In a world faced with the prospect of energy shortages due to increasing demand, limited petroleum supplies, and environmental constraints placed on energy production, it is essential that our energy utilization be as efficient as possible. One way in which improved efficiency may be achieved is through the effective transfer of waste heat between exhaust and supply gas streams. Many such heat exchanger applications exist in practice, recuperators in steam power plants, regenerators in gas turbine power plants and ventilation waste heat recovery units. This latter application has been receiving considerable attention since the recent energy crisis. Large quantities of fresh ventilation air are required in factories where contaminated air from industrial processes must be removed, in hospitals for safety, in convention halls and restaurants to control odors due to people, smoke, and food and in swimming pools to control the humidity. Vast amounts of energy are lost with this ventilation air if it is not reclaimed to preheat incoming air.

Several different classes of gas-to-gas heat exchangers are currently used for waste heat recovery. If the two gas streams are or can economically be brought adjacent to one another direct compact heat exchangers, regenerative heat exchangers or heat pipe heat exchangers may. be effectively used. If the two gas streams cannot economically be brought into close proximity because of physical limitations or the possibility of contamination then liquid coupled forced circulation run around loops may be utilized. The latter system is a highly flexible one but requires a power supply and pump to circulate the intermediate fluid.

The waste heat recovery system discussed in this paper evolved from the concept of combining the physical flexibility of the liquid coupled heat exchanger configuration with the natural-circulation characteristics of a gravity return heat pipe. The resulting system ill trated in Fig. 1 has been termed a two-phase thermosiphon loop to characterize its shape and distinguish this system from gravity heat pipes and other counter-flowing thermosiphon Heat exchangers utilizing such thermosiphon loops to transfer energy between two regions the same uni-directional or bi-directional characteristics as heat pipes in that they maY installed in such a way that they will transfer heat equally well in either direction or wi operate in one direction only.

In order to better understand the characteristic operating behavior of thermosiphon systems it was decided that the initial investigation, sponsored by ASHRAE through RP 140 would be limited to a study of the performance of single tube loops. The extension to tube evaporator and condenser coils with bypass liquid recirculation is the subject of ASHRAE sponsored research through RP 188.