Cooling performance of a variable speed CO2 cycle with an electronic expansion valve and internal heat exchanger

The cooling performance of a CO₂ cycle must be improved to develop a competitive air-conditioning system with the conventional air-conditioners using HFCs. In this study, the cooling performance of a variable speed CO₂ cycle was measured and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). The basic CO₂ system without the IHX showed the maximum cooling COP of 2.1 at the compressor discharge pressure of 9.2 MPa and the optimum normalized charge of 0.282. The cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. Simultaneous control of EEV opening and compressor frequency allowed optimum control of the compressor discharge pressure. The optimal compressor discharge pressure of the modified CO₂ cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP of the CO₂ cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60 Hz.     

Modeling of rotary vane compressor applying artificial neural network

The thermal modeling of rotary vane compressor (RVC) was performed in this paper by applying Artificial Neural Network (ANN) method. In the first step, appropriate tests were designed and experimental data were collected during steady state operating condition of RVC in the experimental setup. Then parameters including refrigerant suction temperature and pressure, compressor rotating speed as well as refrigerant discharge pressure were adjusted.With these input values, the operating output parameters such as refrigerant mass flow rate and refrigerant discharge temperature were measured. In the second step, the experimental results were used to train ANN model for predicting RVC operating parameters such as refrigerant mass flow rate and compressor power consumption. These predicted operating parameters by ANN model agreed well with the experimental values with correlation coefficient in the range of 0.962-0.998, mean relative errors in the range of 2.79-7.36% as well as root mean square error (RMSE) 10.59 kg h⁻¹ and 12 K for refrigerant mass flow rate and refrigerant discharge temperature, respectively. Results showed closer predictions with experimental results for ANN model in comparison with nolinear regression model.     

A study on an online measurement method to determine the oil discharge ratio by utilizing Coriolis mass flow meter in a calorimeter

In this paper, an online measurement method of oil discharge ratio (ODR) is experimentally investigated based on a compressor calorimeter with the Coriolis mass flowmeter (CMF). In this method, the oil concentration is determined by a linear mixing law of the specific volume model using the density of the refrigerant, oil and refrigerant-oil mixture obtained by CMF. During the testing, the light absorption method is also conducted simultaneously for the comparison. The results indicate that the presented CMF based density method has an average relative deviation of 4.28%. The accuracy of the CMF based density method is superior to the light absorption method, which has the relative deviation of −8∼23% compared with the standard sampling method. The presented CMF-based method can be expected to be used for future ODR measurement due to its non-sampling, online, and accurate features.     

Experimental study of a mixed refrigerant Joule–Thomson cryocooler using a commercial air-conditioning scroll compressor

Mixed refrigerant Joule–Thomson (MR J–T) cryocoolers have been used to create cryogenic temperatures and are simple, efficient, cheap, and durable. However, compressors for MR J–T cryocoolers still require optimization. As the MR J–T cryocooler uses a commercial scroll compressor developed for air-conditioning systems, compressor overheating due to the use of less optimized refrigerants may not be negligible, and could cause compressor malfunction due to burn-out of scroll tip seals. Therefore, in the present study, the authors propose procedures to optimize compressor operation to avoid the overheating issue when the MR J–T cryocooler is used with a commercial oil lubricated scroll compressor, and the present experimental results obtained for a MR J–T cryocooler. A single stage 1.49 kW (2 HP) scroll compressor designed for R22 utilizing a mixture of nitrogen and hydrocarbons was used in the present study. As was expected, compressor overheating and irreversible high temperatures at a compressor discharge port were found at the beginning of compressor operation, which is critical, and hence, the authors used a water injection cooling system for the compressor to alleviate temperature overshooting. In addition, a portion of refrigerant in the high-pressure stream was by-passed into the compressor suction port. This allowed an adequate compression ratio, prevented excessive temperature increases at the compressor discharge, and eventually enabled the MR J–T cryocooler to operate stably at 121 K. The study shows that commercial oil lubricated scroll compressors can be used for MR J–T cryocooling systems if care is exercised to avoid compressor overheating.     

