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

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 10-80 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 LabVIEWTM (Elliott et al., 2007), 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 control validation tests were performed, using a 40 ton scroll compressor, to validate the control scheme, and to evaluate the resulting quality of primary measurements. The initial results suggested that the load stand was able to minimize the contribution of the random uncertainty to the total over the duration of the test. Upon further investigation, it was found that the time required for testing could be reduced by three times, with the optimization of the control strategy.

A 30 ton spool compressor and a 75 ton screw compressor were used perform commissioning tests on the load stand. The results of the commissioning tests showed that the load stand is capable of collecting valid test data, in accordance with the compressor testing standards set by ASHRAE-23.1 (2010). A comparative analysis of the data to data provided by the manufacturers of each of these compressors suggested that the load stand is capable of measuring accurate performance data. The results suggest that the load stand is considered a fully commissioned compressor testing facility.