The Scotch Marine style of boiler has become so popular in the last 40 years that it frequently is referred to simply as a “firetube boiler.” Firetube boilers are available for low- or high-pressure steam, or for hot-water applications. Typically used for applications ranging from 15 to 1,500 horsepower, a firetube boiler is a cylindrical vessel, with the flame in the furnace and the combustion gases inside the tubes. The furnace and tubes are within a larger vessel, which contains the water and steam.

The arrangement provides some characteristics that differentiate it from other boiler types. Because of its vessel size, the firetube contains a large amount of water, allowing it to respond to load changes with minimum variation in steam pressure.

Steam pressure in a firetube boiler is generally limited to approximately 350 psig. To achieve higher pressure, it would be necessary to use very thick shell and tube sheet material. For this reason, a watertube boiler is generally used if pressure above 350 psig design is needed. Firetube boilers are usually built similar to a shell and tube heat exchanger. A large quantity of tubes results in more heating surface per boiler horsepower, which greatly improves heat transfer and efficiency.

Firetube boilers are rated in boiler horsepower (BHP), which should not be confused with other horsepower measurements. The furnace and the banks of tubes are used to transfer heat to the water. Combustion occurs within the furnace and the flue gases are routed 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.

Firetube boilers 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, thus making maintenance easier and reduces associated costs.

The wetback 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 water side sludge buildup, because of the restricted flow areas near the turn around chamber.

Learn more about Cleaver-Brooks’ complete line of firetube boilers.