The operational restrictions on several current interferometer/radiometer instruments are such that focal planes, optical components, and telescope baffles must be maintained at temperatures near 8°K, 20°K, and 80°K respectively. One reliable method of achieving this is to expel supercritical helium from a supply dewar at constant pressure by applying energy to a dewar heater; the resulting flow then passes serially through three (or more) heat exchangers to achieve the desired cooling before being vented through a pressure regulator valve. A convenient set of approximate, time-dependent equations that can be programmed on a hand-held calculator for the preliminary design of a supercritical helium cooling system are presented in this paper. Predicted temperature and mass flow rates based on the simplified equations agree within 5 percent of those resulting from analyses by other researchers. The equations are applied to a particular cooler system for the CIRRIS 82 instrument scheduled to be flown on the Space Shuttle in 1982.
Edward W. Vendell,
"Supercritical Helium Cooling Of Optical Sensors", Proc. SPIE 0245, Cryogenically Cooled Sensor Technology, (3 November 1980); doi: 10.1117/12.959346; https://doi.org/10.1117/12.959346