1) La-Mont boiler
The La-Mont boiler is a first forced circulation boiler introduced by La-Mont in 1925 in which pump is used for forced circulation.
Type of boiler: Water tube, forced circulation, high pressure boiler
Steam generating capacity: 45 to 50 tones/hr
Pressure and temperature: 120 atm. and 500˚ C
Working of La-Mont boiler:
The feed water from the hot well is pumped to boiler drum through economizer. The economizer as shown in Fig. below is located in the boiler where hot combustion gases pass. In economizer, the feed water absorbs the sensible heat from flue gases passes over the economizer. The water from the drum is pumped to radiant and convection evaporator, and again to drum, The feed water in radiant evaporator absorb radiant heat and in convective evaporator absorb convective heat and it evaporates.
The evaporated water (steam) collected in the upper part of separating steam drum. Then it passes through superheater where it becomes superheated. The superheated steam is supplied to the prime mover. The atmospheric cold air is preheated in the air-preheater by the flue gases before they are discharged through the chimney. The heated air from air preheater is supplied to the combustion chamber.
The flue gases sequence:
Combustion chamber -> Convective evaporator -> Superheater -> Economizer -> Air preheater -> Chimney -> Atmosphere
The water/steam flow sequence:
Hot well -> Feed water pump -> Economizer -> Steam drum Steam circulating pump -> Radiant evaporator -> Convective evaporator -> Steam drum -> Superheater -> Steam turbine
Main difficulties(disadvantages) experienced in La-Mont boiler are:
(1) The deposition of salt and sediments on the inner surfaces of the water tubes. It leads to reduce heat transfer and ultimately the steam generating capacity. This further increases the danger of overheating of tubes due to salt deposition.
(2) The formation and attachment of bubbles on the inner surfaces of the heating tubes. It leads to increase the heat flow resistance and steam generation.
2) Benson boiler (drumless boiler)
The Benson boiler is introduced by Benson and he argued that if the boiler pressure is raised above the critical pressure (225 atm.), the steam and water have the same density; therefore bubbles would not be formed. Hence, the difficulty experienced in case of La-mont boiler as danger of bubbles formation can be easily eliminated. This boiler also known once through boiler as in this case water is fed to the boiler at one end and superheated steam discharged through the other end. Hence, entire operation of feed water heating, steam formation (evaporation) and superheating are taken place in a single tube without use of drum. In case of non-once through boilers only fraction of water flowing in the steam generating tubes is evaporated and remains water is recirculated through the drum & evaporating tube.
Type of boiler: High pressure, forced circulation, once through boiler and water tube boiler.
Working of Benson boiler:
The feed water is supplied by feed pump at a pressure higher than 225 atm. This water passes through economizer and then radiant evaporator where most of water is evaporated as shown in Fig. above. The remaining water is evaporated in convective evaporator and converted into steam. The steam is passed through the superheater and finally supplied to the prime mover.
Advantages of Benson boiler:
(1) The generation of steam in evaporating tubes at pressure higher than critical pressure (225 atm). Therefore, do not require any evaporator drum.
(2) There is no drum; total weight of boiler is 20 % less than other boiler. It reduces the cost of boiler.
(3) Natural circulation boiler requires expansion joints but it does not require in Benson boiler. Hence Benson boiler can be started very quickly because of welded joints.
(4) The erection of the Benson boiler is easier and quicker as all the parts are welded at Site.
(5) The superheater of Benson boiler is integral part of the forced circulation and hence no special starting arrangement for superheater is required.
(6) At part and overload the Benson boiler can be operated economically by varying temperature and pressure. The desired temperature of steam can be maintained constant at any pressure.
(7) Benson boiler is very suitable for fluctuating demand because there is no any formation of bubbles when demand (load) is suddenly decreased.
(8) High generation rate of steam. Hence very suitable for power generation plant.
(9) It requires less space.
Disadvantages of Benson Boiler:
(1) Salt deposition and sediment on the inner surface of water tubes.
(2) Danger of overheating.
3) Loeffler boiler
In La-Mont and Benson boiler, the major difficulty experienced is the deposition of salt and sediment on the inner surface of the water tubes. This difficulty is solved in Loeffler boiler by preventing the flow of water into the boiler tubes. The most of the steam is generated Outside the tubes and in evaporating drum using parts of superheated steam coming from the boiler. In this boiler more steam circulation is used rather than that of water in other boiler. Hence, steam is heat carrying and heat absorbing medium. This boiler incorporates a radiant superheater which absorbs heat from combustion chamber (most heated zone of boiler).
Type of boiler: Forced circulation, high pressure and water tube boiler.
Steam generating capacity: 100 tones/hours
Pressure: 140 bar
Working of Loeffler boiler:
The high pressure feed pump draws water through the economizer and deliver it into the evaporating drum as shown in Fig. below. An evaporator drum is outside the boiler. In the evaporator drum, feed water is evaporated by mixing superheated steam coming from superheater. The steam circulating pump draws saturated steam from the evaporator drum. Then steam passes through radiant and convective superheaters where it is heated to required temperature. Steam coming from superheater, about 1/3 of superheated steam is supplied to steam turbine and the remaining 2/3 superheated steam is supplied to evaporator drum to evaporate the feed water.
Advantages of Loeffler boiler:
(1) The evaporator tubes in this boiler caries superheated steam, therefore there is no salt deposition on the surface of tubes.
(2) Boilers are compact in design.
(3) High steam generating capacity.
(4) This boiler is suitable for marine applications.
4) Schmidt-Hartmann boiler
This boiler having multi circuit as primary and secondary circuit which reduces the problem of overheating and salt deposition in the tubes. The operation of boiler is similar to an electric transformer.
