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Firetube Boilers

A firetube boiler is a cylindrical vessel, where the flame is contained in the furnace and the combustion gases pass through the tubes. The furnace and tubes are within a larger shell, which contains the water and steam. Firetube boilers are built similar to a shell and tube heat exchanger. The large number of tubes installed in the boiler produces more heating surface per boiler horsepower. This design greatly improves
heat transfer and efficiency. Firetube boilers are used for applications ranging from 15 to 1500 horsepower and are available for low or high-pressure steam and hot water applications.

The firetube construction has characteristics that differentiate it from other types of boilers. The boiler contains a large amount of water, which allows it to respond to load changes with minimal variation in steam pressures. Steam pressures in a firetube boiler are normally limited to approximately 350 psig. Higher pressures with this design are viable, but require the use of heavier gauge shell and tube sheet material. For this reason alone, a watertube boiler is generally used if pressure above 350-psig design is required.

The furnace and banks of tubes transfer heat to the water. Combustion occurs within the furnace and the flue gases exhaust through the tubes to the stack outlet. Firetube boilers are available in two, three and four pass designs. A single "pass" is defined as the area where combustion gases travel the length of the boiler. Generally, boiler efficiencies increase with the number of passes.

Firetube boilers are available in either dryback or wetback design. In the dryback boiler, a refractory-lined chamber, outside of the vessel, is used to direct the combustion gases from the furnace to the tube banks. Easy access to all internal areas of the boiler, including tubes, burner, furnace, and refractory, is available from either end of the boiler.

The wetback boiler design has a water-cooled turn around chamber used to direct the flue gases from the furnace to the tube banks. The wetback design requires less refractory maintenance; however, internal pressure vessel maintenance, such as cleaning, is more difficult and costly. In addition, the wetback design is more prone to waterside sludge buildup because of the restricted flow areas near the turn around chamber.