A study on the performance of multi-stage heat pumps using mixturesEtude sur la performance de pompes à chaleur multi-étagées utilisant des mélanges

Multi-stage heat pumps composed of a condenser, evaporator, compressor, suction line heat exchanger, and low and/or high stage economizers are studied by computer simulation. Their thermodynamic performance and design options are examined for various working fluids. In the simulation, HCFC22/HCFC142b and HFC134a are studied as an interim and long term alternatives for CFC12 while HFC32/HFC134a and HFC125/HFC134a are studied as long term alternatives for HCFC22. The results indicate that the three-stage super heat pump with appropriate mixtures is up to 27.3% more energy efficient than the conventional single-stage system with pure fluids. While many factors contribute to the performance increase of a super heat pump, the most important factor is found to be the temperature matching between the secondary heat transfer fluid and refrigerant mixture, which is followed by the use of a low stage economizer and suction line heat exchanger. The contribution resulting from the use of a high stage economizer, however, is not significant. With the suction line heat exchanger, the system efficiency increases more with the fluids of larger molar liquid specific heats. From the view point of volumetric capacity and energy efficiency, a 40%HCFC22/60%HCFC142b mixture is proposed as an interim alternative for CFC12 while a 25%HFC32/75%HFC134a mixture is proposed as a long term alternative for HCFC22.     

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.     

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.     

Experimental evaluation of refrigerant mixtures as substitutes for CFC12 and R502

Several selected refrigerant mixtures are tested as potential short- and mid-term substitutes for CFC12 and R502. HCFC22 and some hydrocarbons are considered as components of retrofit mixtures. Their influence on the solubility of various lubricant oils is investigated by measuring critical solubility temperatures. The performance of the CFC12 and R502 refrigerants and of their proposed alternatives is compared by testing two different refrigerating units.     

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.     

Comparative assessment of HFC134a and some refrigerants as alternatives to CFC12

A composite plot relating evaporating temperature T_EV, condensing temperature T_CO, pressure ratio (PR) and theoretical Rankine coefficient of performance (COP)_RR is presented for HFC134a. The theoretical performance of HFC134a has been comparatively assessed along with HCFC22, HFC134, HFC152a, HCFC124 and HCFC142b as alternatives to CFC12 by using the standard refrigeration parameters including pressure ratio, specific compressor displacement, theoretical Rankine coefficient of performance, shaft power per ton of refrigeration. A discussion of the practical implications of the choice of the alternatives to CFC12 is also presented.     

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.     

三氟碘甲烷作为冰箱制冷剂的理论循环分析

通过对环保工质三氟碘甲烷(CF3I)的饱和蒸汽压曲线、冰箱名义工况和变工况下循环性能等三方面的理论分析,发现CF3I和CF3I的摩尔组成在50%-65%范围的CF3I/HC290混合工质,理论循环性能与CFC12接近,具有作为冰箱中CFC12灌注式替代物的潜力.     

Performance evaluation of environmentally benign refrigerants in heat pumps 1: A simulation study

An overview of the performance characteristics of possible working fluids in vapour-compression industrial heat-pump systems for medium to high temperature applications is presented. The refrigerants studied include HFC12, HCFC22 HFC134a, HCFC142b, HFC152a, a tenary blend of HCFC22 (40%), HCFC124 (43%) and HFC152a (17%), and NH₃. The calculations are made for all the refrigerants for the same operating conditions and are compared with each other. For high-temperature applications, a compression- absorption heat-pump cycle (with NH₃-H₂O as the working fluid) is described. Its performance characteristics are discussed and compared with vapour-compression cycles with HCFC142b as the working fluid by using the concept of ‘thermodynamic temperature’.     

