Dynamic simulation of natural convection bypass two-circuit cycle refrigerator–freezer and its application: Part I: Component models
Introduction
The two-evaporator in series natural convection refrigerator–freezers (RFs) charged with pure refrigerants, such as R-600a and R134a, are widely used because of the simple design and low costs. For this kind of products, it is difficult to control the temperatures of both compartments in the required values in the same time. The inherent problem is that temperature can be specified in only one compartment. The temperature in either the freezer compartment (FZC) or the fresh food compartment (FFC) controls the operation of the compressor and, ultimately, the amount of heat extracted from both evaporators. Thus, the evaporator that does not control the compressor often receives more or less cooling than it requires. Also, since the evaporating temperature in FFC is the same as the evaporating temperature in FZC, the thermodynamic irreversible loss is large in FFC. For this cycle, extra electric heating is often necessary when the food compartment temperature is specified and ambient temperature is very low.
One method termed “dual-loop system” has been suggested to overcome challenge of the unique two-temperature application of RFs [1], [2], [3], [4]. The dual-loop cycle RFs had independent cycles for FFC and FZC. Every cycle has itself compressor, condenser, capillary tube, and evaporator. One shortcoming for dual-loop cycle is that the initial cost is high since two compressors are used.
Another method termed “bypass two-circuit cycle” has been used to overcome the shortcomings of two-evaporator in series RFs. Bypass two-circuit cycle is achieved by adding an additional path in the two-evaporator in series cycle, which can bypass the former evaporator of in series cycle. The schematic diagrams of bypass two-circuit cycle are shown in Fig. 1(a) and (b). Obviously, the initial cost for this cycle is low since only single compressor is employed. For this cycle, since an additional path is added, the design and the characteristics are more complicated than two-evaporator in series cycle and dual-loop system, researches on this two-circuit cycle are necessary and urgent.
Computer simulation is widely used in refrigerators and other refrigeration system [5], [6], [7], [8], [9], [10], [11], [12], which is a cheap and time-saved means to study characteristics of refrigeration system and improve or optimize refrigeration units. For refrigerators simulation, it includes steady-state simulation [5], [6], [7] and dynamic simulation [8], [9], [10]. For steady-state simulation, the thermal capacity of foam insulation is neglected. For dynamic simulation, not only the refrigeration system, but also the cabinet is considered to be dynamic, so the simulation is complicated. Furthermore, since bypass two-circuit cycle RF is complicated, computation stability of dynamic simulation is difficult to keep, thus reliable and stable models and its solutions are necessary.
Many works on refrigeration simulation were done in our research team, such as RFs, air-conditioners, and liquid chillers [9], [10], [11], [12]. Compared with air-conditioners and liquid chillers, the dynamic cabinet load model is necessary in dynamic simulation of RFs, it is more difficult to build and solve the models for dynamic simulation of refrigerator. In this paper, dynamic simulation of bypass two-circuit cycle RFs is presented as a further research on the early simulation of two-evaporator in series cycle RFs [10]. In Part I, only the component models, including the compressor model, condenser and evaporator model, capillary tube model, and cabinet load model, are presented in details.
Section snippets
Bypass two-circuit cycle RF
As shown in Fig. 1(a) and (b), there are two types of bypass two-circuit cycle RFs. The cycle system of Fig. 1(a) can bypass the freezer evaporator, and the cycle system of Fig. 1(b) can bypass the food evaporator. In bypass two-circuit cycle RFs, the three-way solenoid valves only control the refrigerant flow direction. The operation of compressor and three-way solenoid valve is controlled not only by FFC temperature, but also by FZC temperature. In the example shown in Fig. 1(a), FZC controls
Compressor
The compressor used in the RF is hermetic reciprocating one. It is composed of two parts: cylinder and shell.
The mass flow rate and input work are calculated bywhere, m′com is the mass flow rate of compressor; λ is the coefficient of compressor capacity; Vth is the theoretical piston displacement of compressor; vsuc is the specific volume of suction vapor; W′com is the input work of compressor; ψ is the polytropic exponent; η is
Conclusions
The component models for natural convection bypass two-circuit cycle RFs are presented here. In order to make the simulation program run fast and its accuracy acceptable, the efficiency model that required a single calorimeter data point at the standard test condition is adopted for the compressor; the multi-zone models are employed for condenser and evaporator, with its wall thermal capacity considered by effective metal method; the approximate integral analytic model is employed for adiabatic
Acknowledgements
The research is supported by the State Key Fundamental Research Program of China under the contract No. 2000026309. Part of the research was financed by Refrigerator/Freezer Ltd Company (R/FLC), Haier Group, China. Helps of Mr. Dongning Wang and Mr. Linfei Xu in R/FLC, Haier Group are greatly appreciated.
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