Influence of compressor oil admixtures on theoretical efficiency of a compressor system

This paper presents a theoretical evaluation of the influence of compressor oil admixtures on the thermodynamic performance of a vapor compression system using natural refrigerant R600a. The performance determination is based on the developed pressure–enthalpy diagrams (P–h) for the refrigerant oil solution (isobutane-mineral compressor oil Azmol). A method for calculating the enthalpy of refrigerant–oil solutions has been proposed and the influence of the compressor oil admixtures to isotherms the pressure–enthalpy diagrams has been analyzed. The change of enthalpy at the different oil concentrations in the working fluid in an evaporator has been investigated. An application of the developed refrigerant–oil P–h diagrams for the theoretical evaluation of the efficiency of the vapor compression systems is also demonstrated.     

润滑油循环率对活塞式制冷压缩机性能影响的实验研究

论文用实验的方法研究了润滑油循环率对活塞式制冷压缩机工作性能的影响。为了确保实验分析的准确性,测试系统采用了双油分的装置,使排气后的制冷剂和润滑油尽量完全分离,制冷剂以纯冷媒的形式再次进入压缩机的吸气。实验中采用Sanyo某型号压缩机进行测试,其结果表明润滑油循环率随着压缩机注油量的增加而增加,油循环率对压缩机的制冷量、功率、COP以及排气温度均有影响,较低的油循环率使得压缩机具有较高的制冷量和COP,同时排气温度也比较高,较高油循环率则使压缩机的制冷量和COP下降。因此一定要选择与压缩机相匹配的注油量,控制油循环率在一个合理的范围内,这样既能使润滑油起到有效的润滑作用,保证压缩机正常工作,又能使压缩机获得最佳的性能指标。     

Influence of liquid refrigerant injection on the performance of an inverter-driven scroll compressor

The operation of a scroll compressor at high compression ratios can cause excessively high discharge temperatures, which can be detrimental to the reliability and efficiency of the compressor. In the present study, the performance of an inverter-driven scroll compressor with liquid refrigerant injection was measured with a variation of compressor frequency, injection pressure, and injection location. The influence of the liquid injection on the performance is presented as a function of operating parameters and injection location by comparing the results with those for the non-injection case. It was found that liquid injection under high frequency was very effective at attaining higher performance and reliability of the compressor, but injection under low frequency showed some disadvantages. For high frequency at a given injection ratio, the injection at α=180°, for an injection angle at an injection port, yielded slightly better performance of the compressor as compared to that at α=90°.     

Experimental testing of an oil-flooded hermetic scroll compressor

In this work, a residential air conditioning compressor designed for vapor injection has been modified in order to inject large quantities of oil into the working chamber in order to approach an isothermal compression process. The compressor was tested with oil injection mass flow fractions of up to 45%. At an evaporating temperature of ?10 °C and condensing temperature of 30 °C, the overall isentropic efficiency was up to 70% at the highest oil injection rate. Overall, over the testing envelope investigated, there are no significantly negative effects experienced for the compressor and the compressor isentropic efficiency and refrigerant mass flow rate improve monotonically as the oil injection rate is increased.     

An experimental and numerical study on dynamic characteristic of linear compressor in refrigeration system

This paper presents experimental and numerical results of the dynamic characteristic and COP of a linear compressor in a refrigeration system using R600 refrigerant. The numerical analysis consists of a model and a simulation that includes the linear compressor. In this study, the dynamic characteristic of the natural frequency of the linear compressor is validated by comparing the simulation results with the experimental results. To investigate the effect of system resonance on the performance of linear compressor, COP is evaluated under evaporator pressure in the range of 48.3–63.2 kPa abs, and condenser pressure in the range of 439.0–573.3 kPa abs. Based on the results, the system resonance at the TDC was varied within a range of 3% under the test conditions. COP and its sensitivity were found to vary within 3% according to the operating frequency of the system ranging from 48.5 to 51.5 Hz.     

Theoretical and experimental study on indicator diagram of twin screw refrigeration compressor

In this paper a new mathematical model for calculating the indicator diagram of twin screw refrigeration compressor is presented firstly. The geometric parameters related to the rotation angle of male rotor are used in the model such as groove volume, suction and discharge port area, slide valve bypass port area, leakage area etc. The effects of internal leakage through five paths, oil injection, gas–oil heat transfer, refrigerant property and partial loading etc. are taken into account simultaneously and separately in the theoretical study. To verify the model and the calculated p–V indicator diagram, experimental recording of p–V indicator diagram of twin screw refrigeration compressor is carried out. Various indicator diagrams are recorded successfully by a small pressure sensor embedded in the bottom of female rotor at the discharge side. The results of theoretical calculation are in good agreement with the measured data, which lead to conclusion that the model can be used as a powerful tool for performance prediction and product development.     

