Practical methods for measuring refrigerant mass distribution inside refrigeration system

The purpose of this paper is to present methods for measuring refrigerant mass distribution inside a refrigeration system conveniently and accurately. The quasi on-line measurement method (QOMM) was presented for measuring refrigerant mass inside heat exchangers. Compared with the existing liquid nitrogen method (LNM), QOMM can avoid the refrigerant waste and accelerate the measurement process. For measuring refrigerant mass inside the compressor, QOMM was used together with the oil level observation method. The liquid level method (LLM) was used to measure the refrigerant mass inside the accumulator and the receiver. In order to verify the accuracy of the measurement methods, not only the deviation of the measurement method for refrigerant in single component was analyzed, but also the prediction of the total refrigerant charge in an air conditioner was verified. The results showed that the maximal prediction deviation of the refrigerant charge in the whole refrigeration system is 1.7%.     

A method to estimate the circulated composition in refrigeration and heat pump systems using zeotropic refrigerant mixtures

In recent years a number of refrigerant mixtures have appeared on the market. Some of these refrigerant mixtures are zeotropic, e.g. R407C and R417A. When zeotropic refrigerant mixtures are used in a system, the circulated composition may change from the nominal to a different composition. The changes in composition may be due to leakage or hold-ups of liquid or vapour phase, different solubility in the oil by the different components, or the fact that the system has been charged in an incorrect way. For the understanding of performance measurements made on systems, it is important to know the composition of the circulated refrigerant mixture. A promising method to estimate the circulated composition has been developed. The method has been applied and evaluated on a well equipped lab rig at the Department of Energy Technology at the Royal Institute of Technology. The tests show that it is possible to estimate the composition of the circulated refrigerant mixture to within 2%, by measuring only two temperatures and pressures.     

An investigation of the performance of a scroll compressor under liquid refrigerant injection

In this study, fundamental and practical influence of liquid refrigerant injection on the performance of a refrigerant scroll compressor has been investigated experimentally and theoretically. In the theoretical analysis, a compression model of vapor/liquid mixture is developed by taking account of heat transfer from the cylinder wall to suction, compression and injection refrigerant. An experiment has been done under the condition of keeping the oil temperature constant in order to investigate the fundamental influence of the liquid refrigerant injection on the compressor performance, and the results were compared with the theoretical ones. It was found that the injection basically increases the compression power and decreases the compressor efficiency, though the situation depends on the condition of the heat transfer to the injection refrigerant. And furthermore, the performance of the liquid refrigerant injection compressor under practical operating condition without controlling the oil temperature has been investigated. Under this condition, the compressor showed recovery and slight improvement of performance due to the decrease of the oil and cylinder temperatures by the injection. In addition, influence of the refrigerant injection on the oil viscosity and refrigerant solubility in the oil, which relate mechanical loss and reliability of the compressor, have been discussed.     

Modeling absorption of pure refrigerants and refrigerant mixtures in lubricant oil

This paper addresses the problem of absorption of refrigerant vapor in a stagnant layer of lubricant oil. The bulk motion of the solute is described in terms of apparent diffusion coefficients that encompass both molecular diffusion and possible macroscopic motion induced by liquid density instability and surface tension. In absorption of refrigerant mixtures, diffusion in the vapor and liquid phases are coupled with a thermodynamic model for interfacial equilibrium. Results are compared with experimental data available in the literature for absorption of several refrigerants in polyol ester oil (POE68). The adequacy of the formulation is assessed in the light of its basic assumptions and performance of the model.     

Foaming of refrigerating oil in a rolling piston type rotary compressor

This paper is concerned with foaming of refrigerating oil in a rolling piston type rotary compressor for an air conditioner. Factors causing foaming during the starting operation were investigated experimentally. It was proved that foaming was caused by the stirring of the refrigerant-oil mixture with a motor rotor, a vane and a discharge refrigerant flow. The foaming was correlated with the solubility of refrigerant in oil and with the amount of refrigerant contained in the stirred region of the liquid mixture.     

Experimental investigation on cold startup characteristics of a rotary compressor in the R290 air-conditioning system under cooling condition

R290 is one of the most alternative refrigerants for the air-conditionings for its negligible environmental impact and high efficiency. This experimental study 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 2.21 MPa in 1.3 seconds. 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.     

