A Fresh Look At Vortex Tubes Used As Expansion Device In Vapor Compression Systems

The vortex tube is an intriguing device that separates an incoming high-pressure fluid stream into a two low-pressure streams. Work interaction during the expansion process causes a temperature decrease in one of the two exit streams, while the other one experiences a temperature increase. The overall expansion process in a vortex tube therefore approaches isentropic rather than isenthalpic expansion, and the internal flow separation is achieved without any moving parts, resulting in robust and inexpensive designs. Commercially available vortex tubes are almost exclusively used for spot cooling in industrial applications and use compressed air as the working fluid. In addition, vortex tubes have been gaining lots of attention in air-conditioning and refrigeration research, because of the possibility to replace the expansion valve of vapor compression systems with this low-cost device that can recover expansion work that would otherwise be lost in the isenthalpic throttling process. Most of the work on vortex tubes used for refrigeration has been on numerical studies, and many of them predict very optimistic energy efficiency improvements. However, the few papers available that describe experimental validation of vortex tubes in HVAC&R systems are far less optimistic, which is often caused by the selection of cycle architectures that seem inappropriate for vortex tubes. This paper takes a fresh look at vortex tubes used as the expansion device in refrigeration systems. Vortex tube performance is assessed on a fundamental level for different working fluids, including air and R134a. Suitable vortex tube geometries and operating conditions have been identified and actual work recovery effects have been demonstrated experimentally for both air and R134a. Based on these new findings it is possible to devise novel vortex tube cycles that are able to utilize the demonstrated improvement potentials when applied to vapor compression systems.