Optimization of an oil charge amount on electric driven scroll compressor for eco-friendly vehicle

The purpose of this study is to optimize the amount of oil charge in the electric driven scroll compressor for eco-friendly vehicles. R134a is used as the refrigerant in the vehicle and Polyolester (POE) oil as the compressor oil. The initial amount of oil was increased at 20 g intervals from 40 g to 120 g, and at each initial amount of oil charge, the back pressure is measured at each step of the complex durability test and under each condition of the performance test. Throughout the test, it is found out that the optimum amount of oil is determined by the back pressure, which is 80 g in this test. The performance of a system is compared with pull-down tests in the actual air conditioning system, and the optimum amount of oil was confirmed. A complex durability test was also evaluated and verified for the durability of the compressor with the confirmed amount of oil.     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

Effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger

Liquid refrigerant injection technique can be a very effective method for controlling subcooling and the compressor discharge temperature of a refrigeration system at high ambient temperatures. In this study, the effects of liquid refrigerant injection on the performance of a refrigeration system with an accumulator heat exchanger were investigated by varying the liquid injection rate at the conditions of constant expansion valve opening in the evaporator and constant total flow rate. During the tests, the ambient temperature was maintained at 43 °C. With the increase of the liquid injection rate, the subcooling at the inner heat exchanger outlet increased and the superheat at the accumulator outlet decreased. However, unacceptable results such as the increase of the compressor discharge pressure and decrease of the system performance were also observed depending on the control method applied. To obtain high system performance and reliability, optimum control methods for liquid injection in the accumulator heat exchanger are suggested. The liquid injection technique for the refrigeration system with an accumulator heat exchanger was found to be an effective method for controlling adequate subcooling and the compressor discharge temperature of the refrigeration system at high ambient temperatures.     

An experimental investigation and modelling of the thermodynamic properties of isobutane–compressor oil solutions: Some aspects of experimental methodology

This paper presents experimental data for the solubility, density and capillary constant for solutions of natural refrigerant isobutane with commercial mineral compressor oil Azmol over a wide range of temperatures and concentrations. Based on information for the capillary constant, the surface tension of the solutions isobutane/Azmol is determined. The experimental data were obtained in the temperature range from 303 K to 363 K and at pressures up to 1.7 MPa using static methods. The experimental data obtained for the solutions of the natural refrigerant isobutane with the commercial mineral compressor oil Azmol are sufficiently described with the help of correlations based on the theory of thermodynamic similarity. The paper reports variation of the vapor pressure, density, capillary constant and surface tension as a function of concentration for the isobutane/Azmol solutions. The enthalpy of liquid phase of the isobutane/Azmol solutions is calculated. The analysis of the behaviour of the excess thermodynamic functions is carried out. The paper examines experimental and methodical uncertainties in the investigation of thermodynamic properties of the refrigerant/oil solutions (ROS). The influence of the time taken to establish thermodynamic equilibrium in the experimental cell on the uncertainty of the experimental data for gas-saturated mixtures such as ROS is discussed. Information about the changing concentration of refrigerant in the liquid phase of the ROS and in the surface layer of the liquid phase of the ROS at increasing temperature is presented. In addition, the experimental data for the density, surface tension and refractive index of the mineral compressor oil Azmol are reported.     

Mixing and migration rule of binary medium in vibrated dense medium fluidized bed for fine coal separation

The vibrated dense medium fluidized bed is an efficient waterless dry coal separation technology. In order to reduce the interference of the macroscopic migration of the entire bed with the coal settlement process, the material composition and diffusion behavior of the binary medium are studied. The results show that the maximum mass fraction of fine coal particles is 18% in the uniformly mixed binary medium. Bubbles and vibration are the main factors influencing the mixing process. At low vibration intensity and high gas velocity (f = 15 Hz, U_v = 15 cm/s), the fine coal particles rapidly rise and back mixing under the entrainment of a large number of bubbles. If the vibration frequency is increased to 25 Hz, the excessive kinetic energy causes the entire bed to flow circularly in the vertical direction, interfering with the normal settlement of coal. After reducing the gas velocity (f = 25 Hz, U_v = 11 cm/s), the fine coal particles in the wall region rise rapidly, which is mainly driven by the vibration, whereas the fine coal particles in the central region migrate upward at a relatively low velocity, dragged by the bubbles. The bed impact forces in the opposite directions differ slightly, which promotes the directional settlement of coal.     

