Solubility Of Refrigerant Research Articles

Solubility prediction of refrigerants in PEC lubricants based on back-propagation neural network combined with genetic algorithm

In the present study, a backpropagation neural network combined with genetic algorithm (GA-BP) model was constructed for prediction the solubility of refrigerants in linear chained precursors of POE lubricants (PECs). A total of 2248 experimental solubility data of refrigerants in PECs reported in literature were collected with temperatures from 243.15 K to 363.15 K and pressures up to 10 MPa. The input variables of the model were optimized using non-dominated sorting genetic algorithm with elite strategy (NSGA-II). The optimized inputs include temperature, pressure, molecular weight, critical temperature, and acentric factor. Results indicate that the GA-BP model using the optimized inputs can correlate the solubility data with good accuracy, the average absolute relative deviation between calculated results from the model and the literature is 0.98 %. Moreover, in order to validate the predictive ability of the established GA-BP model, the solubility of R1243zf in PEC4 and PEC5 was measured at the temperature range from 278.15 K to 343.15 K. The calculated values from the GA-BP model were compared with the experimental data, and the average absolute relative deviation is 8.85 %. Finally, sensitivity analysis was performed by Partial Derivatives (PaD) method to assess the contribution of input variables. Leverage approach was used for outlier detection to ensure the robustness of the model.

Vapor-liquid equilibrium measurements of ionic liquid [DEME][TFSI] with different hydrofluorocarbon refrigerants R152a, R32, and R143a

To overcome the shortages of traditional working pairs (LiBr/H2O and NH3/H2O) for absorption-refrigeration systems (ARSs), novel working pairs consisting of low global warming potential (GWP) hydrofluorocarbons difluoroethane (R152a) and difluoromethane (R32) plus low-viscosity ionic liquid N,N‑diethyl-2‑methoxy-N-methylethan-1-aminium bis((trifluoromethyl)sulfonyl)amide ([DEME][TFSI]) are studied. Firstly, solubilities of R152a and R32 in [DEME][TFSI] are measured by the isochoric saturation method in the temperature range of 293 K ∼ 343 K and pressure range of 0.14 MPa ∼ 1.664 MPa with the relative expanded uncertainty of 0.03. A high-GWP hydrofluorocarbon 1,1,1-trifluoroethane (R143a) is involved in the experiment for further comparison. The non-random two-liquid model is used to correlate the vapor-liquid equilibrium data with average absolute relative errors of 0.55 % (R152a), 0.58 % (R32), and 0.94 % (R143a), respectively. [DEME][TFSI] shows its low viscosity and good solubility of hydrofluorocarbons by contrast with conventional ionic liquids. A refrigerant solubility rank order in [DEME][TFSI]: R152a > trans-1,3,3,3-tetrafluoropropene (R1234ze(E)) > R32 > 2,3,3,3-tetrafluoro-1-propene (R1234yf) > R143a is obtained from the comparison. This work provides the possibility to apply low-GWP working pairs R152a/[DEME][TFSI] and R32/[DEME][TFSI] in ARSs.

Numerical Prediction of Refrigerant Oil Two-Phase Flow from Scroll Compressor Discharge to the Suction Side via Back Pressure Chamber

Oil lubricates the contact between the orbiting and stationary scroll in the refrigerant scroll compressor, while the sealing between the scrolls is achieved through the refrigerant vapour pressure in the sealed back pressure chamber. The back pressure should be adjusted using the refrigerant oil two-phase flow from the oil separator at the compressor discharge to the back pressure chamber and the refrigerant oil flow from the back pressure chamber to the compressor suction side. Both of the flows are conducted through connecting tubes with corresponding high-pressure and low-pressure nozzles with small diameters. Models for predicting the refrigerant oil critical and subcritical flows through the nozzles were developed and applied in enable the prediction of the back pressure. The models are original, because the slip between the oil and the refrigerant as well as the refrigerant solubility in the oil are taken into account. The critical flow model is validated against the experimental data that are available in the literature. The back pressure is predicted by equating the mass flow rates of refrigerant and oil two-phase mixtures through the high- and low-pressure nozzles. The results show that the critical flow takes place through the high-pressure nozzle, while the subcritical flow through the low-pressure nozzle can also exist in cases with a small pressure difference between the back pressure chamber and the compressor suction side. The refrigerant solubility in the oil has a small influence on the critical and subcritical refrigerant oil mixture mass flow rates, while the influence on the back pressure is more pronounced.

