Determination criterion of defrosting condition for variable refrigerant flow air conditioning system

► The defrosting criterion model for VRF AC has been built. ► The defrosting accumulative coefficient has been introduced in the model. ► The defrosting judgment time has been introduced in the model. ► The minimum defrosting cycle has been determined by experiments. ► The coil defrosting judgment temperatures has been determined by experiments. The disadvantage of the defrosting determination criterion of time-coil temperature was theoretically analyzed and experimentally verified. Considering the different characteristics of variable refrigerant flow air conditioning system (VRF AC) from the common air conditioners, the defrosting criterion model of time-coil temperature-condensation temperature has been built. In this model, there exist four temperature regions based on different frosting characteristics at different ambient temperatures, i.e., Tao(°C)∈(−∞, −9], (−9, 0], (0, 4], (4, 21]. The relations between the defrosting judgment temperatures and ambient temperatures have been determined by experiments. And the minimum defrosting cycle when Tao(°C)∈(−9, 21] and Tao(°C)∈(−∞, −9] were respectively determined to be 60min and 90min by experiments. Moreover, the defrosting accumulative coefficient, the defrosting judgment time constant and the restriction of the condensation temperature have been introduced into the model. Thus the complete criterion model can avoid delayed and premature defrosting, even false defrosting and inaccessible defrosting. A large number of experiments have verified the feasibility of the model.