An experimental study on HC290 and a commercial liquefied petroleum gas (LPG) mix as suitable replacements for HCFC22

This paper presents an experimental study on the performance of hydrocarbon refrigerants, namely propane and a liquefied petroleum gas (LPG) mix as suitable replacements for the widely used refrigerant HCFC22 in refrigeration and heat pump applications. A cylinder of commercially available LPG from New Zealand market was obtained for this study. The composition of the specific LPG mix (by mass fraction) was propane (HC290)—98.95%, ethane (HC170)—1.007%, iso-butane (HC600a)—0.0397% and other constituents in small proportions. Experiments were carried out in a laboratory heat pump test facility with maximum condenser capacity of approximately 15 kW. Condensing temperatures were held constant at 35, 45 and 55°C, while evaporating temperatures were varied over a wide range from − 15 to + 15°C. All tests were carried out at constant degree of superheat (about 1 K) and subcooling (about 8 K). All appropriate precautions were observed against any leaks or fire.The analysis revealed that the hydrocarbon refrigerants performed better than HCFC22 but with a small loss of condenser capacity. The mass flow rate and compressor discharge temperature were found to be significantly lower than HCFC22. The performance of the specific LPG mix tested was found to be better than HC290 at higher condensing temperatures but poorer at a lower condensing temperature. No adverse effects were found with the LPG mix despite the presence of little moisture (less than 0.01%) in its composition. The study reveals that LPG of the tested composition (i.e. predominantly a mixture of propane, ethane and iso-butane) can be an excellent refrigerant in heat pump/refrigeration applications.     

Transferts de chaleur lors de la condensation du mélange HFC23/HFC134a à l'intérieur d'un tube lisse horizontal

The environmental effects of the depletion of stratospheric ozone due to refrigerants containing chlorine, have resulted in international treaties, laws and amendments (Copenhagen, 1992, to the Montreal protocol, 1987) to phase out and eliminate many common refrigerants. HCFC22 is one of these refrigerants and no such single component alternative has been discovered for this fluid. Zeotropic refrigerant mixtures (binary or ternary) are being considered as potential replacements for HCFC22. Evaporation and condensation heat-transfer characteristics, and inside tubes of heat exchangers, due to the use of zeotropic refrigerant mixtures, have been a subject of fundamental importance in evaluating the heat exchanger performances in the refrigeration and air-conditioning industry.In this study, it is proposed to determine the heat transfer and pressure drop coefficients during in-tube condensation of zeotropic mixture HFC23/HFC134a in a smooth copper tube with an inside diameter of 8.92 mm. The test section of three passes of 2 m each; it is a counter flow double-pipe heat exchanger with water flowing in the annulus and refrigerant in the inner tube. This test section is instrumented with temperature and pressure sensors. We have tested HCFC22, HFC134a, and three refrigerant mixtures of HFC23/HFC134a at different compositions to appreciate the effect of glide on heat transfer. The quality was from 1 to 80%, the heat flux ranged from 2 to 50 kW m⁻² and mass flux varied from 80 to 480 kg m⁻²s⁻¹. In these conditions, no effect of a glide on the heat-transfer coefficient was observed; this result was confirmed by using an equilibrium condensation curve analysis. The pressure drop can be calculated with classical correlations but with physical properties of the mixture.     

Comparison of the refrigerants HFC 134a and CFC 12

A comparison of the refrigerants HFC 134a and CFC 12 has been carried out and the results from a theoretical analysis and from tests with an open piston compressor are reported in this paper. The results indicate that the tested compressor will give a greater refrigerating capacity with HFC 134a than with CFC 12 for certain operating conditions. However, the results also indicate an increased operating power for the compressor over the entire temperature range. As a result the coefficient of performance is decreased. Another noticeable result is dependency of the compressor's isentropic efficiency on temperature when using HFC 134a. This might be explained by the properties of the polyalkene glycol oil which is used with HFC 134a. The increased cost of using HFC 134a is justified if the environmental aspects are considered and the practical problems, such as the influence on the material in the refrigeration cycle, can be solved.     

