Control and Commissioning of a Hot Gas Bypass Compressor Load Stand for Testing Light-Commercial Compressors on Low-GWP Refrigerants

Abstract

Because of changing efficiency standards for HVAC&R equipment, hydrofluorocar-bon 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 replace-ments. This creates a need for design changes to compressors. Recent work by Schmidt et al. (2018) presented a hot-gas bypass compressor load stand constructed at Oklahoma State University to facilitate testing of compressors with their new design changes.The load stand has a design capacity range of 10-80 tons. This wide spectrum of testing conditions necessitates a comprehensive controls package to maintain stability and control over the simulated operating condition. This controls package was designed in LabVIEW and allows independent control over the compressor suction pressure and temperature and the discharge pressure using PI controllers. The controllers operate to both set the desired operating condition as well as minimize the variation of the parameters with time (i.e. random uncertainty).A series of commissioning tests were performed to validate the stand operation and performance metric measurements using a 40 ton scroll compressor. The initial results suggested that appropriately balanced cooling water flow and appropriately tuned controllers are critical to minimize random uncertainty of the data. Without appropriate tuning or cooling water control the load stand was unstable. Adjusting the water flow provided stability but yielded random uncertainty of 0.014% compared with both water control and appropriately tuned controller which yielded random uncertainty of 0.0041%.