a heat exchanger extracts heat from a two - phase fluid coolant so that the coolant changes from a vapor state to a liquid state . two valves have respective inlets which communicate with the coolant in the heat exchanger , and which are physically spaced from each other . valve control structure responds to the presence of liquid at the inlet to either valve by opening that valve , so that the liquid coolant flows through the valve to a discharge section . a different feature involves a housing with a heat exchanger therein , the heat exchanger having a plurality of coolant conduits that are axially spaced . a flow of air travels axially within the housing , then flows transversely past the conduits to the other side thereof , and then resumes flowing axially on the other side of the conduits .