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Boiler Types and Selection

Types of Boilers

There are a number of types of boilers produced by Cleaver-Brooks, including firetube, watertube, commercial, condensing, flextube, electric, and heat recovery.

For more on Boiler Selection  


Non-condensing firetube boilers are available for low- or high-pressure steam, or for hot water applications. Typically used for applications ranging from 15 to 2,200 horsepower, a firetube boiler uses a cylindrical vessel, with the flame in the furnace and the combustion gases inside the tubes. The furnace and tubes are contained within this vessel, which also houses the water and steam. Optimal operating efficiencies are reached when the boiler, burner, and control scheme are designed and engineered to work as an integrated system, like Cleaver-Brooks Elite firetube line. Cleaver-Brooks has introduced EX Technology in its new firetube boilers, allowing substantial increases in efficiency while reducing boiler footprint.

Firetube boilers are available in either dryback or wetback design. In the dryback boiler, a large and swingable refractory-lined rear door, integral with the vessel, is used to direct the combustion gases from the furnace to the multiple tube banks. The wetback boiler design has a water-cooled turnaround chamber that is 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 and tube removal, is more difficult.


The commercial watertube boiler typically produces steam or hot water for commercial or industrial process applications, and is used extensively for comfort heating applications. In the watertube design, tubes contain steam and/or water, and the products of combustion pass around the tubes. Typically, watertube designs consist of multiple drums. A steam drum (upper) and mud drum(s) (lower) are connected by the tubes, forming both the convection section and the furnace areas. Ranging in size from 500,000 to more than 20,000,000 BTU input, these boilers provide fast steaming capability because of the relatively low water content, allowing quick response to changing load demands.

The industrial watertube boiler (Type D, O, or A) is primarily a steam boiler used for various process applications requiring higher pressures, large quantities of steam, and the possibility of additional heat through integrally mounted superheaters. Ranging in size from 10,000 to 1,000,000 pounds of steam per hour, these packages deliver high-quality steam (less than 0.5% moisture) at pressures exceeding 1,000 psig.

Cleaver-Brooks has recently introduced the Steam-Ready line of D-type boilers to augment our custom D-style boilers. Steam-Ready boilers incorporate popular features and specs, cutting time off the design process.


Commercial boilers include small watertube, firetube, and electric resistance designs for use in schools, office buildings, apartments, libraries, universities, research labs, dry cleaning/laundries, government buildings, airports, hospitals, and the like. These units can fire natural gas, propane, light oil, or non-fueled electric only, that will provide steam or hot water for comfort heating or process requirements. Depending upon the application and product selected, fuel efficiency can be as high as 99%.


Condensing boilers can achieve up to 98% thermal efficiency, compared to 70%-80% with conventional designs (based on the higher heating value of fuels). Typical models offer efficiencies over 90% when the return water temperature is at 110 ºF or less; the lower the return water temperature, the higher the efficiency gain.


Available as packaged or field-erected, flexible watertube boilers are an excellent choice for heating applications because of their resistance to thermal shock. Flexible watertube boilers are available in size ranges from 1.5 to 12 MMBTU/hr input, and are applicable for low-pressure steam or hot water applications.  They have low annual maintenance requirements and typically come with easy access to the fireside and tubes.


Electric boilers are noted for being clean, quiet, easy to install, and compact. Because there are no combustion considerations, an electric boiler has minimal complexity (no fuels, fuel handling equipment, or exhaust stack) with easily replaceable heating elements. An electric boiler may be the perfect alternative to supply low- or high-pressure steam or hot water where the customer is restricted by emissions regulations. Electrode-type boilers are also available, and these units use water to carry electric current and generate steam. Because all the electrical energy flows through the water and is converted to hot water or steam, they can achieve nearly 100% efficiency. In areas where the cost of electric power is favorable, the electric boiler could be the best choice. Sizes range from 9 kW to 3,375 kW output.

Heat Recovery

A heat recovery steam generator, or HRSG, is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine. HRSGs consist of three major components: the Evaporator, Superheater, and Economizer. The different components are put together to meet the operating requirements of the unit. Common applications for an HRSG include combined-cycle power stations, where hot exhaust from a gas turbine is fed to an HRSG to generate steam, which in turn drives a steam turbine. This combination produces electricity more efficiently than either the gas turbine or steam turbine alone. The HRSG is also an important component in cogeneration plants. Cogeneration plants typically have a higher overall efficiency in comparison to a combined cycle plant.