In this boiler a feed pump is used to supply water to a drum which discharged saturated steam to the superheater. This main feed water (impure water) is evaporated as pressure about 60 bar in the secondary circuit. The pure distilled water is evaporated at pressure about 100 bar in the primary circuit. In this boiler energy (heat) exchanges between two fluids at different pressures.
In the primary circuit steam is produced from distilled water, this steam passes through a submerged heating coil which is located in an evaporator drum as shown in Fig. below. This high pressure (100 bar) steam in this coil of primary circuit has a sufficient energy (heat) to generate steam from impure water in the evaporator drum at pressure 60 bar. The steam produced in the evaporator drum passes through the superheater and then supplied to the steam turbine.
Due to heat transfer from steam of primary circuit to water of evaporating drum, the steam of primary circuit is condensed and falls down into the low pressure feed preheater on its way give up heat to the impure feed water. Hence the temperature of feed water increases to its saturation temperature. Therefore, only latent heat is supplied in the evaporator drum. The water and steam in the primary circuit flow is taken place due to density difference (natural circulation)
Advantages of Schmidt-Hartmann boiler:
(1) Salt deposited in the evaporator drum due to the circulation of impure water can be easily brushed off just by removing the submerged coil from the drum or by blowing off the water.
(2) No danger of salt deposition as well as there is no chance of interruption to the circulation either by rust or any other material in the primary circuit. Therefore, there is rare chance of overheating or burning the highly heated components of the primary circuit. The highly heated parts run very safe throughout the life of the boiler.
(3) The wide fluctuations of load are easily taken by this boiler without undue priming or abnomal increase in the primary pressure due to high thermal and water capacity of the boiler.
(4) The evaporation of feed water takes place in the evaporator drum without priming due to moderate temperature difference across the heating coil
5) Supercritical boiler
The prime duty of power plant designer is to reduce the cost of electric power generation. The cost of power generation increases with the increase in the fuel cost. To compensate the rising fuel price the designers continuously try to find the ways. The most recent method to produce economical electric power is by the use of supercritical boiler in the thermal power plant. In the super-critical boiler the boiler working under pressure above critical pressure (221.05 bar), hence the heat transfer rates are considerably large compared to subcritical (pressure <221.05 bar) boiler.
A Subcritical boiler (conventional boiler) consists of three heating surfaces as economizer, evaporator and superheater while the supercritical boiler consists only preheater (economizer) and superheater. In the conventional boiler water is heated below critical pressure, hence heat added to water is utilized to increase temperature of water up to their saturation temperature corresponding to pressure of boiler. If more heat added to this water boiling will begin at constant temperature. Bubbles of steam begin to form in the water and rise above the water level in the drum. Further heating of saturated steam, steam becomes superheated. As we know that pressure of water or steam is raised, the enthalpy of evaporation is reduced. At critical pressure (221.05 bar) the enthalpy of evaporation becomes zero. It means that if the water is pressure sed with the help of feed pump upto critical pressure and then heat added to IL, the water will be directly converted to steam at the temperature of 374°C and there is no bubbling action as water changes to steam.
The once through boiler is only type suited to supercritical boiler because there is no latent heat of evaporation involved. When water is heated at constant supercritical pressure Suddenly it is con verted into steam. As shown in Fig. below, the high pressure (above critical point) water enters the tube inlets and leaves at the outlet as the superheated steam. Since, here is no drum, but there should be a transition section where the water is likely to flash in order to accommodate the large increase in volume,
Advantages of supercritical boiler:
(1) Heat transfer rates are considerably large compared to subcritical boilers.
(2) There is no drum, less heat capacity of the generator and hence more stable and gives better response.
(3) There is no two phase mixture and hence the problem of erosion and corrosion are minimized
(4) The turbo-generator connected to super-critical boilers can generate peak loads by changing the pressure of operation.
(5) There is great ease of operation and their comparative simplicity and flexibility made them adaptable to load fluctuations.
(6) Higher thermal efficiency (about 40 to 42%) of power station can be achieved with use of supercritical boiler.
Disadvantages of supercritical boiler:
(1) The high pressure and temperature of supercritical boiler restricts use of boiler due to availability of material and difficulties experienced in the turbine and condenser operation due to large volumes.
(2) The additional problem is created due to the separation of solid impurities as phase changes. These solids remain in the tubes and block the passage for the flow of feed water. Therefore it is necessary to treat water thoroughly before supply to the boiler.
By Referring above boilers You will be able to answer following questions
Q1. Explain with neat construction and working of sketch LaMont boiler.
Q2. Explain with neat sketch construction and working of Benson boiler.
Q3. Draw and explain ‘Schmidt-Hartmann Boiler’, and discuss the merits and demerits.
Q4. Explain a high pressure boiler which can carry higher salt concentration in feed water than any other type with neat sketch.
Q5. Explain the construction, working of one supercritical boiler with diagram.
Q6. Explain construction, working and advantages of Loeffler boiler with neat sketch.
Q7. Explain Drumless boiler with a sketch.(Hint- Benson)
Q8. Part of superheated steam is only used in power generation in case of Loeffler boiler. Explain.
Q9. Explain in brief supercharged boilers are equipped with air compressor and gas turbine
Q10. Draw neat line diagram of a high pressure boiler in which steam is generated by mixing the superheated steam with feed water. Discuss its relative merits and demerits.(Hint- Loeffler)
Q11. What do you mean by supercritical boiler?
References
1. Dr. S. Suyambazhahan, “Power Plant Engineering”, Laxmi Publication Pvt. Ltd.
2. A.K. Raja, “Power Plant Engineering”, New Age International