Experimental evaluation of R22 and R407C evaporative heat transfer coefficients in a vapour compression plant

The mean heat transfer coefficients of R22 and R407C in the coaxial counterflow evaporator (20 mm ID) of a refrigerating vapour compression plant have been experimentally measured. The experimental conditions under which heat transfer coefficients were determined reflect a typical working situation for small-scale refrigeration systems. The heat flux ranged from 1.9 to 9.1 kW/m² and the mass flux was varied from 30 to 140 kg/m² s. The results illustrate that the R22 heat transfer coefficient is always greater than that of R407C. Furthermore, a comparison carried out between the experimental data and those predicted by means of the most credited literature relationships showed a strongly overprediction for R407C coefficients.     

Flow condensation heat transfer coefficients of R22, R134a, R407C, and R410A inside plain and microfin tubes

Flow condensation heat transfer coefficients (HTCs) of R22, R134a, R407C, and R410A inside horizontal plain and microfin tubes of 9.52 mm outside diameter and 1 m length were measured at the condensation temperature of 40 °C with mass fluxes of 100, 200, and 300 kg m⁻² s⁻¹ and a heat flux of 7.7–7.9 kW m⁻². For a plain tube, HTCs of R134a and R410A were similar to those of R22 while HTCs of R407C are 11–15% lower than those of R22. For a microfin tube, HTCs of R134a were similar to those of R22 while HTCs of R407C and R410A were 23–53% and 10–21% lower than those of R22. For a plain tube, our correlation agreed well with the present data for all refrigerants exhibiting a mean deviation of 11.6%. Finally, HTCs of a microfin tube were 2–3 times higher than those of a plain tube and the heat transfer enhancement factor decreased as the mass flux increased for all refrigerants tested.     

Condensation heat transfer coefficients of R22, R407C, and R410A on a horizontal plain, low fin, and turbo-C tubes

In this study, condensation heat transfer coefficients (HTCs) were measured on a horizontal plain tube, low fin tube, and Turbo-C tube at the saturated vapor temperature of 39 °C for R22, R407C, and R410A with the wall subcooling of 3–8 °C. R407C, a non-azeotropic refrigerant mixture, exhibited a quite different condensation phenomenon from those of R22 and R410A and its condensation HTCs were up to 50% lower than those of R22. For R407C, as the wall subcooling increased, condensation HTCs decreased on a plain tube while they increased on both low fin and turbo-C tubes. This was due to the lessening effect of the vapor diffusion film with a rapid increase in condensation rate on enhanced tubes. On the other hand, condensation HTCs of R410A, almost an azeotrope, were similar to those of R22. For all refrigerants tested, condensation HTCs of turbo-C tube were the highest among the tubes tested showing a 3–8 times increase as compared to those of a plain tube.     

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.     

Experimental investigation on the drop-in performance of R407C as a substitute for R22 in a screw chiller with shell-and-tube heat exchangers

A drop-in test of a mixed refrigerant R407C is performed in a commercial screw chiller with shell-and-tube heat exchangers originally designed for R22. The test results show a severe performance reduction when substituting the refrigerant from R22 to R407C. The reason for the performance reduction is analyzed comprehensively, and the influence of thermodynamic properties, compressor efficiency, and heat transfer is evaluated quantitatively. The major factor causing the performance reduction is assessed as the degradation of the heat transfer in using the mixed refrigerant, R407C. The heat transfer degradation in the evaporator is found to be larger and influences more on the chiller performance reduction. The performance reduction caused by the evaporator is approximately two times compared with that of the condenser.     

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.     

Flow-boiling of R22, R134a, R507, R404A and R410A inside a smooth horizontal tube

Flow boiling heat transfer coefficients of R22, R134a, R507, R404A and R410A inside a smooth horizontal tube (6 mm I.D., 6 m length) were measured at a refrigerant mass flux of about 360 kg/m² s varying the evaporating pressure within the range 3–12 bar, with heat fluxes within the range 11–21 kW/m². The experimental data are discussed in terms of the heat transfer coefficients as a function of the vapour quality. The experimental results clearly show that the heat transfer coefficients of R134a are always higher than those pertaining to R22 (from a minimum of +6 to a maximum of +45%).     

Evaporating heat transfer of R22 and R410A in horizontal smooth and microfin tubes

An experimental investigation of condensation heat transfer in 9.52 mm O.D. horizontal copper tubes was conducted using R22 and R410A. The test rig had a straight, horizontal test section with an active length of 0.92 m and was cooled by the heat transfer fluid (cold water) circulated in a surrounding annulus. Constant heat flux of 11.0 kW/m² was maintained throughout the experiment and refrigerant quality varied from 0.9 to 0.1. The condensation test results at 45 °C were reported for 40–80 kg/h mass flow rate. The local and average condensation coefficients for seven microfin tubes were presented compared to those for a smooth tube. The average condensation coefficients of R22 and R410A for the microfin tubes were 1.7–3.19 and 1.7–2.94 times larger than those in smooth tube, respectively.     

