Some complex refrigeration and heat pump systems with several condensers and evaporators have been developed for different kinds of application. Traditional simulation models were developed for systems in certain operating modes and they failed in modeling the complex refrigeration systems with uncertainties of heat exchangers function and refrigerant flowing direction. In order to predict the performance of complex refrigeration systems, a simulation model is presented based on the two-phase fluid network. The model is consisted of distributed-parameter model of heat exchangers and connecting tubes, map-based model of inverter compressor and electronic expansion valve (EEV). Based on the characteristic of refrigeration system and fluid network, the three conservation equations, i.e. energy, momentum and mass equations, are solved iteratively. This model can deal with the uncertainty of refrigerant flow direction by separating the solving process of the components and the fluid network model, and therefore can simulate different kinds of complex refrigeration systems in different operating modes and conditions. The model is validated by the experimental data of an inverter air conditioner in heating/cooling operating modes and it shows the error of the model is mainly determined by the error of submodels of components in calculating heat transfer and pressure loss. The model is applied for performance analysis of three kinds of complex refrigeration systems in the accompanying article [Shi W.X., Shao, S.Q., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network: part II – model applications, International Journal of Refrigeration 31 (3), 500–509.].
Heat pump systems have been widely used in the last decade since they have been the necessities of life at home and in public areas due to the large demand for comfort in modern society. The conception of refrigeration or heat pump system has now developed from simple system with only one evaporator and one condenser to complex systems with several evaporators and condensers, even compressors. To facilitate more flexible and multi-purpose uses, complex refrigeration systems with a variable speed compressor or a group of constant speed compressors connected in parallel, an outdoor unit with a group of heat exchangers, and/or several indoor units have been developed (Qureshi and Tassou, 1996, Field, 2002, Masuda et al., 1991, Lijima et al., 1991, Ito and Miura, 2000, Shao et al., 2003a, Shao et al., 2004a, Ji et al., 2003 and Chua et al., 2002). A multi-unit air conditioner is the system that could distribute cooling/heating capacity to different spaces, which is consisted of a number of indoor units and only one outdoor unit as shown in Fig. 1 (Masuda et al., 1991 and Lijima et al., 1991). Some heat pumps were developed to use different kinds of heat sources simultaneously, such as a heat pump using water and air as heat sources in parallel (Ito and Miura, 2000). A heat pump with several heat exchangers in series or in parallel to produce domestic hot water was also developed (Shao et al., 2004a and Ji et al., 2003), where some heat exchangers are for domestic hot water and the others are for cooling or heating. Another such kind of complex system named multi-unit heat pump dehumidifier (Chua et al., 2002 and Shao et al., 2003a) was developed, which has two condensers in parallel. One condenser is placed outdoor and the other is placed at back of the evaporator to recover part of the condensing heat and to keep the temperature of air passed through the evaporator.
A simulation model based on the two-phase fluid network is presented in this paper, which can describe different kinds of complex refrigeration systems in different operating modes. The mathematical model of the components and the system is developed with distributed-parameter method for heat exchangers and pipes, and network model for the mass and energy balance. A universal structure of network is used to adapt different kinds of refrigeration systems in different operating modes. The solution algorithm of the system is separated from the flow direction of refrigerant. Therefore, the model is convenient to calculate the refrigerant distribution and solve the uncertainty of refrigerant flow direction. The calculation of each condition is within 2 min for complex refrigeration system with four heat exchangers and only several minutes for the inverter air conditioner, which shows that the system solution method is efficient.
The comparison between the simulation results and the experimental data of an inverter air conditioner in heating/cooling operating modes shows that the error of the model is acceptable, which is mainly determined by the error of correlations for heat transfer and pressure loss calculation and the error of the solving process can be ignored.
This model will be used to predict the performance of different complex refrigeration systems such as a multi-unit inverter air conditioner, heat pump with domestic hot water, multi-unit heat pump dehumidifier in part II of the paper (Shi et al., 2008).
