An Oil-Free Compressor for Gifford McMahon Cold Heads
Caughley, A. J.
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Industrial Research Ltd (IRL) has developed a novel metallic-diaphragm pressure wave generator technology for large high-frequency (30-60 Hz) pulse tube cryocoolers. A diaphragm pressure wave generator, using a conventionally lubricated motor-crank drive, provides a clean pressure wave for a cryocooler. High frequency pulse tubes are at their most efficient when closely coupled to pressure wave generators. There are, however, many applications which require the physical separation of the compressor from the cold head such as a Gifford McMahon (GM) system. Furthermore, GM cryocoolers perform well at low temperatures due in part to their low frequency (~1Hz) operation. GM cryocoolers typically employ compressors related to air conditioning compressors which need filters to remove the compressor oil from the helium flow, thus increasing maintenance. This paper describes the development of an oil free compressor for GM cryocoolers fabricated by fitting a reed valve assembly to the outlet of a 200ml swept-volume diaphragm pressure wave generator. The system was designed to charge a high pressure tank to 16 bar drawing gas from a low pressure tank held at 5 bar. Helium mass flow rate was predicted to be 3.5 g/s. Results of testing the compressor are presented.