Two-phase lubrication characteristics of journal bearing in refrigerant-oil system under high-pressure environment considering gaseous cavitation

The journal-bearing system is crucial for the reliability of refrigerant compressors that operate under high-ambient-pressure environments. The pressure changes rapidly, which hereby results in the variation in solubility of refrigerant in oil, and causes gaseous cavitation of the refrigerant-oil mixture in the bearing. However, the cavitation mechanism under high-pressure environments is not well understood, and its impact on lubrication characteristics has been rarely discussed. In this paper, a numerical model is developed to simultaneously solve the gaseous cavitation and two-phase lubrication problems. The first systematic comparison of lubrication characteristics for constant-eccentricity journal bearings, in such a special environment, between the classical Reynolds boundary and gaseous cavitation boundary is presented. Results indicate the load-carrying capacity is reduced by about 3% at a lower eccentricity of 0.3, but is greatly enhanced by about 21% at a higher eccentricity of 0.9 due to the gaseous cavitation. Meanwhile, the ‘reverse’ flow rate is found to reach -0.057 at the end side of the bearing owing to sub-ambient pressure up to -1 MPa at the eccentricity of 0.8. The cavitation-induced sub-ambient pressure should be addressed for the design of the bearing operating in such pressurized refrigerant environments as ambient gas might be sucked into the bearing.

Molecular dynamics investigation on the interaction of nitrile-based ionic liquids in the separation of azeotropic refrigerant R-513A

The reclamation of mixed refrigerants stands as a potent measure to substantially mitigate the emission of fluorinated gases (F-gases). In the separation of the azeotropic refrigerant R-513A (comprising 56 wt% R-1234yf, 2,3,3,3-Tetrafluoropropene, and 44 wt% R-134a, 1,1,1,2-Tetrafluoroethane), the extractive distillation process using ionic liquids (ILs) was employed. The molecular structure of nitrile-based ILs ([C4C1im][SCN], [C4C1im][N(CN)2], [C4C1im][C(CN)3], [C4C1im][B(CN)4]) exerts a regular impact on the solubility of refrigerants, thereby necessitating molecular dynamics simulation to ascertain the law of refrigerant solubility in ILs concurrent with the augmentation in the number of –CN in the anion. Additionally, an analysis was conducted to elucidate the mechanism underlying the high selectivity of nitrile-based ILs. The results revealed pronounced interactions between R-1234yf and R-134a, predominantly occurring between carbon atoms (-CF3 groups in R-134a and R-1234yf), and chiefly van der Waals interactions. An enhancement in the number of –CN in anion led to the fortification of the van der Waals interaction between R-134a/R-1234yf and the anion. The stronger nonbonding interaction energy between R-134a and the anion, as well as the higher radial distribution function (RDF) peak height between R-134a and the anion, signified a more robust interaction between R-134a and the anion compared to R-1234yf and the anion. Concurrently, the variance in the RDFs in the anion-C atom (-CH2F group in R-134a and -CF group in R-1234yf) revealed the selectivity of the four ILs to R-134a and R-1234yf. Among the four anions, [B(CN)4]− exhibited the most interaction sites with R-134a. This investigation contributes significantly to comprehending the interaction mechanism of azeotropic refrigerants and the interaction mechanism between refrigerants and nitrile-based ILs.

Dynamic desorption characteristics of propane and polyalkylene glycol lubricants

Natural refrigerant propane has a saturated-liquid density that is only about half that of HFC refrigerants. Compared with traditional refrigerants, the absorption/desorption characteristics of propane with lubricants show great differences. Studies on the dynamic characteristics of refrigerant/lubricant mixtures have mainly focused on the absorption process, especially the absorption process of high-density refrigerants. The desorption of refrigerant from the mixture has been less studied. In this paper, an experimental apparatus was built to observe and measure the dynamic characteristics of refrigerant desorption. The effects of the refrigerant/lubricant combinations, the depressurization rate, the ambient temperature, and the initial concentration on the desorption process driven by depressurization were analyzed. The depressurization rate is the main factor influencing the desorption mass flow rate. As the pressure dropped, the refrigerant solubility decreased fast, causing the supersaturation degree to rise rapidly, which is the main driver of refrigerant desorption. Moreover, the correlation between the desorption mass flow rate and the supersaturation degree was obtained by the experimental results. The absolute deviations between the experimental and simulated concentrations were less than 2.5%.

