Benson Boiler

BENSON BOILER

Principle 

The presence of steam bubbles in contact with the surface of water tubes seriously impairs heat transmission from the flue gases to water. By raising the boiler pressure to the critical pressure of steam (225 kgf/sq.cm.), this difficulty is overcome as suggested by Mark Benson in 1922. At the critical pressure, water and steam have the same density and no bubbles form.

The first modern high pressure drumless boiler developed by Benson was put into operation in 1927 in West Germany power station.

Absence of Drum

Benson boiler is a water tube type, forced circulation, high pressure boiler. The unique characteristic of this boiler is that it does not use any drum at all. The entire process of heating, steam generation and superheating is done in a single continuous tube. Hence, it is also known as Once-Through Boiler. It withstands very high pressure, even higher than the critical pressure of steam.

Description [Fig. 10]

Fig. 10 shows the flow-circuit of Benson boiler. Feed pump is connected to the Economiser. Radiant Evaporator is placed just above the furnace. It is connected to the economiser at one end and to the convection evaporator at the other end.

Convection Evaporator is connected to the radiant evaporator at one end and to the convection superheater at the other end. Convection Superheater is connected to the convection evaporator at one end and to the steam turbine at the other end."

A Blower draws atmospheric air and supplies compressed high pressure air (forced draught) to the Air Preheater.

Working

The feed water from the feed pump is circulated through the Economiser Tubes. Hot flue gases pass over the economiser tubes and the feed water is preheated. Economiser is used to preheat the feed water using the waste hot gases before going to the chimney. Thus, some of the heat in the hot gases, which otherwise gets wasted, is used to preheat the feed water. This results in an increase in the boiler thermal efficiency.

The preheated feed water from the economiser flows into the Radiant Evaporator with radiant parallel tube sections. The radiant evaporator receives heat from the burning fuel in the furnace through radiation process. Thereby, the major part of water is converted into steam in it.

The remaining water is evaporated in the Convection Evaporator, absorbing the heat from the hot gases by convection. Thus, the saturated high pressure steam at a pressure of 230 bar is produced. The saturated steam available from the convection evaporator is passed through the Convection Superheater, where the saturated steam is superheated to 650°C.

Note that the radiant evaporator, the convection evaporator and the convection superheater are all arranged in the path of the flue gases. The superheated steam is then supplied to the steam turbine.

The heat of the exit gases cannot be fully extracted through the economizer. These exit gases preheat the air from the blower in the Air Preheater. The preheated air is supplied to the furnace for combustion.

Capacity: Capacity of Benson boiler is about 150 Tonnes/hr at a pressure of 230 bar and at a temperature of 650°C. (Efficiency may be improved by running the boiler at a pressure slightly lower than the critical pressure).

1. SALIENT FEATURES

1. High Thermal Efficiency

No higher limit for higher steam pressure. Therefore, highest steam pressure can be used to achieve high thermal efficiency.

2. Less Weight and Less Cost

As there are no drums, the total weight of Benson boiler is 20% less than other boilers. This also reduces the cost of the boiler.

3. Load Fluctuations

Sudden fall of demand creates circulation problems due to bubble formation. This never occurs in Benson boiler.

4. Easy Transportation

As no drums are required, the transfer of Benson parts is easy. Majority of the parts may be carried to the site without pre-assembly.

5. Once-through Boiler

Since no drum is used, this is an once-through boiler. The feed water entering at one end is discharged as superheated steam at the other end.