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BOILER
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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.
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 |
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