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Experimental investigation on cold startup characteristics of a rotary compressor in the R290 air-conditioning system under cooling condition
Accepted Manuscript Title: Experimental investigation on cold startup characteristics of a rotary compressor in the r290 air-conditioning system under cooling condition Author: Jianhua Wu, Jie Lin, Ze Zhang, Zhenhua Chen, Jing Xie, Jun Lu PII: DOI: Reference:
S0140-7007(15)00371-0 http://dx.doi.org/doi: 10.1016/j.ijrefrig.2015.11.009 JIJR 3201
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
30-7-2015 30-9-2015 25-11-2015
Please cite this article as: Jianhua Wu, Jie Lin, Ze Zhang, Zhenhua Chen, Jing Xie, Jun Lu, Experimental investigation on cold startup characteristics of a rotary compressor in the r290 airconditioning system under cooling condition, International Journal of Refrigeration (2015), http://dx.doi.org/doi: 10.1016/j.ijrefrig.2015.11.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation on cold startup characteristics of a rotary compressor in the R290 air-conditioning system under cooling condition Jianhua Wu a, *, Jie Lin a, Ze Zhang a, Zhenhua Chen a, b, Jing Xieb, Jun Lub a School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China b Guangdong Meizhi Compressor Co. Ltd, Guangdong 528333, PR China *
Corresponding author. Tel.: +82 029 8266 3786, +82 13689206050. E-mail address: [email protected] (Jianhua Wu). Postal address: No. 28, Xian-ning West Rd., Xi’an City 710049, Shaanxi, PR China. Highlights
1. Cold startup characteristics of a rotary compressor in the R290 air-conditioning system were experimentally investigated 2. Pressures and temperatures in the R290 air-conditioning system during startup process were measured. 3. Temperatures in the rotary compressor shell and the pressure in cylinder during cold startup process were measured 4. The oil viscosity and oil level of the oil sump in the rotary compressor during cold startup process were also considered 5. The solubility of R290 in the oil during cold startup was discussed. abstract：R290 is one of the most alternative refrigerants for the air-conditionings for its negligible environmental impact and high efficiency. This study experimental investigated the cold startup characteristics of the rotary compressor in a R290 air-conditioning system under cooling condition. The characteristics include the pressures and temperatures in the system and the rotary compressor respectively, the mixture of oil and refrigerant viscosity and oil level of the oil sump. The measurements showed that the startup time for the pressures and the temperatures were much longer than that of R410A and R22 systems. A slight liquid slugging happened in the cylinder at the initial time of startup for the pressure during the later exhaust process increasing rapidly to 2.21MPa in 1.3 second. After startup of the system, both the mixture of oil and refrigerant viscosity and oil level of the oil sump in the compressor were within a proper range to guarantee a steady startup of the air-conditioning system. keywords ： R290; startup characteristics; rotary compressor; experimental investigation; air-conditioning system 1. Introduction With the increasing attention to environmental problems such as global warming, ozone depletion and atmospheric pollution, a large number of studies related to the selection of environmentally friendly refrigerants as working fluids were conducted in the air-conditioning industry in recent years. Most of CFCs and HCFCs as short-term alternative refrigerants, more or less, do harm to the environment or have thermophysical properties. For example, the alternative refrigerant
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R410A and R32 have high global warming potential. Current research and industry trends show that HCFCs and HFCs will gradually be replaced by natural refrigerants such as CO2, R290, NH3 and so on. R290 as the working fluid used in small heat pumps, room air-conditionings and other commercial refrigeration equipment is considered as one of excellent long-term alternative refrigerants owing to its negligible environmental impact and excellent thermosphysical properties, what’s more, it can help to reduce the electric energy consumption of the system(Joudi & Al-Amir, 2014; Palm, 2008; Zhou & Zhang, 2010). Owing to the flammability of R290, it is necessary to control the refrigerant charge amount in the air-conditioning system(Padalkar, Mali, & Devotta, 2014). A number of studies have been conducted on the R290 charge amount in room air-conditionings in recent years. (Fernando, Palm, Lundqvist, & Granryd, 2004) designed a system to minimize the charge of R290 mainly by use of mini-channel aluminium heat exchangers. And the results showed that the system could be run with 200 g of propane at typical Swedish operating conditions without reduction of the COP compared to a traditional design. (Hrnjak & Hoehne, 2004) found that it is possible to create a system that can use less than 150 g of charge and produce between 1 kW and 2 kW of cooling capacity by using microchannel heat exchanger technology. (Li et al., 2015) experimentally investigated the R290 mass distributions in a split type air condition by the liquid nitrogen method (LNM). Much research on the performances of R290 systems has also been carried out. (Devotta, Padalkar, & Sane, 2005) experimentally studied an original R22 window air-conditioning with R290 as refrigerant, and they found that The COP of the R290 system was 7.9% higher under the lower operating conditions and 2.8% higher under the higher operating conditions. (J. Wu, Yang, & Hou, 2012) experimental study the performance of a small wall room air-conditioning retrofitted with R290. The results showed that, with the decrease of the outdoor temperature, alternative systems have higher increase rate and greater increment in both cooling capacity and EER than the original R22 system. (Tian et al., 2015) theoretically and experimentally investigated the performance of the air conditioners working with both R32/R290 and R410A. Experimental results show that the refrigerant charge amount of R32/R290 is reduced by 30.0% to 35.0%; the cooling and heating capacities are increased by 14.0% to 23.7%.(Cai et al., 2015) explored the leakage characteristics of R290 in rolling piston type rotary compressor. And the results showed that, to obtain relatively high efficiency, smaller radial clearance was required for R290 compressor comparing to that of R22 and R410A compressors under the same condition. As well know, to keep the temperature in the rooms unchanged, a great many of air-conditionings have to turn on and off frequently, which will lead the air-conditionings working in the unstable state. Therefore, Startup characteristics of air-conditioning systems are very important and have been investigated by many researchers. T. Yanagisawa develops a mathemat1cal model of a rotary compressor which can predict its transient behavior and can be easily integrated into a simulation program of a heat pump cycle(Yanagisawa, Shimizu, Fukuta, & Suzuki, 1990). For the transient characteristics during speed up of inverter heat pump, (Hwang & Kim, 1998)carried out that the transient cycle migration of the liquid state refrigerant caused significant dynamic change in system. (Kim & Bullard, 2001) experiment studied the dynamic characteristics of a single speed R410A split air-conditioning system during shut-down and start-up. (Kapadia, Jain, & Agarwal,
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2009) studied the transient characteristics of split air-conditionings that use R22 and R410A as refrigerants by simulation methods, and measured and published experimental data were used to validated the program. (Andrade & Negrão, 2013) proposed a semi-empirical model applied to domestic refrigeration systems for devising real time predictions of test results. The results showed that the model can predict fairly well the results within the calibration period and that the model accuracy increases with the calibration time. The liquid slugging consequently threaten the reliability of the compressor as the cylinder pressure could shoot to very high value(Singh, Nieter, & Prater Jr, 1986), which often occurs when liquid refrigerant enters the compressor or during the cold startup process. Thus the liquid slugging in the compressors has been studied by many investigators. (Palm, 2008; Tadashi, Takashi, & Mitsuhiro, 1985) experimentally examined liquid compression characteristics at starting in a sliding vane type rotary compressor. (Liu & Soedel, 1994) presented a mathematical model to simulate the compression processes of two-phase saturated refrigerant mixture and analyzed the factors leading to slugging. (Dutta, Yanagisawa, & Fukuta, 2001; Park, Kim, & Cho, 2002)investigated the performance of a scroll compressor under liquid refrigerant injection. To identify slugging-induced overpressures in reciprocating compressors, (Laughman;, Foy;, Wichakool;, Armstrong;, & Leeb;, 2008)proposed a method by analyzing the electrical power flowing into the compressor motor. Though much research has been devoted to the performances of R290 air-conditioning systems and dynamic characteristics and liquid slugging of the rotary compressor (Devotta et al., 2005; Kapadia et al., 2009; Kim & Bullard, 2001; Liu & Soedel, 1994; Tadashi et al., 1985; Tian et al., 2015) , little attention has been paid to the startup characteristics of the rotary compressor in R290 air-conditioning systems. As the saturated vapor line of R290 was closed to the line of compression process in the P-h graph, the liquid slugging is likely to happen during the compression process. Especially, during the cold startup process, the temperature of the cylinder is so low as to make the vapor refrigerant in the cylinder into liquid by transferring heat from refrigerant to cylinder or exhaust valve. In this study, an experimental investigation was done to characterize the cold startup behaviors of a rotary compressor in a R290 air-conditioning system. This study aims to present the variations of temperatures and pressures in the R290 air-conditioning system and the rotary compressor under standard cooling conditions to identify the happening of the liquid slugging. Meanwhile, the mixture of oil and refrigerant viscosity and the oil level of the oil sump during startup of the system were studied as to confirm if the lubrication of the rotary compressor could be guaranteed during cold startup process. Through this study, we hope that it could provide some useful data for the promising development of the R290 air-conditioning system with a rotary compressor in the future.
