Oil Circulation Ratio Research Articles

Oil circulation ratio prediction in a vapor compression system using a discharge side oil separator and mass flow correction

Oil circulation ratio (OCR) is defined as the ratio of the mass flow rate of oil to the total mass flow rate of refrigerant-oil mixture in a vapor compression system. The standard method for measuring OCR uses liquid line sampling as described in ASHRAE Standard 41.4. Sampling is tedious, alters the steady state operation of the system, depends on different parameters, and only applies to miscible refrigerant-oil pairs. A potential method for measuring real-time OCR is by using an oil separator to separate the refrigerant flow from the oil flow and using the individual flow rates to calculate OCR. Neither a liquid line, nor refrigerant-oil miscibility are necessary for this separation-based method. No oil separator is perfect as some oil always escapes with the separated refrigerant, and some refrigerant, dissolved in oil, always escapes with the separated oil. This can significantly reduce the accuracy of the procedure. The present study investigates OCR measurements using an oil separator-based approach for a full vapor compression cycle working with R134a and PAG ISO 46 oil. A full cycle allows sampling to also be performed in parallel for validation. Mass flow corrections were performed to account for refrigerant dissolved in separated oil, and for oil entrained by separated refrigerant. OCR values from the oil separator-based approach, upon mass flow correction, were within 6% of the sampling results. The usefulness of the oil separation efficiencies at the oil and vapor outlet ports for the oil separator-based approach is discussed.

Experimental Study on the Impact of Lubricant on the Performance of Gravity-Assisted Separated Heat Pipe

Gravity-assisted separation heat pipes (GSHPs) are extensively utilized in telecommunications base stations and data centers. To ensure year-round cooling, integrating GSHPs directly with a vapor compression refrigeration system is a viable solution. It is unavoidable that the refrigeration system’s lubricant will infiltrate the heat pipe loop, thereby affecting its thermal performance. This paper examines the performance of a GSHP, which features a water-cooled plate heat exchanger as the condenser and a finned-tube heat exchanger as the evaporator, when the working fluid (R134a) is contaminated with a lubricant (POE, Emkarate RL-46H). The findings are compared with those from a system free of lubricant. The experimental outcomes indicate that the presence of lubricant degrades the heat transfer efficiency, particularly when the filling ratio is adequate and no significant superheat is observed at the evaporator’s outlet. This results in a 3.86% increase in heat transfer resistance. When the charge of the working fluid is suboptimal, the average heat transfer resistance remains relatively constant at a 3% lubricant concentration yet increases to approximately 5.27% at a 4–6% lubricant concentration, and further to 12.32% at a 9% lubricant concentration. Concurrently, as the lubricant concentration fluctuates between 3% and 9%, the oil circulation ratio (OCR) varies from 0.02% to 0.11%.

Effect of oil concentration, viscosity, and surface tension on tube temperature and heat transfer coefficients of R1234ze(E)/oil spray evaporation on an enhanced tube bundle

Shell-and-tube evaporators are widely used in desalination, refrigeration, and petrochemical industries. The heat transfer process in spray-type evaporators combines the droplets impingement at the top rows of the bundles and falling film evaporation for the bottom rows, with a transitional region in the center of the bundle. Spray evaporators are a promising alternative to flooded-type shell and tube heat exchangers because they run with significantly lower refrigerant charge amounts.This study presents new data for the spray evaporation of low-GWP refrigerant/oil mixtures on an enhanced tube bundle. The experiments involved R134a (HFC) and R1234ze(E) (HFO) paired with synthetic Polyolester (POE68 and POE220) lubricants. The tests utilized a boiling enhanced tube (re-entrant cavity tube) and were conducted at saturation temperatures of −6.7 °C, 2.2 °C and 10 °C (20°F, 36°F and 50°F), with heat flux varying from 3 to 35 kW/m2 and oil circulation ratio (OCR) ranging from 0.4 % to 2 %. With the addition of oil, the bundle heat transfer coefficient (HTC) of refrigerant/oil mixtures decreased by 20–50 % at an OCR of 1.2 % and a heat flux of 13.5 kW/m2, across the tested saturation temperatures. The average wall superheat across the bundle exhibited at least a 48 % increase for all the refrigerant/oil mixtures tested at an OCR of 1.2 %. The viscosity of the refrigerant/oil mixture showed a limited influence on heat transfer compared to surface tension, mainly because the oil concentrations were quite small. Oil promoted a partial dry-out of the tubes, especially at the bottom of the bundle. Foaming was observed and increased with heat flux and oil concentration. Foam sheets on the tube bundle created an additional barrier for the liquid refrigerant to reach the tube heat transfer surfaces and thus decreased the overall bundle heat transfer coefficient.

