Research On Reasonable Scale Of Heating Capacity And Defrosting Performance Of Multi Air-conditioning System In Shanghai

Multi air-conditioning system develops quickly these years for its excellent performance under part load. But its heating capacity drops dramatically due to the frosting and defrosting problem under bad weather conditions. The unitary outdoor unit of multi air-conditioning system increase its capacity continually, however, the increment of the heat exchange area increase the possibility of frosting in winter, which in turn restricts the unitary outdoor unit capacity in winter. In this paper, the relationship between unitary outdoor unit capacity and frosting and defrosting performance in Shanghai is studied to optimize the system.Hot gas reverse cycle is commonly used in multi air-conditioning system for defrosting, during which heat will not be brought into the room, so the heating capacity decreases with the frequency of defrosting. The defrosting time is related to the quantity of frost on the tube-fin which is influenced by several factors such as weather conditions, coil dimensions, and the geometric character of coils. According to the test result of a DVM (digital variable multi) air-conditioning system, the digital scroll compressor is found to be on full time load while defrosting. The unitary unit outdoor heat exchanger's model was set up into 4 stages using the lumped parameter method. All units could be merged as one heat exchanger model without considering their coupling. As the indoor units were merged into one, the EEV(electronic expansion valve) are also considered to be single one.Based on a field test on a DVM air conditioning system, the model was validated. So the model could estimate the influencing factors of defrosting performance. With the rise of heat exchange area of unitary outdoor unit, the defrosting time rises, and the total heat output firstly rises and then goes down. For the DVM system, when heat exchange area of outdoor unit increased to be twice, the actual heat output increased to be maximum 138.19%. It is also analyzed that, the increase of unitary outdoor unit capacity would be limited by defrosting. The defrosting time increases with the unitary outdoor unit capacity, so the capacity is limited by the Standard GB/T18837-2002.