·         Used to preheat combustion air for fired process heaters, industrial boilers, and central station boilers

·         Isothermal device eliminates problems due to acid gas or cold end corrosion

·         Consist of three elements:

o        A sealed pipe

o        A capillary wick structure

o        A working fluid

·         Working principle:

o        Heat from flue gas heats one end of the pipe – causes instantaneous evaporation of the working fluid

o        Rapid generation of vapor creates pressure – forces excess vapor to the cold (combustion air) end of
the pipe

o        Vapor condenses at the cold (combustion air) end transferring heat to the combustion air