Thermodynamic limits of 4He and 3He Joule-Thomson cryocoolers

Joule-Thomson (JT) cryocoolers working at liquid helium temperatures are essential in a variety of space- and ground-based applications. However, the optimal working conditions maximizing the coefficient of performance (COP) for helium JT cryocoolers remain unclear. This study presents a method from the thermodynamic point of view to determine the maximum COP and the corresponding working conditions. The specific cooling capacity of a helium JT cryocooler can be enhanced by using multi-stage throttling when the pinch point of the single-stage JT unit is at its low-temperature end. The performances of 3 He JT cryocoolers working at 0.5-3.2 K and 4 He JT cryocoolers working at 1.0-4.2 K are compared in terms of COP, compression ratio, specific mass flow rate and volumetric flow rate. Fora given low pressure of 100 kPa, JT cryocoolers with three-stage precooling using the state-of-the-art cryocoolers as precoolers have maximum COPs of 0.2829% at 4.2 K using 4 He and 0.2124% at 3.2 K using 3 He. Moreover, sensitivity analysis has been performed to assess the effects of compressor efficiency, precooler COP, heat exchanger effectiveness and pressure drop on the COP of 4 He and 3 He JT cryocoolers. This study provides a reference in designing working conditions for helium JT cryocoolers.