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.     

An analytical equation of state for refrigerant mixtures

We have extended our previous work on the equation of state for refrigerants to their mixtures successfully. The temperature-dependent parameters of the equation of state have been calculated using our previous corresponding-states correlation based on the normal boiling point temperature and the liquid density at the normal boiling point. We have applied a simple combining rule for the normal boiling point constants to extend our previously proposed equation of state to mixtures of refrigerants. In this work the liquid densities of a large number of refrigerant mixtures have been calculated and the results are compared both with experimental data and a recent correlation by Nasrifar et al. (1999). The agreement is good.     

The viscosity surfaces of R152a in the form of multilayer feed forward neural networks

A multilayer feedforward neural network (MLFN) technique is adopted for developing a viscosity equation for R152a. The results obtained are very promising, with an average absolute deviation (AAD) of 0.36% for the currently available 300 primary data points, and they are a significant improvement over those of a corresponding conventional equation in the literature. The method requires a high accuracy equation of state for the fluid in order to convert the experimental P,T into the independent variables ρ,T, but such equation may not be available for the target fluid. Aiming at overcoming this difficulty, two viscosity explicit equations in the form , avoiding the density variable, are also developed, one for the liquid surface and the other for the vapor one. The reached accuracy levels are equivalent to that of the former equation. The trend of the reduced second viscosity virial coefficient is correctly reproduced in the data range. The proposed technique, being heuristic and non theoretically founded, is also a powerful tool for experimental data screening.     

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%.     

The prediction of vapour–liquid equilibrium behaviour of HFC blend–oil mixtures from commonly available data

This paper addresses the problem of achieving accurate calculations for vapour–liquid equilibrium behaviour in the presence of the (new) oils in use with HFC refrigerants by introducing a correlation which depends only on the gas constant, critical temperature and molecular weight of the blend constituent and the molecular weight of the oil; i.e. the method depends on commonly available data and not a specialised measurements. The work is believed to be of value to plant designers and research workers.     

Comparison of thermophysical properties for oil/refrigerant mixtures by use of the pulse heating method

The method of pulse heating for the study of thermophysical properties for oil/refrigerant solutions in a wide temperature range and for monitoring of an actual state of these systems has been developed. The regimes of linear heating and thermostabilization of the superheated probe are applied for solving our task. The objects of study are as follows: synthetic oils Mobil EAL Arctic 22, PLANETELF ACD22, XMPA, and solutions of carbon dioxide in these oils. The upper boundary, with respect to temperature, of the two-phase equilibrium region including the vicinity of the liquid–vapour critical curve of these systems, gas solubility in oils at various temperatures, short-time thermostability, and thermal conductivity of oils are considered. Inclusion of the thermally unstable states of a substance in investigation allows one to essentially extend the set of compared data.     

Phase and viscosity behaviour of refrigerant–lubricant mixtures

The understanding of thermophysical properties and phase behaviour of refrigerant–lubricant oil mixtures is highly important for the optimal design of refrigeration and air-conditioning systems. Refrigerant–lubricant mixtures, which are likely to have strong asymmetry, can show complex phase behaviour such as closed miscibility gaps, open miscibility gaps, liquid–liquid–vapour equilibrium, and even barotropic phenomena (density inversions). In fact, the type of phase behaviour that refrigerant–lubricant mixtures may show is linked to the transition between different types of phase diagrams, mainly as a function of the molecular asymmetry. This also has a profound effect in the mixture transport properties. Thus, in this work the general aspects of phase and viscosity behaviour linked to the type of asymmetry found in refrigerant–lubricant mixtures are discussed in the context of phase behaviour phenomenology.     

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.     

Analysis of the gas compressibility effects on the constant-entropy reversible processes of refrigerants and refrigerant mixtures

Thermally and calorically real gas modelling based on the Martin–Hou equation of state is assumed for pure and mixed refrigerants in the superheated vapour phase. It allows the constant-entropy reversible processes which take place within the work transfer components of ideal vapour compression cycles to be properly analysed. These processes, in fact, occur in a region of the Mollier diagram close to the saturated vapour curve where covolume and molecular forces alter the equation of state of an ideal gas. Thus, real gas effects are significant and cannot be ignored. They give a more accurate indication of the refrigerant end temperature associated with an isentropic compression as well as of the corresponding work exchanged and volumetric efficiency. In particular, it is shown that the gas compressibility effects play a ‘favourable’ role during the isentropic compression processes since they allow the work transferred to be reduced up to 10% for HFC-refrigerant 134a, and HFC-refrigerant mixtures 407C and 410A. But, at the same time, they play an ‘unfavourable’ role since they can reduce the compressor volumetric efficiency (i.e. refrigerant mass flow rate) and, consequently, the cooling (or heating) capacity of the vapour compression system up to 7%.     

