Capillary tube selection for HCFC22 alternativesSélection de capillaires pour les frigorigènes de remplacement du HCFC22

In this paper, pressure drop through a capillary tube is modeled in an attempt to predict the size of capillary tubes used in residential air conditioners and also to provide simple correlating equations for practicing engineers. Stoecker's basic model was modified with the consideration of various effects due to subcooling, area contraction, different equations for viscosity and friction factor, and finally mixture effect. McAdams' equation for the two-phase viscosity and Stoecker's equation for the friction factor yielded the best results among various equations. With these equations, the modified model yielded the performance data that are comparable to those in the ASHRAE handbook. After the model was validated with experimental data for CFC12, HFC134a, HCFC22, and R407C, performance data were generated for HCFC22 and its alternatives, HFC134a, R407C, and R410A under the following conditions: condensing temperature; 40, 45, 50, 55°C, subcooling; 0, 2.5, 5°C, capillary tube diameter; 1.2–2.4 mm, mass flow rate; 5–50 g/s. These data showed that the capillary tube length varies uniformly with the changes in condensing temperature and subcooling. Finally, a regression analysis was performed to determine the dependence of mass flow rate on the length and diameter of a capillary tube, condensing temperature, and subcooling. Thus determined simple practical equations yielded a mean deviation of 2.4% for 1488 data obtained for two pure and two mixed refrigerants examined in this study.     

Condensation heat transfer coefficients of enhanced tubes with alternative refrigerants for CFC11 and CFC12Coefficients de transfert de chaleur de condensation de tubes à surface augmentée fonctionnant avec des frigorigènes de remplacement de CFC11 et de CFC12

In this study, condensation heat transfer coefficients (HTCs) of a plain tube, low fin tube, and Turbo-C tube were measured for the low pressure refrigerants CFC11 and HCFC123 and for the medium pressure refrigerants CFC12 and HFC134a. All data were taken at the vapor temperature of 39°C with a wall subcooling of 3–8°C. Test results showed that the HTCs of HFC123, an alternative for CFC11, were 8.2–19.2% lower than those of CFC11 for all the tubes tested. On the other hand, the HTCs of HFC134a, an alternative for CFC12, were 0.0–31.8% higher than those of CFC12 for all the tubes tested. For all refrigerants tested, the Turbo-C tube showed the highest HTCs among the tubes tested showing almost an 8 times increase in HTCs as compared to the plain tube. Nusselt's prediction equation yielded a 12% deviation for the plain tube data while Beatty and Katz's prediction equation yielded a 20.0% deviation for the low fin tube data.     

Reducing running costs in air conditioningRéduire les coûts de fonctionnement en conditionnement d'air

Previous studies using wide-boiling refrigerant mixtures were reported in the literature as a method of reducing power consumption in air conditioning. A novel air conditioning test unit incorporating this concept was tested at the Department of Chemical Engineering at Leeds University. Experimental tests with a refrigerant mixture, R407C, produced COPs in excess of 4. Also, a simulation program was used to model the behaviour of other mixtures. It yielded a COP of about 5 for mixture R32/R134a. A financial study was made to calculate the total annual costs of a unit using R32/R134a. It showed that annual savings of up to 41 per cent were possible compared to a unit using R134a.     

Performance des mélanges de frigorigènes utilisés pour remplacer le HCFC22

In this study, 14 refrigerant mixtures composed of R32, R125, R134a, R152a, R290 (propane) and R1270 (propylene) were tested in a breadboard heat pump in an attempt to substitute HCFC22 used in residential air-conditioners. The heat pump was of 3.5 kW capacity with water as the heat transfer fluid (HTF) in the evaporator and condenser that are in a counter current flow configuration. All tests were conducted with the HTF temperatures fixed to those found in the ARI test A condition. Test results show that ternary mixtures composed of R32, R125, and R134a have a 4–5% higher coefficient of performance (COP) and capacity than HCFC22. On the other hand, ternary mixtures containing R125, R134a and R152a have both lower COPs and capacities than HCFC22. R32/R134a binary mixtures show a 7% increase in COP with the similar capacity to that of HCFC22 while R290/R134a azeotrope shows a 3–4% increases in both COP and capacity. The compressor discharge temperatures of the mixtures tested are much lower than those of HCFC22, indicating that these mixtures would offer better system reliability and longer life time than HCFC22. Finally, test results with a suction line heat exchanger (SLHX) indicate that SLHX must be used with special care in air-conditioners since its effect is fluid dependent.     

