Classification Steam Boiler Based on Tube Type

A steam boiler is closed vessel in which water, is transformed into steam by application of heat. Classification of steam boiler generally can be seen here and the heating transfer through tubes can be classified as:

§         Fire Tube Steam Boiler
Fig.1 : Fire Tube Steam Boiler
(source: reprinted from ref. 6)
Fire Tube Steam Boilers consist of a series of straight tubes that are housed inside a water-filled outer shell. The tubes are arranged so that hot combustion gases flow through the tubes. As hot gases flow through the tubes, they heat the water that surrounds the tubes. The water is confined by the outer shell of the steam boiler. To avoid the need for a thick outer shell, Fire Tube Steam Boilers are used for lower-pressure applications. Steam pressure is limited to 450 psig.

Advantages of Fire Tube Steam Boiler:
Þ    Easy to clean
Þ    Relatively inexpensive
Þ    Compact in size
Þ    Well suited for industrial applications

Disadvantages of Fire Tube Steam Boiler:
Þ    Not suitable for pressure higher than 450 psig
Þ    Limitation for high-capacity steam generation 

§         Water Tube Steam Boiler
Fig.2 : Water Tube Steam Boiler
(source: reprinted from ref. 6)
Water Tube Steam Boilers are designed to circulate hot combustion gases around the outside of a large number of water-filled tubes. The tubes extend between an upper header, called a steam drum, and one or more lower headers or drums. In older designs, the tubes are either straight or bent into simple shapes. Newer boilers have tubes with complex and diverse bends. Because the pressure is confined inside the tubes, Water Tube Steam Boilers can be fabricated in larger sizes and used for higher-pressure applications.

Water Tubes Boilers have the following advantages:
Þ    Available in larger capacities
Þ    Able to handle higher pressures
Þ    Faster response to changing loads
Þ    Able to reach very high temperatures
Þ    Provide an adequate furnace to ensure complete combustion

Disadvantages of Water Tube Steam Boiler:
Þ    High initial capital cost
Þ    Cleaning is difficult due to design structure
Þ    No commonality between tubes
Þ    Bigger in physical size

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