Performance of Multi-System Air Conditioner With Variable Frequency Control Based On Units

Because there are many indoor units and outdoor units, the measurement and simulation of multiple air conditioning system are very difficult. From the viewpoint of similarity between simple system and multi-system air-conditioner, the refrigeration system is separated into several components by their function and location. By the location there are two types, indoor units and outdoor units. When the room needs to be heated, the indoor unit acts as a condenser. On the other hand, when the room needs to be cooled, the indoor unit acts as an evaporator. In order to predict the performance of multiple air conditioning system, the components’ mathematical model in air conditioning system are set up such as variable frequency compressor, heat exchanger, electronic expansion valve and connecting pipe by distributed-parameter. The performance of each unit is analyzed by simulation under different conditions. The heating/cooling capacity of each indoor unit is determined by the electronic expansion valve and the variable fan. In the outdoor unit, the compressor meets the total heating/cooling load of the system by adjusting the frequency and the variable speed fan keeps proper heat exchanged in the outdoor heat exchanger. Generally speaking, the heat exchanged in the heat exchanger is determined by the inlet air temperature, the air volume, the refrigerant mass flow rate and the refrigerant states at inlet. The refrigerant mass flow rate is controlled by the frequency of compressor and the opening rate of electronic expansion valve, and the air volume is controlled by the variable speed fan. Each unit has enough controlling means to keep the performance by itself, so the system can also be controlled at high efficiency based on the independent control of each unit. The opening rate of EEV or the frequency of compressor regulate the cooling/heating capacity of the unit by adjusting the refrigerant mass flow rate. And the air volume is adopted to keep the refrigerant states, subcooling degree or superheating degree, at the outlet of the heat exchanger in the unit. NOMENCLATURE EEV electronic expansion valve FWV four-way valve Ga air volume (m/h) Gr mass flow rate of refrigerant (g/s) Ha air enthalpy (KJ/Kg) HE heat exchanger HPL high pressure liquid pipe HPG high pressure gas pipe Hr refrigerant enthalpy (KJ/Kg) LPG low pressure gas pipe MVRV multiple variable refrigerant volume refrigeration system Pc condensing pressure (Pa) Pe evaporating pressure (Pa) Qc heat exchanged in condenser (W) Qe heat exchanged in evaporator(W) SH superheating degree(C) SL subcooling degree(C) SVRV simple variable refrigerant volume refrigeration system Tc condensing temperature(C) Te evaporating temperature(C) TWV three-way valve VRV variable refrigerant volume refrigeration system Subscripts 1 inlet 2 outlet o outdoor unit INTRODUCTION With the development of national economy and the improvement of living standard, the requirement of air conditioner increases very quickly to make a comfortable environment for living and working. Some multi-system air conditioners have been developed to satisfy the need of several rooms in residential and light commercial buildings (Lijima et al 1991; Masuda et al 1991; Shao et al 2001a). VRV (Variable Refrigerant Volume air conditioning system) is a typical model of multi-system air conditioners from the viewpoint of decreasing the energy consumption and increasing the human comfort. The multiple VRV system (MVRV) is shown in Figure 1 with several outdoor and indoor units. The simplest VRV system (SVRV) with only one indoor unit and one outdoor unit is shown in HPG LPG HPL TWVO HEO