2. Experimental setup and test procedure
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2.1 Experimental unit /apparatus The object of the present experiment was to investigate the startup characteristics of the R290 rotary compressor and its corresponding air-conditioning system, and thus to provide fundamental information for the application of R290 in the refrigeration industry. The schematic diagram of experimental unit is shown in Fig.1. It is composed of a wall room air-conditioning system and a psychometric chamber. The air-conditioning system fundamentally consisted of a rotary compressor, a condenser, an evaporator, a manual expansion valve and such attachments as an accumulator and fans. The specifications of heat exchangers and a inverter rotary compressor typed with are shown in Table 1 and Table 2 respectively. The psychometric chamber consists of two separate constant temperatures and humidity rooms the indoor side and outdoor side. Two air handling units are installed in both rooms to guarantee the temperature and humidity exactly satisfying the strict requirements in experiments. The dry and wet bulb temperatures of air entering the experiment units are maintained within ±0.2℃, and the accuracies of thermocouples in both rooms are maintained within ±0.1℃. Meanwhile, an air measurement device is installed in indoor side to measuring the temperature, air mass flow rate and other parameters. To investigate the startup characteristics of the R290 rotary compressor and its corresponding air-conditioning system, pressures and temperatures tests in the compressor and the system are necessary. The pressures and temperatures in the system were measured by some pressure transducers and T-type thermocouples fixed at proper sites as shown in Fig.1. The accuracy of pressure transducers is ±0.25%FS and the accuracy of thermocouples is ±0.5 K. Furthermore, some sensors are installed in the compressor to measure pressures and temperatures in the compressor, as shown in Fig.2, thus the shell and some other parts of the original compressor were modified. It is considered that the modification does not influence the performance of the compressor to a notable extent. The temperatures in the compressor are measured by 5 K-type sheathed thermocouples installed in the shell of compressor as shown in Fig.2(a). The pressures in the suction chamber or compression chamber of the compressor are measured by two minitype piezoresistive dynamic pressure sensors XTL-190M produced by Kulite Semiconductor Products, Inc. The installation positions of the pressure sensors are shown in Fig.2(b). Pressure sensor #1 was installed beside the suction port and formed a 142°angle to the center line of the vane groove. Pressure sensor #2 was installed on the discharge side and formed a 10°angle to the center line of the vane groove. The linearity of pressure sensors are less than ±0.5%. A data acquisition system PXI-6255 of NI Company (resolution 16 bits; sampling rate 1.25 MS/s) was used to collect the rapidly changing pressure data in the cylinder during the startup process. A VISCO pro 2000 viscometer with a SPL501 probe, made by Cambridge Company of
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American, is installed at the bottom of the rotary, shown in the red dotted lines in Fig.2(a), to measure the variation of mixture of oil and refrigerant viscosity during the startup of the rotary compressor. And the connection layout of the SPL501 probe and rotary compressor was shown in the left of Fig.2(a). The accuracy and relative error of the viscometer are within ±0.1% and ±0.8%, respectively. A sight glass is installed at the side of the rotary compressor shell to observe the variation of the oil level of the oil sump during startup process. To facilitate the observation, a ruler was stuck to side of the sight glass, of which the scale “0” was as high as the upper surface of the cylinder. 2.2 Experimental condition and method The experiments were performed according to the standard cooling capacity rating condition of which the dry and wet bulb temperatures for the indoor and outdoor are 27/19 and 35/24℃, respectively. The lubricant was a kind of mineral oil. The opening of the expansion valve was 376 degree. The air volume flow rate of the air measurement device was 385m3/min, and the operating frequency of the compressor was 61Hz. All the three operating parameters were obtained by an experiment on the inverter compressor operating under the standard cooling capacity rating condition. In order to observe its cold startup characteristics, the air-conditioning system placed into the environment for a long time before the startup until the temperature of the air-conditioning system was approximately equal to environment temperature. The temperatures and pressures in the air-conditioning system were recorded every 5 seconds after startup of the system, and the temperatures measured by the five thermocouples in the rotary compressor was recorded per second. In order to obtain the pressure in the cylinder for each instant of the operating, the sampling rate of one channel for the data acquisition system is 30 kHz in the test. All data-collecting devices and their corresponding transducers were operating before startup of the system until reaching the steady state. 3. Results and discussion A R290 air-conditioning operating with a rotary compressor was selected for the study to investigate the characteristics during cold startup process. The performances of pressures and temperatures in the air-conditioning system and the rotary compressor were investigated respectively. Meanwhile, the mixture of oil and refrigerant viscosity and oil level of the oil sump in the rotary compressor during cold startup process were also discussed. 3.1 Variations of pressures and temperatures
3.1.1 Performances in the air-conditioning system Fig.3 shows the pressures and temperatures characteristics of the air-conditioning system during cold startup process. As shown in the Fig.3(a), after startup of the system, the equilibrium between the high and low pressure side of the air-conditioning was reached in 6.5 minutes owing to the refrigerant movement from the high pressure side to the low pressure side through the expansion valve. The suction pressure dropped to the lowest point 149kPa in 25 seconds then
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gradually increased until reaching the equilibrium state 478kPa. After reaching the lowest point, the outlet pressures of expansion valve and evaporator increased rapidly then gradually increased to the equilibrium state, 513KPa and 688KPa, after a slight decrease. The evaporator outlet pressure was slightly larger than the discharge temperature in the equilibrium state owing to the pressure loss of refrigerant through the evaporator. While the discharge pressure increased rapidly in 10 seconds then they gradually increased after a slight decrease to the equilibrium state 1413KPa. The expansion valve inlet pressure was a little lower than the discharge pressure, and the variation of it was consistent with that of the discharge pressure. As shown in the Fig.3(b), the variations of the suction and discharge temperatures were in close agreement with that of the suction and discharge pressures shown in Fig.3 (a) , but for temperatures the time from startup to the steady state was longer than that of pressures. After startup of the system, the temperatures equilibrium between the high and low pressure side of the air-conditioning was reached in 11 minutes. The discharge temperature increased rapidly in 20 seconds then they gradually increased after a slight decrease to the steady state 59℃. The suction temperature dropped to the lowest point in 20 seconds then they gradually decreased after reaching a transient maximum in 65 seconds until reaching the steady state 15℃. The slower temperatures response could be attributed to the reconditioning of the thermal mass of the system.
The variations of pressures and temperatures in the R290 air-conditioning system agree well with (Kapadia et al., 2009)’s findings that use R22 and R410A as refrigerants. While the time from startup to the steady state was much longer that of R.G. Kapadia’s findings and the suction pressure drops more sharply before gradually increasing than that of R.G. Kapadia’s findings. The differences between the results are possible because that the physical property of R290 is much different from that of R22 and R410A, and the R290 charge in this system was less than that of the R22 and R410A air-conditioning system.
