A method for tuning refrigerant charge in modeling off-design performance of unitary equipment (RP-1173)

Existing models do not accurately predict the effect of off-design refrigerant charge even when tuned at the design operating condition. Based on experimental testing and theoretical analyses, multiple factors that cause inaccuracies in charge prediction are identified in this paper. The factors include unaccounted liquid volumes, refrigerant dissolved in the compressor lubricant, inaccurate void fraction models, and an inaccurate estimate of the subcooled liquid length. Some of these factors lead to errors that change with operating conditions and amount of charge. This paper presents a method for adjusting refrigerant charge using a charge correction equation that requires data for two operating points for parameter tuning. The approach associates the variable charge errors with the subcooled liquid length and considers all other errors to be constant offsets. The ability of a system model to predict off-design charge effects is significantly improved through the use of this tuning approach. This was confirmed though comparisons with measured results and predictions obtained with existing tuning approaches. The tuning approach was tested for two R-410A units and different operating conditions, using two public domain simulation models.