CLASSIFICATION OF BOILERS

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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