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’.     

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.     

冰淇淋机制冷系统性能及新型环保制冷剂热力学分析

针对软冰淇淋机首先确定了其计算工况，在分析冰淇淋机制冷循环的基础上，应用NIST的REFPROPV6.0软件编写了通用的计算程序，对不同工质的冰淇淋机循环过程进行了计算。经过计算可以发现，纯质HFC134a,HFC125,HFC143a,HFC32和HFC152a等中没有一种工质的热工性能，安全性能和环境性能可以完全满足替代要求，根据蒸气压相似原则，考虑各纯质的优缺点及混合制冷剂配对原则，确定了混合工质HFC134a/HFC32(70/30mass%)作为冰淇淋机的替代制冷剂。     

离心式冷水机组制冷剂选择的探讨

阐述了离心式冷水机组的两种替代制冷剂HFC134a和HCFC123的性能,综合比较了这两种制冷剂的安全性、效率和其它一些因素。从安全性能来讲,HFC134a占有优势;在效率方面,HCFC123略高于HFC134a;由于HCFC123的ODP不为零,根据蒙特利尔议定书这一强制性条约,它将限制禁用;同时京都议定书也要求限制HFCs的排放量。最后文中结合了中国国情对HCFC123的使用进行了风险分析,认为就目前而言,HFC134a应当成为替代剂的主流。     

HFC 134a as a substitute refrigerant for CFC 12

In response to international protocol agreements and national regulatory actions promoted by the increasing concern for ozone depletion and the greenhouse effect, HFC 134a has emerged as a leading candidate for CFC 12 substitution in automotive air conditioners, centrifugal chillers and residential refrigerators and freezers. This Paper discusses compressor and refrigeration system requirements and information gaps for HFC 134a application as a CFC 12 substitute.     

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 heat transfer of a pure fluid and binary mixture outside a bundle of smooth horizontal tubes. Comparison of experimental results and a classical model

The condensation of pure HFC134a and different zeotropic mixtures with pure HFC134a and HFC23 on the outside of a bundle of smooth tubes was studied. The local heat transfer coefficient for each row was experimentally determined using a test section composed by a 13×3 staggered bundle of smooth copper tubes, measuring cooling water temperature in the inlet and the outlet of each tube, and measuring the vapour temperature along the bundle. All data were taken at the inlet vapour temperature of 40°C with a wall subcooling ranging from 4 to 26 K. The heat flux was varied from 5 to 30 kW/m² and the cooling water flow rate from 120 to 300 l/h for each tube. The visualisation of the HFC134a condensate flow by means of transparent glass tubes reveals specific flow patterns and explains the difference between the measured values of the heat transfer coefficient and the calculated values from Nusselt's theory. On the other hand, the experimental heat transfer data with the binary mixtures HFC23-HFC134a show the important effects of temperature glide and the strong decrease of the heat transfer coefficient in comparison with the pure HFC134a data. The measured values with the different zeotropic mixtures were compared with the data calculated with the classical condensation model based on the equilibrium model. An improvement of this model is proposed.     

Measurement of the viscosity of HFC 134a in the temperature range 213–423 K and at pressures up to 30 MPa

The viscosity of HFC 134a was measured over the range of temperatures from 213 to 423 K and pressures up to 30 MPa. The experimental method was that of the capillary flow and a closed-circuit high-pressure viscometer was used. The sample fluid was circulated through a stainless-steel capillary from a highpressure plunger system. The constant of the capillary was calibrated against the reference standard, pure water. The viscosity of the sample was calculated from the flow rate, the pressure drop at the capillary, and the capillary constant using the Hagen-Poiseuille equation. Measurements were made at a total of 39 points on eight isotherms. The measurement uncertainty of the viscosities was estimated as ±1.3%. Based on the present results, an empirical equation for the viscosity of HFC 134a has been correlated. The viscosity on the saturation line calculated by the equation compares with experimental viscosity data in other previous studies. There are rather considerable differences among these measurements. Comparisons of the data for HFC 134a with those for CFC 12 show that the viscosity of HFC 134a is similar in magnitude to that of CFC 12 at temperatures around 300 K but is higher at lower temperatures and lower at higher temperatures. The pressure gradients for these two corresponding substances are similar over the entire temperature range.     

Retrofitting HFC134a into existing CFC12 systems

The CFC phasing-out regulations call for the phasing out of CFCs before the end of the decade. The process of revision of these regulations continues and the dates for phasing-out may be brought forward even more. After the phasing out of CFCs it will be important for the refrigeration industry to be able to use the existing equipment currently running on CFC12. This paper gives a brief outline of ICI developmental work on the new range of ester oils suitable for the alternative refrigerants. The simple procedure developed to enable the refrigeration industry to convert from CFC12 to HFC134a is described. The flushing procedure and the determination of residual mineral-oil content are set out. Five case studies where HFC134a has been retrofitted in five different application sectors are given at the end of the paper.     

The thermal conductivity of liquid 1,1,1,2-tetrafluoroethane (HFC 134a)

The thermal conductivity of HFC 134a was measured in the liquid phase with the polarized transient hot-wire technique. The experiments were performed at temperatures from 213 to 293 K at pressures up to 20 MPa. The data were analyzed to obtain correlations in terms of density and pressure. This study is part of an international project coordinated by the Subcommittee on Transport Properties of Commission 1.2 of IUPAC, conducted to investigate the large discrepancies between the results reported by various authors for the transport properties of HFC 134a, using samples of different origin. Two samples of HFC 134a from different sources have been used. The thermal conductivity of the first sample was measured along the saturation line as a function of temperature and the data were presented earlier. The thermal conductivity of the second one, the round-robin sample was measured as a function of pressure and temperature. These data were extrapolated to the saturation line and compared with the data obtained, previously in order to demonstrate the importance of the sample origin and their real purity. The accuracy of the measurements is estimated to be 0.5%. Finally, the results are compared with the existing literature data.     

