Benson Boiler

The presence of steam bubbles in contact with the surface of tubes seriously impairs heat transmission from the flue gases to water. By rising 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 are formed.

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

Working principle of Benson Boiler:
This boiler has a unique characteristic of absence of steam separating drum. The entire process of heating, steam generation and superheating is done in a single continuous tube.

Economiser
The feed water by means of the feed pump is circulated through the economiser tubes. Hot flue gases pass over the economiser tubes and the feed water is preheated.

Radiant evaporator
The feed water from the economiser flows into the radiant evaporator with radiant parallel tube sections. The radiant evaporator receives heat from the burning fuel through radiation process and majority of water is converted into steam in it.

Convection Evaporator
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 210 kg/sq.cm is produced.

Convection superheater
The saturated steam is now passed through the convection superheater where the saturated steam os superheated to 650’C. The radiant evaporator, the convection evaporater and the convection superheater are all arranged in the path of the flue gases.

Steam outlet
The superheated steam is supplied to the steam turbine through the steam outlet.

Capacity
Capacity of benson boiler is about 150 tonnnes/hr at a pressure of 210 kgf/sq.cm. and at a temperature of 650’C. (Efficiency may be improved by running the boiler at a pressure slightly lower than the critical pressure).

Salient features of Benson Boiler

1. As there are no drums, the total weight of benson bolier is 20% less than other boilers. This also reduces the cost of the boilers.
2. As no drums are required, the transfer of the benson parts is easy. Majority of the parts may be carried to the site without pre-assembly.
3. Since no drum is used, this is an once-through boiler and the feed water entering at one end is discharged as superheated steam at the other end.
4. Circulating pump and downcomers are dispensed with.

Read more...

Loeffler Boiler

This is also a modern high pressure water tube boiler using the forced circulation principle and named after Prof.Loeffler.

Salient features of Loeffler Boiler
The novel feature of the Loeffler Boiler is to evaporate water solely by means of superheated steam. The furnace heat is supplied only to economiser and superheater. In other words, steam is used as a heat absorbing medium.

The major difficulty experienced in La-Mont boiler is deposition of salt and sediment on the inner surfaces of water tubes. The deposition reduces the heat transfer, ultimately, the generating capacity. This difficulty was solved in Loeffler boiler by preventing the flow of water into the boiler tubes. Feed water is evaporated in the drum using part of the superheated steam coming out from the water-heater. Thus only the dry saturated steam passes through the tubes. Poor feed water can, therefore, be used without any difficulty in the boiler, which is great advantage of this boiler.

Working principle of Loeffler Boiler
The image shows the outline diagram of Loeffler Boiler.

Economiser
The feed water from the feed tank is supplied to the economiser by feed pump. In the economiser the feed water is made to flow through a number of tubes surrounding which the hot gases leaving the furnace pass over. There is a heat exchange from the hot gases to the feed water, which is preheated in the economiser.

Evaporated Drum
It is housed away from the furnace. It contains a mixture of steam and water. The feed water from the economiser tubes enters the evaporator drum into which is also passed two-thirds of the superheated steam generated by the boiler. The superheated steam gives its superheat to the water in the drum and evaporates it to saturated steam.

Mixing Nozzles
The nozzles distribute and mix the superheated steam throughout the water in the evaporator drum.

Steam circulating pump
A steam circulating pump forces this saturated steam from the evaporator drum to the radiant superheater through the tube of the furnace wall.

Radiant superheater
The radiant superheater is placed in the furnace. The hot gases in the furnace are used for superheating the saturated steam from the drum. The radiant superheater receives heat from the burning fuel through radiation process.

Convection superheater
Steam from the radiant superheater enters the convection superheater where it is finally heated to the desired temperature of 500’C. The convection superheater receives heat from the flue gases entirely by convective heat transfer. Both radiant and convection superheater are arranged in series in the path of the flue gases.

Steam outlet
About one-third of the superheated steam from the convection superheater passes to the steam turbine while the remaining two-thirds is passed on to evaporator drum to evaporated the feed water to saturated steam.

Capacity
Capacity of the Loeffler boiler is about 100 Tonnes/Hr of superheated steam generated at a pressure of 140 kgf/sq.cm and at a temperature of 500’C.

Read more...