Screening of pure fluids as alternative refrigerants

Hydrofluorocarbons are now well established as refrigerants because of their zero ozone depletion potential. Since they have a high global warming potential, other alternatives as, e.g. fluorinated ethers or cyclic hydrocarbons are considered as next-generation refrigerants. Screening of alternative refrigerants is difficult because mostly no or only few data are available. To evaluate, e.g. the cycle performance, the thermodynamic properties of the refrigerants must be known and described accurately by an equation of state. Here, the physically based BACKONE equations are used to describe alternative refrigerants, such as natural refrigerants, hydrofluorocarbons, fluorinated cyclic hydrocarbons, and fluorinated ethers. BACKONE needs only a few substance specific parameters to describe thermodynamic properties with high accuracy. Thus, even alternative refrigerants, with very few available experimental data can be described. Calculations with BACKONE of the performance of many refrigerants show that some hydrocarbons and fluorinated ethers are a good alternative.     

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.     

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.     

Prediction of the volumetric and thermodynamic properties of some refrigerants using GMA equation of state

The density of 11 refrigerants (hydrochlorofluorocarbon (HCFCs) and hydrofluorocarbon (HFCs)) in the extended ranges of temperature and pressure has been calculated using Goharshadi–Morsali–Abbaspour equation of state (GMA EoS) and the results have been shown as the three-dimensional surfaces of density–temperature–pressure. A wide comparison with experimental data was made. The accuracy of the equation of state in the prediction of density was determined by statistical parameters. The results show that the GMA EoS can reproduce the experimental PVT data of HCFCs and HFCs within experimental errors throughout the liquid phase. The thermodynamic properties such as isobaric expansion coefficient, isothermal compressibility, and vapor–liquid equilibrium (VLE) prediction for these HCFC and HFC refrigerants have been performed using GMA EoS. GMA EoS can predict the characteristic feature of pressure behavior of isobaric expansion and isothermal compressibility coefficients.     

A predictive density model in a corresponding states format. Application to pure and mixed refrigerants

A three parameters density model based on Corresponding States (CS) technique is proposed as a means of predicting the density of pure fluids and their mixtures on the entire PρT (PρTx) surface. The studied fluids belong to two conformal families of the new refrigerant fluids generation: the halogenated alkanes (HA) and the hydrofluoroethers (HFE). The new model is based on an original scaling factor parameter that is determined only on a saturated liquid density experimental value. Using two accurate dedicated equations of state (EoS) as references, the same structure of the Teja CS model is maintained, substituting the classical acentric factor with the new defined scaling parameter. Through this model, the density of the refrigerant fluids considered can be calculated on the whole surface with an accuracy level similar to that of the dedicated equations. The model is validated against experimental data for HFC refrigerants including fluoropropanes, fluorobutanes and fluoroethers. A comparison is also proposed with available density models regarded of high accuracy level.     

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.     

Extension of the applicable range of the implicit curve-fitting method for refrigerant thermodynamic properties to critical pressure

The method of implicit curve-fitting and explicit-calculation has been used for fast and stable calculations of thermodynamic properties of subcritical refrigerants. In order to extend that method to the critical pressure, a method of sectional implicit curve-fitting and explicit-calculation for refrigerant thermodynamic properties is introduced in this paper. The whole data range is divided into several subsections. The requirements on the continuity of thermodynamic properties and the first order derivative of thermodynamic properties in the intersection points of subsections are indicated, and the methods to meet the requirements are presented. Quadric equations are constructed instead of curve-fitting when no data can be given. With the source data obtained from REFPROP 7.1, explicit fast calculation formulae for thermodynamic properties of R410A, covering the saturated temperature of 213.15–344.51 K and superheat of 0–65 K, are given as an example. The calculation speeds of the formulae of R410A are more than 7000 times faster than those of REFPROP 7.1 while the total mean relative deviation of the fast calculation formulae from REFPROP 7.1 is only 0.04%.     

An excess function method to model the thermophysical properties of one-phase secondary refrigerants

This paper describes an easy-to-use and accurate method to calculate some of the thermophysical properties of aqueous solutions which are used as secondary refrigerants. This method is based on the correction of the ideal behaviour of aqueous solutions by excess functions. This method allows to determine the following properties: freezing points, densities, heat capacities, thermal conductivities and dynamic viscosities. As an illustration, it is applied to aqueous solutions of (i) ethyl alcohol, (ii) ammonia, (iii) sodium chloride, (iv) ethylene glycol and (iii) propylene glycol.     

Propriétés thermodynamiques et physiques des mélanges de fluides frigorigènes et d'huilesThermodynamic and physical properties of mixtures of refrigerants and oils

The influence of lubricants circulating within refrigerating plant on boiling and convective condensation mechanisms, and the lack of data supplied by manufacturers mean that predictive models have to be used in order to determine the thermodynamic and transport properties of lubricating oils and of mixtures of refrigerants and oils. This study provides a series of correlations making it possible to calculate the properties of oils. This article also compares literature references to a few methods used to determine the properties of such mixtures.     