3.1.2 Performances in the rotary compressor a.
Pressures in the cylinder Fig.4 shows the variations of the pressure in the cylinder during cold startup process. From
Fig.4(a), it can be observed that, after startup of the compressor, the pressure in position 1 increased first then gradually decreased. While, as shown in Fig.4(b), after the startup of the compressor the pressure in position 2 increased sharply then it decreased rapidly to 1.2MPa. Subsequently, it remained unchanged after a gently increase. The pressure difference between the discharge and suction process was gradually formed, as Fig.4(b) shows. In order to further reveal the variations of pressures in the cylinder during cold startup process, the variation of pressures at 5 different points in time in position 2 were shown in Fig.5. Fig.5(a) shows the 5 points in the discharge pressure curve. Figs.5(b)-(f) show the variations of the
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pressure at the 5 different points in time during the startup process. As shown in Fig.5(b), the pressure during the later exhaust process increase rapidly to 2.21MPa in 1.3 second, while the discharge pressure as shown in Fig.5(a) was only 0.91MPa. As shown in Fig.5(c)-(f), with time goes by, the maximum pressure during the later exhaust process was gradually reduced to 1.6MPa, and the discharge pressure increased from 1.18Mpa to 1.41MPa. These results suggest that there were liquid refrigerant or oil at the exhaust port of the cylinder at point 1 in time and the liquid slugging is likely to happen, while the liquid slugging happens just at the initial time of cold startup process in the R290 air-conditioning system with a rotary compressor. A possible explanation for the liquid refrigerant could be as following. The temperatures of the cylinder and the exhaust valve were as cold as environment at the initial time of the cold startup. And he saturated vapor line of R290 was closed to the compression process line in the P-h graph, thus the liquid may formed during the compression process. But as the high inlet specific volume of the R290, the total quantity of the liquid refrigerant was small. Subsequently, with the increase of the discharge temperature, the liquid refrigerant evaporates rapidly by absorbing heat from the cylinder and the exhaust valve.
b. Temperatures in the rotary compressor Fig.6 shows the variations of the temperatures in the compressor after cold startup of the system. The muffler temperatures and the motor stator temperatures increased rapidly in 25 seconds after startup then they gradually increased to the equilibrium state，59.1℃ and 56.5℃ respectively, after 180 seconds. Meanwhile, the cylinder temperature and the suction pipe temperature decreased rapidly in 25 seconds after startup then they gradually increased to the equilibrium state, 54.9℃ and 20.1℃ respectively, after 300 seconds, and the oil temperature remained 36.5℃ unchanged in 120 seconds after startup then it gradually increased to the equilibrium state 50℃ after 500 seconds. It can also be observed that the oil temperature was higher than the cylinder temperature at the beginning of startup thus the heat was transferred from the oil to the cylinder. Then subsequently, the heat was transferred from the cylinder to the oil because the oil temperature was higher than the cylinder temperature. When it operated in steady state, all temperatures except the suction pipe temperature were higher than the temperatures before startup. And the cylinder temperature was lower than the muffler temperature, motor stator temperature and oil temperature owing to the heat transfer from the cylinder to refrigerant. 3.2 Variations of the mixture of oil and refrigerant viscosity and solubility in the rotary compressor Low oil viscosity in the rotary compressor during operating not only increase wear of bearings, but also has a negative influence on generating oil films on bearings, increasing friction losses and the reliability of rotary compressors. The oil viscosity is closely related to the oil molecular structure, temperature and pressure. In addition, it is also greatly affected by the solubility of refrigerant in the oil. So the variations of mixture of oil and refrigerant viscosity and
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the corresponding solubility of the R290 with time after startup were studied. Fig.7 shows the variation of mixture of oil and refrigerant viscosity in the oil sump and the solubility of R290. At the beginning of the startup, the mixture viscosity was as high as 8cP. While it decreased and reached the minimum 1.9cP after 180 seconds. Then it increased gradually to a steady state of 4cP after 500 seconds. The variation of mixture viscosity in this experiment agrees well with(Fukuda;, Hayano;, & Hayano, 1996)’ systems with HFC/POE as refrigerant. However, the minimum of mixture viscosity in this experiment is much higher than that of the HFC/POE refrigerant system. This result suggests that the R290 with the lubricants is beneficial for the reduction of the frictional loss and the oil supply to guarantee a stable startup process. According to the mixture of oil and refrigerant viscosity and the temperature in the oil sump measured before, the variation of R290 solubility in the mineral oil can be referred as show in Fig.7. The variation tendency of the solubility is in contrast with that of mixture viscosity. It can be observed that the solubility of R290 in the mineral oil increased firstly then decreased before it remaining unchanged during the startup. From 0 to 250s, the discharge pressure increased rapidly (see Fig.5(a)),while the oil temperature just slightly increased. Thus the R290 solubility in the mineral oil increased sharply（blue curve in Fig.6). From 250 to 400s, the discharge pressure remained approximately constant, while the oil temperature rapidly increased. As a result, the R290 solubility in the mineral oil decreased sharply. The initial value of the solubility was 15%. The maximum and stable value of solubility was 42% in 250 seconds and 20% in 500 seconds, respectively. This finding appears that the solubility of R290 was negative correlated with the mixture viscosity, and the solubility of R290 in the oil was low before startup of the system. 3.3 Variations of the liquid height in the rotary compressor The oil level of the oil sump has a huge effect on the total oil flow rate(J. H. Wu & Wang, 2013). A drastic change or too low value of the oil level of the oil sump affects the oil supply system, and significantly reduces the reliability of rotary compressors during startup greatly. Fig.8 shows the variation of the oil level of the oil sump in the compressor shell with time during cold startup of the system. The scale “0” of the ruler stuck to the side of the sight glass was as high as the upper surface of the cylinder. The oil level of the oil sump before startup was 9 mm(Fig.8(a)). It can be observed that the oil level of the oil sump decreased firstly with lots of bubble forming on the oil surface in 6 seconds as shown in Fig.8(b). Then it remained the minimum -6mm unchanged from 16 seconds to 76 seconds as shown in Fig.8(c). After that, it increased gradually to a stable value 3 mm after 106 seconds as shown in Fig.8(d). The minimum oil level was -6mm lower than the upper surface of the cylinder, and the stable oil level was 3mm higher than the upper surface of the cylinder. The cylinder height is 24mm, therefore, the oil level of the oil sump is always higher than the height of the lower surface of cylinder, a continuous steady oil supply to the cylinder and other positions during the startup could be guaranteed, which indicates that the R290 air-conditioning system has a good oil return though the viscosity of the mineral oil in the system was high. 4. Conclusions An experimental investigation on cold startup characteristics of a rotary compressor in an air-conditioning system using R290 as refrigerant under cooling condition has been performed. A series of experiment were carried out in a psychometric chamber with constant temperatures and
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humidity. Pressures and temperatures in the R290 air-conditioning system during startup process were measured, and the temperatures in the rotary compressor shell and the pressure in cylinder during cold startup process were also measured by sensors and an acquisition system. Meanwhile, the mixture of oil and refrigerant viscosity and oil level of the oil sump in the rotary compressor during startup process were also discussed. The following conclusions could be made from the experiments: 1)
The variations of pressures and temperatures in the R290 air-conditioning system during
cold startup process in this study agreed well with (Kapadia et al., 2009)’s findings using R22 and R410A as refrigerants. While the time from startup to the steady state was much longer than that of R.G. Kapadia’s findings and the suction pressure in this study drops more sharply before gradually increasing than that of R.G. Kapadia’s findings. 2)
A slight liquid slugging happened at the initial time during startup process in the R290
rotary compressor. But the pressures rapidly decreased after reached the maximum value during the startup process. These indicated that the liquid slugging happened only at the beginning of the startup processes for the R290 rotary compressor, but it is within the bearing capacity of rotary compressor. 3)
At the initial time after startup, the cylinder temperature decreased as the suction
temperature sharply decreased. Meanwhile, the motor stator temperature and the muffler temperature increased rapidly. But the oil temperature kept unchanged before a slow increase in 120second. After reaching steady state, the temperatures in the compressor from the highest to the lowest were the muffler temperature, the motor stator temperature, the oil temperature, the cylinder temperature and the suction pipe temperature. 4)
After startup of the system, the mixture of oil and refrigerant viscosity decreased to a
minimum value 1.9cP in 180 seconds, then it increased gradually to a steady state of 4cP after 500 seconds. The variation of the solubility is in contrast with that of mixture viscosity. The maximum and stable value of solubility was 42% in 250 seconds and 20% in 500 seconds, respectively. 5)
After startup of the system, the oil level of the oil sump decreased firstly then remained
the minimum unchanged for 60 seconds before increasing gradually to a stable value. And a continuous steady oil supply to the cylinder and other positions during the startup could be guaranteed.
