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.     

Impact of uncertainties on estimations of heat pump cycle performance

Sensitivity analysis is used to show how uncertainties in basic factors, such as heat transfer, pressure drop, compressor efficiency, and thermophysical properties, influence the result of heat pump cycle performance estimations. By comparing the results of this analysis with uncertainty levels of estimations and experimental determinations, it is established to what extent improvements in accuracy are needed. The investigation was carried out for several different applications and, as working fluids, both pure substances and non-azeotropic mixtures were examined. It was found that evaporation heat transfer and compressor efficiency are two areas where improvements are needed. It is also shown that estimates using very accurate, substance-specific, equations of state differ from those using more general types by up to 1 % for COP and 2% for capacity.     

Replacement of R22 in tube-and-shell condensers: experiments and simulations

An investigation of the change in condenser overall heat transfer coefficient when replacing R22 with one of the three mixtures R407C, R404A and R410B was made, both experimentally and theoretically. Measurements have been carried out on a full-scale test plant consisting of a horizontal shell-side condenser. According to the measurements the decrease in overall heat transfer coefficient for the non-azeotropic mixture R407C was very large, up to 70% compared to R22, while for the near-azeotropic mixture R404A the decrease was less than 15%. Simulations of the condenser were done with a comprehensive computer program, calculating the condensation heat transfer with an approximate method including a correction for mass resistance. The calculation model was not able to predict this large degradation for the non-azeotropic mixture, while the predictions agreed rather well with the measurements for the pure fluid and the near-azeotropic mixtures.     

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.     

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.     

Heat transfer and pressure drop in a plate-type evaporator

A plate-type evaporator, working with natural refrigerant circulation, has been investigated both experimentally and theoretically. Motivated by the phase-out of ozone-depleting substances, HCFC22 was compared to HFC134a and two zeotropic refrigerant mixtures. The effect of different separator liquid levels, i.e. refrigerant flows, and its influence on heat transfer was also studied. The investigated plate-type evaporator consists of thirteen vertical flow channels and its size is 3.0 m × 0.5 m. The heat source for the evaporator is a falling water film on the outside of the plate. Experimental studies have been carried out using a test facility that enabled detailed measurements of heat transfer and pressure drop. Experiments were compared to results from a calculation method that simultaneously calculates heat transfer and pressure drop in a variable number of steps along the evaporator. The calculation method is based on a pressure drop correlation proposed by the VDI-Wärmeatlas and a heat transfer correlation for vertical tubes proposed by Steiner and Taborek. For different evaporator duties, heat transfer was over predicted by 12% for pure fluids by 15% for mixtures. Calculated pressure drops were well within ±5% of the measured values. Changes in heat transfer due to different flows were closely predicted by the proposed calculation method.     

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.     

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.     

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.     

Drop-in replacement of R22 in heat pumps used for district heating — influence of equipment and property limitations

In Swedish district heating systems several large (25 MW) turbo-compressor driven heat pumps using R22 are installed. The only commercially available alternative is R134a, but its use could decrease the heating capacity by 35%. In this paper a method for finding the best working fluid for a specific heat pump plant is presented, and applied to a district heating plant. First, a screening is made among almost 2000 mixtures, using criteria such as condenser pressure, Mach number and temperature glide. Simulations of the plant are then made to investigate the change in heating capacity and COP when using a mixture instead of R134a. The results show that there are mixtures that can offer a substantially higher heating capacity than R134a, but there is a decrease in COP. The importance of considering the limiting parameters of the heat pump, such as maximum volume flow to each compressor stage and minimum evaporator pressure, is shown.     

HCFC123与HFC134a离心式冷水机组特性比较

从离心式冷水机组的特性角度出发,阐述冷水机组2种替代制冷剂HFC134a和HCFC123的选择使用,比较分析这2种制冷剂的性能,及其机组性能和其他一些因素。其中制冷剂的环境特性包括毒性、GWP和ODP等。机组性能特性包括产品能耗比及单位质量制冷量比的比较等。其他特性包括产品尺寸、使用维护等。综合考虑上述因素,尤其应考虑2007年9月修订的《蒙特利尔议定书》加速淘汰HCFCs的现实要求,认为在我国对新的离心机组使用HCFC123应持十分谨慎的态度。     

Condensation and evaporation heat transfer of R410A inside internally grooved horizontal tubes

Heat transfer coefficient and pressure drop were measured for condensation and evaporation of R410A and HCFC22 inside internally grooved tubes. The experiments were performed for a conventional spiral groove tube of 8.01 mm o.d. and 7.30 mm mean i.d., and a herring-born groove tube of 8.00 mm o.d. and 7.24 mm mean i.d. To measure the local heat transfer coefficients and pressure drop, the test section was subdivided into four small sections having 2 m working length. The ranges of refrigerant mass flow density was from 200 to 340 kg/(m² s) for both condensation and evaporation of R410A and HCFC22, and the vapour pressure was 2.41 MPa for condensation and 1.09 MPa for the evaporation of R410A. The obtained heat transfer data for R410A and HCFC22 indicate that the values of the local heat transfer coefficients of the herring-bone grooved tube are about twice as large as those of spiral one for condensation and are slightly larger than those of spiral one for the evaporation. The measured local pressure drop in both condensation and evaporation is well correlated with the empirical equation proposed by the authors.     

