Control and commissioning of a hot-gas bypass compressor load stand for testing light-commercial compressors on low-GWP refrigerants

Abstract

Abstract As a result of changing efficiency standards for HVAC&R equipment, hydrofluorocarbon refrigerants such as R134a and R410A are in the process of being phased out because of their high Global Warming Potential (GWP). Many low-GWP refrigerants, such as R1234yf, R1234ze(E), R1234ze(D), R32, and several blends of these, are being considered as replacements. This creates a need for design changes to compressors. Recent work by Schmidt et al. (2019) presented a hot-gas bypass compressor load stand constructed at Oklahoma State University to facilitate testing of compressors deployed with these refrigerants ranging in capacity from 10 to 80 tons. This work extends the previous by developing a comprehensive controls package to maintain stability and control over the wide range of capacities and operating conditions. This controls package, implemented in LabVIEW, allows independent control over the compressor suction pressure and temperature and discharge pressure using Proportional-Integral (PI) controllers. The controllers were tuned to maintain a set point as well as minimize the random uncertainty of the measurements. A series of commissioning tests were then performed, using a 40 ton scroll compressor, to validate the control scheme, and to evaluate the resulting quality of primary measurements. The initial results suggest that an appropriately balanced cooling water flow and tuned controllers are critical to minimize random uncertainty of the data and maintain stability of the load stand. Adjusting the water flow alone provided stability but yielded random uncertainty that was roughly 3.4 times higher than with both flow adjustment and an appropriately tuned controller.