Acknowledgement This work is supported by the HCFC phase out management plane in room air conditioning sector technical assistance fund (China).
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Reference
Andrade, D. E., & Negrão, C. O. (2013). Analysis of start-up tests of household refrigeration systems. A potential for on-line predictions of test results. Applied Thermal Engineering, 54(1), 255-263. doi:10.1016/j.applthermaleng.2013.02.008 Cai, D., He, G., Yokoyama, T., Tian, Q., Yang, X., & Pan, J. (2015). Simulation and comparison of leakage characteristics of R290 in rolling piston type rotary compressor. International Journal of Refrigeration, 53, 42-54. doi:10.1016/j.ijrefrig.2015.02.001 Devotta, S., Padalkar, A., & Sane, N. (2005). Performance assessment of HC-290 as a drop-in substitute to HCFC-22 in a window air conditioner. International Journal of Refrigeration, 28(4), 594-604. doi:10.1016/j.ijrefrig.2004.09.013 Dutta, A. K., Yanagisawa, T., & Fukuta, M. (2001). An investigation of the performance of a scroll compressor under liquid refrigerant injection. International Journal of Refrigeration, 24(6), 577-587. doi:10.1016/S0140-7007(00)00041-4 Fernando, P., Palm, B., Lundqvist, P., & Granryd, E. (2004). Propane heat pump with low refrigerant charge: design and laboratory tests. International Journal of Refrigeration, 27(7), 761-773. doi:10.1016/j.ijrefrig.2004.06.012 Fukuda;, T., Hayano;, M., & Hayano, M. (1996). HFC-POE Lubricity Evaluation on the Rotary Compressor in System Operation. International Compressor Engineering Conference at Purdue, 121-126. Hrnjak, P., & Hoehne, M. (2004). Charge minimization in systems and components using hydrocarbons as a refrigerant: ACRC TR-224. Hwang, Y., & Kim, H. (1998). Experimental and theoretical studies on the transient characteristics during speed up of inverter heat pump. Joudi, K. A., & Al-Amir, Q. R. (2014). Experimental Assessment of residential split type air-conditioning systems using alternative refrigerants to R-22 at high ambient temperatures. Energy Conversion and Management, 86, 496-506. doi:10.1016/j.enconman.2014.05.036 Kapadia, R., Jain, S., & Agarwal, R. (2009). Transient characteristics of split air-conditioning systems using R-22 and R-410A as refrigerants. HVAC&R Research, 15(3), 617-649. Kim, M.-H., & Bullard, C. W. (2001). Dynamic characteristics of a R-410A split air-conditioning system. International Journal of Refrigeration, 24(7), 652-659. Laughman;, C. R., Foy;, R. R. L., Wichakool;, W., Armstrong;, P. R., & Leeb;, S. B. (2008). Electrical and Mechanical Methods for Detecting Liquid Slugging in Reciprocating Compressors. International Compressor Engineering Conference at Purdue, 29(8), 143-149. Li, T., Lu, J., Chen, L., He, D., Qiu, X., Li, H., & Liu, Z. (2015). Measurement of refrigerant mass distribution within a R290 split air conditioner. International Journal of Refrigeration, 57, 163-172. doi:10.1016/j.ijrefrig.2015.05.012 Liu, Z., & Soedel, W. (1994). An investigation of compressor slugging problems. Padalkar, A. S., Mali, K. V., & Devotta, S. (2014). Simulated and experimental performance of split packaged air conditioner using refrigerant HC-290 as a substitute for HCFC-22. Applied Thermal Engineering, 62(1), 277-284. doi:10.1016/j.applthermaleng.2013.09.017
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Palm, B. (2008). Hydrocarbons as refrigerants in small heat pump and refrigeration systems–A review. International Journal of Refrigeration, 31(4), 552-563. doi:10.1016/j.rser.2011.10.015 Park, Y. C., Kim, Y., & Cho, H. (2002). Thermodynamic analysis on the performance of a variable speed scroll compressor with refrigerant injection. International Journal of Refrigeration, 25(8), 1072-1082. doi:10.1016/S0140-7007(02)00007-5 Singh, R., Nieter, J., & Prater Jr, G. (1986). An investigation of the compressor slugging phenomenon. ASHRAE Trans, 92, 250-258. Tadashi, Y., Takashi, S., & Mitsuhiro, F. (1985). A study on liquid compression characteristics at starting in a sliding vane type rotary compressor. Transactions of the Japan Society of Mechanical Engineers Series C, 51(472), 4301-4305. Tian, Q., Cai, D., Ren, L., Tang, W., Xie, Y., He, G., & Liu, F. (2015). An experimental investigation of refrigerant mixture R32/R290 as drop-in replacement for HFC410A in household air conditioners. International Journal of Refrigeration , 57(9), 216-228. doi:10.1016/j.ijrefrig.2015.05.005 Wu, J., Yang, L., & Hou, J. (2012). Experimental performance study of a small wall room air conditioner retrofitted with R290 and R1270. International Journal of Refrigeration, 35(7), 1860-1868. doi:10.1016/j.ijrefrig.2012.06.004 Wu, J. H., & Wang, G. (2013). Numerical study on oil supply system of a rotary compressor. Applied Thermal Engineering, 61(2), 425-432. doi:10.1016/j.applthermaleng.2013.07.047 Yanagisawa, T., Shimizu, T., Fukuta, M., & Suzuki, H. (1990). Mathematical Model of Rotary Compressor to Simulate its Transient Behavior. Zhou, G., & Zhang, Y. (2010). Performance of a split-type air conditioner matched with coiled adiabatic capillary tubes using HCFC22 and HC290. Applied Energy, 87(5), 1522-1528. doi:10.1016/j.apenergy.2009.10.005
Fig.1 Schematic diagram of experimental setup
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Fig.2(a) Thermocouples in the compressor
Fig.2(b) Pressure sensors in the cylinder Fig.2 Installation positions of sensors in the compressor
(a) Variations of pressures (b) Variations of the suction and discharge temperature Fig.3 Variations of pressures and temperatures in the air-conditioning system during cold startup process
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（a）Variation of the pressure in position 1 (b) Variation of the pressure in position 2 Fig.4 Variations of the pressures with time in the cylinder
（a）5 different time points in position 2
(c) Variations of pressures at point 2
(b) Variations of pressures at point 1
(d) Variations of pressures at point 3
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(e) Variations of pressures at point 4 (f) Variations of pressures at point 5 Fig.5 Variation of pressures at 5 different points in time in position 2 of the cylinder
Fig.6 Variations of the temperatures in the compressor
Fig.7 Variations of the mixture of oil and refrigerant viscosity and the solubility of R290 during startup
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(a)Before startup
(b) 6 seconds
(c) 20 seconds (d) Steady state Fig.8 variation of the oil level of the oil sump with time
Table 1 Dimensions of heat exchangers Parameters Length (mm) Tube type Number of U-tubes Number of tube rows Total tube length (mm) Tube outside diameter (mm) Fin geometry Fin spacing (mm) Flow path
Heat exchanger Evaporator
Condenser
607 Rifled U-tube 15 2 607 7.00 Louver 1.25 1in 2out
895 Rifled U-tube 12 1 895 7.00 Louver 1.2 3in 3out
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Table 2 specifications of the ratary compressor Parameters
Cylinder diameter/mm
Cylinder height/mm
Crank shaft eccentricity/mm
Stroke volume /cm3
Motor efficiency/%
Values
Φ54
24
5.3
18.1
86.1
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S0140-7007(15)00371-0 http://dx.doi.org/doi: 10.1016/j.ijrefrig.2015.11.009 JIJR 3201
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
30-7-2015 30-9-2015 25-11-2015
Please cite this article as: Jianhua Wu, Jie Lin, Ze Zhang, Zhenhua Chen, Jing Xie, Jun Lu, Experimental investigation on cold startup characteristics of a rotary compressor in the r290 airconditioning system under cooling condition, International Journal of Refrigeration (2015), http://dx.doi.org/doi: 10.1016/j.ijrefrig.2015.11.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation on cold startup characteristics of a rotary compressor in the R290 air-conditioning system under cooling condition Jianhua Wu a, *, Jie Lin a, Ze Zhang a, Zhenhua Chen a, b, Jing Xieb, Jun Lub a School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China b Guangdong Meizhi Compressor Co. Ltd, Guangdong 528333, PR China *
Corresponding author. Tel.: +82 029 8266 3786, +82 13689206050. E-mail address: [email protected] (Jianhua Wu). Postal address: No. 28, Xian-ning West Rd., Xi’an City 710049, Shaanxi, PR China. Highlights
