A coil loop thermosiphon exchanges heat by virtue of the natural circulation of a two-phase intermediate working fluid between an evaporator coil in the hot gas duct and a condenser coil in the cooler gas duct as shown in Fig. 1. The vapor flows from the evaporator to the condenser due to a vapor pressure difference between these regions, which arises because the liquid vapor interface temperature and hence the saturation pressure in the warmer evaporator coil will always exceed that in the cooler condenser coil. The configuration of the coils and interconnecting piping must be such that the condensate will.return by gravity to the warmer evaporator coil. If the coils are arranged as shown in Fig. lb, where no liquid intermediate fluid resides in one of the coils, then that coil can only act as a condenser and the system will transfer heat in only one direction. Such a system is said to be unidirectional. If both coils contain liquid intermediate fiuid, as in Fig. la, then the system is capable of trans'ferringheat in both directions and is said to be bidirectional.

This paper, which is based on the results of a larger study carried out for ASHRAE, designated as "Research Project 188", is concerned only with the performance of unidirectional coil loop thermosiphon heat exchangers. An earlier study, supported by ASHRAE as RP 140, dealt with single tube thermosiphon loops such as that illustrated in Fig. 2. In this study it was found that a thermosiphon loop achieves its best performance when no resident liquid is present in the condenser and when near dryout occurs at the top of the evaporator. Unfortunately this ideal condition cannot be maintained for a given system if the temperature difference between the evaporator and the condenser tubes changes. In the current study this limitation has been minimized by the addition of a separator and a liquid recirculation tube to the evaporator as illustrated in Fig. 3. This system suppresses the onset of dryout in two ways: first, by permitting a higher charge to be carried in the evaporator without the penalties associated with liquid carryover (substantially higher pressure drops in the vapor pipe and reduced condensation coefficients resulting from thicker liquid film), and second,by permitting a higher flow rate through the evaporator tubes than was previously possible when all the working fluid had to circulate around the entire system.

In order to investigate the performance of thermosiphon loops of the type illustrated in Fig. 3, a computer simulation program was developed, tested against experimental results, then used to study loop behavior under various conditions. This paper outlines how this was .doneand shows the characteristic behavior of unidirectional coil loop thermosiphons.