Evaluation of supplementary/retrofit refrigerants for automobile air-conditioners charged with CFC12Evaluation des frigorigènes supplémentaires / de remplacement pour des systèmes de conditionnement d'air automobiles utilisant du CFC12

In this study, thermodynamic performance of supplementary/retrofit refrigerant mixtures for CFC12 used in existing automobile air-conditioners was examined. A thermodynamic computer analysis of an automobile air-conditioner was carried out for the initial screening of possible mixture candidates, and refrigerant mixtures composed of HCFC22, HFC134a, HCFC142b, RE170 (dimethylether), HC290 (propane), and HC600a (iso-butane) were proposed to supplement CFC12. Also a breadboard type refrigeration test facility was manufactured to verify the performance of the alternative refrigerant mixtures proposed through the computer analysis. Test results showed that HFC134a/RE170 mixture with zero ozone depletion potential is the best long term candidate to supplement CFC12. On the other hand, HCFC22/HFC134a/RE170 and HCFC22/HFC134a/HCFC142b mixtures are good only as short term supplementary/retrofit alternatives since they contain HCFC22. A hydrocarbon mixture of HC290/HC600a showed a good performance but its use in existing automobile air-conditioners should be carefully considered due to its flammability.     

An NTU-ε model for alternative refrigerants: Un modèle NTU-ε pour les frigorigènes de remplacement

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-ε 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.     

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.     

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%.     

Performance of an HCFC22-based vapour absorption refrigeration system

A single-stage vapour absorption refrigeration system (VARS) is tested with monochlorodifluoromethane (HCF22) as refrigerant and different absorbents: dimethylether of tetraethylene glycol (DMETEG) and dimethyl acetamide (DMA). The influence of generator temperatures in the range 75–95°C, which represents low-grade heat sources, is studied. Cooling water temperatures were varied between 20 and 30°C. Two cases of cooling water flow paths are considered, i.e. water entering either absorber or condenser, which are connected in series. For HCFC22-DMETEG, COP values in the range 0.2–0.36 and evaporator temperatures between 0 and 10°C are obtained. For HCF22-DMA, COP values in the range 0.3–0.45 and evaporator temperatures between −10 and 10°C are obtained. It is observed that HCFC22-DMETEG can work at lower heat source temperatures than HCFC22-DMA. However, at the same operating conditions HCFC22-DMA is better from the viewpoints of circulation ratio and COP. Experiments also show that at low heat source temperature, cooling water temperature has strong influence on circulation ratio but does not affect COP significantly. Preferably, cooling water should first flow through the condenser and then through the absorber in order to achieve improved overall performance.     

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.     

Performance assessment of HC-290 as a drop-in substitute to HCFC-22 in a window air conditioner

As per the Montreal Protocol, CFCs and HCFCs are being phased out. HCFC-22 is used in window air conditioners. This paper presents the experimental performance study of a window air conditioner with propane (HC-290), a natural refrigerant, as a drop-in substitute to HCFC-22. Experimental results showed that HC-290 had 6.6% lower cooling capacity for the lower operating conditions and 9.7% lower for the higher operating conditions with respect to HCFC-22. The coefficient of performance for HC-290 was 7.9% higher for the lower operating conditions and 2.8% higher for the higher operating conditions. The energy consumption of the unit with HC-290 was lower in the range 12.4–13.5% than HCFC-22. The discharge pressures for HC-290 were lower in the range 13.7–18.2% than HCFC-22. For HC-290, the pressure drop was lower than HCFC-22 in both heat exchangers.This paper also presents simulation results for the heat exchangers of an HCFC-22 window air conditioner with HC-290 as a drop-in substitute. The simulation has been carried out using EVAP-COND, a heat exchanger model developed by NIST [National Institute of Standards and Technology. EVAP-COND: simulation models for finned-tube heat exchangers, Maryland, USA (2003). http://www2.bfrl.nist.gov/software/evap-cond/ [18]]. The simulated evaporator capacities are within ±4% of the experimentally measured cooling capacities for both refrigerants. Simulation results for HC-290 and HCFC-22 are compared. The exit temperatures of HC-290 are lower by 0.3–1.2 °C in the condenser and are higher by 2.1–2.4 °C in the evaporator than HCFC-22. Evaporating pressures of HC-290 are lower by 2.1–3.3% as compared to HCFC-22. The pressure drops of HC-290 are lower in both the evaporator and the condenser as compared to HCFC-22. The outlet temperatures of air for HCFC-22 and HC-290 in both heat exchangers are nearly the same.     

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.     

