surfaces 98 of the tubes 94 within the first stage absorber 24 which encloses the second stage of the second loop generator 32 the second loop refrigerant within the interior surfaces 96 of the tubes 94 is further heated by the first loop absorption taking place adjacent to the exterior surfaces 98 of the same tubes this heating distills more refrigerant vapor from the less volatile constituent of the second loop refrigerant within the tubes 94 the rising bubbles of vapor formed within the interior surfaces 96 expel both themselves and the entrained less volatile liquid refrigerant through the tops 116 of the tubes 94 and into the second header 92 referring now to fig2 and 3 together the vapor component of the second loop refrigerant leaving the tubes 94 is captured by the headspace in the second header 92 the less volatile liquid constituent of the second loop refrigerant expelled from the same tubes 94 is deflected to and collects in a sump 118 defined by the top of the upper bulkhead 84 from the sump 118 the less volatile second loop refrigerant component is conveyed by the line 58 to the second loop absorber 46 in this embodiment the second loop condenser 50 is also located within the second header 92 the second loop condenser 50 throws off heat to the heat sink 14 fig1 then collects the condensed second loop refrigerant vapor for transport to the second loop evaporator 44 via the line 52 several distinguishing features of the illustrated apparatus are these the tubes 94 in aggregate have a large wall area providing a high degree of heat exchange between the interior surfaces 96 and exterior surfaces 98 of the tubes 94.