Effects of Refrigerant Injection on the Scroll Compressor

Purdue University Purdue e-pubs International Compressor Engineering Conference School of Mechanical Engineering 2006 Effects of Refrigerant Injection on the Scroll Compressor Baolong Wang Xianting Li Wenxin Shi Qisen Yan Follow this and additional works at: http://docs.lib.purdue.edu/icec Wang, Baolong; Li, Xianting; Shi, Wenxin; and Yan, Qisen, "Effects of Refrigerant Injection on the Scroll Compressor" (2006). International Compressor Engineering Conference. Paper 1770. http://docs.lib.purdue.edu/icec/1770 This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html

C091, Page 1 Effects of Refrigerant Injection on the Scroll Compressor Baolong WANG 1 *, Xianting LI 2, Wenxin SHI 3, Qisen YAN 4 1 Department of Building Science,, Beijing 100084 P.R.China Phone: +86-10-62785860 Fax: +86-10-62773461 Email: Wangbl2004@gmail.com 2 Department of Building Science,, Beijing 100084 P.R.China Phone: +86-10-62785860 Fax: +86-10-62773461 Email: Xtingli@tsinghua.edu.cn 3 Department of Building Science,, Beijing 100084 P.R.China Phone: +86-10-62796114 Fax: +86-10-62773461 Email: Wxshi@mail.tsinghua.edu.cn 4 Department of Building Science,, Beijing 100084 P.R.China Phone: +86-10-62785860 Fax: +86-10-62773461 Email: Yanqs@tsinghua.edu.cn ABSTRACT Based on simulations and experiments, the effects of refrigerant injection on the general performance and inner compression process of scroll compressor has been researched. As a result, it is found that the refrigerant injection process can be considered as a continual parameter-varying adiabatic throttling + isostatic mixture time-varying process. The indicated efficiency of the injection scroll compressor will acquire the maximum when the ratio of inner compression ratio and outer compression ratio is a right value. When the compression is an isentropic process, the value is 1. The effects of injection pressure on the power, refrigerant flow rate, p-h diagram, volume efficiency, and indicated efficiency is studied detailedly. 1. INTRODUCTION The scroll theory was brought forward at the beginning of the previous century and the scroll compressor became more and more popular recently because of its unique advantages, such as low level of noise, high efficiency and high reliability. Technology of refrigeration injection has been applied in scroll compressor widely in recent years. For low temperature heat pump system and high condensing temperature refrigeration system, refrigerant injection can not only increase capacity and EER/COP of the system but also decrease the discharge temperature. Because of the variation of the inner pressure in injection process, the injection processes of screw and scroll compressors are more complex than that of two/multi-stage compression, such as the injection process of the centrifugal compressor. According to the state of the injected refrigerant, the refrigerant injection system of the scroll compressor can be divided into liquid injection system and gas injection system. To review the published papers (Beenon and Pham, 2003; Ding et al., 2004; Ma and Chai, 2004; Ma et al., 2003) about refrigerant injection, most researches focuses on the effects of injection parameters, such as injection pressure, on the general performance of heat pump or refrigeration system, such as COP, EER, heating capacity and discharge temperature. The research results can not display the effect of refrigerant injection on the inner compression and are hard to reveal the essence of refrigerant injection, which make it difficult to use those results to direct the optimal design of the injection compressor and injection system.

C091, Page 2 This paper intends to develop a general model of scroll compressors and setup a test plant, which can measure both the dynamic pressure in working chamber and system performance. Use it and the effects of refrigerant injection on the general performance and inner compression process of scroll compressor has been researched detailedly. 2. RESEARCH METHODS Measuring the dynamic pressure in the compression pocket is the fundamental and critical work of the experimental research on refrigerant injection system. Because the scroll is rotated in a high frequency (30Hz~100Hz), which lead to the pressure in the measuring pocket fluctuating quickly, the sensor must have a good dynamic performance to follow the variation of the pressure. So every component in the measurement system of the dynamic pressure must be seriously considered, include pressure-leading system, pressure sensor, signal transducer, detecting circuit, filter and so on. Before all of them, the power spectral density (PSD) of the pressure signal must be analyzed to decide the maximum effective frequency of the signal. The installation position of pressure sensors is effected by both measuring range of the sensor and the installation space. Figure 1 shows the real measuring positions in this test. The measuring system of dynamic pressure is illuminated in Figure 2. Suction port 1# Sensor 4# Sensor 2# Sensor 3# Sensor Cable Shielded pipe Transducer Sensor Shell Fixed scroll Discharge port Figure 1 Location of measurement ports Figure 2 Design of the sensor configuration Figure 3 shows the schematic of the system experimental bench. Two tube-in-tube heat exchangers are used as evaporator and condenser, respectively. A mass flow meter is installed in the liquid line of injection tube to measure the injection refrigerant flowrate. A plate heat exchanger is equipped between the evaporator and the condenser to reduce the cooling and heating requirement of bench to service system. A lot of static pressure sensors and thermocouples are fixed in the bench to measure the stable system performance. HX T111 P202 T107 T112 P203 Comp. T110 P201 T117 P206 T108 Condenser T106 T113 EEV Mass flow rate meter T114 P204 T120 T115 T116 P205 T109 Evaporator Outer cooling source Heater Hot water bulk Filter Samples Inter. HX TEV Cool water bulk Heater Glycol pump Temperature&Pressure Temperature Figure 3 Schematic of the test bench

