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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., T ao (• 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 T ao (• C) ∈ (− 9, 21] and T ao (• C) ∈ (− ∞ , − 9] were respectively determined to be 60 min and 90 min 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.

Defrosting is an important controlled event of variable refrigerant flow air conditioning system (VRF AC). The whole course includes defrosting preparation phase, defrosting execution phase, defrosting end preparation phase and pump-down residual running phase. The defrosting control function can make a great influence on the performance of variable refrigerant flow air conditioning system, whose elements are composed of a certain controlled event, controlled components, executed conditions, controlled time, controlled courses and control logic. A sequence diagram is adopted to express the defrosting dynamic process and the control models of execution conditions on defrosting arrival, defrosting end and pump-down residual running end have been built. Thus, the defrosting control function has been formed based on the model and sequence diagram. The defrosting experiments demonstrate that the built model and sequence diagram can scientifically and accurately express the defrosting control function. Practical application: Defrosting is an important controlled event of variable refrigerant flow air conditioning system. It is difficult to describe the complex controlled course, which is composed of defrosting preparation phase, defrosting execution phase, defrosting end preparation phase and pump-down residual running phase. A sequence diagram can be adopted to express the defrosting dynamic process and the control models of executed conditions on defrosting arrival, defrosting end and pump-down residual running end have been built. Thus, the defrosting control function has been formed based on model and sequence diagram.

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