Dynamic Characteristics of Air¡Mto¡MWater
Heat Pump Under Frosting£¯Defrosting Conditions
Ji Changfa, Huang Dong, Yuan Xiuling
(School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China)
Abstract: The dynamic characteristics of an air-to-water heat pump under the frosting£¯defrosting
conditions were investigated. The experiments were conducted on a heat pump with the
nominal heating capacity of 55kW. The results show that the thermal expansion valve (TEV)
started to have the intermittent instability or hunting in a feed-back control loop, for
64 minutes during the middle of the frosting period due to the airflow maldistribution at
the face of finned tube heat exchangers. Moreover, the TEV had the continuous hunting for
the last 32 minutes of the frosting period. The combination of the airflow maldistribution
and the drop in airflow rate owing to the frost accumulation resulted in the flooding of
the evaporator and the unceasing hunting of the heat pump. Furthermore, a ¡®critical
point¡¯ of the defrost period appeared, after which the system parameters increased
quickly because the modes of heat transfer changed outside the tubes of finned tube heat
exchangers. Before the critical point, the phase change heat transfer occurred and had
higher heat transfer performance outside the tubes when the frost was melting. But after
the critical point, the air natural convection occurred and had lower heat transfer
performance outside the tubes because the frost was cleared and the heat exchanger fans
were turn off. Finally, the discharge pressure had the sudden increase dramatically after
the defrost termination or during the start-up of the frosting period. The phenomenon
seemed to be due to both the small inner volume in the plate heat exchanger and larger
refrigerant flow rate after the defrost termination or during the start-up of the frosting
cycle.
Keywords: frosting;defrosting; hunting;critical point; air¡Mto¡Mwater heat pump