Benson Boiler is a high pressure, drum less, supercritical, water tube steam boiler with forced circulation. This boiler was invented in the year 1922 by Mark Benson. This boiler is a super critical boiler in which the feed water is compressed to a supercritical pressure and this prevents the formation of bubbles in the water tube surface. The bubbles do not form because at supercritical pressure the density of water and steam becomes same. It was Mark Benson who first proposed the idea to compress the water at supercritical pressure before heating into boiler and due to this the latent heat of water reduces to zero. As the latent heat of water reduces to zero the water directly changes into steam without the formation of bubbles.
Features Of Benson Boilers :
The main difficulty experienced in the La Mont boiler is the formation and attachment of bubbles on the inner surfaces of the heating tubes. The attached bubbles reduce the heat flow and steam generation as it offers higher thermal resistance compared to water film :
(a) If the boiler pressure was raised to critical pressure (225 atm), the steam and water would have the same density and therefore the danger of bubble formation can be completely avoided.
(b) Natural circulation boilers require expansion joints but these are not required for Benson as the pipes are welded. The erection of Benson boiler is easier and quicker as all the parts are welded at site and workshop job of tube expansion is altogether avoided.
(c) The transport of Benson boiler parts is easy as no drums are required and majority of the parts are carried to the site without pre-assembly.
(d) The Benson boiler can be erected in a comparatively smaller floor area. The space problem does not control the size of Benson boiler used.
(e) The furnace walls of the boiler can be more efficiently protected by using small diameter and close pitched tubes.
(f) The superheater in the Benson boiler is an integral part of forced circulation system, therefore no special starting arrangement for superheater is required.
(g) The Benson boiler can be started very quickly because of welded joints.
(h) The Benson boiler can be operated most economically by varying the temperature and pres- sure at partial loads and overloads. The desired temperature can also be maintained constant at any pressure.
(i) Sudden fall of demand creates circulation problems due to bubble formation in the natural circulation boiler which never occurs in Benson boiler. This feature of insensitiveness to load fluctuations makes it more suitable for grid power station as it has better adaptive capacity to meet sudden load fluctuations.
(j) The blow-down losses of Benson boiler are hardly 4% of natural circulation boilers of same capacity.
(k) Explosion hazards are not at all severe as it consists of only tubes of small diameter and has very little storage capacity compared to drum type boiler. During starting, the water is passed through the economiser, evaporator, superheater and back to the feed line via starting valve. During starting, first circulating pumps are started and then the burners are started to avoid the overheating of evaporator and superheater tubes.
Construction or Main Parts
1. Air Preheater- It preheats the air before entering into the furnace. The preheated air increases the burning efficiency of the fuel.
2. Economiser- It heats the water to a certain temperature.
3. Radiant Superheater-It is super heater which heats the water with radiation produced by the burnt fuel. It raises the temperature to supercritical temperature.
4. Convection Evaporator-It evaporates the superheated water and converts them into steam. It does so by the convection mode of heat transfer to the water from the hot flue gases.
5. Convection Superheater-It superheats the steam to the desired temperature (nearly 650 degree Celsius).
6. Furnace-It is the place where the fuel is burnt.
7. Feed Pump-It is used to supply the water inside the boiler at supercritical pressure of 225 bars.
It works on the principle that the pressure of the water is increased to the supercritical pressure (i.e. above critical pressure of 225 bar). When the pressure of water is increased to the super critical level, the latent heat of water becomes Zero and due to this, it directly changes into steam without boiling. And this prevents the formation of bubbles at tube surface.
In Benson Boiler, the feed pump increases the pressure of the water to the supercritical pressure and then it enters into the economiser. From economiser, the water the water passes to the radiant heater. Here the water receives the heat through radiation and partly gets converted into steam. The temperature raises almost to the supercritical temperature. After that mixture of steam and water enters into convective evaporator where it is completely converted into steam and may superheated to some degree. Finally it is passed through the superheater to obtained the desired superheated steam. This superheated steam is then used by turbines or engine to produce the electricity.
It is light in weight.
Occupy smaller floor area for its erection.
Explosion hazard is almost negligible because of use of smaller diameter tubes.
It can be started very easily within 15 minutes.
It avoids bubble formation due to the super critical pressure of water.
Transportation is easy.
This boiler may achieve thermal efficiency upto 90 %.
This supercritical boiler is used in different industries to generate steam for the production of electricity or mechanical power. The average operating pressure, temperature and capacity of benson boiler is 650 degree Celsius, 250 bar and 135 tonnes/h.