Interaction parameter, k12, for non-azeotropic refrigerant mixtures

The interaction parameter, k₁₂, is determined from the experimental equilibrium data obtained by other authors. Vapour-liquid equilibria for binary mixtures of halocarbon refrigerants are predicted using the Redlich-Kwong-Soave equation of state. The mixtures considered are: R14-R23, R23-R12, R13-R12, R13-R11, R13B1-R22, R13B1-R152a, R22-RC318, R12-RC318, R12-R11.     

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures in horizontal tubes

A study on the prediction of heat transfer coefficient and pressure drop of refrigerant mixtures is reported. Heat transfer coefficients and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis assuming evaporation in horizontal tubes. Results indicate that nucleate boiling is suppressed at qualities greater than 20% for all mixtures, and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8–10% increase in heat transfer coefficients. Some mixtures with large volatility difference exhibit as much as 55% reduction compared to R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32%) and reduced mass flow rates (23%). Other mixtures with moderate volatility difference exhibit 20–30% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall heat transfer coefficient of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10% in transport properties causes a change of less than 6% in heat transfer prediction.     

Liquid dynamic viscosity: A general prediction method with application to refrigerants and refrigerant mixtures

This paper discusses prediction methods which are able to provide the dynamic viscosity, μ, of liquids along the saturation line. The best empirical or semi-empirical correlations existing in the literature are critically presented and checked to outline the usefulness of the new prediction method presented in this paper. Fifty substances (organic compounds, inorganic compounds and pure elements) are examined to show the reliability of the new simple equation which contains three factors (A, B and C) related to the molecular structure and the most important physical properties. The general scheme of prediction is then applied to the particular case of refrigerant fluids belonging to the methane and ethane families and to their binary mixtures. The accuracy of the proposed prediction method is checked using the most recent and reliable experimental dynamic viscosity data available in literature, and the mean and the maximum deviations between predicted and experimental μ values are shown to be less than 3 and 8%, respectively.     

The use of propane in domestic refrigerators

The implications of using propane in domestic refrigerators are examined in relation to energy consumption, compressor lubrication, costs, availability, environmental factors and safety. It is concluded that propane is an attractive and environmentally friendly alternative to CFCs used currently.     

The temperature performance of domestic refrigerators

Chilled foods are stored for periods of between a few hours and many days in domestic refrigerators. However, there are little published data on the temperature performance of domestic refrigerators within the home. A survey has been taken in 252 households in the UK and some of the results are presented in this paper. The refrigerators investigated in the survey were found to have an overall mean temperature of approximately 6°C, which ranged from 11.4 to −0.9°C. Temperature ranges over the whole refrigerator varied from 4.5 to 30.5°C with 3.7% of the total being warmer than 20°C. On average 29.9% of refrigerators operated below 5°C and 66.7% operated below 7°C. Few refrigerators (7.3%) ran, on average, above 9°C. No refrigerator characteristic (apart from type) could be related to temperatures or temperature distribution in the refrigerators investigated.     

Local composition modelling of the thermodynamic properties of refrigerant and oil mixtures

Six local composition models of the thermodynamic behaviour of mixtures are described. Using data from the literature and a non-linear regression analysis, a comparison of the predictive abilities of the models is undertaken for R12, R22, R134a and R125 with various oils. The Wilson and Heil equations provide the most consistent results, with the Heil equation providing a modest improvement over the Wilson model. Using a 95% confidence interval, the Heil equation predicted the behaviour of R12 with a paraffinic mineral oil to within 3.1%; its worst-case 2-σ error was 10.4% (R22 with a polyol ester oil), and its average 2-σ error for all of the mixtures was 6.2%. Using model parameters and error estimates from the regression analyses, pressure-temperature-concentration behaviour for these mixtures can be predicted for system design and simulation.     

Performance evaluation of a heat pump cycle using NARMs by a simulation with equations of heat transfer and pressure drop

Simulation analyses for a vapour compression heat pump cycle using nonazeotropic refrigerant mixtures (NARMs) of R22 and R114 are conducted under the condition that the heat pump thermal output and the flow rate and inlet temperatures of the heat sink and source water are given. The heat transfer coefficients of the condensation and evaporation are calculated with empirical correlations proposed by the authors. The validity of the evaluation method and the correlations is demonstrated by comparison with experimental data. The relations between the coefficient of performance (COP) and composition are shown under two conditions: (1) the constant heat transfer length of the condenser and evaporator; and (2) the constant temperature of refrigerant at the heat exchanger inlet. The COP of the NARMs is higher than that of pure refrigerant when the heat transfer lengths of the condenser and evaporator are sufficiently long.     

