2.4 Regenerator surfaces. Regenerators, used exclusively in gas-to-gas heat exchanger applications, can have a higher surface area density (a more compact surface) than 1hat of plate-fin or tube- fin surfaces. While rotary regenerators have been designed for a surface area density 3 of up to about 8800 m2 m3 (2700 ft ft'). the fixed-matrix regenerators have been designed for 3 of up to about 16,000 m n' (5000 f ft Regenerators are usually designed for low-pressure applications with operating pressures limited to near- atmospheric pressures for rotary and fixed-matrix regenerators; an exception is the gas turbine rotary regenerator, having an inlet pressure of 615 kPa gauge or 90 psig on the air side. The regenerators are designed to cover an operating temperature range from low cryogenic to very high temperatures. Metal regenerators are used for operating temperatures up to about 870 C (1600 F); ceramic regenerators are used for higher temperatures, up to 2 C (3600 F). The maximum inlet temperature for paper and plastic regenerators is 50 C (120 F).