Heat exchanger

摘要

A combustion air preheater for heating combustion air with heat energy provided from flue gas includes both a heat pipe type heat exchanger for effecting low temperature heat transfer between the flue gas and the combustion air and a tube-and-shell type heat exchanger for effecting higher temperature heat transfer. The preheater includes a first heat exchanger, having first and second heat transfer compartments with a plurality of heat pipes extending therebetween and a second, connected heat exchanger which includes a plurality of heat tubes supported by tube sheets within a shell. Ducting is provided to direct flue gas over the tubes of the second heat exchanger and then through one of the compartments of the first heat exchanger and additional ducting is provided to direct combustion air through the other compartment of the first heat exchanger and through the tubes of the second heat exchanger. Heat energy is transferred through the walls of the tubes in the second heat exchanger to the combustion air in a first, higher temperature range with additional heat energy transferred to the combustion air in the first heat exchanger in a second, lower temperature range through the heat pipes. The preheater advantageously uses the heat pipes in the lower temperature range and the tube-and-shell heat exchanger in the higher temperature range to effect high overall preheater thermal efficiency over a wide temperature range with minimal corrosive attack on the heat exchange surfaces.