A simplified cycle simulation model for the performance rating of refrigerants and refrigerant mixtures

A simulation program, CYCLE11, which is useful for the preliminary evaluation of the performance of refrigerants and refrigerant mixtures in the vapour compression cycle is described. The program simulates a theoretical vapour-compression cycle and departures from the theoretical cycle that occur in a heat pump and in a refrigerator. The cycles are prescribed in terms of the temperatures of the external heat-transfer fluids with the heat exchangers generalized by an average effective temperature difference. The isethalpic expansion process is assumed. The program includes a rudimentary model of a compressor and a representation of the suction line and liquid line heat exchange. Refrigerant thermodynamic properties are calculated by using the Carnahan-Starling-DeSantis equation of state. Refrigerant transport properties are not included in the simulations. The program can generate merit ratings of refrigerants for which limited measured data are available. An example of simulation results stresses the need for careful application of simplified models and consideration for the assumptions involved.     

Condensation heat transfer of binary refrigerant mixtures of R22 and R114 inside a horizontal tube with internal spiral grooves

An experimental study of the condensation of pure and mixed refrigerants of R22 and R114 inside a spirally grooved horizontal copper tube has been carried out. A double-tube counterflow condenser in the pressure range 3–21 bar and at a mass flow-rate 26–70 kg h⁻¹ was used. The axial distributions of refrigerant, tube wall and cooling water temperatures, wall heat flux density and vapour quality are shown graphically. The variation of tube wall temperature around the circumference of the tube is also shown. The local Nusselt number depends on the molar fraction, whereas the average Nusselt number can be correlated by an equation which is modified from a previously established equation for pure refrigerants inside a horizontal smooth tube. The frictional pressure drop evaluated is correlated well by the Lockhart-Martinelli parameters and is independent of the concentration of the mixture.     

Condensing heat transfer for R114/R12 mixtures on horizontal finned tubes

Two titanium tubes with external fins were tested in the horizontal orientation to determine heat transfer performance with R114, R12, and selected non-azeotropic mixtures of the two condensing on the outside surface. For the single-component situation, data were in excellent agreement with predictions from a modified Katz-Keller method, and little performance distinction was found between the tubes or between the pure refrigerants. All mixtures depressed performance below single-component levels, with even low second-component concentrations causing substantial degradation (up to 55% performance reduction for 5% R12). Gas chromatograph composition analyses of vapour from the condenser shell showed elevated concentrations of the more volatile component (R12), evidence that an added transport resistance contributed to the observed mixture performance reductions. If previously suggested benefits of mixtures in heat pump applications are to be realized, the associated condensers should be in a configuration so as to mitigate these performance penalties.     

Performance characteristics correlation for round tube and plate finned heat exchangers: Equations relatives aux performances d'échangeurs de chaleur constitués de tubes ronds et de plaques à ailettes