An evaluation of options for replacing HCFC-22 in medium temperature refrigeration systems

Due to the ongoing global phase-out of R-22, which is still the most widely used refrigerant around the world, there is a need to replace this refrigerant in many different applications. This paper focuses on a thorough evaluation of the R-22 replacement options for medium-temperature refrigeration applications. It includes a thermodynamic analysis, comparison of heat transfer and pressure drop characteristics, system performance comparisons using a validated detailed system model, safety issues, and determination of the environmental impact of refrigerant selection. Three potential alternatives to the R-22 were studied: two HFCs (404A and R-410A) and one HC (R-290). An HFC refrigerant, R-410A, is shown to be an efficient and environmentally acceptable option to replace R-22 in medium temperature applications.     

Performances d'une machine tritherme à éjecteur utilisant des mélanges de fluides frigorigènesPerformance analysis of a jet cooling system using refrigerant mixtures

The optimisation of a jet cooling system using refrigerant mixtures as substitutes of pure refrigerants has been investigated. A steady-state simulation program, for given temperatures of the sources, integrating simple models of each component has been developed. A Peng-Robinson equation of state assuming equality of the fugacities of the two phases was used to model the thermodynamic properties of the vapour and liquid-vapour equilibrium. The refrigerants investigated in this study are: the pure refrigerants R142b, R152a, RC318, R124, R134a, R22 and the binary refrigerants R22/RC318, R22/R142b, R22/R124, R22/R152a, R22/R134a, R134a/R142b, R152a/R142b and R134a/R152a. Results show that the use of a binary mixture does not always increase the performance of system. Generally, when the mixture is strongly zeotropic (e.g.: R22/RC318), the cooling efficiency of the system decreases. However, when the mixture is mildly zeotropic (e.g. R134a/R142b) or almost azeotropic (e.g. R134a/R152a), efficiency and energetic efficiency increase.     

Testing of propane/isobutane mixture in domestic refrigeratorsEtude sur un mélange propane/isobutane dans les réfrigérateurs domestiques

The performance of a propane/isobutane (R290/R600a) mixture was examined for domestic refrigerators. A thermodynamic cycle analysis indicated that the propane/isobutane mixture in the composition range of 0.2 to 0.6 mass fraction of propane yields an increase in the coefficient of performance (COP) of up to 2.3% as compared to CFC12. For the actual tests, two commercial refrigerators of 299 and 465 l were used. For both units, all refrigeration components remained the same throughout the tests, except that the length of the capillary tube and amount of charge were changed for the mixture. Each refrigerator was fully instrumented with more than 20 thermocouples, two pressure transducers, and a digital watt/watt-h meter. For each unit, both ‘energy consumption test’ and ‘no load pull-down test’ were conducted under the same condition. The experimental results obtained with the same compressor indicated that the propane/isobutane mixture at 0.6 mass fraction of propane has a 3–4% higher energy efficiency and a somewhat faster cooling rate than CFC12. The mixture showed a shorter compressor on-time and lower compressor dome temperatures than CFC12. In conclusion, the proposed hydrocarbon mixture seems to be an appropriate long term candidate to replace CFC12/HFC134a from the viewpoint of energy conservation requiring minimal changes in the existing refrigerators.     

Performance characteristics of a small-capacity directly cooled refrigerator using R290/R600a (55/45)

In this study, the performance of a small-capacity directly cooled refrigerator was evaluated by using the mixture of R290 and R600a with mass fraction of 55:45 as an alternative to R134a. The compressor displacement volume of the alternative system with R290/R600a (55/45) was modified from that of the original system with R134a to match the refrigeration capacity. Both systems with R290/R600a (55/45) and R134a were tested, and then optimized by varying the refrigerant charge and capillary tube length under experimental conditions for both the pull-down test and the power consumption test. The refrigerant charge of the optimized R290/R600a system was approximately 50% of that of the optimized R134a system. The capillary tube lengths for each evaporator in the optimized R290/R600a system were 500 mm longer than those in the optimized R134a system. The power consumption of the optimized R134a system was 12.3% higher than that of the optimized R290/R600a system. The cooling speed of the optimized R290/R600a (55/45) system at the in-case setting temperature of −15 °C was improved by 28.8% over that of the optimized R134a system.     