Condensation of R407C in a horizontal microfin tube

This paper describes experimental results that show the effects of mass velocity and condensation temperature difference on the local heat transfer characteristics during condensation of R407C in a horizontal microfin tube. The experiments were performed at the saturation temperature of 40 °C, the refrigerant mass velocity of 50, 100, 200 and 300 kg m⁻² s⁻¹, and the condensation temperature difference of 1.5, 2.5 and 4.5 K. A superficial heat transfer coefficient for the vapor phase was obtained by subtracting the heat transfer resistance of condensate film estimated by using a previously developed theoretical model of film condensation of pure vapor from the overall heat transfer resistance. On the basis of the analogy between heat and mass transfer, an empirical equation for the superficial vapor phase heat transfer coefficient was developed. The heat transfer coefficient predicted by the combination of the previously developed theoretical model of film condensation of pure vapor and the empirical equation of the superficial vapor phase heat transfer coefficient agreed with the measured values with the r.m.s. error of 9.2%.     

Evaporation of R407C and R410A in a horizontal herringbone microfin tube: heat transfer and pressure drop

Based on experimental data for R134a, the present work deals with the development of a prediction method for heat transfer in herringbone microfin tubes. As is shown in earlier works, heat transfer coefficients for the investigated herringbone microfin tube tend to peak at lower vapour qualities than in helical microfin tubes. Correlations developed for other tube types fail to describe this behaviour. A hypothesis that the position of the peak is related to the point where the average film thickness becomes smaller than the fin height is tested and found to be consistent with observed behaviour. The proposed method accounts for this hypothesis and incorporates the well-known Steiner and Taborek correlation for the calculation of flow boiling heat transfer coefficients. The correlation is modified by introducing a surface enhancement factor and adjusting the two-phase multiplier. Experimental data for R134a are predicted with an average residual of 1.5% and a standard deviation of 21%. Tested against experimental data for mixtures R410A and R407C, the proposed method overpredicts experimental data by around 60%. An alternative adjustment of the two-phase multiplier, in order to better predict mixture data, is discussed.     

R22制冷系统采用R407C后的设计改进

主要介绍了环保工质R407C在实际生产中的应用,从制冷循环的四大件(压缩机、冷凝器、膨胀阀、蒸发器)的设计和选择上介绍了设计思路,并对测试结果进行了分析.     

R404A和R407C在水平强化管外的凝结换热实验研究

实验研究了近共沸制冷工质R404A与非共沸制冷工质R407C在水平强化换热管管外的凝结换热性能。采用"Wilson图解法"对实验数据进行处理。结果表明:对于R404A和R407C,强化管外的凝结换热系数随着壁面过冷度的增加而增大,呈现出与纯工质冷凝时不同的变化趋势,这主要是近共沸或非共沸工质凝结过程中,某些组分的凝结会遇到其它组分的凝结气膜热阻所造成的;随着过冷度增加,易挥发组分开始凝结,气膜变薄,冷凝传热系数增大。R407C在强化换热管管外的凝结换热系数比R404A要小70%左右,这是由于R407C的温度滑移较R404A要大,管外形成的凝结扩散气膜造成的影响更大。R407C在高热流密度工况下的换热效果提升明显,故应尽量工作在高热流密度区域。     

Two-phase heat transfer coefficients of three HFC refrigerants inside a horizontal smooth tube, part II: condensation

This paper presents local heat transfer results obtained during the condensation of Isceon 59, R407C and R404A in a smooth horizontal tube. The results have been compared with existing correlations for condensation heat transfer to assess the validity of these models for refrigerant mixtures. Two correlations (Dobson MK, Chato JC. Condensation in smooth horizontal tubes. Journal of Heat Transfer, Transactions of ASME 1998; 120: 193–213, Shah MM. A general correlation for heat transfer during film condensation inside pipes. Int J Heat & Mass Transfer 1979; 22: 547–56) have been considered because they deal with refrigerant blends and their range of applicability suited the experimental test conditions. The Dobson and Chato correlation provided the best prediction for these refrigerant mixtures. The Shah correlation fitted the measurements of the local heat transfer coefficients well and seem to cope well with refrigerant mixtures.     

