Modern High Pressure Boilers

MODERN HIGH PRESSURE BOILERS

High Pressure Boilers

For pressures less than 140 bar, the natural circulation boilers are suitable. For pressures higher than 140 bar and in the supercritical range, i.e., around 221 bar and temperature 375°C, forced circulation boilers are used. Forced circulation boilers are also known as High Pressure Boilers.

Water tube boilers are generally preferred for high pressure and high output. But, fire tube boilers are used for low pressure and low output.

1. ADVANTAGES OF HIGH PRESSURE BOILERS

1. Method of Water Circulation

Water circulation through the boiler may be either natural circulation due to the density difference of water and steam 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.

Forced circulation increases the rate of heat transfer and hence increases the steam generating capacity of boilers.

2. Size of Drum

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

3. 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 frictional resistance to the flow of water, the high pressure boilers have a parallel set of arrangement of tubes.

4. Compactness

High head required for natural circulation is eliminated by using forced circulation. The space required is less and the arrangement is compact.

5. Foundation Cost

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

6. Efficiency

Overall 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.

7. Cost of Electricity

The cost of electricity production is less.

8. Overheating

All the parts are uniformly heated. Therefore, the danger of overheating is reduced. Also, thermal stress problem is avoided.

9. Scale Formation

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

10. Forced Draught using Blower

The flow of flue gases through the boiler furnace, economizer, pre-heater and chimney require a difference of pressure equal to that necessary to accelerate the hot gases. The purpose of draught is to supply required quantity of air for combustion. Also, it removes the burnt gases from the system.

Draught can be obtained by the use of chimney or blower. The former is called Natural Draught and the later is Forced Draught. In the forced draught, a blower is located near the base of the chimney and accelerates the flow of hot gases through the economizer, pre-heater and chimney, thus improving the efficiency of the system.

2. TYPES OF HIGH PRESSURE BOILERS

(i) La-Mont Boiler and (ii) Benson Boiler