Mutual diffusivities of difluoromethane and 2,3,3,3-tetrafluoropropylene dissolved in polyalkylene glycol lubricant

In this work, the mutual diffusivities of difluoromethane (R32) and 2,3,3,3-Tetrafluoropropylene (R1234yf) dissolved in Polyalkylene glycol (PAG) lubricant were studied experimentally based on the principle of dynamic light scattering. The pressure range of mutual diffusivity measurement is from 0.142 MPa to 3.371 MPa, and the temperature range is from 293.15 K to 353.15 K. The experimental data were correlated as the equation of temperature and pressure. The average relative deviation between the experimental data and the model is less than 1.34%. The results show that when the temperature is constant, mutual diffusivities of R32/PAG and R1234yf/PAG mixture decrease with the increasing of the solubility of refrigerants. The mutual diffusivity of R1234yf/PAG is less than that of R32/PAG at the same temperature and solubility. This may be due to the longer length of the R1234yf molecule and greater polarity, which results in the difficult diffusion in the lubricant. The molecular radius (0.164 and 0.299 nm) for the mutual diffusivity of R32 and R1234yf in PAG lubricants were obtained according to the Einstein-Stokes equation. The obtained radius values are within a reasonable order of magnitude compared to literature values. Since the molecular radius of R32 is smaller than that of R1234yf, which is also consistent with the experimental results in this work that the mutual diffusivity of R32 dissolved in PAG is larger than that of R1234yf.

Numerical Study on Liquid-Return Characteristics in the Accumulator of the R290 Rotary Compressor

Liquid often exists in the accumulator of the rotary compressor during the process of startup or defrost of air-conditioning systems. Too much liquid entering the compressor cylinder would result in excessive pressure caused by the liquid compression, which is a great threat to the compressor. The liquid return through the liquid-return hole is the key to ensure the stable operation of the compressor. In this paper, the liquid-return characteristics in the liquid-return holes of the accumulator, including the mass-flow rate, liquid velocity and pressure difference between the liquid-return holes, during the startup process of the R290 rotary compressor are numerically investigated. The numerical simulation using the fluent volume of fluid (VOF) method was experimentally validated with the error of 1.55%. The comparison of liquid-return characteristics using different refrigerants is conducted. Effects of refrigerant solubility in the oil, refrigerant/oil-mixture type, liquid-return-hole diameter and compressor frequency on the liquid-return characteristics and liquid shape trough the liquid-return hole are discussed. The results show that the surface tension and viscosity of the liquid are the main factors affecting the liquid-return speed. The liquid-return rate of the refrigerant R290 is slower than that of other refrigerants R22 and R410A. The liquid-return rate increases with the increase in the compressor frequency. We conclude that for air-conditioning systems using R290 as refrigerant, increasing the number of return holes or the hole diameter is necessary to improve the liquid-return characteristics of the compressor. This research will provide theoretical guidance for the optimization of liquid return of rotary compressors using new refrigerants.

Thermal-Elastohydrodynamic Contacts Lubricated With Oil/Refrigerant Solutions: A New Cavitation Modeling Approach Based on Refrigerant Solubility

Abstract A first cavitation modeling with thermal effects for oil/refrigerant solutions lubricated elastohydrodynamic point contacts is reported in this work. The solubility of the oil/refrigerant system is introduced into the generalized Reynolds equation coupled with the elasticity equation and the energy conservation equation. The numerical results show a very good agreement with the published experimental results concerning film thickness prediction. Moreover, the present model describes the cavitation region on a physical basis. A discussion with other cavitation models from the literature is proposed. It puts into light the necessity of taking into account the solubility of the refrigerant into oil for such problems. Compared to pure oil, oil/refrigerant solutions can potentially reduce the amount of liquid oil for the next contact due to its higher cavitation intensity.

Refrigerant migration and oil sump viscosity change of R290 high-pressure rotary compressor during shutdown process

Propane (R290) has excellent thermophysical properties and development potential. The dynamic characteristics of R290 rotary compressors and room air conditioners have been studied mainly on the start-up and defrost processes. There is little research on the shutdown process. Unlike hydrofluorocarbons (HFCs), the density of liquid propane is lower than that of lubricant. It is difficult for propane to dissolve into the oil sump and downward after the refrigerant separation after shutdown. The shutdown state affects the distribution of refrigerant in the oil sump and the mixture viscosity of the restart-up, which is related to the compressor reliability. In this paper, an R290 room air conditioner system was built and the variations of system pressures, temperatures, and oil sump parameters of an R290 high-pressure rotary compressor with three different lubricants were measured after shutdown in two conditions. As the experiments indicated, driven by the refrigerant supersaturation (the difference between the actual refrigerant solubility and saturation solubility) resulting from the rapid pressure drop after shutdown, a large amount of refrigerant separated from the oil sump, and the refrigerant solubility dropped fast. As the compressor temperature decreased, the bottom viscosity reached a peak. For the low-temperature heating condition, the peak was about 120.4∼160.1mPa•s. However, when the degree of supersaturation decreased to a certain value, the refrigerant no longer separated out. There is always a solubility gradient of refrigerant in the oil sump of R290 high-pressure compressor after shutdown. Its size was related to the pressure drop and the miscibility of lubricants and refrigerants.