A number of correlation equations describing the performance characteristics of round tube and plate fin have been published in the open literature. However, many of these correlations are restricted to flat finned heat exchangers and a limited number of geometrical configurations. In this study, 28 heat exchanger samples were tested in an open circuit thermal wind tunnel over a velocity range of 1 to 20 m/s for a number of geometries. The geometrical variations include the number of tube rows, fin thickness and the spacing between fins, rows and tubes. Both flat and corrugated fins were tested and the results were correlated in terms of j and f factors as a function of Reynolds number and the geometrical parameters of the heat exchangers. An important feature of this correlation is the novel way in which the geometric parameters are expressed in the correlation. Ratios of these parameters are derived from consideration of the physics of the flow and heat transfer in the heat exchangers. This results in a more accurate and physically meaningful correlation which can be applied to a broader range of geometries. The correlation was validated against test data in the literature for round tube and plate fin with good agreement. It was found that the fin type affects the heat transfer and friction factor, and that the number of tube rows has a negligible effect on the friction factor. The number of tube rows effect was found to be influenced by the fin and tube geometries as well as the Reynolds number.Un certain nombre d'équations pour des caractéristiques du rendement des échangeurs de chaleur à tubes ronds plaques à ailettes ont été publiés dans le littérature. Cependant, dans bien des cas, ces corrélations se limitent aux échangeurs à ailette plate dans un nombre limité de configurations géométriques. Dans cette étude, 28 échangeurs de chaleur ont été testés utilisant une soufflerie à circuit ouvert avec une vitesse d'air de 1 à 20 m/s pour plusieurs formes géométriques. Les variations géométriques portaient sur le nombre de rangées de tubes, l'épaisseur des ailettes et la distance séparant des ailettes, des rangées et des tubes. Les ailettes plates et ondulées ont été testées et les corrélations en termes de facteurs j et f en fonction du nombre de Reynolds et les paramètres géométriques des échangeurs de chaleur. Un aspect important de cette corrélation est le façon originale d'exprimer des paramètres géométriques. Les rapports de ces paramètres sont obtenus à partir des flux et transferts de chaleur dans des échangeurs de chaleur. Ce procedé permet d'obtenir une corrélation plus précise et utilé qui s'applique à une gamme de formes géomátriques plus large. La corrélation a été validée en fonction des données concernant des échangeurs à tube et à plaque à ailettes dans la littérature: les données expérimentales et théoriques concordent bien. On a montré que le type d'ailette exerce une influence sur le transfert de chaleur et le facteur de frottement. Cependant, le nombre de rangées de tubes a un effet negligeable sur le coéfficient de frottement. On a démontré que l'effet nombre de rangées de tube est influencé par les géométries des ailettes et des tubes ainsi que par le nombre de Reynolds.     

Experimental study on forced convective boiling heat transfer of pure refrigerants and refrigerant mixtures in a horizontal tube

Convective boiling heat transfer coefficients of pure refrigerants (R22, R32, R134A, R290, and R600a) and refrigerant mixtures (R32/R134a, R290/R600a, and R32/R125) are measured experimentally and compared with Gungor and Winterton correlation. The test section is made of a seamless stainless steel tube with an inner diameter of 7.7 mm and is uniformly heated by applying electric current directly to the tube. The exit temperature of the test section was kept at 12°C ± 0.5°C for all refrigerants in this study. Heat fluxes are varied from 10 to 30 kW m⁻² and mass fluxes are set to the discrete values in the range of 424–742 kg m⁻² s⁻¹ for R22, R32, R134a, R32/R134a, and R32/R125; 265–583 kg m⁻² s⁻¹ for R290, R600a, and R290/R600a. Heat transfer coefficients depend strongly on heat flux at a low quality region and become independent as quality increases. The Gungor and Winterton correlation for pure substances and the Thome-Shakil modification of this correlation for refrigerant mixtures overpredicts the heat transfer coefficients measured in this study.     

Thermodynamic properties of HFC-32 (difluoromethane)

An 18-coefficient modified Benedict–Webb–Rubin equation of state of HFC-32 (difluoromethane) has been developed, based on the updated available PVT measurements, heat capacity measurements and speed of sound measurements. Correlations of vapor pressure and saturated liquid density are also presented. The correlations have been developed based on the reported experimental saturation properties data. This equation of state is effective both in the superheated gaseous phase and compressed liquid phase at pressures up to 70 MPa, densities to 1450 kg/m³, and temperatures from 150 to 475 K, respectively.     

An experimental comparison of evaporation and condensation heat transfer coefficients for HFC-134a and CFC-12

Experimental heat transfer coefficients are reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation. These heat transfer coefficients were measured in a horizontal, smooth tube with an inner diameter of 8.0 mm and a length of 3.67 m. The refrigerant in the test-tube was heated or cooled by using water flowing through an annulus surrounding the tube. Evaporation tests were performed for a refrigerant temperature range of 5–15°C with inlet and exit qualities of 10 and 90%, respectively. For condensation tests, the refrigerant temperature ranged from 30 to 50°C, with et and exit qualities of 90 and 10%, respectively. The mass flux was varied from 125 to 400 kg m⁻² s⁻¹ for all tests. For similar mass fluxes, the evaporation and condensation heat transfer coefficients for HFC-134a were significantly higher than those of CFC-12. Specifically, HFC-134a showed a 35–45% increase over CFC-12 for evaporation and a 25–35% increase over CFC-12 for condensation.     

