Experimental study on applying standardized frost load to evaporators in a secondary test chamber (ASHRAE RP-1831)

This study investigates a frosted coil defrost test method on a unit cooler inside a secondary box within an environmental chamber and attempts to empirically identify reliable frost load indicators and a full frost load, using existing AHRI 1250 test standard measurements. Four low-temperature round-tube-plate-fin unit coolers were tested. Measurements through dual instrumentation are verified using energy balance of the secondary box. The multi-cycle frosted coil defrost test method shows repeatable frost load (in terms of defrost drainage water mass) no later than Cycle 3 for the studied coils under selected conditions. Results show that the measured frost mass has strong correlations with net cooling capacity reduction, fan power increase, and saturated suction temperature reduction. Coil temperature reduction shows moderate correlation with the frost load. To search for a universal frost load for different unit sizes, ten normalization parameters based on physical dimensions of heat exchangers are investigated. Although none of the studied normalization parameters provide a perfect correlation between frost indicators and normalized frost loads across the four test units, fin surface area or total heat exchanger surface area appear to be the best parameters to normalize frost mass when comparing different unit sizes, leading to overall satisfactory results.