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.     

Oil concentration in liquid refrigerants: in situ measurementConcentration d''huile dans les frigorigènes liquides : mesures sur le terrain

The concentration of oil in refrigerants (while in liquid state) can be measured with an acoustic velocity sensor. The transit time for an acoustic signal can be related to the oil concentration and temperature of the liquid mixture. The performance of the sensor is dependent on the properties of the oil and refrigerant, and their miscibility. In general, a thorough calibration becomes necessary. It is shown in this paper that for concentrations less than 10%, an approximation can be made for the estimate of concentration hence eliminating the need for an elaborate calibration procedure.     

Performance and heat transfer characteristics of hydrocarbon refrigerants in a heat pump systemPerformance et caractéristiques de transfert de chaleur des frigorigènes hydrocarbures utilisés dans un système de pompe à chaleur

Performance of a heat pump system using hydrocarbon refrigerants has been investigated experimentally. Single component hydrocarbon refrigerants (propane, isobutane, butane and propylene) and binary mixtures of propane/isobutane and propane/butane are considered as working fluids in a heat pump system. The heat pump system consists of compressor, condenser, evaporator, and expansion device with auxiliary facilities such as evacuating and charging unit, the secondary heat transfer fluid circulation unit, and several measurement units. Performance of each refrigerant is compared at several compressor speeds and temperature levels of the secondary heat transfer fluid. Coefficient of performance (COP) and cooling/heating capacity of hydrocarbon refrigerants are presented. Experimental results show that some hydrocarbon refrigerants are comparable to R22. Condensation and evaporation heat transfer coefficients of selected refrigerants are obtained from overall conductance measurements for subsections of heat exchangers, and compared with those of R22. It is found that heat transfer is degraded for hydrocarbon refrigerant mixtures due to composition variation with phase change. Empirical correlations to estimate heat transfer coefficients for pure and mixed hydrocarbons are developed, and they show good agreement with experimental data. Some hydrocarbon refrigerants have better performance characteristics than R22.     

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.     

Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubesTransfert de chaleur et chute de pression lors de la condensation de frigorigènes à l'intérieur de tubes horizontaux à surface augmentée

This paper presents a critical review of correlations to compute heat transfer coefficients and pressure drop, for refrigerants condensing inside commercially available tubes with enhanced surfaces of various types, and a theoretical analysis of the condensation phenomenon. Predictions from some of the above equations are compared with experimental data. In addition, information is presented about the influence of small amounts of compressor oil on the condensation of refrigerants in enhanced tubes.     

Transport properties of refrigerants R32, R125, R134a, and R125+R32 mixtures in and beyond the critical regionPropriétés de transport des frigorigènes R32, R125, R134a et des mélanges de R125+R32 dans et au-delà la région critique

A practical representation for the transport coefficients of pure refrigerants R32, R125, R134a, and R125+R32 mixtures is presented which is valid in the vapor–liquid critical region. The crossover expressions for the transport coefficients incorporate scaling laws near the critical point and are transformed to regular background values far away from the critical point. The regular background parts of the transport coefficients of pure refrigerants are obtained from independently fitting pure fluid data. For the calculation of the background contributions of the transport coefficients in binary mixtures, corresponding-states correlations are used. The transport property model is compared with thermal conductivity and thermal diffusivity data for pure refrigerants, and with thermal conductivity data for R125+R32 mixtures. The average relative deviations between the calculated values of the thermal conductivity and experimental data are less than 4–5% at densities ρ0.1ρ_c and temperatures up to T=2T_c.     

Empirical correlation for ejector designCorrélation empirique pour la conception des éjecteurs

In the present study, two empirical correlations from the test results of 15 ejectors are derived for the performance prediction of ejectors using R141b as the working fluid. The ratio of the hypothetical throat area of the entrained flow to the nozzle throat area A_e/A_t, the geometric design parameter of the ejector A₃/A_t, and the pressure ratios P_g/P_e and P_c*/P_e are used to correlate the performance of the ejector. The prediction of the entrainment ratio ω using the correlations is within ±10% error. A method of calculation for the ejector design using the correlations is also developed. R141b is shown in the present study to be a good working fluid for an ejector. The measured ω for the ejectors used in the present study can reach as high as 0.54 at P_g=0.465 MPa (84°C), P_c^*=0.087 MPa (28°C) and P_e=0.040 MPa (8°C). For P_g=0.538 MPa (90°C), P_c^*=0.101 MPa (32°C) and P_e=0.040 MPa (8°C), ω reaches 0.45.