Flow fields in shell-and-tube condensers: comparison of a pure refrigerant and a binary mixture

Detailed 2D CFD calculations for vapour flow field and rate of condensation are carried out for a geometry similar to a real shell-and-tube condenser with 100 tubes, with condensation on the shell-side. The differences in vapour flow behaviour are investigated for pure R22 and for a binary mixture of R32 and R134a, which has a gliding temperature difference of 5.5 K. It is shown that, the flow field for a zeotropic mixture is significantly different from that for a pure fluid. The nature of the mixture flow causes the vapour and condensate to flow counter-currently in part of the condenser. Adjustments of the inlet design turn out to influence the rate of heat transfer by up to 24% for the conditions tested, with greater influence on heat transfer for lower driving forces.     

HFC134a/HC600a/HC290 mixture a retrofit for CFC12 systems

The environmental concerns with the impact of refrigerant emissions lead to the importance in identifying a long-term alternative to meet all requirements in respect of system performance and service. Even though HFC134a and HC blend (containing 55.2% HC600a and 44.8% HC290 by weight) have been reported to be substitutes for CFC12, they have their own drawbacks in respect of energy efficiency/flammability/serviceability aspects of the system. In this present work, experimental investigation has been carried out on the performance of an ozone friendly refrigerant mixture (containing HFC134a/HC blend) in two low temperature systems (a 165 l domestic refrigerator and a 400 l deep freezer) and two medium temperature systems [a 165 l vending machine (visi cooler) and a 3.5 kW walk-in cooler]. The oil miscibility of the new mixture with mineral oil was also studied and found to be good. The HFC134a/HC blend mixture that contains 9% HC blend (by weight) has better performance resulting in 10–30% and 5–15% less energy consumption (than CFC12) in medium and low temperature system, respectively.     

Solubility, density and viscosity of a mixture of R-600a and polyol ester oil

Experimental data on solubility, liquid phase density and viscosity of a mixture of R-600a and a POE ISO 7 lubricant oil are presented. A specially designed experimental facility for simultaneous measurements of the physical properties was used in the experiments at temperatures ranging from 10 to 60 °C. The VLE data were correlated with the Heil–Prausnitz and Flory–Huggins activity models and the Peng–Robinson equation of state (EoS). Liquid density was correlated with the Peng–Robinson EoS and with a first-order Redlich–Kister expansion for the excess molar volume. Liquid viscosity was correlated with an excess-property approach based on the classical Eyring liquid viscosity model. Satisfactory agreement was obtained between models and experiments; maximum root mean square (RMS) deviations of models used in the VLE, density and viscosity predictions were 1.1% (VLE EoS), 0.2% (Redlich–Kister) and 3.0% (Grunberg–Nissan), respectively.     

Real-time measurement of mixing ratio of refrigerant/refrigeration oil mixture

The mixing of refrigeration oil with refrigerant in a refrigeration cycle has great influence on cycle performance. A sampling method is the most general way to measure the mixing ratio of refrigerant and refrigeration oil. Since the sampling method is time-consuming and reduces the amount of refrigerant and oil in the cycle, a real-time measurement is desirable. In this study, a refractive index measurement was applied to measure the mixing ratio of refrigerant/oil mixture. A laser displacement sensor was used to detect any change in optical path which results from changes of the refractive index of refrigerant/oil mixture. For the practical application of real-time measurement of the oil circulation ratio (OCR) in the refrigeration cycle, a correlation between the refractive index and the mixing ratio was developed. In addition, the changes of the refractive index in a range of a few percentages of the oil concentration and under subcooled conditions were measured. Finally, a transient measurement of the OCR in a practically operating refrigeration cycle was carried out successfully.     

Comparison of CFD analysis to empirical data in a commercial vortex tube

This paper presents a comparison between the performance predicted by a computational fluid dynamic (CFD) model and experimental measurements taken using a commercially available vortex tube. Specifically, the measured exit temperatures into and out of the vortex tube are compared with the CFD model. The data and the model are both verified using global mass and energy balances. The CFD model is a two-dimensional (2D) steady axisymmetric model (with swirl) that utilizes both the standard and renormalization group (RNG) k-epsilon turbulence models. While CFD has been used previously to understand the fluid behavior internal to the vortex tube, it has not been applied as a predictive model of the vortex tube in order to develop a design tool that can be used with confidence over a range of operating conditions and geometries. The objective of this paper is the demonstration of the successful use of CFD in this regard, thereby providing a powerful tool that can be used to optimize vortex tube design as well as assess its utility in the context of new applications.     

