Performance Simulation Of Multi-split Air-conditioning System Under Different Running Conditions

In recent years, more and more multi-split air-conditioning systems are widely used in public places such as office buildings, shopping malls, for its simple systems, reliable operation, and high-SEER. However the study on variable operation condition characteristics of multi-connected air-conditioning systems is not thorough enough, though the study on the system has important theoretical and practical significance on optimizing the system, energy-saving design and the operation methods.A MDV system is taken as the study object, which is installed in School of Environmental & Municipal Engineering of Xi’an University of Architecture & Technology. The mathematical simulation models were developed for the compressor and the heat exchangers in MATLAB language.Based on the developed simulation model, the heat transfer characteristics under the standard refrigeration and heating conditions were studied and analyzed. Under the standard refrigeration conditions, the apparent heat transfer coefficient of two phase area on refrigerant side in indoor unit increases with the increasing of the length of coil pipe, and then decreases. It reaches the minimum value when the length is 80% of the pipe length. The pressure drop is large in evaporation process with the values from 625.4 kPa to 553.4 kPa. The apparent heat transfer coefficient of two phase area on refrigerant side in outdoor unit is larger than that in the superheated area and the subcooled area. Meanwhile, the heat transfer coefficient reduces with the decrease of the coil pipe length. The coil pipe areas of the indoor and outdoor units in the standard refrigeration condition are less than that in the standard heating condition. Simultaneously, the heat transfer coefficients of indoor unit and the surface on refrigerant side are larger than that in the standard heating condition.The indoor unit load changes with the outdoor temperature which leads to the variation of operation parameters is simulated when the indoor temperature is designated. The coil pipe areas of indoor unit are reduced to 91.8%, 80.5% and 68.2%, when the load is reduced to 80%,60% and 40% respectively in the standard refrigeration. The outlet temperature of indoor unit increases from 13.93℃to 21.68℃.The heat transfer coefficients of indoor unit and the surface on refrigerant side are reduced with the load ratio.At the same time, the operation parameters of outdoor unit are simulated when the numbers of running indoor units change which results in the variation of load ratio. The load of outdoor unit decreases with the decline of numbers of running indoor units, which leads to the reduction of refrigerant flow. The coil pipe areas of condenser reduce to 82.3%,66.1% and 58.4% when the refrigerant flow is 1/2,1/3 and 1/4 of standard conditions, and the heat transfer coefficients of indoor unit and the surface on refrigerant side decrease correspondingly.From the above analyses, in order to achieve the aim of energy saving more effectively, we can appropriately control the openness of the expansion valve to maintain the suitable superheat degree of the evaporator outlet to provide the amount of heat required when the refrigerant passes the whole indoor coil pipe.