Type T91, T22, and T11 alloy steel tube
materials are typically used in HP superheaters
and reheaters. Carbon steel is used for intermediate
and LP superheaters and also in evaporators
and economizers. The feedwater economizers
operate at 45 to 150 °C (110 to 300 °F) and
are located immediately before the exhaust
stack. Economizers generally receive special
attention in the materials selection processes,
due to the potential of acid dew-point corrosion
under normal operating conditions. Some
HRSG units use stainless steel alloys 430, 439,
or 2205 in this location to avoid dew-point corrosion,
while others simply use carbon steel.