Abstract: An air preheater wherein heated combustion gas is caused to flow in a straight path through the full lengths of preheater tubes while controlling the velocity and temperature of such gas to sweep liquid droplets from the preheater. The temperature of the preheated air is regulated by mixing inlet air and preheated air to control the temperature of the metal of the preheater at or near the dew point of the combustion gas to thereby inhibit the formation of large liquid droplets which cannot be removed with the gas moving at its velocity through the preheater, whereby essentially all corrosive liquid is removed with the gas to enable the use of carbon steel in the preheater. The air preheater has a bundle of substantially vertical preheater tubes through which combustion gases pass in a substantially straight flow path. The system normally operates with forced draft but a stack disposed integrally with the tubes enables the system to operate with natural draft when the forced draft is not operational.