Dynamic characteristics of an air-to-water heat pump under frosting/defrosting conditions

The dynamic characteristics of a medium air-to-water heat pump with multi-circuit evaporator controlled by the thermostatic expansion valve (TEV) under the frosting/defrosting conditions were investigated experimentally. The airflow maldistribution often occurs if the fin-and-tube heat exchangers in a medium heat pump are arranged in V-type or W-type position with the fan at the top. The experimental results show that the airflow maldistribution will result in the intermittent or unceasing hunting during the frosting period, which is not published in public yet. The airflow uniformity causes every circuit to see the different airflow rates. The liquid refrigerant was not evaporated completely in some circuits with the less airflow rate and a little liquid refrigerant flowed out at the outlet, which resulted in less superheat than the necessary MSS (minimum-stable-signal) of the TEV at the outlet of the evaporator. The TEV had the intermittent hunting during the first two-thirds of the frosting period. Afterwards, the combination of the airflow maldistribution and the airflow drop caused by the frost blocking resulted in more liquid refrigerant flowing out of the evaporator. Hence, the TEV had the unceasing hunting during the last one-third of the frosting period. Finally, the operating parameters for both the low-pressure and the high-pressure sides have the intermittent or unceasing hunting during the frosting period, which synchronized with that of the TEV.