1. Cold startup characteristics of a rotary compressor in the R290 air-conditioning system were experimentally investigated 2. Pressures and temperatures in the R290 air-conditioning system during startup process were measured. 3. Temperatures in the rotary compressor shell and the pressure in cylinder during cold startup process were measured 4. The oil viscosity and oil level of the oil sump in the rotary compressor during cold startup process were also considered 5. The solubility of R290 in the oil during cold startup was discussed. abstract：R290 is one of the most alternative refrigerants for the air-conditionings for its negligible environmental impact and high efficiency. This study experimental investigated the cold startup characteristics of the rotary compressor in a R290 air-conditioning system under cooling condition. The characteristics include the pressures and temperatures in the system and the rotary compressor respectively, the mixture of oil and refrigerant viscosity and oil level of the oil sump. The measurements showed that the startup time for the pressures and the temperatures were much longer than that of R410A and R22 systems. A slight liquid slugging happened in the cylinder at the initial time of startup for the pressure during the later exhaust process increasing rapidly to 2.21MPa in 1.3 second. After startup of the system, both the mixture of oil and refrigerant viscosity and oil level of the oil sump in the compressor were within a proper range to guarantee a steady startup of the air-conditioning system. keywords ： R290; startup characteristics; rotary compressor; experimental investigation; air-conditioning system 1. Introduction With the increasing attention to environmental problems such as global warming, ozone depletion and atmospheric pollution, a large number of studies related to the selection of environmentally friendly refrigerants as working fluids were conducted in the air-conditioning industry in recent years. Most of CFCs and HCFCs as short-term alternative refrigerants, more or less, do harm to the environment or have thermophysical properties. For example, the alternative refrigerant
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R410A and R32 have high global warming potential. Current research and industry trends show that HCFCs and HFCs will gradually be replaced by natural refrigerants such as CO2, R290, NH3 and so on. R290 as the working fluid used in small heat pumps, room air-conditionings and other commercial refrigeration equipment is considered as one of excellent long-term alternative refrigerants owing to its negligible environmental impact and excellent thermosphysical properties, what’s more, it can help to reduce the electric energy consumption of the system(Joudi & Al-Amir, 2014; Palm, 2008; Zhou & Zhang, 2010). Owing to the flammability of R290, it is necessary to control the refrigerant charge amount in the air-conditioning system(Padalkar, Mali, & Devotta, 2014). A number of studies have been conducted on the R290 charge amount in room air-conditionings in recent years. (Fernando, Palm, Lundqvist, & Granryd, 2004) designed a system to minimize the charge of R290 mainly by use of mini-channel aluminium heat exchangers. And the results showed that the system could be run with 200 g of propane at typical Swedish operating conditions without reduction of the COP compared to a traditional design. (Hrnjak & Hoehne, 2004) found that it is possible to create a system that can use less than 150 g of charge and produce between 1 kW and 2 kW of cooling capacity by using microchannel heat exchanger technology. (Li et al., 2015) experimentally investigated the R290 mass distributions in a split type air condition by the liquid nitrogen method (LNM). Much research on the performances of R290 systems has also been carried out. (Devotta, Padalkar, & Sane, 2005) experimentally studied an original R22 window air-conditioning with R290 as refrigerant, and they found that The COP of the R290 system was 7.9% higher under the lower operating conditions and 2.8% higher under the higher operating conditions. (J. Wu, Yang, & Hou, 2012) experimental study the performance of a small wall room air-conditioning retrofitted with R290. The results showed that, with the decrease of the outdoor temperature, alternative systems have higher increase rate and greater increment in both cooling capacity and EER than the original R22 system. (Tian et al., 2015) theoretically and experimentally investigated the performance of the air conditioners working with both R32/R290 and R410A. Experimental results show that the refrigerant charge amount of R32/R290 is reduced by 30.0% to 35.0%; the cooling and heating capacities are increased by 14.0% to 23.7%.(Cai et al., 2015) explored the leakage characteristics of R290 in rolling piston type rotary compressor. And the results showed that, to obtain relatively high efficiency, smaller radial clearance was required for R290 compressor comparing to that of R22 and R410A compressors under the same condition. As well know, to keep the temperature in the rooms unchanged, a great many of air-conditionings have to turn on and off frequently, which will lead the air-conditionings working in the unstable state. Therefore, Startup characteristics of air-conditioning systems are very important and have been investigated by many researchers. T. Yanagisawa develops a mathemat1cal model of a rotary compressor which can predict its transient behavior and can be easily integrated into a simulation program of a heat pump cycle(Yanagisawa, Shimizu, Fukuta, & Suzuki, 1990). For the transient characteristics during speed up of inverter heat pump, (Hwang & Kim, 1998)carried out that the transient cycle migration of the liquid state refrigerant caused significant dynamic change in system. (Kim & Bullard, 2001) experiment studied the dynamic characteristics of a single speed R410A split air-conditioning system during shut-down and start-up. (Kapadia, Jain, & Agarwal,
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2009) studied the transient characteristics of split air-conditionings that use R22 and R410A as refrigerants by simulation methods, and measured and published experimental data were used to validated the program. (Andrade & Negrão, 2013) proposed a semi-empirical model applied to domestic refrigeration systems for devising real time predictions of test results. The results showed that the model can predict fairly well the results within the calibration period and that the model accuracy increases with the calibration time. The liquid slugging consequently threaten the reliability of the compressor as the cylinder pressure could shoot to very high value(Singh, Nieter, & Prater Jr, 1986), which often occurs when liquid refrigerant enters the compressor or during the cold startup process. Thus the liquid slugging in the compressors has been studied by many investigators. (Palm, 2008; Tadashi, Takashi, & Mitsuhiro, 1985) experimentally examined liquid compression characteristics at starting in a sliding vane type rotary compressor. (Liu & Soedel, 1994) presented a mathematical model to simulate the compression processes of two-phase saturated refrigerant mixture and analyzed the factors leading to slugging. (Dutta, Yanagisawa, & Fukuta, 2001; Park, Kim, & Cho, 2002)investigated the performance of a scroll compressor under liquid refrigerant injection. To identify slugging-induced overpressures in reciprocating compressors, (Laughman;, Foy;, Wichakool;, Armstrong;, & Leeb;, 2008)proposed a method by analyzing the electrical power flowing into the compressor motor. Though much research has been devoted to the performances of R290 air-conditioning systems and dynamic characteristics and liquid slugging of the rotary compressor (Devotta et al., 2005; Kapadia et al., 2009; Kim & Bullard, 2001; Liu & Soedel, 1994; Tadashi et al., 1985; Tian et al., 2015) , little attention has been paid to the startup characteristics of the rotary compressor in R290 air-conditioning systems. As the saturated vapor line of R290 was closed to the line of compression process in the P-h graph, the liquid slugging is likely to happen during the compression process. Especially, during the cold startup process, the temperature of the cylinder is so low as to make the vapor refrigerant in the cylinder into liquid by transferring heat from refrigerant to cylinder or exhaust valve. In this study, an experimental investigation was done to characterize the cold startup behaviors of a rotary compressor in a R290 air-conditioning system. This study aims to present the variations of temperatures and pressures in the R290 air-conditioning system and the rotary compressor under standard cooling conditions to identify the happening of the liquid slugging. Meanwhile, the mixture of oil and refrigerant viscosity and the oil level of the oil sump during startup of the system were studied as to confirm if the lubrication of the rotary compressor could be guaranteed during cold startup process. Through this study, we hope that it could provide some useful data for the promising development of the R290 air-conditioning system with a rotary compressor in the future.