Condensation heat transfer coefficients of flammable refrigerants

In this study, external condensation heat transfer coefficients (HTCs) of six flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), dimethylether (RE170), and HFC32 were measured at the vapor temperature of 39 °C on a plain tube of 19.0 mm outside diameter with a wall subcooling of 3–8 °C under a heat flux of 7–23 kW m⁻². Test results showed a typical trend that external condensation HTCs decrease with the wall subcooling. No unusual behavior or phenomenon was observed for these flammable refrigerants during experiments. HFC32 and DME showed 28–44% higher HTCs than those of HCFC22 due to their excellent thermophysical properties. Propylene and butane showed the similar HTCs as those of HCFC22 while propane and isobutane showed 9% lower HTCs than those of HCFC22. Finally, a general correlation was made by modifying Nusselt's equation based upon the measured data of eleven fluids of various vapor pressures including halogenated refrigerants. The general equation showed an excellent agreement with all data exhibiting a deviation of less than 3%.     

Refrigeration system performance using liquid-suction heat exchangersPerformance d''un système frigorifique utilisant des échangeurs liquide-vapeur à l''aspiration

Heat transfer devices are provided in many refrigeration systems to exchange energy between the cool gaseous refrigerant leaving the evaporator and warm liquid refrigerant exiting the condenser. These liquid-suction or suction-line heat exchangers can, in some cases, yield improved system performance while in other cases they degrade system performance. Although previous researchers have investigated performance of liquid-suction heat exchangers, this study can be distinguished from the previous studies in three ways. First, this paper identifies a new dimensionless group to correlate performance impacts attributable to liquid-suction heat exchangers. Second, the paper extends previous analyses to include new refrigerants. Third, the analysis includes the impact of pressure drops through the liquid-suction heat exchanger on system performance. It is shown that reliance on simplified analysis techniques can lead to inaccurate conclusions regarding the impact of liquid-suction heat exchangers on refrigeration system performance. From detailed analyses, it can be concluded that liquid-suction heat exchangers that have a minimal pressure loss on the low pressure side are useful for systems using R507A, R134a, R12, R404A, R290, R407C, R600, and R410A. The liquid-suction heat exchanger is detrimental to system performance in systems using R22, R32, and R717.     

Experiment and simulation on the performance of an autocascade refrigeration system using carbon dioxide as a refrigerant

The main purpose of this study is to investigate the performance of an autocascade refrigeration system using zeotropic refrigerant mixtures of R744/134a and R744/290. One of the advantages of this system is the possibility of keeping the highest pressure of the system within a limit by selecting the composition of a refrigerant mixture as compared to that in the vapor compression system using pure carbon dioxide. Performance test and simulation have been carried out for an autocascade refrigeration system by varying secondary fluid temperatures at evaporator and condenser inlets. Variations of mass flow rate of refrigerant, compressor power, refrigeration capacity, and coefficient of performance (COP) with respect to the mass fraction of R744 in R744/134a and R744/290 mixtures are presented at different operating conditions. Experimental results show similar trends with those from the simulation. As the composition of R744 in the refrigerant mixture increases, cooling capacity is enhanced, but COP tends to decrease while the system pressure rises.

Résumé

The main purpose of this study is to investigate the performance of an autocascade refrigeration system using zeotropic refrigerant mixtures of R744/134a and R744/290. One of the advantages of this system is the possibility in keeping the highest pressure of the system within a limit by selecting the composition of a refrigerant mixture as compared to that in the vapor compression system using pure carbon dioxide. Performance test and simulation have been carried out for an autocascade refrigeration system by varying secondary fluid temperatures at evaporator and condenser inlets. Variations of mass flow rate of refrigerant, compressor power, refrigeration capacity, and coefficient of performance (COP) with respect to the mass fraction of R744 in R744/134a and R744/290 mixtures are presented at different operating conditions. Experimental results show similar trends with those from the simulation. As the composition of R744 in the refrigerant mixture increases, cooling capacity is enhanced, but COP tends to decrease while the system pressure rises.     

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.