CLASSIFICATION OF BOILERS

WATER-TUBE BOILERS

 In the water tube boilers, water is in the tubes and the hot gases surround the tubes. Generally, the high capacity, high-pressure boilers are water tube boilers. Power boilers are water boilers. All water tube boilers are erected and assembled at the site. 

SMOKE TUBE BOILERS

As the name suggests the water is outside the tubes and the hot gases are inside. These boilers are used as process boilers and are comparatively have lower pressures and capacities. Generally, they come as package boilers. These are much more efficient than the smoke tube coil type boilers since the heat in the flue gases can be utilized completely by providing more passes.

COMPARISON OF SMOKE TUBE BOILERS WITH WATER TUBE BOILERS

 

Generally, water tube boilers are suitable for large gas flows and high steam pressures and temperatures. Fire tube boilers are suitable for low steam pressures, generally below 500psig (34.5 Barg).

In water tube boilers, extended surfaces can be made compact if the gas stream is clean. Flue gas pressure drop will also be lower than for an equivalent fire tube boiler owing to the compactness of the design. Water-tube boilers can be smaller and weigh less, particularly if the gas flow is large, exceeding 45,000 kg/h. Superheaters can be used in both types. In a water tube boiler, they can be in an optimum gas temperature zone. A shield screen section or a large convection section precedes the superheater. In a fire tube boiler, the superheater must be located at either the gas inlet or exit, making the design less flexible and vulnerable to slagging or corrosion. If the waste gas is slagging in nature, a water tube boiler is desired because the surfaces can be cleaned by using retractable soot blowers.

Variable	Fire-tube boiler	Water-tube boiler
Gas flow rate	Small—less than 22,500 Kg/h	22,500 Kg/h to millions of Kg/h
Gas inlet temperature	Low to adiabatic combustion	Low to adiabatic combustion
Gas pressure	High	Generally, less than 0.5 Barg
Fuel Firing	Possible	Possible
Type of heating surface	Plain tube	Plain and finned tubes
Superheater location	At inlet or exit of the boiler	Anywhere in the gas path using screen section
Water inventory	High	Low
Heat flux-steam side	Generally low	Can be high with finned tubes
Multiple steam pressure	No	Yes
Soot blower location	Inlet or exit of the boiler	Anywhere inside boiler surfaces
Multiple modules	No	Yes