Binary mixed-refrigerants for steady cooling temperatures between 80 K and 150 K with Joule-Thomson cryocoolers

Joule-Thomson (JT) cryocoolers operating with pure gases attain vapor-liquid equilibrium in the evaporator and yield a steady cooling temperature which depends solely on the pressure. Mixed gases often substitute the pure gases as the working fluid in order to allow lower pressure ratios and improved coefficient of performance; however, mixed refrigerants are known for their cooling temperature dependence on the ambient temperature and heat load. It is possible to control the cooling temperature via the compressor operating parameters, heating device at the evaporator, bypass ports for the fluid, and more. The present research aims to achieve a steady cooling temperature with mixed refrigerants in a manner similar to pure refrigerants; meaning, having enough phases to leave a single degree of freedom of the fluid thermodynamic state. Thus, by stabilizing the pressure a stable temperature is attained. The current study focuses on binary mixtures since they can be analytically investigated in a more convenient manner relative to multi-component mixtures. Nitrogen-ethane and nitrogen-propane mixtures are considered based on former research conclusions. The present research includes an analytical method for calculating the cooling temperature, a numerical method for calculating the cooling power of the JT cryocooler in order to evaluate whether the cooling temperature is feasible, and experimental data are presented to verify some of the calculated results. (C) 2014 Elsevier Ltd. All rights reserved.