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.
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.
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.
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.
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