Condensation heat transfer of R410A inside internally grooved horizontal tubes

Heat transfer coefficients were measured for the condensation of R410A and R22 inside internally grooved horizontal tubes. The experiment was performed for five different kinds of internally grooved tubes of about 8.00 mm o.d. the shapes of which were conventional helical grooved and herring-bone grooved ones. To measure the local heat transfer coefficients, the test section was subdivided into 10 small sections having 1 m working length. The ranges tested are as follows: the refrigerant mass velocity was from 130 to 400 kg/(m²/s) for R410A and R22, and the vapour pressure was 2.4l MPa for R410A and 1.53 MPa for R22. The obtained heat transfer data of R410A and R22 indicate that the values of the local heat transfer coefficients of the herring-bone grooved tube are about twice as large as those of helical one. All measured local heat transfer coefficients of condensation were compared with the predicted values from previous correlations proposed by other researchers, and were well correlated with the empirical equation using the frictional coefficients for each tube proposed by the author.     

Experimental investigation of the performance of R22, R407C and R410A in several capillary tubes for air-conditioners

The objective of this study is to present test results and to develop a dimensionless correlation on the basis of the experimental data of adiabatic capillary tubes for R22 and its alternatives, R407C (R32/125/134a, 23/25/52 wt.%) and R410A (R32/125, 50/50 wt.%). Several capillary tubes with different length and inner diameter were selected as test sections. Mass flow rate through the capillary tube was measured for several condensing temperatures and various degrees of subcooling at the inlet of each capillary tube. Experimental conditions for the condensing temperatures were selected as 40, 45 and 50°C, and the degrees of subcooling were adjusted to 1.5, 5 and 10°C. Mass flow rates of R407C and R410A were compared with those of R22 for the same test conditions. The results for straight capillary tubes were also compared with those of coiled capillary tubes. A new correlation based on Buckingham π theorem to predict the mass flow rate through the capillary tubes was presented based on extensive experimental data for R22, R407C and R410A. Dimensionless parameters were chosen considering the effects of tube geometry, capillary tube inlet conditions, and refrigerant properties. Dimensionless correlation predicted experimental data within relative deviations ranging from −12% to +12% for every test condition for R22, R407C and R410A. The predictions by the developed correlation were in good agreement with the results in the open literature.     

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.     

Heat transfer, pressure drop, and flow pattern recognition during condensation inside smooth, helical micro-fin, and herringbone tubes

This paper presents a study of flow regimes, pressure drops, and heat transfer coefficients during refrigerant condensation inside a smooth, an 18° helical micro-fin, and a herringbone tubes. Experimental work was conducted for condensing refrigerants R-22, R-407C, and R-134a at an average saturation temperature of 40 °C with mass fluxes ranging from 400 to 800 kg m⁻² s⁻¹, and with vapour qualities ranging from 0.85 to 0.95 at condenser inlet and from 0.05 to 0.15 at condenser outlet. These test conditions represent annular and intermittent (slug and plug) flow conditions. Results showed that transition from annular flow to intermittent flow, on average for the three refrigerants, occurred at a vapour quality of 0.49 for the smooth tube, 0.29 for the helical micro-fin tube, and 0.26 for the herringbone tube. These transition vapour qualities were also reflected in the pressure gradients, with the herringbone tube having the highest pressure gradient. The pressure gradients encountered in the herringbone tube were about 79% higher than that of the smooth tube and about 27% higher than that of the helical micro-fin tube. A widely used pressure drop correlation for condensation in helical micro-fin tubes was modified for the case of the herringbone tube. The modified correlation predicted the data within a 1% error with an absolute deviation of 7%. Heat transfer enhancement factors for the herringbone tube against the smooth tube were on average 70% higher while against the helical micro-fin tube it was 40% higher. A correlation for predicting heat transfer coefficients inside a helical micro-fin tube was modified for the herringbone tube. On average the correlation predicted the data to within 4% with an average standard deviation of 8%.     

R32/R134a和R407C用于变浓度热泵系统的实验

为了解决R32/R134a应用于变浓度热泵系统存在的排气温度过高问题,提出使用三元混合工质R407C用于该系统中.以R32/R134a和R407C作为工质在变浓度容量调节热泵系统中进行了吸气压力不变时的变浓度实验.实验结果表明,R407C在本系统中变浓度范围低于R32/R134a,但R407C的排气温度和耗功均低于R32/R134a,具有良好的变浓度调节潜力.     

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.     

Reasons for drop in shell-and-tube condenser performance when replacing R22 with zeotropic mixtures. Part 1. Analysis of experimental findings

An experimental and theoretical investigation was made to find out the reasons for the drop in shell-and-tube condenser performance when replacing R22 with a zeotropic mixture R407C. Measurements show that at lower condenser loads the reduction in performance can be as large as 70% compared to the full condenser load. Calculation results are compared with experimental results for two different condensers, one with micro-finned tubes and one with 3-D finned tubes. Calculations show that the degree of mixing of the newly formed condensate on a tube and the drained condensate is a factor influential enough to explain the performance drop. 3-D finned tubes seem to have better mixing in the condensate than integral finned tubes.