A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications. Boilers are classified into different types based on their working pressure, design, fuel type, circulation type, and capacity. Low-pressure boilers are limited to 15 psig design, and are typically used for heating applications. Medium-pressure boilers have design pressures between 15-300 psig and are used for industrial process steam generation. High-pressure boilers have design pressures above 300 psig and are typically used for power generation. Fire-tube boilers are boilers that have tubes through which the hot gases from a furnace pass and around which the water to be converted to steam circulates. Water-tube boilers have water running through the tubes and the hot gases from a furnace or burner pass around the outside of the tubes. Heat recovery steam generators (HRSGs) are a special type of boiler that utilizes the heat from the exhaust of a gas turbine or gas engine to produce steam. HRSGs are typically used in combined cycle power plants where a gas turbine and a steam turbine are used to produce electricity.