Influence of fluorinated self-assembled monolayer on wetting dynamics during evaporation of refrigerant–oil mixture on metal surface

Reducing wettability of a metal surface is a promising method for enhancing boiling heat transfer of refrigerant–oil mixture on the metal. As fluorinated self-assembled monolayer (F-SAM) coating is effective for wettability reduction, its influence on wetting dynamics including meniscus shape, contact angle, contact line velocity and rising liquid height during evaporation of refrigerant–oil mixture on metal surface were experimentally investigated. The refrigerant–oil mixture was prepared by R141b and NM56, the oil mass fraction ranged from 0 to 10 wt%, and the surface roughness ranged from 0.028 to 1.166 µm. The results show that during evaporation of refrigerant–oil mixture, the presence of F-SAM changes the evaporation mode to be constant contact line velocity followed by both constant contact angle and contact line velocity, while decreases the rising liquid height. The results suggest that larger surface roughness and higher oil mass fraction are preferred when using F-SAM to reduce surface wettability.     

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.     

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.     

垂直U形管内含油制冷剂流动的数值分析

利用EES软件对直接膨胀式地源热泵系统中U形埋管换热器内含油制冷流动状态进剂行了模拟分析。模拟分析结果表明:含油制冷剂压力沿U形埋管先缓慢增加后减少;温度沿U形埋管先增加后减少,进入过热区后急剧增加;在U形埋管上升管段中含油制冷剂不存在泡状流,以弹状流和环状流形式存在,且以环状流为主;最小回油速度随U形埋管管径的增加而增加。计算结果和实验结果的对比表明实验系统能够正常回油。     

Oil presence in an evaporator: experimental validation of a refrigerant/oil mixture enthalpy calculation model

In this paper, the impact of the oil presence on the performances of a refrigerating machine is investigated both experimentally and numerically. To highlight the effect of oil, particularly on the evaporator behaviour, a theoretical model of enthalpy calculation for a refrigerant/oil mixture has been previously developed [Int J Refrigeration, 26(2003), 284]. In order to validate this model, tests were carried out on an industrial refrigerating machine working with R-407C. The lubricant is a polyol-ester oil whose solubility curves are given by the oil manufacturer. The oil circulating mass fraction is measured by a sampling technique and by an on-line density measurement method, whose advantages and drawbacks are presented. Both the model and the experiments show that the ratio of enthalpy change through the evaporator with to without the oil presence increases when the apparent superheat at the evaporator outlet increases. This is due to the presence of a non-evaporated amount of liquid refrigerant dissolved in the oil at this location, which is confirmed by visual observations. The numerical and experimental results are found to be in a good agreement as the maximum deviation is about 2.2%.     

Design of experimental bench and internal pressure measurement of scroll compressor with refrigerant injection

Experiments on the inner compression process of scroll compressor with refrigerant injection can reveal the essence of refrigerant injection. The difficulty of the experiment is the design of location of measuring ports, measuring system of dynamic pressure and design of the injection system. Focusing on the dynamic pressure measurement of inner compression process during refrigerant injection, an integrated bench design method for refrigerant injection research in scroll compressor is presented in this paper. The location design of injection ports and measuring ports, frequency spectrum analysis of pressure signal, selection of the sensor type and configuration, and design of the pressure-leading system are expressed, respectively. Finally, a test bench is set up. Based on it, several elementary experiments were carried out. The results show that: this design method solves most problems in the experimental research of scroll compressor with refrigerant injection and works reliably; the refrigerant injection effects the majority of the inner compression process and should not be considered as a transient process; gas injection can increase the system performance greatly and there is an optimal injection pressure for a certain scroll compressor.     

An experimentally validated model for microchannel heat exchanger incorporating lubricant effect

This paper presents the further development of Li and Hrnjak's (2013) microchannel heat exchanger model which includes the thermodynamic and transport properties of refrigerant–oil mixture. Effect of lubricant is accounted in boiling heat transfer, pressure drop and refrigerant distribution. A newly proposed infrared thermography based method is implemented in the model to describe the liquid refrigerant distribution in the inlet header of the microchannel heat exchanger. The new model is validated against experimental results (R134a-PAG 46 oil) at various oil circulation ratios (0.1%–8.3%). Simulation results also indicate that lubricant addition improves refrigerant distribution which is in agreement with experiments and the infrared thermography based method enables the model capture lubricant effect on capacity more comprehensively.     

