The Basics of Boiler & HRSG Design

Boilers

• In watertube boilers, air-filled furnaces contain water-filled tubes. When the air is heated, it causes the water in the tubes to boil, which produces steam. In firetube boilers, water-filled tanks contain air-filled tubes. When the air is heated, it circulates through the tubes. According to P.C. McKenzie Company, firetube boilers are smaller and able to handle pressures of up to 250 pound-force per square-inch gauge (psig) and below. Watertube boilers are much larger and capable of handling higher pressures of up to 5,000 psig.

HRSGs

• Instead of discharging hot flue gases into the atmosphere, HRSGs recover the gases and repurpose them to power other applications. For example, the Chicago Bridge and Iron Company installs HRSGs in power plants where their waste heat powers turbines. According to PG Environmental and Thermal Technologies, although HRSGs have different designs, common to all designs are three basic sections: evaporator, superheater and economizer.

CHP Plants

• Combined heat and power (CHP) plants blend boiler and HRSG technologies to maximize efficiency and conserve energy. According to the U.S. Environmental Protection Agency, CHP plants offer energy-efficient solutions to generate power and thermal energy from one fuel source. This technology is found in industries that use steam and electricity, such as chemical plants, refineries and paper mills. The EPA credits CHP technology with making significant reductions of carbon dioxide gas, which is the greatest contributor to collective greenhouse gases.

Considerations

• Because steam generators emit pollutants, emission controls must be implemented to reduce health risks. Carbon monoxide (CO) gas is one byproduct of boiler combustion, with highest emissions from wood-burning fuel sources. According to individual boiler specifications, proper ventilation is critical to reduce health risks from CO poisoning regardless of fuel source, and exhaust chimneys must be sealed to prevent noxious gases from leaking through joints. For example, according to Rutgers University, using a low-nitrogen fuel oil reduces emissions.