Economizers are mechanical devices that are designed to reduce energy consumption, or to perform other useful, energy-related functions (such as preheating fluids). There are two types of products: bare tube and extended surface. In basic terms, economizers are heat exchangers. Applications include the preheating of boiler feed water, waste heat recovery, and cogeneration. Economizers are also used in waste heat boilers and other heating and cooling equipment. By definition, waste heat is heat produced by machines and industrial processes for which there is no useful application. Typically, economizers are used with waste heat boilers that feature shell or water tube designs. Such waste heat equipment may use gases from diesel exhaust, furnaces, gas turbines, and incinerators. Selecting economizers requires a basic understanding of industrial boilers, closed vessels that use a fuel source such as electricity or oil to heat water or generate steam for heating and humidification. In boilers, economizers function as heat exchangers that heat a fluid, usually water, up to but not normally beyond the fluid’s boiling point. Economizers are so named because they use the enthalpy in fluid streams that are hot, but not hot enough for use in a boiler. In this way, economizers can recover more enthalpy and improve the boiler’s efficiency. In thermodynamics and molecular chemistry, enthalpy or heat content can be used to calculate the useful work that can be obtained from a closed system under constant pressure. Economizers can use the exhaust gases from an industrial boiler to preheat the cold water (feed water) that is used to fill the boiler. In this way, economizers can serve as feedwater heaters or feedwater pre-heaters. There are two types of feedwater heaters: open and closed. Open feedwater heaters are direct-contact heat exchangers that allow the extracted stream to mix with the feedwater. Normally, these economizers require a feedwater pump at both the feed inlet and outlet. Deaerators, a type of open feedwater heater, are designed to remove non-condensable gases from the boiler feed water. Closed feedwater heaters are shell and tube heat exchangers that do not require pumps to boost the water pressure. Note that economizers that use a closed design also pass the boiler feedwater through tubes.