A semi-theoretical model for predicting refrigerant/lubricant mixture pool boiling heat transfer

This paper outlines the framework of a semi-theoretical model for predicting the pool boiling heat transfer of refrigerant/lubricant mixtures on a roughened, horizontal, flat pool-boiling surface. The predictive model is based on the mechanisms involved in the formation of the lubricant excess layer that exists on the heat transfer surface. The lubricant accumulates on the surface in excess of the bulk concentration via preferential evaporation of the refrigerant from the bulk refrigerant/lubricant mixture. As a result, excess lubricant resides in a thin layer on the surface and influences the boiling performance, giving either an enhancement or degradation in heat transfer. A dimensionless excess layer parameter and a thermal boundary layer constant were derived and fitted to data in an attempt to generalize the model to other refrigerant/lubricant mixtures. The model inputs include transport and thermodynamic refrigerant properties and the lubricant composition, viscosity, and critical solution temperature with the refrigerant. The model predicts the boiling heat transfer coefficient of three different mixtures of R123 and lubricant to within ±10%. Comparisons of heat transfer predictions to measurements for 13 different refrigerant/lubricant mixtures were made, including two different refrigerants and three different lubricants.     

A new three-parameter cubic equation of state for refrigeration engineering calculations

A new three-parameter cubic equation of state is proposed in which the repulsion pressure term of the semiempirical van der Waals equation has been modified based on the hard-sphere potential and a functional form similar to Redlich-Kwong attraction term is adopted for the attraction term. All three parameters are treated as functions of temperature. For the purpose of examining the applicability of the new equation, it was applied to describe the thermodynamic properties of difluoromethane (R-32) and pentafluoroethane (R-125). It has been found that the essential thermodynamic properties are represented by the new equation as accurately as a conventional far-complicated modified Benedict-Webb-Rubin equation in the working range of refrigeration equipment. The new cubic equation was also compared with the Peng-Robinson equation and the Carnahan-Starling-De Santis equation.     

Study of domestic refrigerator temperature and analysis of factors affecting temperature: a French survey

A survey was carried out in France from April to June 1999. Temperatures were recorded at three levels (top, middle and bottom) of the refrigerator compartment using a data logger. A questionnaire was filled in, enabling the following information to be obtained: characteristics of the family, characteristics of the refrigerator and the use conditions. One hundred and forty-three domestic refrigerators were surveyed, but due to various technical problems, only 119 sets of recorded temperatures were exploitable. The temperatures of the surveyed refrigerators were: average 6.6 °C, minimum 0.9 °C and maximum 11.4 °C. Statistical analysis such as clustering and segmentation were used. It was found that the heterogeneousness of temperature inside each refrigerator seems to be dependent on the type. This study shows the influence of the use conditions on the temperature. No one factor exerts a single direct effect; a combination of the effects of all factors is observed.     

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures

A study on the prediction of heat transfer coefficient (HTC) and pressure drop of refrigerant mixtures is reported. HTCs and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis. Results indicate that nucleate boiling is suppressed at qualities greater than 20.0% for all mixtures and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8.0–10.0% increase in HTCs. Some mixtures with large volatility difference exhibit as much as 55.0% reduction compared with R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32.0%) and reduced mass flow rates (23.0%). Other mixtures with moderate volatility difference exhibit 20.0–30.0% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall HTC of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10.0% in transport properties causes a change of less than 6% in heat transfer prediction.     

A generalised mathematical modelling methodology for design of horticultural food packages exposed to refrigerated conditions: Part 3, mass transfer modelling and testing

A modelling methodology developed to simulate a wide range of heat and mass transfer processes in refrigerated storage of horticultural foods was used for prediction of mass transfer of water vapour in packaged horticultural products. The mass transfer pathways, which are considered the most significant in industrial practice for a range of products and packaging systems, were modelled. Experimental testing was conducted for 10 product-package systems. Even though the model supports multi-zone package representation, a single zone was found to be adequate for all test data. Agreement between prediction and measurement was within likely data uncertainties for mass loss, packaging moisture uptake and relative humidity difference to the external environment. Sub-models for which further development may be justified are identified. Coupled with the heat transfer results presented in Part 2, the present work verifies the validity of the overall model structure presented in Part 1.