Résumé

Dans cette étude, on a établi deux corrélations empiriques à partir des résultats expérimentaux obtenus utilisant 15 éjecteurs; ces corrélations ont été utilisées ensuite pour prédire la performance d'éjecteurs utilisant le R141b comme fluide frigorigène. Les rapports A_e/A_t (section de passage du fluide entraîné rapporté à la section théorique du col de l'éjecteur), et A₃/A_t (section de sortie de l'éjecteur rapporté à la section théorique du col de l'éjecteur) et les relations entre pressions P_g/P_e et P_c*/P_e sont utilisés pour trouver la corrélation de la performance de l'éjecteur. La prévision du taux d'entraînement à partir des corrélations est précise à la hauteur de ±10%. Les auteurs ont également développé une méthode de calcul permettant de concevoir des éjecteurs à partir des corrélations. On a montré dans cette étude que le R141b s'avère être un fluide actif efficace pour cette utilisation. Le ω mesuré des éjecteurs utilisés dans cette étude peuvent atteindre 0.54 à P_g=0.465 MPa (84°C), P_c^*=0.087 MPa (28°C) et P_e=0.040 MPa (8°C). Pour P_g=0.538 MPa (90°C), P_c^*=0.101 MPa (32°C) et P_e=0.040 MPa (8°C), ω atteint 0.45.     

Flow condensation heat transfer coefficients of pure refrigerants

Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m⁻² s⁻¹ and heat flux of 7.3–7.7 kW m⁻². Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.     

State of the art in pool boiling heat transfer of new refrigerantsEtat du calcul pour le transfert de chaleur en ébullition libre de nouveaux frigorigènes

The predictive methods for the calculation of the heat transfer coefficient α with pool boiling are important tools for the optimum design of the evaporator and for the successful operation of refrigeration units. The method given in the VDI Heat Atlas is discussed as an example of the currently available methods/ and results of recent experimental investigations on nucleate boiling of partly fluorinated hydrocarbons (HFCs) and of hydrocarbons (HCs) are added covering those parts where the predictive methods should be improved, namely boiling of mixtures, influence of surface structure and material of the heating wall, and influence of additional flow of bubbles and liquid in tube bundles.     

The influence of a low viscosity oil on the pool boiling heat transfer of the refrigerant R507Influence d'une huile à faible viscosité sur le transfert de chaleur lors de l'ébullition libre du frigorigène R507

This paper describes the influence of a low viscosity polyolester based lubricating oil on the pool boiling heat transfer of the refrigerant R507. The pool boiling heat transfer coefficients for this refrigerant–oil mixture are measured on a smooth tube and on an enhanced tube. The investigation is made for oil mass fractions up to 10% and for saturation temperatures between −28.6°C and +20.1°C. For the smooth tube the heat transfer increases for increasing oil mass fractions up to 3% at lower saturation temperatures. At higher saturation temperatures the heat transfer decreases for increasing oil mass fractions for both tubes. For oil mass fractions greater than 1% at the higher saturation temperatures a range of decreasing heat transfer coefficient is found for increasing heat flux. The effect is caused by the different miscibility of the oil and the components of the refrigerant mixture.     

Performance of a finned-tube evaporator optimized for different refrigerants and its effect on system efficiency

This paper presents a comparable evaluation of R600a (isobutane), R290 (propane), R134a, R22, R410A, and R32 in an optimized finned-tube evaporator, and analyzes the impact of evaporator effects on the system coefficient of performance (COP). The study relied on a detailed evaporator model derived from NIST's EVAP-COND simulation package and used the ISHED1 scheme employing a non-Darwinian learnable evolution model for circuitry optimization. In the process, 4500 circuitry designs were generated and evaluated for each refrigerant. The obtained evaporator optimization results were incorporated in a conventional analysis of the vapor compression cycle. For a theoretical cycle analysis without accounting for evaporator effects, the COP spread for the studied refrigerants was as high as 11.7%. For cycle simulations including evaporator effects, the COP of R290 was better than that of R22 by up to 3.5%, while the remaining refrigerants performed approximately within a 2% COP band of the R22 baseline for the two condensing temperatures considered.