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Introduction to Boilers | Classification Of Boilers

Introduction to Boilers

Boiler is an apparatus to produce steam. Thermal energy released by combustion of fuel is transferred to water, which vaporizes and gets converted into steam at the desired temperature and pressure.

The steam produced is used for:
(i) Producing mechanical work by expanding it in steam engine or steam turbine.
(ii) Heating the residential and industrial buildings
(iii) Performing certain processes in the sugar mills, chemical and textile industries.

Boiler is a closed vessel in which water is converted into steam by the application of heat. Usually boilers are coal or oil fired.

 A boiler should fulfill the following requirements
(i) Safety. The boiler should be safe under operating conditions.

(ii) Accessibility. The various parts of the boiler should be accessible for repair and maintenance.

(iii) Capacity. The boiler should be capable of supplying steam according to the requirements.

(iv) Efficiency. To permit efficient operation, the boiler should be able to absorb a maximum amount of heat produced due to burning of fuel in the furnace.

(v) It should be simple in construction and its maintenance cost should be low.

(vi) Its initial cost should be low.

(vii) The boiler should have no joints exposed to flames.

(viii) The boiler should be capable of quick starting and loading.


The performance of a boiler may be measured in terms of its evaporative capacity also called power of a boiler. It is defined as the amount of water evaporated or steam produced in kg per hour. It may also be expressed in kg per kg of fuel burnt or kg/hr/m2 of heating surface.

Classification Of Boilers

The boilers can be classified according to the following criteria.
1. According to flow of water and hot gases.
1. Water tube.
2. Fire tube.

Water tube boilers are classified as follows.
1. Horizontal straight tube boilers
(a) Longitudinal drum (b) Cross-drum.
2. Bent tube boilers
(a) Two drum (b) Three drum
(c) Low head three drum (d) Four drum.
3. Cyclone fired boilers

Various advantages of water tube boilers are as follows.
(i) High pressure of the order of 140 kg/cm2 can be obtained.
(ii) Heating surface is large. Therefore steam can be generated easily.
(iii) Large heating surface can be obtained by use of large number of tubes.
(iv) Because of high movement of water in the tubes the rate of heat transfer becomes large
resulting into a greater efficiency.

Fire tube boilers are classified as follows.
l. External furnace:
(i) Horizontal return tubular
(ii) Short fire box
(iii) Compact.
2. Internal furnace:
(i) Horizontal tubular
(a) Short firebox (b) Locomotive (c) Compact (d) Scotch.
(ii) Vertical tubular.
(a) Straight vertical shell, vertical tube
(b) Cochran (vertical shell) horizontal tube.

2. According to position of furnace.
(i) Internally fired (ii) Externally fired
In internally fired boilers the grate combustion chamber are enclosed within the boiler shell whereas in case of extremely fired boilers and furnace and grate are separated from the boiler shell.

3. According to the position of principle axis.
(i) Vertical (ii) Horizontal (iii) Inclined.

4. According to application.
(i) Stationary (ii) Mobile, (Marine, Locomotive).

5. According to the circulating water.
(i) Natural circulation (ii) Forced circulation.

6.According to steam pressure.
(i) Low pressure (ii) Medium pressure (iii) Higher pressure.

Boiler Used in Power Plants
Boiler Used in Power Plants 

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