High Performance Scroll Compressor With Liquid Refrigerant Injection

Improvement of compressor efficiency using the alternative refrigerant is demanded strongly to reduce CO2 emission, which causes the global warming. Especially the compressor using in frozen temperature level has long lifetime, and the correspondence to above-mentioned is one of the most important subjects. The scroll compressor is the promising structure to solve this problem. A technical point is the controlling mechanism of the discharge gas temperature. In HCFC-22 scroll compressor, the injection system of the liquid refrigerant to the compression chambers is put in practical use. The numerical calculation program for the performance simulation using the alternative refrigerant was created to develop the new scroll compressor with the liquid injection system. By using this simulation, the development period can be shortened in the conventional half. NOMENCLATURE Cp: Isobaric specific heat (J/kg/K) h: Specific enthalpy (J/kg) G: Mass flow rate (kg/s) T: Temperature (K) H: Latent heat (J/kg) m: Mass (kg) V: Volume (m) v: Specific volume (m/kg) Q: Heat (W) E: Energy (W) INTRODUCTION Recently the protection of environment from the ozone layer destruction by emitting HCFC and the global warming by emitting CO2 has been promoted in the world. The new compressors operated with the alternative refrigeration HFC have been developed by manufacturers. The improvement of their efficiency is required strongly to save the power and to reduce CO2 emission. In this respect, the choice of the compressor type is very important and the most promising is a scroll-type compressor. It offers the higher level of efficiency and reliability. Compared to other type compressors, scroll compressors have lower vibration and noise level. Therefore, they have been applied widely to air conditioning and refrigeration. They are also used in the more demanding applications such as refrigeration and high discharge heating conditions. Moreover, the compressor using in frozen temperature level has long lifetime, and developing the scroll compressor for low temperature can contribute to the solutions of the above-mentioned problems. Its technical point is the controlling the discharge gas temperature. The discharge gas temperature at the high compression ratio is much higher than that at the normal one without controlling the temperature. This can cause the chemical degradation of the oil and refrigerant and the thermally induced mechanical failure. In HCFC-22 scroll compressor, the injection system of the liquid refrigerant to the compression chambers is put in practical use. So, the system was also applied to the development of the new scroll compressor for low temperature using the alternative refrigerant. The experimental works have been carried out to improve the reliability of the compressor by lowering the discharge temperature with the liquid refrigerant injection. Hirano et al. [1] investigated the characteristics of the scroll compressor under liquid refrigerant injection to the compression chamber and under liquid bypass to the suction line. Dutta et al. [2] investigated the influence of liquid refrigerant injection on the performance of a scroll compressor. The numerical calculation program of the performance simulation for the scroll compressor corresponding to the injection system using the alternative refrigerant was created to develop the new scroll compressor. By using the simulation, the development period of the new scroll compressor for low temperature can be shortened in the conventional half. This paper shows the analysis model of the liquid injection. The efficiency characteristics of the scroll compressor and the experimental verification are also described. STRUCTURE OF THE SCROLL COMPRESSOR FOR LOW TEMPERATURE The newly developed scroll compressors for low temperature are used in the refrigeration unit for cargo container shown in figure 1. Described previously, scroll compressors have the advantages in efficiency and vibration, and that is why they are often adapted. The detail of the their characteristics are as follows. The scroll compressor has less dead volume than the other type. When driven at the higher compression ratio than the geometrically designed one, the compression power increases due to the re-compression work. The scroll compressor with less dead volume has the advantage in decreasing the re-compression loss. Because the scroll compressor does not need a suction valve, the loss in the suction process is small. In the scroll compressor, the compression process is much smoother and the several pairs of compression chambers are formed simultaneously. Therefore, the fluctuation of the torque in the scroll compressor is smaller than that in the other types, which leads to low vibration and low noise level. Figure 2 shows the cross-sectional view of the scroll compressor. The liquid refrigerant injection was applied in order to cope with the operation at the low temperature, which cause the excessively high discharge gas temperature without the control. Figure 3 shows the refrigeration cycle with liquid refrigerant injection. The normal refrigeration cycle is 1-2-3-4-5-1 and the ideal refrigeration cycle with liquid refrigerant injection is 1-2-2’-3’-4-5-1 in figure 3. Liquid refrigerant is obtained from the point 6 and is injected to the compression chambers at the point 2 during the compression process. After mixing, the temperature in the compression chambers gets lower and the state becomes the point 2’. Therefore, the end of the compression process moves from the point 3 to the point 3’. This is the ideal case in which all amount of the liquid refrigerant is injected to the compression chambers instantly. The amount of the liquid injection varies due to the difference of the pressure between the compression chamber and injected liquid and then the pressure changed smoothly after liquid injection. ANALYSIS MODEL OF THE LIQUID INJECTION Experimental and theoretical works have been carried out on the liquid refrigerant injection. Yanagisawa et al investigated the performance of a scroll compressor under liquid refrigerant injection [2,3]. Figure 4 (a) shows their analysis model. The amount of the vaporization was determined by considering the heat transfer from the high temperature gas to the liquid refrigerant injected into the compression chamber. The evaporation time was considered and the amount of the vaporization was obtained by solving the energy equations for both the vapor and the liquid refrigerant in each time step. Hirano et al. [1] also investigated the operating characteristics of the scroll compressor with liquid refrigerant injection experimentally and theoretically. In their evaporation model, the heat transfer was dealt with around the surface of the liquid refrigerant drop and the evaporation time was considered. In this paper, the prediction of the compressor performance and the discharge gas temperature has been carried out by using the easier analysis model in order to shorten the calculation time. The applied analysis model is shown in figure 4 (b). In the model, the liquid refrigerant is assumed to evaporate immediately after injection. In the analysis, the heat transfer coefficient, which is difficult to be obtained from the fundamental experiments, is not needed. The energy of the liquid and vapor refrigerant is changed to the energy of the cooled vapor refrigerant and the vapor refrigerant obtained by the evaporation of the liquid refrigerant. The energy equation can be obtained as follows. H G T G G C T G C T G C inj gas inj pg gas gas pg inj inj pl + + = + ) ( (1) where the subscripts in the equation are; l: liquid phase g: gas phase inj: liquid refrigerant gas: gas refrigerant By using specific enthalpy, the following can be obtained.