Numerical study of a CO2 swirl ejector with lubricating oil

At present, most simulations of ejectors ignore the presence of lubricating oil, which may affect the accuracy of the results. Additionally, there has been less research conducted on the influence of motive nozzles with swirl generators on the performance of ejectors. In this study, a heterogeneous mixture model was developed for a three-phase swirl ejector equipped with a motive nozzle featuring a swirl generator. This article examines the performance of swirl ejectors under supercritical conditions using three-dimensional numerical simulations conducted in ANSYS. The study explores swirl ejectors with varying helical angles of 77°, 64°, 58°, 50°, 37° . The study also takes into account the influence of lubricating oil to enhance the practicality of the research. Firstly, lubricating oil with a volume fraction of 2 % was introduced into both the motive and suction streams, followed by conducting an ejection analysis. The results indicate that the swirl enhances the entrainment ratio, and the swirl strength can be modified by changing the helical angle. A smaller helical angle results in greater swirl strength and a higher entrainment ratio. When the helical angle is 37° and the helical pitch is 20 mm, a swirl number of 0.42, an entrainment ratio of 0.998, and an effective entrainment ratio of 0.863 is achieved. However, when the oil circulation ratio in the suction stream is increased from 2 % to 10 % while keeping the oil circulation ratio of the motive stream constant, the entrainment ratio decreases.

Spray evaporation of R1234ze(E)/POE-68 refrigerant-oil mixture on enhanced tube bundles

Lubricating oil is used in compressors of vapor compression-based refrigeration systems. The oil that escapes from the compressors inevitably circulates in the heat exchangers, affecting their thermal performances. This paper presents a comprehensive study on the impact of lubricating oil on the heat transfer performance of spray evaporator tube bundles used in air conditioning and refrigeration systems. The research includes valuable data on the heat transfer coefficient of tube bundles when subjected to refrigerant and lubricant mixtures of R134a (HFC) and R1234ze(E) (HFO) with synthetic polyolester (POE) lubricant. The investigation delves into the influence of three crucial factors: heat flux, oil circulation ratio (OCR), and saturation temperature. The saturation temperature was examined across a range of −6.7 to 10 °C (20–50 °F), while the heat flux was varied from 6 kW/m2 to 17 kW/m2. OCR was controlled at 1.2 %. The effects of oil on spray evaporation of R1234ze(E)/POE mixture on enhanced tube bundle was studied by varying tube surface structure and bundle layout, that is, square and triangular configurations.The bundle heat transfer coefficient decreased by at least 10 % when lubricant was present in the mixture but, in some cases, varied by 50 % or more compared to the performances with refrigerant only (i.e., no lubricant). The data suggested that the oil penalized the heat transfer coefficient of the bottom tube rows of the bundle, which in many conditions, starved and experienced a partial-dry out state. Externally enhanced boiling-type tubes were more sensitive to the presence of oil than externally enhanced condenser-type tubes.

