Positive displacement compressor, load stand, refrigerant injection
The development of compressors with refrigerant injection ports provides a less complex and less costly alternative to implementing multi-stage compressors with economization. The ports can be used to inject economized refrigerant during the compression process, which provides the desired cooling effect and decreases the work required to compress the gas per unit mass. Experiments have shown that injecting liquid or low quality refrigerant is effective for reducing the compressor exit temperature and improving system reliability, while injecting refrigerant vapor improves the cooling or heating capacity of the system. However, very little information is available for cycles operating with injection states between these limits of liquid and vapor injection. Theoretical work suggests that cycle performance with two-phase refrigerant injection can provide greater improvements in COP than vapor injection. In addition, experimental work has shown that increasing the number of stages in an economized cycle with a multi-stage compressor improves the cycle performance, and theoretical work suggests that increasing the number of injection ports would have a similar effect. Therefore, this paper presents the design of a compressor load stand for testing compressors with multiple injection ports. The load stand is based on a traditional hot gas bypass configuration but is capable of supplying refrigerant to injection ports at two different pressures between the compressor suction and discharge pressures. In addition, the state of the injected refrigerant can be controlled such that it is either superheated vapor or a saturated liquid-vapor mixture. To guide the design of the bench and size system components, a model was developed to predict the system performance with a commercially available R-410A compressor that has a single injection port. The model is used to predict the range of injection conditions that can be achieved with the load stand over a range of operating conditions. Finally, preliminary test results for the load stand operating without injection are presented, and the experimentally measured compressor performance is compared to the performance data published by the compressor manufacturer.