C091, Page 3 A distributed parameter model of an injected scroll compressor is built to research the influence of refrigerant injection on the compressor performance. Compared to the other published models of the scroll compressor (Cho et al., 2003; Dutta et al., 2001; Park et al., 2002), the unique advantage of this model is that this model can be used in the dry or wet compression as well as refrigerant injection or refrigerant release. The detailed information about this numerical model can be found in the further papers. 3. RESULTS AND DISCUSSION 3.1 Characteristics of Refrigerant Injection In previous researches (Jonsson, 1991; Winandy and Lebrun, 2002), the injection process of the scroll compressor or the screw compressor always is considered as an instantaneous process. Under this assumption, the injection process is treated as an isostatic or isometric mixture. In actual, for the scroll compressor, the refrigerant injection process can not be finished in a short period in the geometrical viewpoint, it will last 3π/2~2π. The experiments and simulations also certify this point. Fig.5 presents the experimental results and the numerical results of dynamic pressure in the working pocket of scroll compressor under an injection condition. The evaporating pressure is 0.33 MPa, the condensing pressure is 1.51 MPa and the injection pressure in 0.65 MPa. It can be seen that the effect of the refrigerant injection on the pocket pressure is long-time and the injection does not make sudden change in the inner compression process. 1.7 Pressure/(MPa) 1.3 0.9 No injection, experimental No injection, numerical Injection, experimental Injection, numerical 0.5 0.1 0 200 400 600 800 1000 1200 Rotating angle/( o ) Figure 4 Pocket pressure VS. rotating angle 410 2 Temperature/(K) 370 330 290 No injection Injection Refrigerant mass/(10-3 kg) 1.6 1.2 0.8 0.4 No injection Injection 250 0 200 400 600 800 1000 1200 Rotating angle/( o ) (a) Pocket temperature VS. rotating angle 0 0 200 400 600 800 1000 1200 Rotating angle/( o ) (b) Pocket mass VS. rotating angle Figure 5 Influence of refrigerant injection on the pocket parameters

C091, Page 4 Fig.5 shows the influence of refrigerant injection on the other parameters in working pocket. The temperature and the mass also change slowly in the whole injection process (3π/2~2π). To sum up, the refrigerant injection process of the scroll compressor will last a long time and it has an obvious timevarying characteristic. For the scroll compressor, the injection ports must be very small to avoid the leakage between adjacent pockets. The injection ports have a very obvious choking effect, which make the injected refrigerant almost has the same pressure when it enters the pocket. After that, the injected refrigerant and the original refrigerant in the pocket will mixed under the equal pressure condition. Because the pressure in the working pocket and the injected refrigerant mass flowrate will vary in the injection process, the injection process of scroll compressor should be considered as a continual parameter-varying adiabatic throttling + isostatic mixture process. The schematic thermodynamic process is in Fig.6: the refrigerant outside of the injected port (A) is adiabatic throttled by the injection port when it passes it and reaches (C), which has a almost same pressure with the refrigerant in the pocket (B) at this time. B and C is mixed at the same pressure condition to get the final station D. After that, the scroll is rotated to compress the D to B1. At this time, the refrigerant A is throttled to C1 and mixed with B1 to get D1. This process will go on until the injection process finishes. p Figure 6 Schematic of refrigerant injection process h Finally, an important conclusion is gotten: the refrigerant injection process is a continual parameter-varying adiabatic throttling + isostatic mixture time-varying process. 3.2 Effects on the Performance of the Scroll Compressor The factors effecting refrigerant injection system include injected refrigerant states, parameters of injection ports and compressor characteristics. The injected refrigerant states refer to the injection pressure (intermediate pressure) and injection enthalpy. The parameters of injection ports consist of area of injection ports, position of ports and one-way valve. For a preset compressor, the compressor characteristic mainly refers to the speed of the compressor. To do the research of refrigerant injection on the compressor performance, these factors should be considered one by one. Only the effect of the injection pressure on the performance is introduced here because of the limitation of the paper length, the effects of the other factors will be presents in future publications. Fig.7 illuminates the influence of injection pressure on the compressor performance by simulation. Fig.7(a) presents the p-v curves under different injection pressures when the evaporating temperature is -10. It can be seen that the polytropic factor increases with the increase of the injection pressure and the polytropic factor will decrease when the injection is reverse. Fig.7(b) shows the relative power of compressor under different injection pressures. The relative power enhances with increase of the intermediate pressure but the speed of increase goes down gradually due to decrease of the injection mass flowrate. When the injection is reverse, the power consumption of the compressor will smaller than uninjected system. The p-h curves is showed in Fig.7(c). Because the enthalpy of injected refrigerant is smaller than the one in the pocket, the discharge enthalpy decreases with the increase of the