The influences of the oil circulation ratio on the performance of a vapor injection scroll compressor in heat pump air conditioning system intended for electrical vehicles

Heat pump air conditioning (AC) based on a vapor injection scroll compressor has proven to be a promising solution to the current driving range limitations of electric vehicles (EV), especially in freezing climates. The lubricating oil inside the compressor is critical in promoting sealing and cooling, but it also exhibits a low performance under extreme working conditions. In addition, the effects of oil retention on compressor behavior and the optimal charge amount at low temperatures require further investigation. This paper adapts a second refrigerant calorimeter method to study the effects of different injection pressures on compressor performance. The results indicate that the mass flow rate, discharge temperature, compressor power consumption, and volume efficiency performance would increase significantly compared with the non-vapor injection (NV). The effects of oil circulation rate (OCR) on the performance of a vapor injection scroll compressor under two low-temperature operating conditions were also studied. The flow and heat transfer parameters were analyzed, and heating capacity was found to increase and decrease with enlarged OCR. The highest coefficient of heating performance (COP) was obtained at an OCR of 3.5% with a vapor injection pressure of 0.28 MPa. Compared with the non-vapor injection (NV) condition, the heating capacity and COP of the system at the same pressure of 0.28 MPa were increased by 6.2% and 1.8%, respectively. At extremely low temperatures, the heating capacity and COP were maximized at an OCR of 5.5% and an injection pressure of 0.45 MPa. Compared with that of NV, the heating capacity and COP of the heating system at the injection pressure were increased by 6.7% and 2.6%, respectively. The compressor discharge temperature decreased obviously with the increasing OCR. An empirical formula was devised to calculate the heating capacity for varying OCR.

Analytical and experimental study of two-phase ejectors considering the influence of lubricant oil

The use of traditional refrigerants such as HCFCs and HFCs has a significant impact on global warming. On the other hand, natural refrigerants have little environmental impact with essential characteristics such as low global warming potential (GWP) and zero ozone depletion potential (ODP). Among various natural refrigerants, carbon dioxide is used as an alternative natural refrigerant due to its excellent heat transfer efficiency in a supercritical state. However, its COP is low due to its inherent high compressor work. To reduce this high compressor work, additional components such as ejectors are used for carbon dioxide cycles. In this study, a two-phase ejector was investigated. In particular, ejector analysis was performed in consideration of the influence of lubricant oil. In vapor compression cycle, lubrication is essential. This lubricant oil is discharged to the system through a compressor. When oil flows to system and enters the ejector, it forms a layer that can obstruct the flow inside the ejector. Oil circulation ratio (OCR) change can affect the mass flow rate of the ejector as if the inner diameter of the ejector is reduced. Therefore, it is essential to consider effects of OCR in a two-phase ejector analysis. As a result of the simulation, considering the effect of oil, a lower mass flow rate was predicted compared to existing analysis. Compared to previous studies without considering oil, there was an average mass flow rate difference of 13.6%. This simulation result was validated through experiments.

An experimental investigation of oil circulation ratio influence on heating performance in an air condition heat pump system for electrical vehicles

Oil, lubricating and cooling the sliding parts of the compressorcomponents exposed to friction, could prevent refrigerant gas leakage in compressor chamber of vapor compression air conditioning system. For air condition heat pump system in electric vehicles, no oil circulating routes and oil separator are used to drive oil from system to compressor, making oil retainedmore difficult to control. Oil accumulated in heat exchanger and expansion valve would enhance pressure drops and reduce heat transfer capacity, leading to decrease of entire heating performance in cold climate. In this paper, a newly designed air conditioning heat pump system with R134a and PAG46 oil is introduced firstly, and oil circulation experiments were conducted on a full-scale electric vehicle. Typical heat transfer characteristics and system performance parameter were recorded and analyzed subsequently under different compressor oil charge conditions, including excessive charge and insufficient charge. Oil circulation rates in system were also measured and exhibited an increasing trend with compressor oil charge mass. Furthermore, heating performances of the system under different oil circulation rate were compared by various heating capacity and pressure drops. Results showed that both excessive and insufficient charge might decrease heating performance, i.e., determination of suitable oil charge is quite important for air condition heat pump system.

