Heat recovery boilers produce steam through the extraction of heat from a hot fluid, usually a gas, that is either the exhaust from an internal combustion engine or a gas turbine, or it is the hot fluid that is formed as a by-product of a chemical reaction and has no intrinsic value as a chemical. Heat recovery boilers are also frequently used in high-temperature operations, like the production of steel and glass.
In order to elevate the boilers inlet gas temperature to a level appropriate with the necessary steam conditions, it is not commonplace for these boilers to receive additional heat through the supplementary firing of other by-products or by the burning of gas or light oil.
Although they are more similar to the back end of a furnace-fired boiler aside from their geometry, the convective elements of boilers burnt by municipal industrial or other waste products are frequently referred to as heat recovery boilers. The gas turbine heat recovery boilers is arguably the most widely used type of heat recovery boilers nowadays.
Low heat capacity fluids and low pressures are the hallmarks of most heat recovery boilers. These factors work together to restrict tube velocities and provide poor heat transfer coefficients. For this reason, finning on the outside of the tubes is virtually always used when the gases are pure. The majority of boilermakers employ helical finning that is serrated, however some still use solid finning in spite of the weight penalty and decreased thermal efficiency.
In order to recover heat from high-temperature gases exhausted from glass-making plants, boilers must have very low pressure drops in the gas stream. Additionally, the tubes in the boilers must be arranged so that deposits—which are inevitable due to the gas’s excess sulfur trioxide—can be regularly removed by blowing off soot. The tri-drum boiler design is one example of this type. If necessary, economizer and super heater surfaces can be added before and after the evaporator tubing to maximize steaming rate or boost boiler efficiency. For the purpose of accommodating changes in the steam production rate during the soot blowing cycles, a large capacity steam drum is provided.
