There are a number of models of boiler having different industrial applications. Boiler can be classified on the following basis:
On the basis of contents inside the tube, boilers can be classified as fire tube boilers and water tube boilers. In fire tube boilers, flue gas passes through the tube and the heat of flue gas is absorbed by tube and transferred to water surrounding the tube, for example, Cochran boiler, locomotive boiler, Lancashire boiler, and Cornish boiler. But, in water tube boilers, water flows inside the tube and tube is kept in the path of flow of flue gas, for example, Babcock and Wilcox Boiler, Benson boiler, Loffler boiler, etc. Most of the high pressure boilers are water tube boiler. In this boiler, tube absorbs the heat of flue gas flowing surrounding the tube and transferred to water passing through the tube. The difference between water tube and fire tube boilers is given in Table 4.1.
On the basis of firing system, boilers can be classified as internal fired boilers and external fired boilers. In internal fired boiler, firing takes place inside the boiler drum, i.e., furnace is located inside the drum, for example, Cochran boiler, Locomotive boiler, Lancashire boiler, etc. whereas in external fired boiler, furnace is outside the boiler drum and water is circulated inside the tube passing through the furnace, for example, Babcock and Wilcox Boiler.
On the basis of position of drum, boilers can be classifies as horizontal boiler, inclined boiler, and vertical boiler. On the basis of pressure, boiler can be classified as low pressure boiler, high pressure boiler, and super critical boiler. Low pressure boilers operate below 80 bar; high pressure boilers operate above 80 bar and super critical boilers operate at 221 bar and above. On the basis of nature of circulation of water, boilers can be classified as natural circulation boilers and forced circulation boilers. In natural circulation boilers, water circulates automatically due to pressure difference created by temperature difference. But, in forced circulated boilers, pumps are used to circulate the water through tubes.
Table 4.1 Differences Between Water Tube Boilers and Fire Tube Boilers
| Water tube boilers | Fir tube boilers |
|---|---|
| Water flows inside the tube.It is more safer than fire tube boilers because a large part of the water is in smaller tubes which if rupture, only a comparatively small volume of water released into flash of steam.It is more efficient and economic.Pressure limit in water tube boilers is much higher than the fire tube boilers.Water tube boilers are most suitable for large sized boiler.In this boiler, steam production rate is very high.Water treatment plant is required due to problem of scaling inside the tube. | Flue gas flows inside the tube.It is more dangerous compared to water tube boiler.It is less efficient and non-economic.Pressure limit is very low. It is approximately 16–20 bar.Fire tube boilers are most suitable for small sized boiler.Steam production rate is low.There is no need of water treatment plant. |
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