A water tube boiler usually has two drums, i.e. a steam drum and a water drum or mud drum. The upper drum supplies water to the lower drum. The upper drum, in the steam chest section, collects the saturated steam and dries it by means of cyclonic separators and chevrons and then discharges the dried steam to the superheater section of the boiler. The water drum supplies water to the team generating tubes and to water distribution headers serving the boiler water walls. The fuel is burned in a combustion chamber arranged so that radiant heat and convection heat is transferred to the outside of the water tubes to heat the water within.
Water tube boilers may be either straight tube boilers or bent tube boilers. The tubes of most straight tube boilers are connected into headers, which in turn are connected to the boiler drums. Water tube boilers are always used when large steam capacities are needed. They are also used for high pressures and temperatures. They have been built in sizes up to 5,000,000 lb. (2,268,000 kg) of steam per hour, at pressures up to 5000 psi gauge (34,474 kPa) and temperatures of up to approximately 1200°F (649°C).
Bent tube boilers are made in a variety of arrangements. They are similar to straight tube boilers, but they are almost always multi-drum, and the tubes are connected directly into the boiler drums. The tubes are bent to allow them to enter the drums radially, to facilitate installation, to allow for expansion and contraction, and to allow for flexibility in design. Bent tube boilers may be either balanced draft boilers or positive pressure boilers.
Some boilers are fired using hot process waste gas streams, including fluid catalytic cracking unit (FCCU) regenerator flue gas as fuel to recover both sensible heat and fuel value. Carbon monoxide boilers can still be found in some refineries. Some refineries also use the combined cycle system, which utilizes the hot exhaust from gas turbines as combustion air in the boilers.