Refrigerant R134a condensation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental tests on HFC-134a condensation inside a small brazed plate heat exchanger: the effects of refrigerant mass flux, saturation temperature and vapour super-heating are investigated.A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m² s. For refrigerant mass flux lower than 20 kg/m² s, the saturated vapour heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [Nusselt, W., 1916. Die oberflachenkondensation des wasserdampfes. Z. Ver. Dt. Ing. 60, 541–546, 569–575] analysis for vertical surface. For refrigerant mass flux higher than 20 kg/m² s, the saturated vapour heat transfer coefficients depend on mass flux and are well predicted by the Akers et al. [Akers, W.W., Deans, H.A., Crosser, O.K., 1959. Condensing heat transfer within horizontal tubes. Chem. Eng. Prog. Symp. Ser. 55, 171–176] equation. In the forced convection condensation region, the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by the Webb [Webb, R.L., 1998. Convective condensation of superheated vapour. ASME J. Heat Transfer 120, 418–421] model. The heat transfer coefficients show weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux.     

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.     

Flow condensation heat transfer coefficients of pure refrigerants

Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m⁻² s⁻¹ and heat flux of 7.3–7.7 kW m⁻². Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.     

Experimental and theoretical study on flow condensation with non-azeotropic refrigerant mixtures of R32/R134a

Experiments on flow condensation have been conducted with both pure R32, R134a and their mixtures inside a tube (10 m long, 6 mm ID), with a mass flux of 131–369 kg m⁻²s⁻¹ and average condensation temperature of 23–40°C. The experimental heat transfer coefficients are compared with those predicted from correlations. The maximum mean heat transfer coefficient reduction (from a linear interpolation of the single component values) occurs at a concentration of roughly 30% R32 for the same mass flux basis, and is approximately 20% at Gr = 190 kg m⁻²s⁻¹, 16% at Gr = 300 kg m⁻²s⁻¹. Non-ideal properties of the mixture have a certain, but relatively small, influence on the degradation. Among others, temperature and concentration gradients, slip, etc. are also causes of heat transfer degradation.     

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.     

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.     

In-tube condensation of mixture of R134a and ester oil: empirical correlations

This paper presents a comparative study of the condensation heat transfer coefficients in a smooth tube when operating with pure refrigerant R134a and its mixture with lubricant Castrol “icematic sw”. The lubricant is synthetic polyol ester based oil commonly used in lubricating the compressors. Two concentrations of R134a-oil mixtures of 2% and 5% oil (by mass) were analysed for a range of saturation temperatures of refrigerant R134a between 35 °C and 45 °C. The mass flow rate of the refrigerant and the mixtures was carefully maintained at 1 g/s, with a vapour quality varying between 1.0 and 0. The effects of vapour quality, flow rate, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. The experimental results were then compared with predictions from earlier models [Int J Heat Mass Transfer (1979), 185; 6th Int Heat Transfer Congress 3 (1974) 309; Int J Refrig 18 (1995) 524; Trans ASME 120 (1998) 193]. Finally two new empirical models were developed to predict the two-phase condensation heat transfer coefficient for pure refrigerant R134a and a mixture of refrigerant R134a with Castrol “icematic sw”.     

Refrigerant charge, pressure drop, and condensation heat transfer in flattened tubes

Horizontal smooth and microfinned copper tubes with an approximate diameter of 9 mm were successively flattened in order to determine changes in flow field characteristics as a round tube is altered into a flattened tube profile. Refrigerants R134a and R410A were investigated over a mass flux range from 75 to 400 kg m⁻² s⁻¹ and a quality range from approximately 10–80%. For a given refrigerant mass flow rate, the results show that a significant reduction in refrigerant charge is possible. Pressure drop results show increases of pressure drop at a given mass flux and quality as a tube profile is flattened. Heat transfer results indicate enhancement of the condensation heat transfer coefficient as a tube is flattened. Flattened tubes with an 18° helix angle displayed the highest heat transfer coefficients. Smooth tubes and axial microfin tubes displayed similar levels of heat transfer enhancement. Heat transfer enhancement is dependent on the mass flux, quality and tube profile.     

