How Gas Steam Boilers Heat

Fire-Tube Boiler

• Fire-tube boilers produce heat by sending hot gases from a fire to the unit's water tank. A fire-tube boiler's firebox contains the fire. The hot gases reach the water tank via a series of tubes and heat conduction. Heat conduction is the transfer of heat energy from the boiler's fire to the water tank. Once hot gases reach the water tank, they warm the water to create steam. The earliest fire-tube boilers ran on coal, but modern models operate on oil or natural gas. Modern fire-tube boilers also have safety features, such as forced draft fans and flame safeguard control systems, on gas-fired models.

Water-Tube Boiler

• A water-tube boiler has a series of water tubes that constantly circulates water. The water tubes receive external heat from a fire located within the boiler. Many water-tube boilers use natural gas to fuel the boiler's fire. The water tubes connect two drums -- the mud drum and steam drum -- and the drums serve as the boiler's water and steam containers. Mud drums are where water from the plumbing system enters the unit, and steam drums contain saturated steam, which is released into a building to create heat. Sediments from the boiler's water settle in the mud drum.

Flash Boiler

• As specialized versions of water-tube boilers, flash boilers have a series of tubes for facilitating constantly running water. However, a flash boiler's metal pipes constantly maintain a high temperature to convert water into steam constantly. Flash boilers do not use an external fire for heat and do not have water drums for containing the unit's water and steam. One problem that sometimes occurs in flash boilers is overheating, since the drums are not present to cool the tubes if necessary. Flash boilers are lighter than other boiler models, though, since they do not have as many parts.

Cast-Iron Boilers

• Also known as a sectional boiler, cast-iron boilers are split into several sections, and each section is made of cast iron. Cast-iron boilers have heat exchangers, which transfer heat energy from one medium (water) to another (steam). The water passes through tubes into the heat exchanger to create steam. Three types of cast-iron boilers are available -- vertical, horizontal and one-piece. Vertical cast-iron boilers resemble a loaf of bread, while horizontal units look like a pancake stack. One-piece boilers are one section and are the smallest of the three types of cast-iron models.