An implicit curve-fitting method for fast calculation of thermal properties of pure and mixed refrigerants

Calculations of refrigerant thermal properties are desired to be very fast and stable in cases of simulation of refrigeration system, etc. The traditional method based on equation of state cannot meet such requirement because of unavoidable iterations in calculation. In this paper, a new calculation method for refrigerant thermal properties is presented. Low order implicit polynomial equations are got by using curve-fitting method at first, and then explicit formulae for calculating refrigerant thermal properties quickly are obtained by getting the analytical solution of these implicit equations. Explicit fast calculation formulae for thermal properties of R22 and R407C, covering the saturated temperature of −6080 °C and superheat of 0–65 °C, are presented as examples. The calculation speeds of the formulae of R22 are about 140 times faster than those of REFPROP 6.01 while the formulae of R407C are about 1000 times faster. The total mean relative deviations of the fast calculation formulae for R22 and R407C are less than 0.02%.     

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 extended corresponding states model for the thermal conductivity of refrigerants and refrigerant mixturesModèle d'états correspondants étendus pour la conductivité thermique de frigorigènes et de mélanges de frigorigènes

The extended corresponding states (ECS) model of Huber et al. (Huber, M.L., Friend, D.G., Ely, J.F. Prediction of the thermal conductivity of refrigerants and refrigerant mixtures. Fluid Phase Equilibria 1992;80:249–61) for calculating the thermal conductivity of a pure fluid or fluid mixture is modified by the introduction of a thermal conductivity shape factor which is determined from experimental data. An additional empirical correction to the traditional Eucken correlation for the dilute-gas conductivity was necessary, especially for highly polar fluids. For pure fluids, these additional factors result in significantly improved agreement between the ECS predictions and experimental data. A further modification for mixtures eliminates discontinuities at the pure component limits. The method has been applied to 11 halocarbon refrigerants, propane, ammonia, and carbon dioxide as well as mixtures of these fluids. The average absolute deviations between the calculated and experimental values ranged from 1.08 to 5.57% for the 14 pure fluids studied. Deviations for the 12 mixtures studied ranged from 2.98 to 9.40%. Deviations increase near the critical point, especially for mixtures.     

Lubrication oil pumping by utilizing vane motion in a horizontal rotary compressor

For a horizontal rotary compressor which utilizes reciprocating motion of the vane for oil supply into lubrication elements, an analytical study has been carried out on the oil pumping mechanism. Energy equation has been applied to the oil flow inside the oil-conveying pipe with oil feeding hole in the middle. Oil distributions into individual lubrication elements such as various bearing elements have also been analyzed by applying electric circuit network theory to the oil flow network. Fairly good agreement between calculations and experiments for the oil pumping rate has been obtained in a wide range of compressor speed.     

Azeotropy in the natural and synthetic refrigerant mixtures

A novel approach for the prediction of azeotrope formation in a mixture that does not require vapour–liquid equilibrium calculations is developed. The method employs neural networks and global phase diagram methodologies to correlate azeotropic data for binary mixtures based only on critical properties and acentric factor of the individual components in refrigerant mixtures. Analytical expressions to predict azeotropy and double azeotropy phenomena in terms of critical parameters of pure components and interaction parameters k₁₂, are derived using global phase diagram conception. Modeling of thermodynamic and phase behavior has been carried out on the base of the Redlich–Kwong–Soave and the Peng–Robinson equations of state (EoS). Local mapping method is introduced to describe thermodynamically consistently an accurate saturation curve of refrigerants by three parameters EoS. Optimized neural network was chosen to achieve a complete coincidence of predicted and experimentally observable azeotropic states for training, validation, and test sets simultaneously. All possible cases of azeotropy appearance/absence in the more than 1500 industrially significant binary mixtures of natural and synthetic refrigerants are presented.     

Numerical simulation of dehumidifying fin-and-tube heat exchangers: Semi-analytical modelling and experimental comparison

The aim of this paper is to present a developed semi-analytical model for the simulation of dehumidifying air–liquid fin-and-tube heat exchangers. The simulation strategy and the mathematical methodology are described in detail. The model is based on -NTU method, and formulated in a compact way for dry and wet surface situations (temperature or enthalpy driven, respectively). Both rating and design procedures have been developed for fully dry, partially wet, or fully wet surface conditions. The model predictions are compared with experimental data obtained on a wavy and a plain finned heat exchanger, giving reasonably accurate results. The limitations of the empirical information used are clearly identified in the work. The aim of this model is to provide a fast but reliable rating and design numerical tool for air–liquid heat exchanger applications.