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Essay # 1. Introduction to High Pressure Boilers:
In simple, a boiler may be defined as a closed vessel in which steam is produced from water by combustion of fuel.
According to American Society of Mechanical Engineers (A.S.M.E) a ‘steam generating unit’ is defined as:
“A combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat so made available to the fluid being heated and vaporised”.
The steam generated is employed for the following purposes:
(i) For generating power in steam engines or steam turbines.
(ii) In the textile industries for sizing and bleaching etc., and many other industries like sugar mills; chemical industries.
(iii) For heating the buildings in cold weather and for producing hot water for hot water supply.
In applications where steam is needed at pressure, 30 bar, and individual boilers are required to raise less than about 30000 kg of steam per hour, shell boilers are considerably cheaper than the water-tube boilers. Above these limits, shell boilers (generally factory built) are difficult to transport if not impossible. There are no such limits to water-tube boilers.
These can be site erected from easily transportable parts, and moreover the pressure parts are of smaller diameter and therefore can be thinner. The geometry can be varied to suit a wide range of situations and furnace is not limited to cylindrical form. Therefore, water- tube boilers are generally preferred for high pressure and high output, whereas, shell boilers for low pressure and low output.
The modern high pressure boilers employed for power generation are for steam capacities 30 to 650 tonnes/h and above with a pressure up to 160 bar and maximum steam temperature of about 540°C.
Essay # 2. Unique Features of the High Pressure Boilers:
Following are the unique features of high pressure boilers:
1. Method of Water Circulation:
The circulation of water through the boiler may be natural circulation due to density difference or forced circulation. In all modern high pressure boiler plants, the water circulation is maintained with the help of pump which forces the water through the boiler plant. The use of natural circulation is limited to sub-critical boilers due to its limitations.
2. Type of Tubing:
In most of the high pressure boilers, the water is circulated through the tubes and their external surfaces are exposed to the flue gases. In Water-tube boilers, if the flow takes place through one continuous tube, the large pressure drop takes place due to friction.
This is considerably reduced by arranging the flow to pass through parallel system of tubing. In most of the cases, several sets of the tubing are used. This type of arrangement helps to reduce the pressure loss, and better control over the quality of the steam.
3. Improved Method of Heating:
The following improved methods of heating may be used to increase the heat transfer:
(i) The saving of heat by evaporation of water above critical pressure of the steam.
(ii) The heating of water can be made by mixing the superheated steam. The mixing phenomenon gives highest heat transfer coefficient.
(iii) The overall heat transfer coefficient can be increased by increasing the water velocity inside the tube and increasing the gas velocity above sonic velocity.
Essay # 3. Advantages of High Pressure Boilers:
The following are the advantages of high pressure boilers:
1. In high pressure boilers pumps are used to maintain forced circulation of water through the tubes of the boiler. This ensures positive circulation of water and increases evaporative capacity of the boiler arid less number of steam drums will be required.
2. The heat of combustion is utilised more efficiently by the use of small diameter tubes in large number and in multiple circuits.
3. Pressurised combustion is used which increases rate of firing of fuel thus increasing the rate of heat release.
4. Due to compactness less floor space is required.
5. The tendency of scale formation is eliminated due to high velocity of water through the tubes.
6. All the parts are uniformly heated, therefore, the danger of overheating is reduced and thermal stress problem is simplified.
7. The differential expansion is reduced due to uniform temperature and this reduces the possibility of gas and air leakages.
8. The components can be arranged horizontally as high head required for natural circulation is eliminated using forced circulation. There is a greater flexibility in the components arrangement.
9. The steam can be raised quickly to meet the variable load requirements without the use of complicated control devices.
10. The efficiency of plant is increased upto 40 to 42 percent by using high pressure and high temperature steam.
11. A very rapid start from cold is possible if an external supply of power is available. Hence the boiler can be used for carrying peak loads or standby purposes with hydraulic station.
12. Use of high pressure and high temperature steam is economical.