Abstract: A heat exchanger including a plurality of heat transfer elements defining hot and cold fluid flow paths with inlet and outlet ends for flow therethrough of a relatively hot fluid and a relatively cold fluid respectively, the hot and the cold fluid flow paths each having at least one portion substantially parallel to each other in which the relatively hot fluid and the relatively cold fluid flow in a common direction such that a thermal resistance between the plurality of heat transfer elements and the flow of the fluids therethrough, and a temperature difference between the hot and the cold fluid flow paths, are maintained over a portion of the heat exchanger to substantially prevent freezing of at least one component of either of the relatively hot fluid and the relatively cold fluid.

Abstract: An air cycle air conditioning system including an air-air heat exchanger defining a first air flow path for air to be cooled and a second air flow path for air to pass therethrough and absorb heat from the air to be cooled and apparatus for supplying air to the second air flow path at sub-atmospheric pressure.