A boiler which generates steam at a pressure of 85 kgf/sq.cm or above is termed as a “high pressure boiler”. The present tendency is towards the use of high pressure boilers in power plants. The modern high pressure boilers used for power generation have capacities of 40 to 1600 tonnes/hr of superheated steam with a pressure upto 210 kgf/sq.cm and a temperature of about 650’C. One of the largest modern steam power plants in the world is in japan with a steam capacity of 1600 Tonnes/hr. In India, the trombay power plant has a steam generating capacity of 550 tonnes/hr, Ramagundam Power power plant with 320 tonnes/hr and bokaro plant with 160 tonnes/hr.

Water tube boilers are generally preferred for high pressure and high output whereas fire tube boilers for low pressure and low output.

Advantages of high pressure boilers:

Method of water circulation

Water circulation through the boiler may be either natural circulation due to density difference or by forced circulation. In high pressure boilers, water circulation is made with the help of a centrifugal pump which forces water through the boiler tubes. This is called “forced circulation of water”. The use of natural circulation is limited upto 120 kgf/sq.cm. Steam pressure and forced circulation upto 210 kgf/sq.cm. Forced circulation increases the rate of heat transfer and hence increases the steam generating capacity of boilers.

 

Size of drums

The high pressure boilers are characterized by the use of very small steam separating drums or by the complete absence of any drum.

 

Type and arrangement of tubes

The heat of combustion is utilized more efficiently by the use of small diameter and light weight tubes in large numbers. To avoid large resistance to the flow of water , the high pressure boilers have a parallel set of arrangement of tubes.

 

Compactness

The boiler components can be arranged horizontally, giving greater accessibility and operational convenience as high head required for natural circulation is eliminated by using forced circulation. The space required is hence less and arrangement is compact.

 

Foundation cost

Due to the light weight tubes and small size drums required and the arrangement being compact, the cost of foundation is reduced.

 

Efficiency

The efficiency of the power plant is increased upto 40%, by using high pressure superheated steam. Also steam can be raised quickly after the boiler is fired.

 

Cost of electricity

Since efficiency of the plant is increased by using high pressure boilers, the cost of electricity production is reduced.

 

Overheating

All the parts are uniformly heated; therefore the danger of overheating is reduced. Also thermal stress probelm is avoided.

 

Scale formation

The tendency of scale formation is eliminated due to the high velocity of water through the boiler tubes.

 

Types of high pressure boilers

 

1.    La-Mont Boiler

2.    Loeffler Boiler

3.    Benson Boiler