There are different types modern high boilers used: 1. La Mont Boiler 2. Benson Boiler 3. Loeffler Boiler 4. Schmidt-Hartmann Boiler 5. Velox Boiler.
Type # 1. La Mont Boiler:
The La Mont boiler is a first forced circulation boiler introduced by La Mont in 1925. This boiler is water tube type. The water circulation and schematic location of different components of the boiler are shown in fig. 4-30.
This boiler incorporates water circulation in tubes surrounded by gases. Water is supplied through an economiser to a separating and storage drum which contains a feed regulator that controls the speed of the feed pump.
Most of the sensible heat is supplied to the feed water passing through the economiser. From the drum a centrifugal pump circulates about 8 to 10 times the quantity of water evaporated. This large quantity of water circulated prevents the tubes from being overheated.
The circulating pump passes water to radiant evaporator or water wall. Then steam and water pass to convective evaporator and again to the drum. From the drum, the released steam then passes to the superheater.
This boiler is capable of generating 40 to 50 tonnes of superheated steam per hour at about 600°C and 150 to 1300 kN/cm2 pressure.
This boiler has the advantage of flexibility of design, compactness and small size of drum.
Type # 2. Benson Boiler:
The formation bubbles on the inner surfaces of the heating tubes of La Moat boiler reduces the heat flow and steam generation as it offers high thermal resistance than water film. Mark Benson developed the boiler where pressure are raised to the critical value (2200 N/cm2 ab), the steam and water would have the same density, and therefore danger of bubble formation can be eliminated.
Fig. 4-31 shows the layout sketch of a Benson boiler. The Benson boilers are drumless type of boilers. The feed water is pumped through the economiser, radiant and convective evaporators, and superheater.
The boiler pressure is maintained as critical pressure and hence water evaporates steam directly without actually boiling. In this boiler if distilled water is not used, heavy deposits of sludge occurs. This difficulty can be avoided if the evaporator is cleaned regularly.
In this boiler the reduced value of entropy at the critical pressure, the steam rapidly becomes wet when it is expanded in a turbine, which causing erosion on the blades. In order to avoid erosion and to provide a more moderate working pressure, the steam is throttled to about 1500 to 1600 N/cm2.
The main advantages of the Benson boiler are:
(1) Total weight of boiler is reduced due to absence of drum
(2) Low cost transport
(3) The boiler can be constructed easily and quickly
(4) Economic operation
(5) Quick starting and can reach full capacity operation in short time.
Type # 3. Loeffler Boiler:
This boiler uses circulation of steam instead of water. Thus, the difficulty experienced in La Mont boiler about deposition of salt and sediment in boiler tubes is avoided.
This boiler has advantages of forced circulation and indirect heating. In this boiler steam is used as heat carrying and heat absorbing medium. This boiler uses circulation of steam rather than that of water.
Fig. 4-32 shows Loeffler boiler. This boiler has economiser and superheater units in the path of gases from furnace to chimney. An evaporator drum is outside the boiler. The portion of main superheated steam is utilized for external use, whereas the reminder passes on to the evaporator drum.
This giving superheat to water coming from economiser, the steam is generated equal to the steam by passed.
The steam circulating pump draws the saturated steam from the evaporator drum and passes it through the radiant and convective superheaters. The nozzles distributing the superheated steam throughout the water in the drum are of special design to avoid priming and noise.
This boiler can carry higher salt concentrations than any other type and is more compact than indirectly heated natural circulation boilers. Therefore it is used for seatransport power generation. The Loeffler boilers of generating capacity 100 tonnes per hour and pressure of 1500 N/cm2.
Type # 4. Schmidt-Hartmann Boiler:
Fig. 4-33 shows arrangement of boiler components. This boiler is high pressure indirectly heated boiler. There are two pressure circuits used to exchange energy. In the primary circuit, steam is passed through submerged heating coil, located in the evaporator drum.
The high pressure steam of primary circuit possesses sufficient thermal head to produce steam at 500 N/cm2 with a heat transfer rate of 2000 W/m2 hr°C. This main steam is passed through a superheater placed in the uptake and then to the application point.
The condensate of high pressure primary circuit steam is circulated through the water drum where feed water is heated to its saturation temperature. In the primary circuit, natural circulation is used.
For purpose of safety a combined pressure gauge and thermometer are fitted to the primary circuit. There is an arrangement is provided for making distilled water for the primary circuit.
Main advantages of the Schmidt-Hartmann boiler are:
(i) Since if use distilled water in primary circuit, there are less chances of overheating or burning of the highly heated components as there is no danger of salt deposition.
(ii) There is no chance of interruption to the circulation either by rust or other material, due to use of distilled water in the primary circuit.
(iii) The feed water outside the heating coil and hence it is easy to remove salt deposits.
(iv) Due to high thermal and water capacity, wide, fluctuations of load are allowed without undue priming or abnormal increase in the primary pressure.
(v) The absence of water risers in the drum, and the moderate temperature difference across the heating coil, allows evaporation to proceed without priming.
Type # 5. Velox Boiler:
In this boiler, the velocity of a gas exceeds the velocity of sound, the heat is transferred from the gas at a much greater rate than the rate achieved with subsonic flow. Therefore in the Velox boiler it is possible achieve the large amount of heat transfer from the given surface area.
In the Velox boiler, air is compressed to 20-50 N/cm2 by an air compressor run by a gas turbine before supplying to the combustion chamber as shown is fig. 4-34. The object of this compression is to secure a supersonic velocity of the gases passing through the combustion chamber and gas tube. As a result of this high rate of heat release are achieved. The capacity of this boiler is limited to about 100 tonnes/hour. Because large power (about 6,000 kw) is required to run the air compressor at this output.
The fuel and air are injected downwards into a vertical combustion chamber which consists of annulus gas tubes and annulus water tubes. The products of combustion are deflected upwards into the evaporator tubes which consist of an outer annulus through which the water evaporated is circulated at a high velocity.
The heat is transferred from gases to the water at a very high rate. The mixture of water and steam formed is then passes into a separator from which the separated steam passes to the superheater. The water removed from steam in the separator is again passed in the water tubes along with preheated feed water coming from economiser.
The advantages of this boiler are:
(i) It has very high rates of heat transfer
(ii) It is compact steam generating unit
(iii) It is capable of quick starting
(iv) It requires low excess air as compressed air is used
(v) The control is entirely automatic, and a thermal efficiency is high.