Compact High Effectiveness Parallel Plate Heat Exchangers

Abstract

Compact high effectiveness heat exchangers are required for recuperative cycles such as Joule-Thomson and Brayton coolers. Higher exchanger effectiveness translates directly to system efficiency. A quick survey of the theoretical heat transfer performance of different geometries leads designers to parallel plates as the most compact surface. Unfortunately, parallel plates in practical exchangers have never lived up to their theoretical performance. The primary difficulty with any multiple flow path exchanger is one of mass flow imbalance between the channels. In order to achieve very high effectiveness, over 98%, the mass flow must be balanced to within 2% or better in each channel; even if the heat transfer is ideal. To reach these high levels of effectiveness, the geometry of each channel must be nearly identical.

We have developed a method of manufacturing parallel plate exchangers to produce more uniform channels. It is based on photo etching and diffusion bonding. This results in very uniform channel geometry, limited by the thickness variations in the original raw material used in photo etching.

An exchanger has been manufactured and tested with a measured effectiveness of 97.3%. While this was short of the design goal of 99.5%, it shows great promise. Future work includes improved measurements, long-life hermetic sealing, header design, pressure drop predictions, and improved modeling.