Miniature Heat Exchangers for Cryogenic Applications

Abstract

Infra Red (I-R) sensors (Mercury Cadmium Telluride) operate most efficiently at newlineliquid Nitrogen temperature as signal to noise ratio is maximum at this temperature. newlineThese are required to be cooled to liquid nitrogen temperature range (~80K) for newlinevarious military and civil applications. The requirement of low temperature of this newlineorder thus necessitates a cooling device, which has to meet the criteria of reliable and newlinerepeatable performance, operational duration, cooling power, space, weight and newlineenvironmental considerations etc. The selection of process cycle (open or close) of the newlinecooling device (mini cooler) generally depends on the specific application and newlineoperational duration. For the application under consideration, a miniature Joule- newlineThomson (J-T) cooler operating in open cycle is best suited despite many demerits newlineviz. higher operating pressure, limited refrigeration capacityand operational time, prone newlineto blockage, logistics etc. J-T coolers have the advantages over famous and most newlineefficient Stirling cycle based coolers on account of nil electrical power requirement, newlineno vibration concerns due to absence of moving parts, ease of operation, negligible newlineweight and bulk and instant cooling. newlineJ T coolers are available in variety of configuration, material, geometry etc, but the newlinescope of the present study is limited to finned tube J T coolers of cylindrical shape. newlineA basic J-T cryogenic cooling system consist of Detector- Dewar -Cooler assembly, newlinehigh pressure gas reservoir, gas cleaner to arrest gas impurities such as moisture, newlinehydrocarbon, CO2, CO etc, electrically operated control valve to facilitate remote newlineoperation and pressure regulator to conserve gas as well as to maintain uniform vapor newlinepressure for temperature stability.