Contribution to the gas chromatographic analysis for both refrigerants composition and cell gas in insulating foams – Part I: Method

In this paper, an easy low cost chromatographic analysis for both refrigerants blend composition and cell gas in insulating foams is presented. This work firstly deals with the measurement protocols for these substances. The presented protocols are simple, likely to be used by non-specialists of chemical analysis (i.e. by refrigeration engineers). Among other methods, gas chromatography was used for separation and detection of halocarbons. For refrigerants extracted from refrigeration plants, the analysis requires a prior oil separation. As for insulating foams, analysis of cell gas composition is processed with the same protocol. Sampling remains of main importance; two possible sampling methods were validated and compared. Finally, the interest in the analysis method is illustrated with the example of a real application: study of the distillation phenomenon of a zeotropic refrigerant in an industrial refrigeration plant with flooded evaporators. The results are discussed for their consequences on the working performances of the refrigerating system. In part II, we propose original results obtained with that method to study the aging of insulating foams.     

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.     

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.     

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%.     

Mixing and separation characteristics of isobutane with refrigeration oil

This paper discusses the transient mixing and separation characteristics of isobutane with/from refrigeration oil. The mixing/separation processes are observed and investigated experimentally in a glass cylindrical vessel. Since liquid isobutane is less dense than refrigeration oil, the mixing process proceeds one dimensionally by diffusion from the interface between isobutane gas and refrigeration oil. The progress of mixing, therefore, is very slow compared with a combination of halocarbon refrigerant and refrigeration oil having convection flow during the mixing process. The diffusion process can be analyzed using a one-dimensional diffusion model with an appropriate diffusion coefficient, which increases linearly with temperature. The separation of isobutane from the oil–refrigerant mixture occurs at the interface and the denser oil from which isobutane is separated causes a convective flow. Bubble generation under the depressurized conditions is unstable, but in the most cases, it tends to start when a high super saturation degree is reached. The temperature change during the separation process is estimated using latent heat as the separation heat of refrigerant.     

Appraising the flammability hazards of hydrocarbon refrigerants using quantitative risk assessment model. Part II. Model evaluation and analysis

Hydrocarbon refrigerants present are fire and explosion hazards due to their flammability. This paper describes a quantitative risk assessment (QRA) model to evaluate the potential for ignition when hydrocarbons are employed in stationary refrigeration and air-conditioning equipment. QRA enables examination of the effects that design, installation of equipment and external conditions on the frequency of ignition of the refrigerant and its consequences. Part I of this study presents the modelling approach for ignition frequencies, sub-models for refrigerant leakage and development of flammable concentration, and the associated consequences, being overpressures and thermal radiation. Part II provides recommended empirical input data and example results generated from the model.     

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.     

An NTU- model for alternative refrigerants

This paper presents a steady state simulation model to predict the performance of alternative refrigerants in vapour compression refrigeration/heat pump systems. The model is based on the NTU- method in analysing the heat exchangers following an elemental approach. The model extends its applicability to new refrigerants including hydrocarbons and uses a large database of REFPROP package for refrigerant properties. The main inputs to the model include the physical details of the heat exchangers, compressor efficiency, mass flow rates of heat transfer fluids and their inlet temperatures to the evaporator and the condenser, the pressure drops across the heat exchangers and the capacity of either the evaporator or condenser (in kW). The model results are validated with a wide range of experimental data of HCFC22 and propane (HC290) on a heat pump test facility for a number of parameters, e.g. coefficient of performance, condenser capacity, mass flow rate of the refrigerant and compressor discharge temperature. Although the model is currently tested for pure refrigerants using compact brazed plate (counter flow type) heat exchangers, it can also be applied to mixture of refrigerants as well as to other types of heat exchangers.

Résumé

Dans cet article, on présente un modèle de simulation de régime permanent pour prédire la performance des frigorigènes de remplacement dans les systèmes frigorifiques ou les pompes à chaleur à compression de vapeur. Fondé sur la méthode NTU- utilisée pour analyser les échangeurs de chaleur, ce modèle emploie une approche élémentaire. Ce modèle étend la méthode aux nouveaux frigorigènes, y compris deees hydrocarbures, et utilise une base de données étendue, celle de REFPROP, pour les propriétés des frigorigènes. Les principaux paramètres du modèle comprennent des détails physiques sur les échangeurs de chaleur, le rendement des compresseurs, et les débits massiques des fluides de transfert de chaleur et leurs températures à l'entrée de l'évaporateur ou du condenseur, la chute de pression à travers les échangeurs de chaleur et la puissance soit de l'évaporateur, soit du condenseur (exprimés en kW). Les résultats obtensus en utilisant ce modèle sont validés pour une large gamme de données expérimentales obtenus avec le HCFC22 et avec le propane (le HC290) sur un banc d'essai de pompe à chaleur et pour un certain nombre de paramètres, par exemple le coefficient de performance, la puissance du compresseur, le débit massique du frigorigène et la température du frigorigène à la sortie du compresseur. En ce moment, le comportement des frigorigènes purs utilisés dans des échangeurs de chaleur compacts à plaques brasées (de type contre-courant) est en train d'être étudié; le modèle peut également être appliqué aux mélanges de frigorigènes et à d'autres types d'échangeurs de chaleur.     

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.     

Predicting the viscosity of liquid refrigerant blends: comparison with experimental data

A method is presented for predicting the viscosity of liquid refrigerant mixtures. The method has no adjustable parameters and, in essence, relies upon the knowledge of the viscosity of the pure components to predict the viscosity of a mixture by means of kinetic theory and rigid-sphere formalism. The predictions have been compared with the available experimental data for a number of refrigerant mixtures. Based on this comparison and previous studies, the accuracy of the proposed method is assessed to be of the order of ±7%.