2. Experimental setup and test procedure
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2.1 Experimental unit /apparatus The object of the present experiment was to investigate the startup characteristics of the R290 rotary compressor and its corresponding air-conditioning system, and thus to provide fundamental information for the application of R290 in the refrigeration industry. The schematic diagram of experimental unit is shown in Fig.1. It is composed of a wall room air-conditioning system and a psychometric chamber. The air-conditioning system fundamentally consisted of a rotary compressor, a condenser, an evaporator, a manual expansion valve and such attachments as an accumulator and fans. The specifications of heat exchangers and a inverter rotary compressor typed with are shown in Table 1 and Table 2 respectively. The psychometric chamber consists of two separate constant temperatures and humidity rooms the indoor side and outdoor side. Two air handling units are installed in both rooms to guarantee the temperature and humidity exactly satisfying the strict requirements in experiments. The dry and wet bulb temperatures of air entering the experiment units are maintained within ±0.2℃, and the accuracies of thermocouples in both rooms are maintained within ±0.1℃. Meanwhile, an air measurement device is installed in indoor side to measuring the temperature, air mass flow rate and other parameters. To investigate the startup characteristics of the R290 rotary compressor and its corresponding air-conditioning system, pressures and temperatures tests in the compressor and the system are necessary. The pressures and temperatures in the system were measured by some pressure transducers and T-type thermocouples fixed at proper sites as shown in Fig.1. The accuracy of pressure transducers is ±0.25%FS and the accuracy of thermocouples is ±0.5 K. Furthermore, some sensors are installed in the compressor to measure pressures and temperatures in the compressor, as shown in Fig.2, thus the shell and some other parts of the original compressor were modified. It is considered that the modification does not influence the performance of the compressor to a notable extent. The temperatures in the compressor are measured by 5 K-type sheathed thermocouples installed in the shell of compressor as shown in Fig.2(a). The pressures in the suction chamber or compression chamber of the compressor are measured by two minitype piezoresistive dynamic pressure sensors XTL-190M produced by Kulite Semiconductor Products, Inc. The installation positions of the pressure sensors are shown in Fig.2(b). Pressure sensor #1 was installed beside the suction port and formed a 142°angle to the center line of the vane groove. Pressure sensor #2 was installed on the discharge side and formed a 10°angle to the center line of the vane groove. The linearity of pressure sensors are less than ±0.5%. A data acquisition system PXI-6255 of NI Company (resolution 16 bits; sampling rate 1.25 MS/s) was used to collect the rapidly changing pressure data in the cylinder during the startup process. A VISCO pro 2000 viscometer with a SPL501 probe, made by Cambridge Company of
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American, is installed at the bottom of the rotary, shown in the red dotted lines in Fig.2(a), to measure the variation of mixture of oil and refrigerant viscosity during the startup of the rotary compressor. And the connection layout of the SPL501 probe and rotary compressor was shown in the left of Fig.2(a). The accuracy and relative error of the viscometer are within ±0.1% and ±0.8%, respectively. A sight glass is installed at the side of the rotary compressor shell to observe the variation of the oil level of the oil sump during startup process. To facilitate the observation, a ruler was stuck to side of the sight glass, of which the scale “0” was as high as the upper surface of the cylinder. 2.2 Experimental condition and method The experiments were performed according to the standard cooling capacity rating condition of which the dry and wet bulb temperatures for the indoor and outdoor are 27/19 and 35/24℃, respectively. The lubricant was a kind of mineral oil. The opening of the expansion valve was 376 degree. The air volume flow rate of the air measurement device was 385m3/min, and the operating frequency of the compressor was 61Hz. All the three operating parameters were obtained by an experiment on the inverter compressor operating under the standard cooling capacity rating condition. In order to observe its cold startup characteristics, the air-conditioning system placed into the environment for a long time before the startup until the temperature of the air-conditioning system was approximately equal to environment temperature. The temperatures and pressures in the air-conditioning system were recorded every 5 seconds after startup of the system, and the temperatures measured by the five thermocouples in the rotary compressor was recorded per second. In order to obtain the pressure in the cylinder for each instant of the operating, the sampling rate of one channel for the data acquisition system is 30 kHz in the test. All data-collecting devices and their corresponding transducers were operating before startup of the system until reaching the steady state. 3. Results and discussion A R290 air-conditioning operating with a rotary compressor was selected for the study to investigate the characteristics during cold startup process. The performances of pressures and temperatures in the air-conditioning system and the rotary compressor were investigated respectively. Meanwhile, the mixture of oil and refrigerant viscosity and oil level of the oil sump in the rotary compressor during cold startup process were also discussed. 3.1 Variations of pressures and temperatures
3.1.1 Performances in the air-conditioning system Fig.3 shows the pressures and temperatures characteristics of the air-conditioning system during cold startup process. As shown in the Fig.3(a), after startup of the system, the equilibrium between the high and low pressure side of the air-conditioning was reached in 6.5 minutes owing to the refrigerant movement from the high pressure side to the low pressure side through the expansion valve. The suction pressure dropped to the lowest point 149kPa in 25 seconds then
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gradually increased until reaching the equilibrium state 478kPa. After reaching the lowest point, the outlet pressures of expansion valve and evaporator increased rapidly then gradually increased to the equilibrium state, 513KPa and 688KPa, after a slight decrease. The evaporator outlet pressure was slightly larger than the discharge temperature in the equilibrium state owing to the pressure loss of refrigerant through the evaporator. While the discharge pressure increased rapidly in 10 seconds then they gradually increased after a slight decrease to the equilibrium state 1413KPa. The expansion valve inlet pressure was a little lower than the discharge pressure, and the variation of it was consistent with that of the discharge pressure. As shown in the Fig.3(b), the variations of the suction and discharge temperatures were in close agreement with that of the suction and discharge pressures shown in Fig.3 (a) , but for temperatures the time from startup to the steady state was longer than that of pressures. After startup of the system, the temperatures equilibrium between the high and low pressure side of the air-conditioning was reached in 11 minutes. The discharge temperature increased rapidly in 20 seconds then they gradually increased after a slight decrease to the steady state 59℃. The suction temperature dropped to the lowest point in 20 seconds then they gradually decreased after reaching a transient maximum in 65 seconds until reaching the steady state 15℃. The slower temperatures response could be attributed to the reconditioning of the thermal mass of the system.
The variations of pressures and temperatures in the R290 air-conditioning system agree well with (Kapadia et al., 2009)’s findings that use R22 and R410A as refrigerants. While the time from startup to the steady state was much longer that of R.G. Kapadia’s findings and the suction pressure drops more sharply before gradually increasing than that of R.G. Kapadia’s findings. The differences between the results are possible because that the physical property of R290 is much different from that of R22 and R410A, and the R290 charge in this system was less than that of the R22 and R410A air-conditioning system.
