A comparative study of water as a refrigerant with some current refrigerants

Water as a refrigerant (R718) is compared with some current natural (R717 and R290) and synthetic refrigerants (R134a, R12, R22, and R152a) regarding environmental issues including ozone depletion potential (ODP) and global warming potential (GWP), safety (toxicity and flammability), operating cost, refrigeration capacity and coefficient of performance (COP). A computer code simulating a simple vapour compression cycle was developed to calculate COPs, pressure ratios, outlet temperatures of the refrigerants from the compressor, and evaporator temperatures above which water theoretically yields better COPs than the other refrigerants investigated. The main difference of this study from other similar studies is that both evaporator temperature and condenser temperature are changed as changing parameters, but the temperature lift, which is the temperature difference between condenser and evaporator, are held constant and the irreversibility during the compression process is also taken into consideration by taking the isentropic efficiency different from 100%. It is found that for evaporator temperatures above 20°C and small temperature lift (5 K), R718 gives the highest COP assuming exactly the same cycle parameters. For medium temperature lifts (20–25 K), this evaporator temperature is above 35°C, whereas for even greater temperature lifts it decreases again. Furthermore, with increased values of polytropic efficiency, R718 can maintain higher COPs over other refrigerants, at lower evaporator temperatures. Copyright © 2005 John Wiley & Sons, Ltd.     

Flow Boiling Heat Transfer Characteristics of R600a in Multiport Minichannel

ABSTRACT

Evaporators of small and medium refrigeration systems, as in commercial and automobile air conditioning applications, are being studied to develop more compact and lighter equipment, that reaches good thermal performance and reliability, with low pressure drop. In this way, evaporators are being designed with small channels and materials like aluminum. Moreover, different refrigerants are being tested to substitute for hydrofluorocarbon (HFC) refrigerants, with different operational temperatures and pressures. Some of them, like hydrocarbons, although they present advantages with respect to their thermodynamic and transport properties, should be used with small charge in the system due to their flammability. This work presents the results of an experimental study to characterize the flow boiling of the refrigerant R600a (isobutane) in a multiport aluminum extruded tube with 7 parallel minichannels of 1.47 mm hydraulic diameter. The effects of mass velocity, heat flux, and vapor quality on heat transfer were investigated for constant saturation temperature and pressure. Heat fluxes in the range from 5 to 30 kW m^?2, mass velocities set to discrete values in the range of 50 to 200 kg m^?2 s^?1, and saturation temperature of 20°C were considered. It was verified a significant effect of heat flux. Moreover, some images of flow patterns, in different conditions, are presented, and the main patterns identified were slug, intermittent, and annular.     

过冷度对蒸气压缩式制冷循环性能的影响分析

在蒸气压缩式制冷循环中,存在高压液体的过冷现象。过冷液体温度与其饱和温度之间的差值称为过冷度。液体过冷可明显提高一些制冷剂的单位质量制冷量和性能系数。本文运用热力学定律,对多种常用工质的蒸气压缩式制冷循环进行了理论分析,并计算了在不同的过冷度下,循环的制冷量、性能系数等参数与蒸发温度、冷凝器出口温度的关系,得出了不同工质在不同过冷度下的表现等规律和结论,为制冷设备循环方式和制冷工质的选择提供一些参考。     

制冷剂环境影响及可持续发展浅析

论述了当前使用的制冷剂以及存在的问题,指出现行制冷剂对臭氧层的破坏作用及引起的温室效应,将严重影响环境的可持续发展。总结了当前制冷剂的替代工作及取得的成果。在论述可持续发展概念的基础上分析了制冷剂的替代研究与环境的可持续发展的关系,得出了环境的可持续发展的要求推动了制冷剂的替代研究工作,并为替代研究指明了方向,同时制冷剂的替代进一步促进了环境的可持续发展。总结了在环境可持续发展要求下的制冷剂的发展趋势。     

制冷工质的安全、环境评价指标和数据

本文综述了当前制冷工质的安全、环境评价指标,给出了130多种制冷工质的安全,环境的最新数据。这些制冷工质不仅涵盖了当前使用的制冷剂,而且还有将来可能使用的制冷剂。可为制冷工质替代的科研人员和工商业界选用的新制冷工质参考。     

Experimental study on heat transfer performance of pulsating heat pipe with refrigerants

The effects of different refrigerants on heat transfer performance of pulsating heat pipe(PHP) are investigated experimentally.The working temperature of pulsating heat pipe is kept in the range of 20℃-50℃.The startup time of the pulsating heat pipe with refrigerants can be shorter than 4 min,when heating power is in the range of 10W-100W.The startup time decreases with heating power.Thermal resistances of PHP with filling ratio 20.55% were obviously larger than those with other filling ratios.Thermal resistance of the PHP with R134a is much smaller than that with R404A and R600a.It indicates that the heat transfer ability of R134a is better.In addition,a correlation to predict thermal resistance of PHP with refrigerants was suggested.     

