Development of a High Efficiency 4K GM Refrigerator

Abstract

This paper describes performance measurements made on a 4 K Gifford-McMahon (GM) refrigerator that incorporates a magnetic regenerator material. The method used to achieve high efficiency was optimization of the pressure ratio. Three types of cold heads were tested: Type #1 with a 32-mm inner diameter of the second stage cylinder, Type #2 with an inner diameter of 40 mm, and Type #3 with an inner diameter of 56 mm. The second stage regenerator materials were HoCu ₂ , Er ₃ Ni,and lead. The pressure ratio was changed by using several types of compressors and regulating the opening of a bypass valve that was connected between the intake and exhaust gas lines from the compressor. The compressor work was estimated by measuring the intake and exhaust pressure and mass flow rate. The highest measured Coefficient of Performance (COP) at 4.2 K was 4.29×10⁻⁴ for Type #1, 4.59×10⁻⁴ for Type #2, and 5.95×10⁻⁴ for Type #3; these were obtained at pressure ratios of 2.21, 2.38 and 2.23, respectively. At these pressure ratios, the cooling power at 4.2 K was 0.335 W, 1.01 W, and 1.99 W, and the compressor work was 0.78 kW, 2.20 kW, and 3.34 kW, respectively. A maximum cooling power of 3.68 W was obtained at 4.2 K using the Type #3 cold head at a high pressure ratio of 3.25, and a power of 7.99 kW.