Component losses in refrigeration compressors

The purpose of this investigation was to examine the relative importance of factors affecting the volumetric efficiency of small refrigeration compressors with particular reference to blowback effects at the suction valve. The overall volumetric efficiency being recognised as an unreliable indication of the relative importance of component losses, the separate losses associated with the action of unclamped, reed type, compressor suction valves were analysed and measured. The main factors affecting volumetric efficiency were assumed to be heat transfer between fluid and cylinder, re-expansion of clearance volume vapour, throttling at the valves, blowback past the valves while closing, leakage past the piston and leakage due to imperfect seating of the valves. A review of previous work indicated that the loss due to blowback was worthy of attention though no experimental results were available owing to the difficulties attending the measurement of this quantity. A theoretical study of the problem of blowback past the suction valve was made and methods of analysis devised by which blowback quantities could be predicted. The approach of previous workers to the dynamics of valve action was not followed as it proved invalid when applied to those parts of the cycle where blowback occurred. New experimental techniques were developed in order to make direct measurements of blowback losses in a small open type compressor. Initial tests were performed in which the suction valve of an otherwise conventional compressor was operated by electro-magnetic means. This method did not prove sufficiently accurate and an A.C. operated constant temperature hot-wire anemometer was developed to indicate gas valocities in the suction port. The anemometer, in conjunction with valve motion measurements, was used to investigate the effects of speed, valve lift, valve thickness, valve end freedom and fluid density on blowback. From the records obtained it became evident that the anemometer diagrams could also be used to give rapid and accurate measurements of re-expansion and throttling losses. The results showed that a combination of light valve reed and low lift produced the greatest efficiencies in practice. The loss due to blowback was also much reduced when pumping dense fluids. Some tests were carried out on a small modem "hermetic" compressor and these indicated that the anemometer could be applied to investigations on such sealed units without undue difficulty. Details are also given of a new type of compressor which was designed on the basis of the data obtained in the investigation. The main features of the design are the reduced throttling and blowback losses