Abstract: The heat exchanger utilizes variable conductance heat pipes to control the temperature of a cooled liquid or condensate, such as in dry cooling towers, chemical processes, air conditioning, and pollution control. Heat pipes, connected to common or separate gas reservoirs, extend between two separate ducts, one of which is for flow of air and the other is for cooling or condensation of a fluid. The fluid gives up heat to the heat pipes and cooled fluid is collected thereafter. In the air duct, heat is removed from the heat pipes, with the gas reservoir controlling the temperature of the heat pipes, as well as maintaining a constant temperature in the cooling side of the heat exchanger to control the cooling process, to maintain the gas front of the working fluid in the heat pipe solely within the air flow chamber, and to prevent solidification of fluids in the cooling side of the heat exchanger.