Use of hydrocarbons as drop-in replacements for HCFC-22 in on-farm milk cooling equipment

Many refrigeration systems on New Zealand dairy farms use HCFC-22 which is being phased out by 2015. Both laboratory and on-farm trials were undertaken to investigate hydrocarbons as drop-in replacements to HCFC-22 in milk silo refrigeration systems. A mixture of propane and ethane (Care-50) reduced energy use by 6–8%, and had similar system cooling capacity relative to HCFC-22. With propane (Care-40), energy use decreased by 5% but cooling capacity was 9% lower. The retrofits were simple and low cost because no alterations to the systems other than change in refrigerant and appropriate safety labelling and documentation were made. For most farms, the outside refrigeration system location and small charge mean that hydrocarbons could meet NZ standards for safe use of refrigerants. The low retrofit cost, improved energy efficiency, low environmental impact, mineral oil compatibility, similar cooling capacity and controllable flammability risks mean that the propane–ethane mixture is an attractive replacement for HCFC-22 on NZ dairy farms.     

A minichannel aluminium tube heat exchanger – Part III: Condenser performance with propane

This paper reports heat transfer results obtained during condensation of refrigerant propane inside a minichannel aluminium heat exchanger vertically mounted in an experimental setup simulating a water-to-water heat pump. The condenser was constructed of multiport minichannel aluminium tubes assembled as a shell-and-tube heat exchanger. Propane vapour entered the condenser tubes via the top end and exited sub-cooled from the bottom. Coolant water flowed upward on the shell-side. The heat transfer areas of the tube-side and the shell-side of the condenser were 0.941 m² and 0.985 m², respectively. The heat transfer rate between the two fluids was controlled by varying the evaporation temperature while the condensation temperature was fixed. The applied heat transfer rate was within 3900–9500 W for all tests. Experiments were performed at constant condensing temperatures of 30 °C, 40 °C and 50 °C, respectively. The cooling water flow rate was maintained at 11.90 l min⁻¹ for all tests. De-superheating length, two-phase length, sub-cooling length, local heat transfer coefficients and average heat transfer coefficients of the condenser were calculated. The experimental heat transfer coefficients were compared with predictions from correlations found in the literature. The experimental heat transfer coefficients in the different regions were higher than those predicted by the available correlations.     

Performance of diffusion absorption refrigeration cycle with organic working fluids

A diffusion absorption refrigeration (DAR) cycle is driven by heat and utilizes a binary solution of refrigerant and absorbent as working fluid, together with an auxiliary inert gas. Commercial DAR systems operate with ammonia–water solution and hydrogen or helium as the inert gas. In this work, the performance of a simplified DAR system working with an organic absorbent (DMAC – dimethylacetamide) and five different refrigerants and helium as inert gas was examined numerically, with the aim of lowering the generator temperature and system pressure along with a non-toxic refrigerant The refrigerants were: chlorodifluoromethane (R22), difluoromethane (R32), 2-chloro-1,1,1,2-tetrafluoroethane (R124), pentafluoroethane (R125) and 1,1,1,2-tetrafluoroethane (R134a). The results were compared with the performance of the same system working with ammonia–water and helium. Similar behavior was found for all systems, regarding the coefficient of performance (COP) and rich and poor solution concentrations as functions of generator temperature. It was found that typical generator temperature with the new substances was 150 °C, yet lower COPs, higher evaporator temperatures and lower condensation temperature of about 40 °C governed these systems.     

A study on the performance of multi-stage condensation heat pumpsEtude sur la performance des pompes à chaleur à condensation en plusieurs étages

In this study, computer simulation programs were developed for multi-stage condensation heat pumps and their performance was examined for CFC11, HCFC123, HCFC141b under the same condition. The results showed that the coefficient of performance (COP) of an optimized ‘non-split type’ three-stage condensation heat pump was 25–42% higher than that of a conventional single-stage heat pump. The increase in COP differed among the fluids examined. The improvement in COP was due largely to the decrease in average temperature difference between the refrigerant and water in the condensers, which resulted in a decrease in thermodynamic irreversibility. For the three-stage heat pump, the highest COP was achieved when the total condenser area was evenly distributed to the three condensers. For the two-stage heat pump, however, the optimum distribution of total condenser area varied with working fluids. For the three-stage system, splitting the condenser cooling water for the use of intermediate and high pressure subcoolers helped increase the COP further. When the individual cooling water for the intermediate and high pressure subcoolers was roughly 10% of the total condenser cooling water, the optimum COP was achieved showing an additional 11% increase in COP as compared to that of the ‘non-split type’ for the three-stage heat pump system.     

A theoretical study of a novel regenerative ejector refrigeration cycle

There has been a demand for developments of the ejector refrigeration systems using low grade thermal energy, such as solar energy and waste heat. In this paper, a novel regenerative ejector refrigeration cycle was described, which uses an auxiliary jet pump and a conventional regenerator to enhance the performance of the novel cycle. The theoretical analysis on the performance characteristics was carried out for the novel cycle with the refrigerant R141b. Compared with the conventional cycle, the simulation results show that the coefficient of performance (COP) of the novel cycle increases, respectively, by from 9.3 to 12.1% when generating temperature is in a range of 80–160 °C, the condensing temperature is in a range of 35–45 °C and the evaporating temperature is fixed at 10 °C. Especially due to the enhanced regeneration with increasing the pump outlet pressure, the improvement of COP of the novel cycle is approached to 17.8% compared with that in the conventional cycle under the operating condition that generating temperature is 100 °C, condensing temperature is 40 °C and evaporating temperature is 10 °C. Therefore, the characteristics of the novel cycle performance show its promise in using low grade thermal energy for the ejector refrigeration system.     

