Classification of Boilers
(1) Depending upon the relative position of water and flue gases :
(a) Smoke tube or fire tube boilers : In these type of boilers the hot gases or the flue gases from the furnace are passed through the tubes surrounded by the water outside to be evaporated. Examples : Cochron, Lancashire, Locomotive boilers etc.
(b) Water tube boilers : In these boilers the water to be evaporated passes through the tubes and the gases from the furnace are passed over the external surface of the tubes. Examples : Babcok-Wilcox, Stirling boilers etc.
(ii) Depending upon the position furnace :
(a) Internally fired boilers : In these the furnace is inside the body of the boiler shell. Examples : Locomotive and Lancashire boilers etc.
(b) External fired boilers : In these the furnace is outside the boiler shell. Examples : Stirling, Babcok-Wilcox and Loeffler boilers etc.
( iii ) Depending upon the position of axis of the boiler :
( a ) Vertical boilers : In which the axis of the boiler shell is in vertical plane.
( b ) Horizontal boilers : In which the axis of the boiler shell is in the horizontal plane.
(iv) Depending upon the service to which the boilers are put, like stationary, portable, marine or locomotive boilers etc.
(v) Depending upon the source of heat :
The source of heat may be due to :
Heat generated due to combustion of solid, liquid or gaseous fuels.
Hot waste gases or electrical energy or atomic energy etc.
(vi) According to the method of circulation of water and steam :
(a) Natural circulation :
In case of natural circulation steam boilers, the water is circulated by natural convection which are set up due to heating of water. The temperature gradient causes the density gradient along the length of the tube due to which water flows from high density water to low density water and steam.
Examples : Babcok-Wilcox boiler.
(b) Forced circulation :
In forced circulation boiler, the water is circulated with the help of centrifugal pump driven by external source of power. Such a method of water circulation is usually employed in high pressure boilers. Examples : LaMont and Loeffet boilers.
(vii) According to pressure of steam generated :
The boiler which produce steam at less than 20 bar pressures are called low pressure boilers.
Examples : Cochron, Lancashire and Locomotive boilers.
The boilers which produce steam above 20 bar pressures are called high pressure boilers. Example : Babcok-Wilcox, Stirling, LaMont boilers.
(viii) According to nature of draught employed :
When the fuel is burnt on the surface due to natural circulation of air, the draught is called natural or chimney draught. If the air is circulated with the help of fan driven by external source of power, the draught used is called artificial draught.