Types of Boilers
Boiler designs can be classified in three main divisions—fire-tube boilers, water-tube boilers, and electric boilers.
Fire-Tube Boilers
Horizontal return fire-tube boiler—Click image to enlarge and view reference
Fire-tube boilers rely on hot gases circulating through the boiler inside tubes that are submerged in water. These gases usually make several passes through the tubes, thereby transferring their heat through the tube walls and causing the water to boil on the other side. Fire-tube boilers are generally available in the range of 20 through 800 boiler horsepower (bhp) and in pressures up to 150 psi.
Water-Tube Boilers
Horizontal return fire-tube boiler—Click image to enlarge and view reference.
Most high-pressure and large boilers are of this type. It is important to note that the small tubes in the water-tube boiler can withstand high pressure better than the large vessels of a fire-tube boiler. In the water-tube boiler, gases flow over water-filled tubes. These water-filled tubes are in turn connected to large containers called drums.
Water-tube boilers are available in sizes ranging from a smaller residential type to very large utility class boilers. Boiler pressures range from 15 psi through pressures exceeding 3,500 psi.
Electric Boilers
Horizontal return fire-tube boiler—Click image to enlarge and view reference.
Electric boilers are very efficient sources of hot water or steam, which are available in ratings from 5 to over 50,000 kW. They can provide sufficient heat for any HVAC requirement in applications ranging from humidification to primary heat sources.