, , , , Y.C. Lee
Lightweight and compact microcoolers are needed for advanced, hand-held infrared systems. A temperature of 150 K is adequate for high sensitivity with some of the latest IR detectors, which simplifies the cooling requirements compared to 80 K detectors. The largest and heaviest component of most gas-cycle cryocoolers has been the compressor. This paper describes how the use of a 150 K, five-stage, cascade Joule-Thomson (JT) cycle significantly reduces the total refrigerant flow rate and total compressor swept volume compared with other types of cryocoolers for this temperature range. The use of vapor compression in each of the five stages results in very high specific refrigeration powers and low flow rates even with no recuperative heat exchangers and with pressure ratios of only about 4:1 Such low pressure ratios are needed for the use of chip-scale compressors. We discuss the thermodynamic design of a five-stage planar polyimide cold head for a range of refrigeration powers up to about 350 mW at 150 K. We compare the use of single-layer and double-layer construction over this range of refrigeration powers. In the double-layer construction, the high-pressure fluid streams are on the bottom and the low-pressure fluid streams are on top. Such construction allows for simple recuperative heat exchangers and leads to a compact planar geometry of about 20 mm by 30 mm for 350 mW refrigeration power at 150 K. The ideal fluid input power for the five-stage system at 150 K with no recuperative heat exchangers is 1.59 W per watt of net refrigeration power. For a heat exchanger ineffectiveness of 0.05 the specific power is 1.29 W/W for an efficiency of 89.2 % of Carnot with perfect compressors. The paper describes the geometry of the condensers, evaporators and Joule-Thomson impedances to achieve the high heat flux required for minimum size.
June 9-12, 2014
cascade cycle, cryogenics, cryocoolers, Joule-Thomson, microcryocooler, refrigeration, polyimide, vapor-compression.