Temperature instability comparison of micro- and meso-scale Joule-Thomson cryocoolers employing mixed refrigerants

Abstract

Previously we demonstrated cryogenic cooling in a Joule-Thomson (JT) microcryocooler (MCC) with mixed refrigerants operating at pressure ratios of 16:1 that achieved stable temperatures of 140 K, with transient temperatures down to 76 K, with precooling of the refrigerant to 240 K. Pre-cooling improves the minimum enthalpy difference, (Δh T)min compared with that of pure fluids. Micro-scale compressors have been unavailable to meet 16:1 ratios. By reducing the ratio to 4:1, mini-compressors become viable in the near term. Utilizing mixed refrigerants optimized for 4:1 pressure ratios we compare the performance stability of this micro-JT employing a 25 mm long multichannel glass fiber heat exchanger (outer low-pressure capillary ID/OD=536 μm/617 μm, inner high-pressure channels ID/OD=75 μm/125 μm) with a scaled up (meso-scopic) version employing a 20 cm long single channel stainless steel heat exchanger (outer low pressure channel ID/OD=580 μm/760 μm, inner high pressure channel ID/OD=150 μm/266 μm). This easy to fabricate and modify meso-scale version was fabricated to investigate the temperature instabilities of mixed refrigerants for similar operating conditions but for proportionally higher flows of ∼ 30 cm/min compared with ∼ 10 cm/min. We compare measured pressures, flow rates, temperatures, and stabilities for both micro- and meso-JT cryocoolers to better understand the causes for the temperature instabilities within the micro-JT cryocooler. © 2012 American Institute of Physics.