Impinging oil separator for compressors

Abstract Oil separation is commonly needed in air conditioning and refrigeration systems to reduce the oil circulation rate and keep the oil inside the compressor. For compactness, the oil separation structure integrated into the compressor is more and more popular than traditional external oil separators. However, a quantitative guideline for the design of oil separation structure is missing for irregular geometry and realistic flow condition at the compressor discharge. This paper presents impinging separation mechanism, one of the basic mechanisms of droplet separation. The separating structures are studied by flow visualization and experimental measurement under realistic compressor discharge conditions. The video of oil mist flowing through the baffles and plates is captured by a high-speed camera and analyzed quantitatively. The effect of the oil flow condition and the effect of separator geometry design are investigated based on the experimental data. With the help of flow visualization, the flow details are analyzed, and a semi-empirical model is proposed to predict the separation efficiency and pressure drop of a certain wave-plates structure. The results and conclusions from this study give useful guidelines about how to reduce the oil circulation ratio by designing oil separators.

Use of peripheral fins for R-290 charge reduction in split-type residential air-conditioners

The use of hydrocarbons as refrigerants has increased substantially in the past decades due to more severe regulations on direct and indirect carbon dioxide emissions. However, safety standards related to flammable fluids limit the amount of propane (R-290) used in air-conditioning (AC) systems. In this study, an experimentally validated numerical model for a split-type AC system is employed and, for the first time, an air-side peripheral fin geometry is used in the condenser in an attempt to reduce the refrigerant charge for a given system cooling capacity. To establish a fair basis for comparison, plain, wavy and louvered fins were also investigated in the model. A more realistic assessment of the system performance was achieved by considering the POE ISO 22 oil circulation ratio (OCR) in the computation of the physical properties of the mixture. The results have shown that, for the simulated range, heat exchangers with peripheral fins provided higher values of thermal conductance (UA) and cooling capacity than other types of fins. A substantial decrease in the R-290 mass (as high as 12.13%) was observed when peripheral fin condensers were employed.

Formation, distribution, and movement of oil droplets in the compressor plenum

Oil is important to compressors, but it brings some side effects on the performance of refrigeration systems. Discharge valve or orifice in the compressor plenum is the gateway for the oil to leave the compressor and circulate in the system. The space of the compressor plenum has the potential to separate the oil mist from the refrigerant so that oil circulation ratio (OCR) is reduced. In this study, the formation, distribution, and movement of oil droplets in the plenum are investigated through visualization and CFD simulation. Videos of oil atomization are captured by the high-speed camera to describe how oil mist is formed. The distribution of droplet size and velocity is quantified by video processing techniques. Discrete phase model gives the numerical approach to study the oil droplet movement inside the compressor plenum. The numerical simulation is verified by comparing visualization results with simulation results. The oil flow phenomenon and its quantification provide important information for the design of separation structure incorporated in the compressor. Also, a set of tools are developed and verified to study the oil droplets behavior in different types of compressor plenums.

Oil flow at the scroll compressor discharge: visualization and CFD simulation

Oil is important to the compressor but has other side effect on the refrigeration system performance. Discharge valves located in the compressor plenum are the gateway for the oil when leaving the compressor and circulate in the system. The space in between: the compressor discharge plenum has the potential to separate the oil mist and reduce the oil circulation ratio (OCR) in the system. In order to provide information for building incorporated separation feature for the oil flow near the compressor discharge, video processing method is used to quantify the oil droplets movement and distribution. Also, CFD discrete phase model gives the numerical approach to study the oil flow inside compressor plenum. Oil droplet size distributions are given by visualization and simulation and the results show a good agreement. The mass balance and spatial distribution are also discussed and compared with experimental results. The verification shows that discrete phase model has the potential to simulate the oil droplet flow inside the compressor.

Quantification of flow and retention of oil in compressor discharge pipe

This paper presents a new method to measure the oil retention and oil circulation ratio (OCR) in the compressor discharge pipe based on oil film thickness, oil film average velocity, oil droplet size, oil droplet velocity, and system mass flow rate. Oil flow parameters are quantified based on visualization using high-speed camera and video processing techniques. The estimated oil retention and oil circulation ratio results are compared quantitatively with the results from sampling measurements under different compressor speed and compressor types. The agreement between video results and sampling measurements verify the accuracy of this innovative method, which can also be applied in other annular-mist flow analysis. It also shows that most of the oil exists in film by mass while oil droplets contributes more to the oil mass flow rate because oil droplets travel in a much higher speed.

