Performance and Operating Characteristics of a Novel Positive-Displacement Oil-Free CO2 Compressor

In this paper, a prototype positive-displacement oil-free carbon dioxide (CO2) compressor with a novel mechanical linkage system is introduced and tested. Preliminary compressor test results of the volumetric efficiency and overall isentropic efficiency are presented. The novel compressor design introduces a new low-friction, variabledisplacement drive mechanism. The displacement of the compressor can be varied mechanically while maintaining a minor constant head clearance, eliminating the need for variable speed motors and variable frequency drives. The compressor was designed and manufactured to provide cooling capacities from 10 kW to 100 kW by changing the displacement of the piston. A test stand was constructed to map the compressor isentropic efficiency, volumetric efficiency, mass flow rate, power consumption and discharge temperature. The test stand is based on a hot gas bypass design, in which a part of the discharged refrigerant flow bypasses the condenser, whereas the other part of the flow changes phase as it flows through the condenser. The two streams are mixed to obtain the desired compressor superheat at the suction side of the compressor. The bypass valve enables control of the suction pressure as the discharge valve controls the discharge pressure. The prototype compressor was tested at pressure ratios (PR) of 1.5, 2, 2.5, and 3 at 25 Hz (≈750 rpm), and additionally at PRs of 1.5, 2, and 3 at 20 Hz (≈600 rpm). Based on the test results, the maximum isentropic efficiency is 76% at 20 Hz (592 rpm) and a PR of 2, while the volumetric efficiency is 88%.

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