3.1.2 Performances in the rotary compressor a.
Pressures in the cylinder Fig.4 shows the variations of the pressure in the cylinder during cold startup process. From
Fig.4(a), it can be observed that, after startup of the compressor, the pressure in position 1 increased first then gradually decreased. While, as shown in Fig.4(b), after the startup of the compressor the pressure in position 2 increased sharply then it decreased rapidly to 1.2MPa. Subsequently, it remained unchanged after a gently increase. The pressure difference between the discharge and suction process was gradually formed, as Fig.4(b) shows. In order to further reveal the variations of pressures in the cylinder during cold startup process, the variation of pressures at 5 different points in time in position 2 were shown in Fig.5. Fig.5(a) shows the 5 points in the discharge pressure curve. Figs.5(b)-(f) show the variations of the
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pressure at the 5 different points in time during the startup process. As shown in Fig.5(b), the pressure during the later exhaust process increase rapidly to 2.21MPa in 1.3 second, while the discharge pressure as shown in Fig.5(a) was only 0.91MPa. As shown in Fig.5(c)-(f), with time goes by, the maximum pressure during the later exhaust process was gradually reduced to 1.6MPa, and the discharge pressure increased from 1.18Mpa to 1.41MPa. These results suggest that there were liquid refrigerant or oil at the exhaust port of the cylinder at point 1 in time and the liquid slugging is likely to happen, while the liquid slugging happens just at the initial time of cold startup process in the R290 air-conditioning system with a rotary compressor. A possible explanation for the liquid refrigerant could be as following. The temperatures of the cylinder and the exhaust valve were as cold as environment at the initial time of the cold startup. And he saturated vapor line of R290 was closed to the compression process line in the P-h graph, thus the liquid may formed during the compression process. But as the high inlet specific volume of the R290, the total quantity of the liquid refrigerant was small. Subsequently, with the increase of the discharge temperature, the liquid refrigerant evaporates rapidly by absorbing heat from the cylinder and the exhaust valve.
b. Temperatures in the rotary compressor Fig.6 shows the variations of the temperatures in the compressor after cold startup of the system. The muffler temperatures and the motor stator temperatures increased rapidly in 25 seconds after startup then they gradually increased to the equilibrium state，59.1℃ and 56.5℃ respectively, after 180 seconds. Meanwhile, the cylinder temperature and the suction pipe temperature decreased rapidly in 25 seconds after startup then they gradually increased to the equilibrium state, 54.9℃ and 20.1℃ respectively, after 300 seconds, and the oil temperature remained 36.5℃ unchanged in 120 seconds after startup then it gradually increased to the equilibrium state 50℃ after 500 seconds. It can also be observed that the oil temperature was higher than the cylinder temperature at the beginning of startup thus the heat was transferred from the oil to the cylinder. Then subsequently, the heat was transferred from the cylinder to the oil because the oil temperature was higher than the cylinder temperature. When it operated in steady state, all temperatures except the suction pipe temperature were higher than the temperatures before startup. And the cylinder temperature was lower than the muffler temperature, motor stator temperature and oil temperature owing to the heat transfer from the cylinder to refrigerant. 3.2 Variations of the mixture of oil and refrigerant viscosity and solubility in the rotary compressor Low oil viscosity in the rotary compressor during operating not only increase wear of bearings, but also has a negative influence on generating oil films on bearings, increasing friction losses and the reliability of rotary compressors. The oil viscosity is closely related to the oil molecular structure, temperature and pressure. In addition, it is also greatly affected by the solubility of refrigerant in the oil. So the variations of mixture of oil and refrigerant viscosity and
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the corresponding solubility of the R290 with time after startup were studied. Fig.7 shows the variation of mixture of oil and refrigerant viscosity in the oil sump and the solubility of R290. At the beginning of the startup, the mixture viscosity was as high as 8cP. While it decreased and reached the minimum 1.9cP after 180 seconds. Then it increased gradually to a steady state of 4cP after 500 seconds. The variation of mixture viscosity in this experiment agrees well with(Fukuda;, Hayano;, & Hayano, 1996)’ systems with HFC/POE as refrigerant. However, the minimum of mixture viscosity in this experiment is much higher than that of the HFC/POE refrigerant system. This result suggests that the R290 with the lubricants is beneficial for the reduction of the frictional loss and the oil supply to guarantee a stable startup process. According to the mixture of oil and refrigerant viscosity and the temperature in the oil sump measured before, the variation of R290 solubility in the mineral oil can be referred as show in Fig.7. The variation tendency of the solubility is in contrast with that of mixture viscosity. It can be observed that the solubility of R290 in the mineral oil increased firstly then decreased before it remaining unchanged during the startup. From 0 to 250s, the discharge pressure increased rapidly (see Fig.5(a)),while the oil temperature just slightly increased. Thus the R290 solubility in the mineral oil increased sharply（blue curve in Fig.6). From 250 to 400s, the discharge pressure remained approximately constant, while the oil temperature rapidly increased. As a result, the R290 solubility in the mineral oil decreased sharply. The initial value of the solubility was 15%. The maximum and stable value of solubility was 42% in 250 seconds and 20% in 500 seconds, respectively. This finding appears that the solubility of R290 was negative correlated with the mixture viscosity, and the solubility of R290 in the oil was low before startup of the system. 3.3 Variations of the liquid height in the rotary compressor The oil level of the oil sump has a huge effect on the total oil flow rate(J. H. Wu & Wang, 2013). A drastic change or too low value of the oil level of the oil sump affects the oil supply system, and significantly reduces the reliability of rotary compressors during startup greatly. Fig.8 shows the variation of the oil level of the oil sump in the compressor shell with time during cold startup of the system. The scale “0” of the ruler stuck to the side of the sight glass was as high as the upper surface of the cylinder. The oil level of the oil sump before startup was 9 mm(Fig.8(a)). It can be observed that the oil level of the oil sump decreased firstly with lots of bubble forming on the oil surface in 6 seconds as shown in Fig.8(b). Then it remained the minimum -6mm unchanged from 16 seconds to 76 seconds as shown in Fig.8(c). After that, it increased gradually to a stable value 3 mm after 106 seconds as shown in Fig.8(d). The minimum oil level was -6mm lower than the upper surface of the cylinder, and the stable oil level was 3mm higher than the upper surface of the cylinder. The cylinder height is 24mm, therefore, the oil level of the oil sump is always higher than the height of the lower surface of cylinder, a continuous steady oil supply to the cylinder and other positions during the startup could be guaranteed, which indicates that the R290 air-conditioning system has a good oil return though the viscosity of the mineral oil in the system was high. 4. Conclusions An experimental investigation on cold startup characteristics of a rotary compressor in an air-conditioning system using R290 as refrigerant under cooling condition has been performed. A series of experiment were carried out in a psychometric chamber with constant temperatures and
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humidity. Pressures and temperatures in the R290 air-conditioning system during startup process were measured, and the temperatures in the rotary compressor shell and the pressure in cylinder during cold startup process were also measured by sensors and an acquisition system. Meanwhile, the mixture of oil and refrigerant viscosity and oil level of the oil sump in the rotary compressor during startup process were also discussed. The following conclusions could be made from the experiments: 1)
The variations of pressures and temperatures in the R290 air-conditioning system during
cold startup process in this study agreed well with (Kapadia et al., 2009)’s findings using R22 and R410A as refrigerants. While the time from startup to the steady state was much longer than that of R.G. Kapadia’s findings and the suction pressure in this study drops more sharply before gradually increasing than that of R.G. Kapadia’s findings. 2)
A slight liquid slugging happened at the initial time during startup process in the R290
rotary compressor. But the pressures rapidly decreased after reached the maximum value during the startup process. These indicated that the liquid slugging happened only at the beginning of the startup processes for the R290 rotary compressor, but it is within the bearing capacity of rotary compressor. 3)
At the initial time after startup, the cylinder temperature decreased as the suction
temperature sharply decreased. Meanwhile, the motor stator temperature and the muffler temperature increased rapidly. But the oil temperature kept unchanged before a slow increase in 120second. After reaching steady state, the temperatures in the compressor from the highest to the lowest were the muffler temperature, the motor stator temperature, the oil temperature, the cylinder temperature and the suction pipe temperature. 4)
After startup of the system, the mixture of oil and refrigerant viscosity decreased to a
minimum value 1.9cP in 180 seconds, then it increased gradually to a steady state of 4cP after 500 seconds. The variation of the solubility is in contrast with that of mixture viscosity. The maximum and stable value of solubility was 42% in 250 seconds and 20% in 500 seconds, respectively. 5)
After startup of the system, the oil level of the oil sump decreased firstly then remained
the minimum unchanged for 60 seconds before increasing gradually to a stable value. And a continuous steady oil supply to the cylinder and other positions during the startup could be guaranteed.