Solubility measurements of refrigerants in polyolesters lubricants at temperature from 323.K to 383.K

In refrigeration systems and heat pumps, lubricant selection is important to avoid issues that can lead to a breakdown of the compressor and the entire system. Daniel plots are a great tool to make the selection. Half of the graph is the solubility of refrigerant in lubricating oil. Isothermal vapor-liquid equilibrium (VLE) data are measured for five refrigerants in four oils. Refrigerants include HFC-32 (dichlorofluoromethane), HFC-134a (1,1,1,2-tetrafluoroethane), HFO-1234yf (2,3,3,3-tetrafluoropropene), HFO-1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene) and HCFO-1233zd(E) (trans-1‑chloro-3,3,3-trifluoroprop-1-ene) in four oils named POE 80, SE 55, SE 170 and SE 220, based on their viscosity grade. The experimental technique used is based on static synthetic methods. All of the data are correlated with the cubic equation of state developed by Yokozeki.

Research on the Effect of Refrigerant Solubility on the Lubrication Characteristics of Connecting Rod Bearings

Research on the Effect of Refrigerant Solubility on the Lubrication Characteristics of Connecting Rod Bearings

Misleading results on the use of artificial neural networks for correlating and predicting properties of fluids. A case on the solubility of refrigerant R-32 in ionic liquids

The ability and limitations of Artificial Neural Networks (ANN) for correlating and predicting properties of fluids are discussed. Common mistakes found in applications of ANN to fluid property estimation presented in the literature are analyzed and a useful and appropriate way of presenting results of these applications is proposed. As a study case the use of ANN for correlating and predicting the solubility of difluoromethane (R-32, C2H2F2), in seventeen ionic liquids is performed and discussed. The dependent variable is the solubility of R-32 and the independent variables are the temperature (T) and pressure (P), in addition to properties that identify each Ionic Liquid, such as critical temperature (Tc), the critical pressure (Pc), the mass connectivity index (λ), the acentric factor (ω), the mass of the cation (M+) and the mass of the anion (M−). Six cases, combining these independent variables in different forms were analyzed. The study shows that although some cases of the proposed artificial neural networks model give good results for training and testing, none of them gives acceptable results for prediction. Finally, some indications and suggestions are given to correctly apply ANN and to present the results in the literature.

Understanding and improving the 9-coefficient pressure viscosity temperature (PVT) model

Data describing the physical properties of a mixed refrigerant-lubricant blend is necessary for compressor and system design and is measured using a pressure, viscosity, temperature (PVT) instrument setup. Measurements are taken at discrete conditions and refrigerant concentrations and a model is regressed which can describe pressure (refrigerant solubility), working viscosity, and density across the range of interest. Understanding the applicability of the model is crucial in choosing a suitable lubricant, since the tested conditions are unlikely to exactly match all operating conditions of interest.This paper describes how the PVT model is created, and how choices in input data can affect the resulting model. Emphasis is placed on the range over which PVT models are valid and what can be done to increase model accuracy.

Molecular modeling of the solubility of low global warming potential refrigerants in polyol ester lubricants

Pentaerythritol esters (PECs) act as the main precursors in polyol ester (POE) lubricants. The solubility of low global warming potential (GWP) refrigerants in linear chained PECs was studied in this work using the perturbed chain form of the statistical associating fluid theory (PC-SAFT). The effect of PEC alkyl tail length on the solubility of various refrigerants was investigated. Results showed that the solubility of HFO refrigerants is weakly dependent on PEC molecular structure. On contrary, the solubility of other natural and paraffin refrigerants significantly increased with increasing the alkyl tail length. Within the SAFT framework, pure component and binary interaction parameters were found to correlate linearly with PEC molecular weight. These correlations were used to predict the solubility of refrigerants in a predefined PEC blend through component lumping. Sensitivity analyses showed that the latter simplification could adequately describe the mixture phase behavior. The proposed model was then applied to fully and accurately predict the solubility of carbon dioxide in commercial POE lubricants. Results demonstrate that the proposed model can be used as a fully predictive tool to determine the solubility of refrigerants in commercial POE oils, provided that the lubricant molecular weight is known, a clear step forward developing adequate low GWP refrigerants-lubricant pairs.

