Multi-Evaporator Refrigeration System with Variable Area Ratio Ejector

Experimental study on a multi-evaporator refrigeration system with variable area ratio ejector

Experimental set up

The multi-evaporator refrigeration system (MERS) developed in the present study is schematically shown in Fig. 1. The main components of this test facility are: a BITZER 4cc-9.2 semi-hermetic type of inverter compressor, an air-cooled condenser, two electronic expansion valves (indicated by EEV1 and EEV2, respectively), two testing chambers, a variable area ratio ejector, and a pressure regulating valve (PRV). The well accepted refrigerant R134a is used as the working fluid. All the equipment and pipes in the present system are well insulated by foam rubber to prevent heat losses. The blender is driven by electrical motor and helps to increase the heat transfer rate in the testing chamber. The power controllable heater is applied so that the cooling load in the testing chamber can be adjusted according to the cooling demand. When the temperature reaches stable state in testing chamber, the cooling capacity is just the same with the heater power.

The system can run in two modes, i.e., the conventional mode and the pressure recovery mode. In the conventional mode, the PRV is used and the refrigerant vapor at higher temperature is first throttled and then compressed by the compressor. In the pressure recovery mode, a variable area ratio ejector characterized by its adjustable area ratio akes the place of PRV. Besides keeping the required pressure difference, and the incentive for application of variable area ratio ejector also includes: (a) to get required allocation of cooling capacity by adjusting the entrainment rate of secondary fluid, and (b) to improve the system energy efficiency by the pressure recovery effect of the ejector and by the higher compressor efficiency resulting from the reduced compression ratio.

All the pressures are measured by pressure transducers with the accuracies of 0.5% of full scales. The temperatures are measured by PT1000 platinum resistance with an error of ±0.3 °C. The flow rates are measured by two metal tube rotameters: one is mounted before EEV1 for refrigerant passing through the high temperature evaporator, and the other is located before EEV2 for low-temperature evaporator. Each rotameter has an accuracy of ±1.6%. The output signals from the measurement devices are transferred to a PC through a data acquisition board.

Effect of ejector primary fluid state on the performance of MERS

It is an important feature of the R134a T-S coordinate diagram that its specific entropy decreases with the rising temperature along the saturated vapor line. This phenomenon could make quite an impact on the performances of ejector and MERS. In this section, superheat degree is considered as an indicator of primary flow physical state, and the dependencies of both system and ejector performances on superheat heat degree are analyzed in order to get a better understanding.

1. System performance

The cooling capacity varies with the physical state of primary fluid. The physical state of ejector primary flow changes from saturated state to superheated state when the ethylene glycol solution in the high-temperature evaporator is subjected to a sustainable temperature rise. In this process, the cooling capacities of both evaporators are strongly influenced. Fig. 5(a) shows a typical view of the cooling capacity variations with primary flow superheat degree, and the ejector geometry involved has a spindle position of 5 mm and an area ratio of 6.25. The first and most obvious feature of these two cooling capacity curves is that the cooling capacity keeps almost constant as the superheat degree varies except close to the saturated point where drastic variation is observed.

To be specific, the cooling capacity of the high-temperature evaporator (Evaporator 1) drops from 2.4 kW to 1.9 kW when the superheat degree increases from 0 to 2°C, and then keeps around 2.0 kW when superheat degree is further increased. The power consumption of compressor varies in a way similar to the cooling capacity of Evaporator 1.

1. Ejector performance

The ejector performance is evaluated by the entrainment ratio which is defined as:

where is the mass flow rate of primary fluid, and is the mass flow rate of secondary fluid. As shown in Fig. 5(b), the entrainment ratio is strongly influenced by the superheat degree of primary flow.

Similar variation of entrainment ratio to superheat degree are also founded for x = 0, 5, 10 (see Fig. 5(c)). It should be noted that the rapid increase of entrainment ratio is caused by the decreasing and the increasing In other words, it is favorable to keep the primary flow under superheat condition from the viewpoint of exergy and energy quality.

Considering the different behavior of MERS and ejector, it is convenient to divide the primary flow states into two categories: the saturated state and the superheated state. The former one refers to the primary flow states close to the saturation curve and has a superheat degree less than 2 C. And the latter refers to the primary flow states with a certain degree of superheat, namely, the dry–vapor state. The differences of system and ejector performances are fairly obvious between superheated and saturated conditions, and can be observed for different ejector configurations.

At saturated primary state, variation of superheat degree causes rapid changes of cooling capacity, power consumption and entrainment ratio. While at superheated primary state, the effect of superheat degree is relatively small. This may be resulting

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