Acknowledgement This work is supported by the HCFC phase out management plane in room air conditioning sector technical assistance fund (China).
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Reference
Andrade, D. E., & Negrão, C. O. (2013). Analysis of start-up tests of household refrigeration systems. A potential for on-line predictions of test results. Applied Thermal Engineering, 54(1), 255-263. doi:10.1016/j.applthermaleng.2013.02.008 Cai, D., He, G., Yokoyama, T., Tian, Q., Yang, X., & Pan, J. (2015). Simulation and comparison of leakage characteristics of R290 in rolling piston type rotary compressor. International Journal of Refrigeration, 53, 42-54. doi:10.1016/j.ijrefrig.2015.02.001 Devotta, S., Padalkar, A., & Sane, N. (2005). Performance assessment of HC-290 as a drop-in substitute to HCFC-22 in a window air conditioner. International Journal of Refrigeration, 28(4), 594-604. doi:10.1016/j.ijrefrig.2004.09.013 Dutta, A. K., Yanagisawa, T., & Fukuta, M. (2001). An investigation of the performance of a scroll compressor under liquid refrigerant injection. International Journal of Refrigeration, 24(6), 577-587. doi:10.1016/S0140-7007(00)00041-4 Fernando, P., Palm, B., Lundqvist, P., & Granryd, E. (2004). Propane heat pump with low refrigerant charge: design and laboratory tests. International Journal of Refrigeration, 27(7), 761-773. doi:10.1016/j.ijrefrig.2004.06.012 Fukuda;, T., Hayano;, M., & Hayano, M. (1996). HFC-POE Lubricity Evaluation on the Rotary Compressor in System Operation. International Compressor Engineering Conference at Purdue, 121-126. Hrnjak, P., & Hoehne, M. (2004). Charge minimization in systems and components using hydrocarbons as a refrigerant: ACRC TR-224. Hwang, Y., & Kim, H. (1998). Experimental and theoretical studies on the transient characteristics during speed up of inverter heat pump. Joudi, K. A., & Al-Amir, Q. R. (2014). Experimental Assessment of residential split type air-conditioning systems using alternative refrigerants to R-22 at high ambient temperatures. Energy Conversion and Management, 86, 496-506. doi:10.1016/j.enconman.2014.05.036 Kapadia, R., Jain, S., & Agarwal, R. (2009). Transient characteristics of split air-conditioning systems using R-22 and R-410A as refrigerants. HVAC&R Research, 15(3), 617-649. Kim, M.-H., & Bullard, C. W. (2001). Dynamic characteristics of a R-410A split air-conditioning system. International Journal of Refrigeration, 24(7), 652-659. Laughman;, C. R., Foy;, R. R. L., Wichakool;, W., Armstrong;, P. R., & Leeb;, S. B. (2008). Electrical and Mechanical Methods for Detecting Liquid Slugging in Reciprocating Compressors. International Compressor Engineering Conference at Purdue, 29(8), 143-149. Li, T., Lu, J., Chen, L., He, D., Qiu, X., Li, H., & Liu, Z. (2015). Measurement of refrigerant mass distribution within a R290 split air conditioner. International Journal of Refrigeration, 57, 163-172. doi:10.1016/j.ijrefrig.2015.05.012 Liu, Z., & Soedel, W. (1994). An investigation of compressor slugging problems. Padalkar, A. S., Mali, K. V., & Devotta, S. (2014). Simulated and experimental performance of split packaged air conditioner using refrigerant HC-290 as a substitute for HCFC-22. Applied Thermal Engineering, 62(1), 277-284. doi:10.1016/j.applthermaleng.2013.09.017
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Palm, B. (2008). Hydrocarbons as refrigerants in small heat pump and refrigeration systems–A review. International Journal of Refrigeration, 31(4), 552-563. doi:10.1016/j.rser.2011.10.015 Park, Y. C., Kim, Y., & Cho, H. (2002). Thermodynamic analysis on the performance of a variable speed scroll compressor with refrigerant injection. International Journal of Refrigeration, 25(8), 1072-1082. doi:10.1016/S0140-7007(02)00007-5 Singh, R., Nieter, J., & Prater Jr, G. (1986). An investigation of the compressor slugging phenomenon. ASHRAE Trans, 92, 250-258. Tadashi, Y., Takashi, S., & Mitsuhiro, F. (1985). A study on liquid compression characteristics at starting in a sliding vane type rotary compressor. Transactions of the Japan Society of Mechanical Engineers Series C, 51(472), 4301-4305. Tian, Q., Cai, D., Ren, L., Tang, W., Xie, Y., He, G., & Liu, F. (2015). An experimental investigation of refrigerant mixture R32/R290 as drop-in replacement for HFC410A in household air conditioners. International Journal of Refrigeration , 57(9), 216-228. doi:10.1016/j.ijrefrig.2015.05.005 Wu, J., Yang, L., & Hou, J. (2012). Experimental performance study of a small wall room air conditioner retrofitted with R290 and R1270. International Journal of Refrigeration, 35(7), 1860-1868. doi:10.1016/j.ijrefrig.2012.06.004 Wu, J. H., & Wang, G. (2013). Numerical study on oil supply system of a rotary compressor. Applied Thermal Engineering, 61(2), 425-432. doi:10.1016/j.applthermaleng.2013.07.047 Yanagisawa, T., Shimizu, T., Fukuta, M., & Suzuki, H. (1990). Mathematical Model of Rotary Compressor to Simulate its Transient Behavior. Zhou, G., & Zhang, Y. (2010). Performance of a split-type air conditioner matched with coiled adiabatic capillary tubes using HCFC22 and HC290. Applied Energy, 87(5), 1522-1528. doi:10.1016/j.apenergy.2009.10.005
Fig.1 Schematic diagram of experimental setup
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Fig.2(a) Thermocouples in the compressor
Fig.2(b) Pressure sensors in the cylinder Fig.2 Installation positions of sensors in the compressor
(a) Variations of pressures (b) Variations of the suction and discharge temperature Fig.3 Variations of pressures and temperatures in the air-conditioning system during cold startup process
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（a）Variation of the pressure in position 1 (b) Variation of the pressure in position 2 Fig.4 Variations of the pressures with time in the cylinder
（a）5 different time points in position 2
(c) Variations of pressures at point 2
(b) Variations of pressures at point 1
(d) Variations of pressures at point 3
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(e) Variations of pressures at point 4 (f) Variations of pressures at point 5 Fig.5 Variation of pressures at 5 different points in time in position 2 of the cylinder
Fig.6 Variations of the temperatures in the compressor
Fig.7 Variations of the mixture of oil and refrigerant viscosity and the solubility of R290 during startup
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(a)Before startup
(b) 6 seconds
(c) 20 seconds (d) Steady state Fig.8 variation of the oil level of the oil sump with time
Table 1 Dimensions of heat exchangers Parameters Length (mm) Tube type Number of U-tubes Number of tube rows Total tube length (mm) Tube outside diameter (mm) Fin geometry Fin spacing (mm) Flow path
Heat exchanger Evaporator
Condenser
607 Rifled U-tube 15 2 607 7.00 Louver 1.25 1in 2out
895 Rifled U-tube 12 1 895 7.00 Louver 1.2 3in 3out
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Table 2 specifications of the ratary compressor Parameters
Cylinder diameter/mm
Cylinder height/mm
Crank shaft eccentricity/mm
Stroke volume /cm3
Motor efficiency/%
Values
Φ54
24
5.3
18.1
86.1
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