Energy and exergy analysis of vapor compression refrigeration system using pure hydrocarbon refrigerants

This study presents a comparison of energetic and exergetic performance of a vapor compression refrigeration system using pure hydrocarbon (HC) refrigerants. In this study, four different pure HCs propane (R290), butane (R600), isobutane (R600a) and isopentane (R1270) are used in theoretical analysis. R22 and R134a are also used in the analysis. For the analysis, EES package program was used for solving thermodynamic equations of the refrigerants. Results have been presented graphically. According to results, differences of coefficient of cooling performance values of these refrigerants are quite small. Energetic and exergetic efficiency values obtained with R1270 and R600 are higher than R600a and R290. Copyright © 2009 John Wiley & Sons, Ltd.     

微尺度条件下反复熄燃火焰特性的实验研究

在可视化微尺度燃烧实验台上进行甲烷和氧气的燃烧试验,利用高速数码照相机捕捉到了火焰面在微通道内的传播过程,测试分析了不同进气流量下反复熄燃火焰的可燃极限、火焰传播速度和火焰间隔时间,获得了反复熄燃火焰(Flames with repetitive extinction and ignition, FREI)的燃烧特性。结果表明,随着甲烷进气流速的增加,可以形成FREI火焰的氧气进气流速范围也在扩大；在甲烷进气流速一定的情况下,随着氧气进气流速的增加,火焰的传播速度也逐渐增加,并且火焰重复点燃的间隔时间呈现先变大后逐渐变小的规律,即火焰重复点燃的频率先变慢后又逐渐变快直至火焰熄灭。     

Performance comparison of CFCs with their substitutes using artificial neural network

In order to decrease global pollution due to chlorofluorocarbons (CFCs), the usage of HFC‐ and HC‐based refrigerants and their mixtures are considered instead of CFCs (R12, R22, and R502). This was confirmed by an international consensus (i.e. Montreal Protocol signed in 1987). This paper offers to determine coefficient of performance (COP) and total irreversibility (TI) values of vapour‐compression refrigeration system with different refrigerants and their mixtures mentioned above using artificial neural networks (ANN). In order to train the network, COPs and TIs of refrigerants and their some binary, ternary and quartet mixtures of different ratios have been calculated in a vapour‐compression refrigeration system with liquid/suction line heat exchanger. In the calculations thermodynamic properties of refrigerants have been taken from REFPROP 6.01 which was prepared based on Helmholtz energy equation of state. To achieve this, a new software has been written in FORTRAN programming language using sub‐programs of REFPROP, and all related calculations have been performed using this software using constant temperature method as reference. Scaled conjugate gradient, Pola–Ribiere conjugate gradient, and Levenberg–Marquardt learning algorithms and logistic sigmoid transfer function were used in the network. Mixing ratios of refrigerants, and evaporator temperature were used as input layer; COP and TI values were used as output layer. It is shown that R² values are about 0.9999, maximum errors for training and test data are smaller than 2 and 3%, respectively. It is concluded that, ANNs can be used for prediction of COP and TI as an accurate method in the systems. Copyright © 2004 John Wiley & Sons, Ltd.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

Flow characteristics of refrigerants flowing through capillary tubes – A review

A comprehensive review of the literature on the flow of various refrigerants through the capillary tubes of different geometries viz. straight and coiled and flow configurations viz. adiabatic and diabatic, has been discussed in this paper. The paper presents in chronological order the experimental and numerical investigations systematically under different categories. Flow aspects like effect of coiling and effect of oil in the refrigerants on the mass flow rate through the capillary tube have been discussed. Furthermore, the phenomenon of metastability and the correlations to predict the underpressure of vaporization have also been discussed. The paper provides key information about the range of input parameters viz. tube diameter, tube length, surface roughness, coil pitch and coil diameter, inlet subcooling and condensing pressure or temperature. Other information includes type of refrigerants used, correlations proposed and methodology adopted in the analysis of flow through the capillary tubes of different geometries operating under adiabatic and diabatic flow conditions. It has been found from the review of the literature that there is a lot more to investigate for the flow of various refrigerants through different capillary tube geometries.     

