Three two-row heat pump evaporators (outdoor coils) were tested on a residential heat pump to determine whether fin staging would slow frost growth and improve heat pump frosting/defrosting performance at heavy frosting conditions. The base coil had a uniform fin density of 20 fins/in. (fpi) (8 fins/cm) on both rows. Twofin staged coils were examined. The first (Case One) had fin spacing on the front and second rows of 15 fpi (6 fins/cm) and 20 fpi (8 fins/cm), respectively. The second (Case Two) had fin spacing on the front and second rows of 15 fpi (6 fins/cm) and 25 fpi (10 fins/cm), respectively. Frost/defrost performance was evaluated at 28ºF (-2.2ºC) with 90% RH and at 35ºF (1.7ºC)with 95% RH. Three airflow rates through the fin staged coils were tested at 35ºF (1.7ºC) with82% RH. Frost growth on the leading edge of the front row was measured. Frost mass was estimated from psychrometric measurements across the evaporator. Cyclic COP was calculated for the frost/defrost cycle. The Case One and Case Two staged coils improved frost/ defrost COP 8.7% over the base case at 35ºF (1.7ºC), while at 28ºF (-2.2ºC), the Case One coil reduced cyclic COP4.5% and the Case Two coil reduced cyclic COP 9.0%. Fin staging increased frost/defrost cycle time from 32% to 100%, depending on outdoor conditions. Both staged coils delayed frost growth on the leading edge of the evaporator fins even though psychrometric measurements indicated an increase in moisture removal from the outdoor airstream. These trends suggested that staging was successful in moving frost growth to locations away from the leading edge of the front row. Decreased airflow reduced COP up to 8% and reduced cycle time by up to 57%. These were less for Case Two.

Units: Dual