Propane heat pump with low refrigerant charge: design and laboratory tests

Independently of the choice of refrigerant, environmental and or safety issues can be minimised by reducing the amount of refrigerant charge per heat pump or refrigeration system. In the investigation reported here, a laboratory test rig was built, simulating a water-to-water heat pump with a heating capacity of 5 kW. The system was designed to minimize the charge of refrigerant mainly by use of mini-channel aluminium heat exchangers. It was shown 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. Additional charge reduction is possible by selecting proper compressor lubrication oils or by using a compressor with less lubrication oil.     

变频制冷机组的回油设计

对8 hp变频制冷机组24 h连续运转的回油进行试验研究,发现制冷机组在低频率运转工况下回油困难。对理论计算的吸气制冷剂流速进行分析,认为低频率运转工况下吸气制冷剂流速低是其难以将冷冻油带回压缩机的主要原因。对低频运转回油的改进措施进行试验验证,结果表明：吸气管管径变细和在排气侧加装油分离器均不能从根本上解决变频制冷机组的回油问题;在加装油分离器的基础上,在电气控制上采用定期强制高频运转回油可以使机组在蒸发温度-5～-40℃,频率为20～75 Hz的工况范围内保证稳定的油平衡;而在实际应用中,出于节能方面的考虑,与定期强制高频运转回油相比,定期停机回油作为优选方案。     

Surface tension measurement of oil/refrigerant mixture by maximum bubble pressure method

Lubrication oil used in refrigerant compressors forms oil mist in the compressor shell. Some of the oil mist is discharged into a refrigeration cycle with refrigerant and causes degradation of heat transfer in heat exchangers. Since the generation of the oil mist is related to the Weber number, it is necessary to measure the surface tension of the oil/refrigerant mixture before discussing the oil mist generation in the compressor. In this study, the maximum bubble pressure method was adapted to measure the surface tension of PAG (polyalkylene glycol) oil/CO₂ mixture. The density of the mixture needed for the measurement was also carried out. It was found that the surface tension of PAG/CO₂ mixture sharply decreases with an increase in the concentration of the refrigerant in the mixture. The surface tension of the mixture under 10 MPa and 100 °C is estimated to be 14.6 mN m⁻¹ by an extrapolation.     

An experimental investigation and modelling of the viscosity refrigerant/oil solutions

This paper presents experimental data for the viscosity of solutions of refrigerant R600a (isobutane) with mineral compressor oils Azmol, Reniso WF 15A, and R245fa (1,1,1,3,3-pentafluoropropane) with polyolester compressor oil Planetelf ACD 100 FY on the saturation line. The experimental data were obtained for solution of R600a with mineral compressor oil Azmol in the temperature range from 294.7 to 338.1 K and the concentration range 0.04399 ≤ w_R ≤ 0.3651, the solution of R600a with mineral compressor oil Reniso WF 15A at the temperatures from 285.8 to 348.4 K and the concentration range 0.03364 ≤ w_R ≤ 0.2911, the solution of R245fa with polyolester compressor oil Planetelf ACD 100 FY at the temperatures from 309 to 348.2 and the concentration range 0.06390 ≤ w_R ≤ 0.3845. The viscosity was measured using a rolling ball method. The method for prediction of the dynamic viscosity for refrigerant/oil solutions is reported.     

Assessment of R-438A as a retrofit refrigerant for R-22 in direct expansion water chiller

The present work aims to evaluate the performance characteristics of a vapor compression refrigeration system using R-438A as a retrofit refrigerant for R-22. In order to achieve this objective, a test facility is developed and experiments are performed over a wide range of chilled water inlet temperature (11:20 °C), condenser water inlet temperature (25:35 °C) and condenser water mass flow rate (363:543 kg h⁻¹). Results showed that as the chilled water inlet temperature changes from 11.5 to 20.5 °C, system COP increases from 1.78 to 2.07 at constant condenser water inlet temperature of 25.5 °C. Cooling capacity and COP of the system using R-438A are lower than R-22 by 11% and 12.5%, respectively. However, compressor discharge temperature using R-438A is slightly lower than R-22 which confirms that R-438A can be used as a retrofit refrigerant for R-22 to complete the remaining life time of the existing plants.