Evaluation of Fin Staging Methods for Minimizing Coil Frost Accumulation

Abstract

Frost formation on heat pump evaporators is a source of degradation in the performance of heat pumps during heating mode operation. This research sought to determine whether staged fins on outdoor evaporators could slow the growth of frost on the fins and improve heat pump frosting/defrosting performance. The staging method involved having larger fin spacing, or a smaller fin density, on the front row with respect to the subsequent rows on the heat exchanger. Prior research emphasized the potential benefit of fin geometry and fin spacing on the overall heat transfer of the outdoor heat exchanger. An experimental plan was developed to examine fin staging on the outdoor evaporator of a residential, 3 ton (10.6 kW), air-source heat pump with a standard two-row coil design and one with a non-standard three-row coil design. For the two-row coil, in addition to the base case, there were two cases of fin staging that were examined. The three-row coil base case coil was tested against one case of fin staging. System performance was measured during steady state cooling operation at outdoor conditions of 95⁰F (35.0⁰C) and 82⁰F (27.8⁰C) with indoor temperature of 80⁰F (26.7⁰C) dry-bulb and 67⁰F (19.4⁰C) wet-bulb. Steady state heating performance was measured at 47⁰F (8.3⁰C) dry-bulb and 43⁰F (6.1⁰C) wet-bulb outdoor temperatures with an indoor dry-bulb temperature of 70⁰F (21.1⁰C). Frosting/defrosting performance was then tested at the standard outdoor condition of 35⁰F (1.7⁰C) and 82% RH with an indoor dry-bulb of 70⁰F (21.1⁰C) [Test 1]. Additional tests were done to examine the impact of outdoor conditions at: (Test 2) 35⁰F (1.7⁰C) with 90% RH, (Test 3) 35⁰F (1.7⁰C) with 95% RH, and (Test 4) 28⁰F (-2.2⁰C) with 90% RH. For the two-row heat exchangers, three airflow rates were examined: 2800 cfm (79.3 m³/min), 2100 cfm (59.5 m³/min), and 1400 cfm (39.6 m³/min). For the three-row heat exchangers, two airflow rates were examined: 2800 cfm (79.3 m³/min) and 2250 cfm (63.7 m³/min). A detailed comparison of the physical processes that occurred during the frosting/defrosting cycles for the stated heat pumps is provided.