Oil droplet behavior with the position of a muffler discharge hole in the shell of a rotary compressor

In this study, high-frequency visualization in the compressor shell via optical glass was performed to confirm inner flow. Visualization experiments were conducted under actual operating conditions (suction pressure = 0.99 MPa, discharge pressure = 2.29 MPa, operating speed = 40, 60, 80 Hz) by using a calorimeter. Most oil droplets transferred to the upper cavity were observed at the coil winding area around the muffler discharge hole. Oil droplets released from the muffler hole collided with the lower face of the motor, because of the high discharge speed and the short distance between the motor and the muffler. In this process, a part of these oil droplets were directly induced into the coil winding area. According to the above-mentioned results, the vane installation to reduce the Oil circulation ratio (OCR) was considered through Computational fluid dynamics (CFD) simulation. The vane was installed inside the coil winding area around the muffler hole. As a result, the oil droplet velocity was decreased by vane, and an OCR reduction effect from 1.2 to 1.8% was predicted.

A numerical study on the oil retention of R410A and PVE oil mixture in multi heat pump system

Predicting an amount of discharged oil from a compressor is necessary to charge the compressor with proper amount of oil. The amount of discharged oil can be predicted by calculating the oil retention amount in each component of a heat pump system. This study suggests a method for calculating the oil retention amount in a heat pump system. In addition, flow pattern of refrigerant and oil mixture in horizontal gas line of refrigerant was ascertained by flow visualization. The oil retention amounts in each component of multi heat pump system were calculated with respect to mass flux of refrigerant, Oil circulation ratio (OCR), length of horizontal and vertical lines. Oil retention amounts in horizontal and vertical gas lines of refrigerant were significant. To validate the model for gas lines of refrigerant, comparison between predicted and experimental oil retention amounts was conducted, and mean absolute percentage error was 15.0%.

A study on the flow characteristics of refrigerant and oil mixture in compressor suction line

Abstract This paper discusses the experimental results of oil retention amount and pressure drop in the gas line of refrigerant. R410A and polyvinylether oil were used as refrigerant and lubricant, respectively. The experiment was conducted under various pipe inner diameters of 14.1, 17.3 and 26.0 mm, and refrigerant mass flux and oil circulation ratio (OCR) were varied from 40 to 250 kg m −2 s −1 and 0.5–4.0%, respectively. The oil retention amount tends to increase as refrigerant mass flux decreases and OCR increases. The oil retention amount in vertical pipe was higher than that in horizontal pipe. The empirical expressions for predicting the oil retention amount and pressure drop were suggested. The mean absolute percentage errors of each expression for predicting the oil retention amount in horizontal and vertical pipes were 18.1% and 14.1%, respectively, and those for predicting the pressure drop in horizontal and vertical pipes were 22.2% and 20.1%, respectively.

A112 音速計を用いたHFO-1234vf冷凍サイクル内のオイル循環率測定(OS-7:次世代ヒートポンプ・冷凍システム(1))

In order to clarify the possibility of measuring an oil circulation ratio (OCR), the speed of sound in a mixture of refrigerant/PAG oil has been measured in an HF01234yf refrigeration cycle. As the result, it is found that the speed of sound of the mixture depends on the value of OCR, although it is also affected by temperature and pressure. We think that it is possible for measuring OCR by means of measuring the speed of sound by taking into account the effected of temperature and pressure. In the case of immiscible oil, on the other hand, the measured value of the speed of sound changes significantly when oil drops pass through the sound velocity sensor. By using a mixing chamber, the variation of the speed of sound becomes small.

CO2압축기의 운전조건과 오일토출의 상관분석에 관한 실험적 연구

A portion of oil for lubrication of compressor flows together with refrigerant in the refrigeration system. If the oil discharge from a compressor is increased in the refrigeration system, not only pressure drop is increased in other components, such as evaporator and gas cooler, but also heat transfer coefficient in the heat exchangers is decreased. Oil discharge rate from a compressor may strongly depend on operating conditions of a compressor. In this study, one stage single rotary compressor is employed for measuring oil circulation ratio(OCR). Carbon dioxide and PAG oil are used as refrigerant and lubricant. Using a U-tube densimeter, mixture density is measured. Oil circulation ratio(OCR) can be estimated by measured mixture density. The results obtained indicate that the oil circulation ratio(OCR) is increased as the suction temperature or compressor operating frequency is increased. Oil circulation ratio(OCR) correlation of the compressor is also suggested.

