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Different Types of Steam Boilers

In order to have an efficient heat transfer from flue gases to water in the boiler,the heat transfer surface should be made as large as possible. To achieve this two methods could be thought of and based on this the boilers are categorized into two categories.

i. Fire-Tube Boilers
First method could be that the water to be evaporated is contained in a vessel,which is provided with large number of tubes. If the hot gases from the furnace are made to flow through the tubes, then a large amount of heating surface of these tubes will be exposed to water to heat the same. Thus considerable amount of water particles will be able to come directly in contact with heating surface. The boilers designed on this principle are known as fire tube boilers. In the fire-tube boilers, the hot flue gases from the furnace are made to pass through the tubes and water surrounds these tubes. The number of tubes varies as per the capacity and design of the boiler. A boiler with large number of tubes is known as multi-tubular boiler. These tubes are placed in the boiler shell.Water in the shell is heated by the conduction of heat from hot gases through the walls of the tubes. Since the shell holds a large amount of water, the evaporation process is slow and it takes more time to raise steam in such boilers. This type of boilers cannot be used to work under very high pressure and are usually made for smaller capacity.

ii. Water-Tube Boilers

In the second case, a chamber consisting of a furnace at the bottom is provided with a number of tubes .Here the hot gases travel from the furnace to the chimney through the space outside the tubes in the chamber, while water, which is to be heated, is circulated through the tubes. The boilers designed on this principle are known as water tube boilers. Thus, in the water-tube boilers, the water is contained inside the tubes and hot flue gases from outside surround these tubes. Since the amount of water contained inside the tubes is small the heating is efficient and the evaporation is faster. This helps in increasing the capacity and the working pressure of the boiler.However, these boilers require more attention and the cleaning of tubes is difficult.Commercial Boilers Amongst the commercially available fire tube boilers are : Cornish boiler, Lancashire boiler and the Locomotive boilers. Various commercially available water tube boilers are : Babcock & Wilcox boiler, Sterling boilers, etc.

iii. Additional classification of boilers

In addition to the above classification of boilers into fire tube or water tube boilers,further classification of conventional type of boilers may be done in many more different ways according to their construction and service conditions. Boilers are classified according to their use as Stationary, Portable and Locomotive. According to position of furnace inside the shell or outside the shell they are termed as internally fired or externally-fire boilers. Depending upon the circulation of water in the boiler they are classified as Natural Convection Circulation type or Forced Pump Circulation type boilers. A boiler is termed as a High Pressure Boiler if the working pressure is over 80 kg/cm 2 and as a Low Pressure Boiler if the working pressure is less than that.

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