Precision Monitoring of Automotive Compressor Capacity Values Determined From Calorimeter Measurements

In routine mobile A/C compressor testing, Secondary Refrigerant Calorimetry and the First Law of Thermodynamics are employed in the determination of calorimeter primary mass flow rate of volatile refrigerants. An energy balance is performed on the secondary refrigerant calorimeter (SRC) to yield the primary system mass flow rate. This term is the ratio of energy transferred as heat into the SRC to the change in specific refrigerant enthalpy across the SRC. It is important that the calculated flow rate be as close as possible to the actual value so that compressor capacity and volumetric efficiency are correctly calculated. The enthalpy terms are determined from thermodynamic property values recorded during system testing, typically several hundred test points. The average flow rate may be calculated by two methods discussed in this work: (1) at each test point (ratio of SRC heat transfer to enthalpy change) and then averaged over all test points or (2) from the SRC test-averaged heat rate divided by the test-averaged enthalpy change. The first method is the average of ratios approach (AOR) and the second method is the ratio of averages approach (ROA). The ROA calculation is straightforward while the AOR result is shown here to be the product of the ROA and a function that depends on the distribution of numerator and denominator values about their respective means. Identification of the averaging technique used in a given work is important because laboratories’ reported values will depend on the technique employed. Also, analysts reporting AOR results will disagree with those reporting ROA results obtained from operations on the same set of data. It is suggested here that for a given set of data the amount of separation between mass flow rate calculated from ROA and AOR indicates the degree of precision of calorimeter measurements. Determination of flow rate ROA and AOR values’ separation is of use (1) to those preparing to modify or specify a calculation procedure and (2) to those charged with monitoring the performance of an operational system. Guidelines for use of these calculations to monitor calorimeter precision are provided. The considerations cited here for mass flow calculations may be applied to any set of calculations employing ratios of measured values.