Modeling solubility of refrigerants in ionic liquids using Peng Robinson-Two State equation of state

In recent years ionic liquids (ILs) are introduced as new absorbents for refrigerants in the refrigeration processes. In this paper, we investigate the phase equilibrium behavior of refrigerants and ILs binary solutions as novel potential systems in refrigeration industries. A simple association model namely Peng Robinson-Two State equation of state (PR-TS EoS), is used to model the phase equilibrium data (P-T-x) of 37 various imidazolium-based ionic liquids and refrigerants binary systems at different temperatures and pressures with a total of 969 data points. In the first step, the PR-TS EoS is applied on pure components and the parameters of the model are determined. The percent of average absolute relative deviation (AARD%) between the calculated results and the experimental data of vapor pressures and liquid densities of the refrigerants are 0.58% and 1.59%, respectively. The similar AARD% for the liquid density of the ionic liquids is 0.20%. In the following, the present model is applied to calculate the vapor-liquid equilibrium of refrigerant + IL binary systems. The results of the current model show a deviation as AARD = 3.54%. Also, the performance of the PR-TS EoS is compared with the Peng-Robinson (PR) EoS and Peng Robinson-Kwak Mansoori (PR-KM) EoS. In summary, the results of the PR-TS EoS represent a good agreement with the experiments and the present model shows superiority with respect to PR and PR-KM EoSs.

Quantitative Elastohydrodynamic Film-Forming for an Oil/Refrigerant System

The first calculations of film thickness for an oil/refrigerant system using quantitative elastohydrodynamics are reported in this work. It is demonstrated that primary measurements of the properties of the oil/refrigerant system can be employed to accurately predict film thickness in concentrated contacts. An unusual response to lubricant inlet temperature is revealed, wherein the film thickness may increase with temperature as a result of decreasing refrigerant solubility in oil when the inlet pressure is high. There is competition between the reduction in viscosity of the oil and the reduction of refrigerant concentration with increased temperature. For high inlet pressure, the dilution effect is dominant, whereas for low inlet pressure, the temperature dependence of the viscosity of the solution dominates over the range of inlet temperatures considered. It seems that only central film thicknesses have been experimentally measured for oil/refrigerant systems leaving these calculations as the only means of assessing the minimum.

Influence of refrigerant solubility and surface geometry on the wetting properties of lubricating oil

Experimental results are reported on the effect of refrigerant solubility on the wetting parameters (apparent contact angle and half-filling angle) of POE ISO 10 lubricating oil on carbon steel surfaces. The basic geometric configuration of the test section consisted of an oil film strained between two vertically aligned spheres. An ancillary configuration consisted of a sphere and a flat horizontal surface (infinite curvature radius). R-134a was used as the refrigerant in all tests. The influence of the diameters of the spheres and the spacing between them was also evaluated. It was found that an increase in the curvature of the surfaces gives rise to a reduction of the contact angle and an increase in the half-filling angle. In the sphere–sphere configuration, the contact angle was observed to be inversely proportional to the refrigerant solubility.

Cooling of a reciprocating compressor through oil atomization in the cylinder

An analysis of the influence of oil atomization in the cylinder of a reciprocating compressor is presented in this paper. During compression, oil atomization enhances heat removal from the refrigerant gas. This cooling effect, which eventually results in a global temperature decrease of the compressor parts, aims primarily at reducing the refrigerant superheating in the suction system and inside the cylinder, which is largely responsible for overall energy losses and a decrease of the volumetric efficiency. A prototype was constructed and tested with R-134a in a hot gas-cycle calorimeter. A significant reduction of the compressor thermal profile has been achieved, with the largest variations of 30.9°C (55.6°F) and 23.6°C (42.5°F) in the discharge chamber and cylinder wall, respectively. A major dependence of the compressor efficiency parameters on the refrigerant solubility in the oil has been observed. A simulation model using an integral control volume formulation for mass and energy conservation in the cylinder and in other compressor components is proposed. Based on this model, the effect of oil atomization on the compressor performance is presented and discussed in terms of oil injection parameters, such as nozzle position, oil temperature and flow rate.

Solubility Measurements and Data Correlation of Carbon Dioxide in Pentaerythritol Tetra(2-ethylbutanoate) (PEBE6)

Carbon dioxide is one of the most promising alternative refrigerants, but its compatibility with compressor lubricants is still almost unknown. The solubility of refrigerants in oils is one of the most important properties in the selection of the proper lubricant. For this reason, in our laboratories a systematic research program on the solubility of carbon dioxide in oils has been started, putting particular attention on its dependence on lubricant chemical structure, temperature, and pressure. In this paper, experimental solubility data and their correlation of carbon dioxide in a precursor of polyol ester oil, that is, pentaerythritol tetra(2-ethylbutanoate), are presented in the temperature range of (283 to 343) K and pressures up to 10 MPa.