Describes frost-buildup tests conducted on a 3-ton (10.6-kW) nominal cooling capacity air-source heat pump with an orifice expansion device. The study was conducted to determine if a simple temperature-based control variable could be used to determine the amount of degradation in the outdoor airflow (and heating capacity) of the unit. Refrigerant pressures and temperatures were monitored throughout the system in addition to power requirements and airflow rates. A temperature-based variable was developed that could be used to predict airflow degradation across the outdoor heat exchanger. This variable was defined using the difference between ambient air temperature and a measured refrigerant temperature. Eight refrigerant temperatures in the system were recorded and evaluated. Plots of airflow as a function of this temperature variable, along with plots of the absolute value percent changes of this temperature variable and airflow were evaluated to determine which refrigerant temperatures could best be used in the variable to predict degradation in airflow. The best fit between the temperature-based variable and airflow degradation occurred with the inclusion of the refrigerant temperature at the outlet from the evaporator. Calculations of percent changes based on values sampled after a defrost showed a polynominal or linear relationship between airflow and the temperature-based variable.

KEYWORDS: monitoring, outdoor, cooler batteries, air flow, air heat pumps, heat pumps, testing, frost, air flow rate, heat exchangers, refrigerants, temperature, experiment, psychrometry