R290冷凝器的设计与制造

针对R290的性能特点和管内两相流的冷凝换热特性，设计并制造了R290／CO2复叠式制冷循环中的R290水冷式冷凝器，为自然工质R290／CO2复叠式制冷循环系统的研究和实际应用奠定基础。     

Thermoeconomic optimisation of the condenser in a vapour compression heat pump

In the paper, the design optimisation of a heat exchanger is discussed, using a thermoeconomic approach. The investigation is referred to the tube-in-tube condenser of a conventional vapour-compression heat pump, with a two-phase refrigerant flowing in the inner tube and a single-phase fluid flowing in the annulus. A cost function to be minimised is introduced, defined as the sum of two contributions: (i) the amortisation cost of the condenser, related to the heat exchange area; (ii) the operating cost of the electric-driven heat pump in which the heat exchanger will work, depending on the overall exergy destruction rate in the system. In the paper, this latter contribution is related to the local irreversibility rate in the condenser, using the so-called structural approach. The optimal trade-off between amortisation and operating cost is therefore investigated, by minimising the above-mentioned cost function. A numerical example is discussed, in which, for a commercial heat exchanger, the design improvements needed to obtain a cost-optimal configuration are investigated. The analysis is carried out for three different refrigerants: R22, R134a and R410A.     

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.     

利用熵产最小法分析R290/CO2复叠式制冷循环

介绍了低温环境下采用自然工质R290和CO2的复叠式制冷循环，用熵产最小法对R290／CO2复叠式低温制冷循环进行了分析，利用熵产最小法确定R290/CO2复叠式低温制冷循环的最佳中间温度。为提高R290／CO2复叠式低温制冷循环的效率，应减少蒸发器、冷凝器和冷凝蒸发器的传热温差，可以看出R290／CO2的复叠式制冷循环在低温制冷条件下有很好的发展前景。     

Experimental and numerical research on condenser performance for R-22 and R-407C refrigerants

An experimental study of a fin and tube condenser was performed using two different configurations of condenser paths (U and Z type) and two kinds of refrigerants (R-22 and R-407C) as working fluids. An integral test facility was constructed to evaluate the heat transfer capacity of the air and refrigerant sides of the condenser. An uncertainty study was also performed. A numerical code was developed, using a section-by-section analysis scheme in which mal-distribution on the air side and temperature gliding on the refrigerant side could be considered along the tube-length direction. Different condenser capacities were obtained from both the experimental and numerical results, depending on the paths and refrigerants used. R-22 performed better than R-407C for the Z-type path configuration, but no significant difference was found between results using either refrigerant in the U-type path configuration. On average, the numerical results obtained with R-22 were 10.1% greater than experiment data; using R-407C, results were 10.7% less than experiment data. The numerical code can be used as a design tool to develop better condenser paths.     

Modelling of reciprocating and scroll compressors

This paper presents simple and thermodynamically realistic models of two types of compressors widely used in domestic heat pumps (reciprocating and scroll compressors). These models calculate the mass flow rate of refrigerant and the power consumption from the knowledge of operating conditions and parameters. Some of these parameters may be found in the technical datasheets of compressors whereas others are determined in such a way that the calculated mass flow rate and electrical power match those given in these datasheets.The two models have been tested on five reciprocating compressors and five scroll compressors. This study has been limited to compressors with a maximum electrical power of 10 kW and for the following operating conditions: evaporating temperatures ranging from −20 to 15 °C and condensing temperatures ranging from 15 to 60 °C.The average discrepancies on mass flow rate and power for reciprocating compressors are 1.10 and 1.69% (for different refrigerants: R134a, R404A, R22, R12 and R407C). For scroll compressors, the average discrepancies on mass flow rate and power are 2.42 and 1.04% (for different refrigerants: R134a, R404A, R407C and R22).     

Numerical modeling of serpentine microchannel condensers

Microchannel (or mini-channel) heat exchangers are drawing more attention because of the potential cost reduction and the lower refrigerant charge. Serpentine microchannel heat exchangers are even more compact because of the minimized headers. Using the serpentine microchannel condenser, some thermodynamically good but flammable refrigerants like R-290 (Propane) can be extended to more applications. To well size the serpentine microchannel condensers, a distributed-parameter model has been developed in this paper. Airside maldistribution is taken into account. Model validation shows good agreement with the experimental data. The predictions on the heating capacity and the pressure drop fall into ±10% error band. Further analysis shows the impact of the pass number and the airside maldistribution on the condenser performance.