Q. How does flame impingement affect the potter? Bollers 41 ANS. Q Does a draft affect boiler operation? Ans, The draft problems that are encountered when a chimney alone is used are eliminated if draft fans are installed.
Practical experiences show that a draft fan is far more economical than a chimney of adequate height sin all cases excep those of small installations. Q Soachimney can be eliminated fdraft fans are instatted. Is it? No, Some kind of chimney or stack is neces- sary despite the installation and operation of draft fans. What is the need for a large water capacity?
Ans, If sudden load-changes are prevalent, boilers having fess of a small water capacity will be more responsive to quicker load fluctuations. Compare warertube and firetube boilers in the light of water capacity? Watertube boilers have a smaller water capacity than firetwbe boilers, Therefore wateriube boilers are more responsive to a rapid load change.
The targer water capacity of firetube boilers im- parts stability but does not permit rapid load fluctua- ton. Q How does water circulation affect a boiler? Firetnbe boilers usually have poor circulation compared to watertube boilers. Q What are she steaming characteristics of a firetube boiler? Firetube boilers in general, are not fast steamers. The steaming rate of a firebox type boiler is much faster than that of externally fired boilers, However, the steaming characteristics ofa firetube boiler vary with differing design and with she ratio of fheating surface to water volume.
Q What are the steaming characteristics ofa pack- aged boiler? Packaged boilers have quicker steaming rate than fitetube boilers because of alarge heating surface with smaller water volume. Whar are she steaming characteristics of water. Fast, Q. Whar are the steaming characteristics of cast- iron botlers?
Stow steaming cate, except those having smaller water volume. This is because alarge quamtily of metal is involved in a cast-iron boiler. Althe start-up, the steaming rate is low but there- after such boilers have riatively stable steaming characteristics, QL What are the effecis of low-water in the case of a steel boiler?
Ans, Replacement of exposed tubes or crown sheets is required. This means a considerable downtime lass as the boiler is shutdown and kept out of service tit the repair jobs are completed Q. Waal are the effecis of low-water operation in the case of cast-iron boilers? Warpuge may sometime invite replacernent of the boiler. How can siudge be removed from the boiler? Periodic blowdown is the method most prac- lised to remove sludge deposited on the mud drum or mud ring.
Tei has become solid cake, a high-pressure water hoke is hamessed to dislodge the cake during shut- down perious, If it sull remains, itis chipped out Due to lack of adequate clearance. How can she boiler ibe scaies be removed?
The boiler is then carefully heated 3. Cold water is sprayed on the tubes to induce themmat stress in the solidified scale aad romove the ruptured scales, Scales inside a boiler tube are turbined reammed out carefully with special equipment. Improper combustion is the cause of soot Tor nuttion in the boiler furnace. Combustion-gas-borne soot gets deposited in the boiler flue passes.
QL What ts the effect of soot deposinon in the flue gas passage of « boiler? Compare the soot blowing characteristics of a steel firetube boiler and w steel warertube boiter? Ans, Annual or biannual soot blowing nay be ade- quate for stee! Large installations fave soot blowers. Ans, Initially there is some soot buildup and with that there occurs initial efficiency reduction.
After that there is little additional soot development be- cause of the large size of passageways. And so 00 further efficiency drop takes place. What about the relative ease of servicing of cast- iran boiler, ste! Forcast-iron boilers, the waterside is inacces- sible whereas the fireside is accessible for inspection and maintenance by simply opening the doors.
The flue ducts are large and lend themselves to easy cleaning. The waterside of watertube steel boilers and pack- aged units is accessible for inspection and main- tenance jobs through manholes, handholes or washout plugs, The mere opening of doors makes the fireside aocessible to inspection and maintenance, Accessibility to tubes for firetube boilers is gained through ttinged, front flue doors. Soot cleaning is ac- complished through the doors set in the rear strioke box, It the fitetube boilers are fitted with smaller steel tubes they present more difficulty in cleaning than cast-iron boilers because the latter provide ample cleaning space.
The waterside of a packaged firctube unit is acees- sible for inspection trough manholes. The rear wall is to be unbolted and removed to get access to the fireside, Hence firemube packaged boilers are not generally considered gasily accessible for main- tenance and cleaning. Why do she present duy boiler designers not recommend maximum flue gas velocities in boilers?
Boller 43 Ans, Ttis due to deteriorating fuel quality. This erosiou problem being propor- tional to an exponential function of flue-gas velocity, the latter is a major consideration in boiler design. Why is chromizing of watervall, superheater and reheater tubes done? This strengthens she tubes in combating flyash erosion problems. Chromium, a powerful corrosion resistant metal, diffuses into the base metal of the tubes of the heat exchangers, viz.. Why are imernally ribbed tubes laid out in the high-heat-absorplion zones of naturai circulation drum-type boilers operating at high supercritical pressures?
Itensures high availability of the boilers. When intemally ribbed tubes are arranged in the high-heat-absorption zones of the combustion chamber, they provide a far greater margin of safety against tube failure from departure from nucteate boiling DNB conditions. They impart a centrifugal action to the working Quid that forces water droplets towards the waterwall tube surface and inhibits the Formation of a steam film,.
Is iturwe then thai the DNB is more possible with smooth tubes than with internally ribbed tubes? Ans, Yes. What do you mean by cycling service? It means boilers must be designed to cycle on and off, For example, a typical requirement may be nightly or weekend shutdowns.
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