Degradation of polyethylene terephthalate films in the presence of lubricants for HFC 134a: a critical issue for hermetic motor insulation systems

Earlier studies conducted in the use of sealed tubes with polyalkylene glycol lubricants and polyethylene terephthalete (PET) films revealed that the PET films exhibited embrittlement and (visual) degradation. This led to an investigation of PET embrittlement mechanisms with the new lubricants used with HFC 134a. The lubricants studied were three polypropylene glycols (the monol, the diol and the completely end-capped glycols), pentaerythritol ester and a blend of monol and ester. The effects of moisture content, temperature and lubricant structure were studied. All lubricants in this study were of viscosity grade ISO-32 (150 SUS). The results were compared to PET film embrittlement in the presence of CFC 12 and mineral oil. This study reconfirmed the earlier findings that the PET films must be dried to lower than 0.1 wt.% moisture content for use in hermetic systems. This paper discusses the effect of the moisture content of the lubricant and the effect of the lubricant structure on PET films. The dependence of the various mechanisms on temperature is shown. Esters and end-capped polyalkylene glycols are recommended for use with HFC 134a.     

Nucleate boiling heat transfer coefficients of mixtures containing HFC32, HFC125, and HFC134a

Nucleate boiling heat transfer coefficients (HTCs) of binary and ternary mixtures composed of HFC32, HFC125, and HFC134a on a horizontal smooth tube of 19.0 mm outside diameter were measured. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature at 7 °C. HTCs of nonazeotropic mixtures of HFC32/HFC134a, HFC125/HFC134a, and HFC32/HFC125/HFC134a showed a reduction of HTCs as much as 40% from the ideal values while the near azeotropic mixture of HFC32/HFC125 did not show the reduction. Four of the well known correlations were compared against the present data for binary mixtures. Stephan and Körner's and Schlünder's correlations yielded a good agreement with a deviation of less than 10% but they can not be easily extended to multi-component mixtures of more than three components. A new correlation was developed utilizing only the phase equilibrium data and physical properties. A regression analysis was carried out to account for the reduction of HTCs and the final correlation, which can be easily extended to multi-component mixtures of more than three components, yielded a deviation of 7% for all binary and ternary mixtures.     

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.     

A simple experimental investigation of saturated vapour pressure for HFC134a-oil mixtures

A new refrigerant , HFC134a, seems to be the most promising substitute for CFC12. The vapour pressure of HFC134a-oil mixtures is one parameter that is important for a proper analysis of the operation of refrigeration systems. This paper presents vapour pressure curves for HFC134a and three kinds of representative oil for different oil percentages, and for the temperature range from -20 to +40°C (253.15–313.15 K).     

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.     

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.     

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.     

风冷螺杆热泵机组制冷剂替换试验研究

对R22、R407c、R134a三种制冷剂的基本物性及热力性能进行了分析比较，并在风冷螺杆热泵机组基础上进行了替换试验研究。结果表明：R407c为最佳替换R22的制冷剂；R134a替换后能效比较高，但制冷（热）量衰减过多，同时R134a的运行压力过低不太适合热泵工况。     

Evaluation of minimum desorption temperatures of thermal compressors in adsorption refrigeration cycles

The purpose of this paper is to identify the minimum desorption temperatures required to operate thermally driven adsorption beds of a solid sorption refrigeration system. The method is based on the evaluation of uptake efficiency of the adsorption bed and estimating there from conditions under which the compressor ceases to provide any throughput. The difference in the densities of the refrigerant between the inlet and outlet, the adsorption characteristics of the adsorbate–refrigerant pair and the void volume in the thermal compressor are the contributors to the manifestation of the desorption state. Among them, the void volume is a controllable parameter whose role is analogous to the clearance volume in a positive displacement compressor. The methodology has been tested out with three systems, namely, silica gel + water, activated carbon fiber + ethanol and activated carbon + HFC 134a systems. It is shown that waste heat at as low as 60 °C can operate these systems which make them good energy conservation devices through recovery of low grade process waste heat.     

Critical parameters for HFC134a, HFC32 and HFC125

The vapour-liquid coexistence curves near the critical point for HFC134a (CF₃CH₂F: 1,1,1,2-tetrafluoroethane), HFC32 (CH₂F₂: difluoromethane) and HFC125 (CHF₂CF₃: pentafluoroethane) have been measured by visual observation of the meniscus disappearance. Three sets of 17 experimental results for the saturated liquid or vapour densities for HFC134a, HFC32 and HFC125 have been obtained in the reduced temperature range T/T_c > 0.96 and in the reduced density range 0.4 < ρ/ρ_c < 1.7. From these measurements, the critical temperature and the critical density for these HFCs have been determined in consideration of the meniscus disappearance level as well as the intensity of the critical opalescence. The critical pressure has been calculated by the extrapolation of the vapour-pressure correlation. The uncertainties of the critical temperature, critical density and critical pressure are estimated to be within ± 10 mK, ± 5 kg m⁻³ and ± 9 kPa, respectively.