Effect of saturation‐temperature on the performance of a vapour‐compression refrigeration‐cycle working on different refrigerants using exergy method

In this study, the behaviour of a vapour‐compression refrigeration cycle, for different refrigerants such as NH₃, R‐12, R‐22 and HFC‐134a was investigated using the exergy method. The cooling load of the plant and the saturation‐temperature of the cold chamber were held constant, whereas the saturation‐temperatures of the evaporator and the condenser were varied from 303 to 313 K and 258 to 248 K, respectively. The irreversibility rates (or exergy destruction rates) of sub‐regions for the whole cycle, using energy and exergy analysis, were determined for each refrigerant. The effects of changes in the saturation‐temperature in the condenser and evaporator on the irreversibility rate of the cycle were obtained for each refrigerant. The relations between the total irreversibility rate of the plant and the irreversibility rate of the condenser and the evaporator were determined for different values of saturation temperatures of the condenser and the evaporator. The COP of the cycle and the rational efficiency were determined for each of the refrigerants and compared with each other. Among the refrigerants used, R‐12 was found to be the most economical refrigerant as compared with the others. Copyright © 2005 John Wiley & Sons, Ltd.     

The combined effect of pressure and oxygen concentration on piloted ignition of a solid combustible

There are a number of situations when fires may occur at low pressures and oxygen concentrations that are different than standard atmospheric conditions, such as in buildings at high elevation, airplanes, and spacecraft. The flammability of materials may be affected by these environmental conditions. Since ignition delay is a measure of material flammability and directly influences whether a fire will occur, experiments were conducted to assess the variation of the ignition delay of PMMA in sub-atmospheric pressures and elevated oxygen concentrations. Three sets of experiments were performed at different pressures and in air, in an atmosphere having 30% oxygen/70% nitrogen by volume, and in a “normoxic” atmosphere (constant oxygen partial pressure). It was observed that as the pressure is reduced, the ignition time decreased, reached a minimum, and then increased until ignition did not occur. Several mechanisms were considered to explain the “U-shaped” dependence of ignition time on pressure, and three regimes were identified each having a different controlling mechanism: the transport regime where the ignition delay is controlled by changes in convection heat losses and critical mass flux for ignition; the chemical kinetic regime where the ignition delay is controlled by gas-phase chemical kinetics; and an overlap region where both the transport and chemistry effects are seen. The results provide further insight about the effect of the environmental conditions on the flammability of materials, and guidance about fire safety in low pressure environments.     

Sustainability with prospective refrigerants

This paper first presents a brief historical summary of the recent past and current status of refrigerants, and then proceeds to suggest a simple Second Law‐based approach using the ideal vapor compression refrigeration cycle to estimate a refrigerant's performance potential. The resulting performance rankings are then supplemented by additional rankings of the exergy losses in the compressor and in the condenser. These methodologies are then applied to several refrigerants: three natural refrigerants (ammonia, propane, and isobutane), five conventional single‐component halocarbon refrigerants, two conventional blends of halocarbon refrigerants, and three newer refrigerants (fluorinated propene isomers). Generally speaking, the lower pressure refrigerants have better COP and lesser volumetric cooling capacity than the higher pressure refrigerants; whereas, the lower pressure refrigerants have higher penalty factor (measure of condenser exergy losses) than the higher pressure refrigerants. The analyses presented in this paper suggest that to minimize a refrigeration system's overall impact on the environment, the choice of refrigerant should not necessarily be based on a single criterion but rather should be chosen based on the particular application. Copyright © 2013 John Wiley & Sons, Ltd.     