C091, Page 5 injection pressure, which makes the discharge temperature decrease and keep the compressor in safe working range. Fig.7(d) is the variation of the volumetric efficiencies of the scroll compressor under different injection pressure. It can be seen that the external leakage increases with the increase of the injection pressure and it reduces the volumetric efficiency. Fig.7(e) presents the indicated efficiencies of compressor under different injection pressure. For different working conditions, the effects of the injection pressure on the indicated efficiencies is quite different: for low evaporating condition, the indicated efficiency increases following the increase of the injection pressure; for the high evaporating condition, the indicated efficiency will decreases with the increase of the injection pressure. Reorganize the indicated efficiency as a function of the ratio of inner compression ratio and outer compression ratio (Fig.7(f)), it s easy to found the indicated efficiency reaches the summit when the ratio of inner compression ratio and outer compression ratio is between 0.78~0.83. p /(MPa) 2.4 2.0 1.6 1.2 0.8 0.4 0.0 p=0.5 MPa p=0.7 MPa p=0.9 MPa p=1.1 MPa p=1.3 MPa No injection 0 20 40 60 80 100 V /(cm 3 ) (a) p-v(te=-10 ) Power/No injection power 1.3 1.2 1.1 1.0 0.9 0.8 1.04 Te=-20 0.5 0.7 0.9 1.1 1.3 Injection pressure/(mpa) (b) relative power η v /No injection η v 1.02 1.00 0.98 0.96 Te=-20 0.94 0.5 0.7 0.9 1.1 1.3 Injection pressure/(mpa) (c) p-h(te=-10 ) (d) relative volumetric efficiency η i /No injection η i 1.10 1.05 1.00 0.95 Te=-20 η i /No injection η i 1.12 1.08 1.04 1.00 0.96 Te=-20 0.90 0.5 0.7 0.9 1.1 1.3 Injection pressure/(mpa) (e) relative indicated efficiency 0.92 0.5 0.7 0.9 1.1 Inner compression ratio/outer compression ratio (f) relative indicated efficiency Figure 7 Effects of injection pressure on the compressor performance

C091, Page 6 Actually, the key point of increase the indicated efficiency is decease the loss of under- or over- compression. Here, the optimal ratio of inner compression ration and outer compression ratio for injection system is brought forward in Equation (1). R p p p dis,, i opt suc dis,, i opt popt, = = (1) pdis psuc pdis Rp,opt stands for the mend situation of refrigerant injection to under- or over- compression and can be used as the index of compressor performance. For the compression process is or is close to an isentropic process, Rp,opt is 1. 4. CONCLUSION Based on simulations and experiments, the effects of refrigerant injection on the general performance and inner compression process of scroll compressor has been researched. As a result, it is found that the refrigerant injection process can be considered as a continual parameter-varying adiabatic throttling + isostatic mixture time-varying process. The indicated efficiency of the injection scroll compressor will acquire the maximum when the ratio of inner compression ratio and outer compression ratio is a right value. When the compression is an isentropic process, the value is 1. The effects of injection pressure on the power, refrigerant flow rate, p-h diagram, volume efficiency, and indicated efficiency is studied detailedly. REFERENCE Beenon W L, Pham H M. Vapor-injected scroll compressor. ASHRAE Journal, 2003, 45(4): p. 22-27 Cho H, Chung J T, Kim Y. Influence of liquid refrigerant injection on the performance of an inverter-driven scroll compressor. International Journal of Refrigeration, 2003, 26: p. 87-94 Ding Y J, Chai Q H, Ma G Y, et al. Experimental study of an improved air source heat pump. Energy Conversion and Management, 2004, 45: p. 2393-2403 Dutta A K, Yanagisawa T, Fukuta M. An investigation of the performance of a scroll compressor under liquid refrigerant injection. International Journal of Refrigeration, 2001, 24: p. 577-587 Jonsson S. Performance simulation of twin-screw compressors with economizer. International Journal of Refrigeration, 1991, 14: p. 345-350 Ma G Y, Chai Q H. Characteristics of an improved heat-pump cycle for cold regions. Applied Energy, 2004, 77: p.235-247 Ma G Y, Chai Q H, Jiang Y. Experimental investigation of air-source heat pump for cold regions. International Journal of Refrigeration, 2003, 26: p. 12-18 Park Y C, Kim Y, Cho H. Thermodynamic analysis on the performance of a variable speed scroll compressor with refrigerant injection. International Journal of Refrigeration, 2002, 25: p. 1072-1082 Winandy E L, Lebrun J. Scroll compressors using gas and liquid injection: experimental analysis and modeling. International Journal of Refrigeration, 2002, 25: p. 1143-1156 ACKNOWLEDGEMENT Financial assistance from Beijing HVAC Principle Laboratory s Open Thesis Program is greatly appreciated.