Steam heat exchangers preheat air to a temperature of about 8.3oC (15oF) below the temperature of the steam used in the heat exchanger. Those heat exchangers normally include backs of fin-walled tubes through which high pressure steam is circulated and a suction fan that pulls ambient and/or recirculated air over the fin-walled tubes. The limitation of steam heat exchangers is that only modest temperatures (150-160oC/300-320oF) can normally be created by steam heat exchangers operated with 8-9 atmosphere (118-132 psia) steam.