Effects of coil diameter and pitch on the flow characteristics of alternative refrigerants flowing through adiabatic helical capillary tubes

This paper presents the effects of various geometries of helical capillary tubes on the flow characteristics of alternative refrigerants flowing through adiabatic helical capillary tubes. The theoretical model is based on the conservation of mass, energy and momentum of fluids in the capillary tube. The two-phase flow model developed was based on a homogenous flow assumption. The model was validated by comparing it with the experimental data of published in literature for R-22, particularly various pairs of refrigerants. It was found conventional refrigerants had lower capillary lengths than alternative refrigerants. For all pairs, the numerical results showed that the traditional refrigerants consistently gave lower pressure drops for both single-phase and two-phase flows, which resulted in longer tube lengths. The results show that coil diameter variation (less than 300 mm) for helical capillary tube geometries affected the length of helical capillary tubes. However, pitch variation (more than 300 mm) had no significant effect on the length of helical capillary tubes. This adiabatic helical capillary tube model can be used to integrate system models working with alternative refrigerants for design and optimisation.     

Thermodynamic analysis of subcooling and superheating effects of alternative refrigerants for vapour compression refrigeration cycles

This paper presents a computer‐based first law and exergy analysis applied to vapour compression refrigeration systems for determining subcooling and superheating effects of environmentally safe new refrigerants. Three refrigerants are considered: R134a, R407c and R410a. It is found that subcooling and superheating temperatures directly influence the system performance as both condenser and evaporator temperatures are affected. The thermodynamic properties of the refrigerants are formulated using artificial neural network (ANN) methodology. Six ANNs were trained to predict various properties of the three refrigerants. The training and validation of the ANNs were performed with good accuracy. The correlation coefficient obtained when unknown data were used to the networks were found to be equal or very near to 1 which is very satisfactory. Additionally, the present methodology proved to be much better than the linear multiple regression analysis. From the analysis of the results it is found that condenser and evaporator temperatures have strong effects on coefficient of performance (COP) and system irreversibility. Also both subcooling and superheating affect the system performance. This effect is similar for R134a and R407c, and different for R410a. Copyright © 2005 John Wiley & Sons, Ltd.     

R245fa阻燃特性理论与试验研究

分析了R245fa的阻燃,试验研究了R245fa在电打火(点火能约为100 J)和普通打火机点火两种条件下对R290、R152a、DME、R600a的阻燃特性,提出了新的评判阻燃组分阻燃特性强弱的指标(阻燃系数K),并对R245fa作用于各可燃工质的阻燃效果进行分类．试验结果表明,点火能大小、点火源形式对可燃工质燃爆极限影响很大；R245fa对碳氢类可燃工质阻燃效果差；对HFC类的阻燃效果相对较好；R245fa的阻燃效果比R134a的差．     

NH3／CO2超市制冷系统的分析研究

介绍了利用自然工质NH_3和CO_2的复叠式超市制冷系统,通过对NH_3/CO_2超市制冷系统与R22和R404A超市制冷系统的分析比较,得出NH_3/CO_2超市制冷系统较R22和R404A超市制冷系统有明显的优势;自然工质NH_3/CO_2复叠式超市制冷系统,不仅节约能源,而且保护环境,是很有发展前景的绿色环保制冷系统。     

Development of a diabatic two-phase flow pattern map for horizontal flow boiling

An improved two-phase flow pattern map is proposed for evaporation in horizontal tubes. Based on new flow pattern data for three different refrigerants covering a wide range of mass velocities, vapor qualities and heat fluxes. The new flow pattern map includes the prediction of the onset of dryout at the top of the tube during evaporation inside horizontal tubes as a function of heat flux and flow parameters and is an extension to the flow pattern map model of Kattan et al. [J. Heat Transfer 120 (1998) 140]. The proposed modifications allow an accurate prediction of the flow pattern for very different fluids which are the substitute refrigerants (HFC-134a and HFC-407C) and the natural refrigerant R-717 (ammonia).     

Effect of low temperature on the flammability limits of methane/nitrogen mixtures

An experimental setup was established to measure the flammability limits of flammable gases at low temperatures. The flammability limits of methane/nitrogen mixtures in air were measured at atmospheric pressure and wide temperature range from 150 to 300 K. The estimated uncertainty of the experimental values was 0.2 vol%. The results reveal that as initial temperature decreases, the upper flammability limits (UFL) decrease while the lower flammability limits (LFL) increase. Therefore, the critical flammability ratio (CFR) decreases as the initial temperature decreases. In addition, the variation in the UFL is more sensitive than that in the LFL as the molar ratio of nitrogen to methane increases. In addition, an extended Le Chatelier’s formula with higher-order and temperature terms was proposed to correlate and predict the flammability limits at each temperature. The calculated values conformed to the experimental results for both the LFL and the UFL.