Its job is to compress the helium working fluid from about 5 atm to about 15 atm and then cool it to room temperature, while maintaining sufficient flow rate for the refrigerator. The refrigerator manufacturers also supply the compressors, but at the NRAO we have usually assembled our own compressors from major components purchased from third parties. Because power consumption has heretofore not been a major concern, and to limit the number of different compressor configurations that must be supported, those built by both the NRAO and the refrigerator manufacturers have tended to be over-designed. This has produced efficiencies lower than what is feasible. Load on each stage is for zero load on the other stage.
Minimum temperature is at second stage with no external loads.
Low flow into 1st stage for J-T circuit. This is usually thought of as a three-stage system. Due to the fairly high compression ratio, a two-stage compressor is usually required. Traditionally, all components of the J-T stage have been built at the NRAO, including recuperative heat exchangers and compressors. Compressors have been assembled from readily available components, leading to over-design and imbalance between stages, and producing lower than attainable efficiency.