Heat transfer and flow characteristics of flow boiling of CO2‐oil mixtures in horizontal smooth and micro‐fin tubes

AbstractExperiments were carried out on the flow pattern, heat transfer, and pressure drop of flow boiling of pure CO2 and CO2‐oil mixtures in horizontal smooth and micro‐fin tubes. The smooth tube is a stainless steel tube with an inner diameter of 3.76 mm. The micro‐fin tube is a copper tube with a mean inner diameter of 3.75 mm. The experiments were carried out at mass velocities from 100 to 500 kg/(m2·s), saturation temperature of 10 °C, and the circulation ratio of lubricating oil (PAG) was from 0 to 1.0 mass%. Flow pattern observations mainly showed slug and wavy flow for the smooth tube, but annular flow for the micro‐fin tube. Compared with the flow patterns in the case of pure CO2, an increase in frequency of slug occurrence in the slug flow region, and a decrease in the quantity of liquid at the top of the tube in the annular flow region were observed in the case of CO2‐oil mixtures. With pure CO2, the flow boiling heat transfer was dominated by nucleate boiling in the low vapor quality region, and the heat transfer coefficients for the micro‐fin tube were higher than those of the smooth tube. With CO2‐oil mixtures, the flow boiling heat transfer was dominated by convective evaporation, especially in the high vapor quality region. In addition, the heat transfer coefficient decreased significantly when the oil circulation ratio was larger than 0.1 mass%. For the pressure drop characteristics, in the case of pure CO2, the homogeneous flow model agreed with the experimental results within ±30% for the smooth tube. The pressure drops of the micro‐fin tube were 0–70% higher than those predicted with the homogeneous flow model, and the pressure drops increased for the high oil circulation ratio and high vapor quality conditions. The increases in the pressure drops were considered to be due to the increase in the thickness of the oil film and the decrease in the effective flow cross‐sectional area. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20287

Heat Transfer and Flow Characteristics of Flow Boiling of CO2-oil Mixtures in Horizontal Smooth and Micro-fin Tubes

Experiments were carried out on the flow pattern, heat transfer, and pressure drop of flow boiling of pure CO2 and CO2-oil mixtures in horizontal smooth and micro-fin tubes. The smooth tube is a stainless steel tube with an inner diameter of 3.76 mm. The micro-fin tube is a copper tube with a mean inner diameter of 3.75 mm. The experiments were carried out at mass velocities from 100 to 500 kg/(m2·s), saturation temperature of 10 °C, and the circulation ratio of lubricating oil (PAG) was from 0 to 1.0 mass%. Flow pattern observations mainly showed slug and wavy flow for the smooth tube, but annular flow for the micro-fin tube. Compared with the flow patterns in the case of pure CO2, an increase in frequency of slug occurrence in the slug flow region, and a decrease in the quantity of liquid at the top of the tube in the annular flow region were observed in the case of CO2-oil mixtures. With pure CO2, the flow boiling heat transfer was dominated by nucleate boiling in the low vapor quality region, and the heat transfer coefficients for the micro-fin tube were higher than those of the smooth tube. With CO2-oil mixtures, the flow boiling heat transfer was dominated by convective evaporation, especially in the high vapor quality region. In addition, the heat transfer coefficient decreased significantly when the oil circulation ratio was larger than 0.1 mass%. For the pressure drop characteristics, in the case of pure CO2, the homogeneous flow model agreed with the experimental results within ±30% for the smooth tube. The pressure drops of the micro-fin tube were 0–70% higher than those predicted with the homogeneous flow model, and the pressure drops increased for the high oil circulation ratio and high vapor quality conditions. The increases in the pressure drops were considered to be due to the increase in the thickness of the oil film and the decrease in the effective flow cross-sectional area. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20287