Design of GM - JT Refrigerators

A general methodology for the optimum design of Gifford-McMahon/Joule-Thomson(GM-JT) refrigerators is presented. The design goal is to obtain maximum refrigeration for a given 2-stage GM refrigerator precooling the heat exchangers of the JT circuit. A computer code to calculate the refrigeration at liquid helium temperature is developed and executed for various design parameters. In the calculation, the real properties of helium, the heat transfer characteristics in the heat exchangers, and the cooling load of the radiation shield are included. It is found that there exist unique optimum values for the sizes of the heat exchangers, the mass flow rate of helium, and the high pressure of the JT circuit. In addition, the effect of surface emissivity of the radiation shield on the refrigeration is discussed.