Influences of miscible and immiscible oils on flow characteristics through capillary tube—part I: experimental study

A capillary tube is widely used as an expansion device for small refrigeration cycles. In a practical refrigeration cycle, some amount of refrigeration oil is discharged from a compressor and refrigerant/oil mixture flows through the capillary tube. This study investigated experimentally the influence of mixing of the refrigeration oil with the refrigerant on the flow through the capillary tube. The experiments are carried out with not only a miscible combination of refrigerant and oil but also an immiscible combination. In both cases, the mass flow rate through the capillary tube and temperature and pressure distributions along the tube are measured under several conditions of subcooled degree and oil concentration. In the case of miscible combination, the mass flow rate of refrigerant decreases with increasing the oil concentration because the viscosity of liquid phase increases by the mixing of viscous oil. Even in the case of the immiscible combination, the oil droplet is so small that it mixes homogeneously in the liquid phase in the capillary tube and the refrigerant mass flow rate decreases by the mixing of immiscible oil. There is no significant influence of the oil concentration on the underpressure, which means pressure difference between saturation pressure and flash inception pressure, in both miscible and immiscible combinations.     

Flow characteristics of refrigerant and oil mixture in an oil separator

The efficiency and pressure drop in the oil separator of a multi heat pump system were experimentally investigated. R410A and polyvinylether (PVE) oil were used as the refrigerant and lubricant of the system, respectively. The refrigerant mass flow rate and the liquid circulation ratio were varied between 30 and 150 g s⁻¹ and between 1.5% and 4.5%, respectively. Five different oil separators were considered, and the measured efficiencies and pressure drops were analyzed with respect to the height and diameter of the oil separators. The analytical results were used to develop empirical equations for predicting the efficiency and pressure drop of the oil separator. The mean absolute percentage errors between the predicted and measured efficiencies and pressure drops for the oil separators were 0.4% and 11.7%, respectively.     

冷库液体冷媒融霜的性能实验研究

介绍了冷库液体冷媒融霜实验装置,对实验结果进行分析研究.与热气融霜相比较,液体冷媒融霜系统具有制冷量大,对换热器翅片效率影响小,室温波动小等优点,是一种可行的融霜方法.     

Experimental investigation of oil retention in air conditioning systems

In air conditioning and refrigeration systems a small amount of oil is carried with the refrigerant and is retained in the system components. Oil retention characteristics in the condenser, evaporator, liquid and suction lines were measured and are presented and discussed here. Refrigerants R22, R410A, and R134a with miscible and non-miscible lubricants were considered to investigate oil retention physics in the widest possible range of transport properties. A parametric analysis in the suction line showed that oil retention depends on the oil mass fraction, vapor refrigerant mass flux, mixture viscosity ratio and orientation of the pipe. In the suction line, an increase in mixture viscosity of about 55% caused a rise in oil retention in the range of 50%, depending on the oil mass fraction. Oil retention in the upward vertical suction line is about 50% higher than in the horizontal line at similar conditions.     

制冷压缩机制冷量测定方法研究及其不确定度分析

介绍制冷压缩机性能测试方法,分析制冷压缩机性能测试系统制冷量的测试方法及测试不确定度,对引起测试不确定度的因素进行论述。     

载冷剂低温热物性测试方法综述

航天器内的温度通过热控制系统中载冷剂的强制对流换热来调节。在研制和选择载冷剂时,根据载冷剂的性能要求,需要对流体工质在低温下的热物性参数进行测量。对现有的载冷剂物性测试方法进行了综述,总结了测量载冷剂密度、凝固点、沸点、黏度等参数的方法,并通过比较得出绝热量热法是低温下测量液体比热容的最佳方法,径向热流法适合于低温下测量液体工质的导热系数。     

制冷系统湿压缩时吸气干度控制方法的实验研究

不完全湿压缩能大幅度降低压缩机排气温度,然而该应用的最大难点是如何控制实时压缩机吸气干度在合适的范围内。本文提出了假拟饱和等熵压缩排气温度控制压缩机吸气该干度的方法,理论分析了在AHRI(空调供暖制冷协会)空调和低温制冷两种典型工况下,R22、R32、R134a和R410A四种制冷剂作为冷媒时,应用该方法控制压缩机吸气带液时系统性能的变化,并通过R32实验验证该结论的正确性。结果表明:利用假拟饱和等熵压缩排气温度可以将压缩机吸气状态控制在少量湿蒸气的状态;在T-s图上具有钟型饱和线形状的R32制冷剂,利用假拟饱和等熵压缩所控制的制冷系统,当吸气干度在0.96~1时,制冷量和COP均能达到最大值。