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.     

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.     

Design considerations for refrigeration cycles

The Carnot COP, which assumes a thermodynamically ideal cycle in which no irreversibilities exist, is often considered to be a design goal for actual cycles. However, the Carnot COP does not consider heat transfer mechanisms. Heat transfer at a finite rate is necessarily an irreversible process and unavoidable in a refrigeration cycle. The lack of consideration of rate processes reduces the usefulness of the Carnot COP as a realistic design goal. In this paper, the limitations of both thermodynamics and heat transfer are considered to identify a more realistic design goal for the COP of refrigeration cycles. The consideration of heat transfer limitations leads to a design rule for the optimum distribution of heat exchange area between the low- and high-temperature heat exchangers.     

基于最小熵增法编程确定非共沸混合工质组分比

以非共沸混合工质在蒸发器中沿程温度分布变化所导致传热不可逆熵增为目标函数,建立混合工质与冷媒水在蒸发器中的稳态换热模型;以换热温差最小值为基准,编程分析计算,得出二元混合工质R290/R600在不同组分比下的相对熵增,选取其中最小值对应组分比为最佳组分比.     

Viscosity of Gaseous Mixtures of HCFC-22 + HCFC-142b at Pressures to 6.3 MPa

The gaseous viscosity of non-azeotropic mixtures of HCFC-22 and HCFC-142b was measured by an oscillating disk viscometer of the Maxwell type from 298.15 K to 423.15 K and at pressures up to 6.3  MPa. The viscosity at approximately atmospheric pressure was predicted with a maximum deviation of 1.84 % and an average deviation of 0.61 % by the Sutherland-Thiesen equation, coefficients of which were determined by the Brokaw method. An empirical equation was developed for the viscosity as a function of composition, temperature, and density. This equation reproduced the observed viscosity with a maximum deviation of 5.61 % and an average deviation of 1.14 %.     

The thermal conductivity of R22, R142b,R152a,and their mixtures in the liquid state

An experimental apparatus for measuring the thermal conductivity of liquids by the transient hot-wire method was constructed and tested with toluene as a standard liquid. Measurements were performed on R22, R142b, and R152a. The thermal conductivities of mixtures of R142b and R152a with R22 were also measured by varying the weight fraction of R22. Experiments were performed in the range from –50 to 50°C and from 2 to 20 MPa and the measured data are analyzed to obtain a correlation in terms of temperature, pressure, and composition of the mixture. While the thermal conductivity of R22 + R152a mixtures varies monotonously with composition, that of R22 + R142b mixtures turned out to go through an extremum value. The accuracy of our measurements is estimated to be within 2%.Paper dedicated to Professor Joseph Kestin.     

Molecular properties of alternative refrigerants derived from dielectric-constant measurements

A review of the current work in Lisbon on the measurement of the dielectric constant of the liquid phase of some environmentally acceptable refrigerants proposed as alternative replacements of the chlorofluorocarbons (CFCs), responsible for the destruction of the ozone layer, is presented. Measurements on HCFC 141b, HCFC 142b, HCFC 123, HFC 134a, HFC 152a, and HFC 32 samples of stated purities of 99.8 mass% or better were performed as a function of pressure and temperature, in the temperature range from 200 to 300 K and at pressures up to 20 MPa. The ratio of the capacitances of a cell filled with the sample and under vacuum was measured with a direct capacitance method. The dielectric-constant measurements have a repeatability of 0.003% and an accuracy of 0.1%. The theory developed by Vedam et al. based on the Eulerian Strain and the Kirkwood equation for the variation of the modified molar polarization with temperature and density were applied to obtain the dipole moments of the refrigegrants in the liquid state, to obtain a physical insight of the molecular behavior, and to understand the equilibrium configuration of these liquids. Invited paper presented at the Fourth Asian Thermophysical Properties Conference. September 5–8, 1995, Tokyo, Japan.     

Pool boiling heat transfer from a horizontal tube coated with oxide ceramics

Pool boiling heat transfer from a horizontal copper tube coated with 0.2 mm of aluminum oxide-titanium oxide ceramics has been investigated for several pure fluids (refrigerants and hydrocarbons) and three propane/n-butane mixtures. The heat transfer coefficient shows a similar dependence on heat flux and normalized saturation pressure as with a metallic heating tube. At normalized saturation pressures p/p_c0.1, the absolute values of the heat transfer coefficient are just as high as for a sandblasted copper tube of similar surface roughness and at lower saturation pressures even higher. The negative influence of the low thermal conductivity of the ceramics is completely compensated or even overcompensated by the positive influence of the microstructure which results in a higher nucleation site density. This is especially effective in pool boiling of mixtures.