Convective boiling performance of refrigerant R-134a in herringbone and microfin copper tubes

This paper reports an experimental investigation of convective boiling heat transfer and pressure drop of refrigerant R-134a in smooth, standard microfin and herringbone copper tubes of 9.52 mm external diameter. Tests have been conducted under the following conditions: inlet saturation temperature of 5 °C, qualities from 5 to 90%, mass velocity from 100 to 500 kg s⁻¹ m⁻², and a heat flux of 5 kW m⁻². Experimental results indicate that the herringbone tube has a distinct heat transfer performance over the mass velocity range considered in the present study. Thermal performance of the herringbone tube has been found better than that of the standard microfin in the high range of mass velocities, and worst for the smallest mass velocity (G=100 kg s⁻¹ m⁻²) at qualities higher than 50%. The herringbone tube pressure drop is higher than that of the standard microfin tube over the whole range of mass velocities and qualities. The enhancement parameter is higher than one for both tubes for mass velocities lower than 200 kg s⁻¹ m⁻². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient.     

Experimental studies on the evaporative heat transfer and pressure drop of CO2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm

Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. The tests were conducted at mass fluxes of from 212 to 656 kg m⁻² s⁻¹, saturation temperatures of from 0 to 20 °C and heat fluxes of from 6 to 20 kW m⁻². The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52 mm OD tube and 170–210% for 5 mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement.     

Heat transfer and flow pattern during two-phase flow boiling of R-134a in horizontal smooth and microfin tubes

Flow pattern and heat transfer during evaporation in a 10.7 mm diameter smooth tube and a micro-fin tube are presented. The tubes were tested in the ranges of mass flux between 163 and 408 kg m⁻² s⁻¹, and heat flux between 2200 and 56 000 W m⁻². The evaporation temperature was 6 °C. Flow maps for both the tubes are plotted in the coordinates of mass flux and vapor quality. The relations of flow pattern and local heat transfer coefficient are discussed. The heat transfer coefficients for intermittent and annular flows in both the smooth tube and the micro-fin tube are shown to agree well with Gungor and Winterton's correlation with modified constants.     

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%.     

Equations of State for Technical Applications. III. Results for Polar Fluids

New functional forms have been developed for multiparameter equations of state for non- and weakly polar fluids and for polar fluids. The resulting functional forms, which were established with an optimization algorithm which considers data sets for different fluids simultaneously, are suitable as a basis for equations of state for a broad variety of fluids. The functional forms were designed to fulfil typical demands of advanced technical application with regard to the achieved accuracy. They are numerically very stable and their substance-specific coefficients can easily be fitted to restricted data sets. In this way, a fast extension of the group of fluids for which accurate empirical equations of state are available is now possible. This article deals with the results found for the polar fluids CFC-11 (trichlorofluoromethane), CFC-12 (dichlorodifluoromethane), HCFC-22 (chlorodifluoromethane), HFC-32 (difluoromethane), CFC-113 (1,1,2-trichlorotrifluoroethane), HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane), HFC-125 (pentafluoroethane), HFC-134a (1,1,1,2-tetrafluoroethane), HFC-143a (1,1,1-trifluoroethane), HFC-152a (1,1-difluoroethane), carbon dioxide, and ammonia. The substance-specific parameters of the new equations of state are given as well as statistical and graphical comparisons with experimental data. General features of the new class of equations of state such as their extrapolation behavior or their numerical stability and results for non- and weakly polar fluids have been discussed in preceding articles.     

Characteristics of evaporative heat transfer and pressure drop of carbon dioxide and correlation development

Carbon dioxide among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In this study, transcritical refrigeration cycle using carbon dioxide is of great interest, and the evaporation process is investigated by experiment and analysis. This paper presents the measured heat transfer coefficients and pressure drop during evaporation process of carbon dioxide in a horizontal smooth tube. The test section was made of a seamless stainless steel tube with the inner diameter of 7.53 mm, and length of 5 m. Heat is provided by a direct heating method to the test section. Experiments were conducted at saturation temperatures of −4 to 20 °C, heat fluxes of 12 to 20 kWm⁻² and mass fluxes of 200 to 530 kgm⁻² s⁻¹. A comparison of different heat transfer correlations applicable to evaporation of carbon dioxide has been made. Based on the experiments for the evaporation heat transfer, useful correlation is developed.     

Condensation heat transfer coefficients of binary HFC mixtures on low fin and Turbo-C tubes

In this study, external condensation heat transfer coefficients (HTCs) are measured for nonazeotropic refrigerant mixtures (NARMs) of HFC32/HFC134a and HFC134a/HCFC123 on a low fin and Turbo-C tubes. All measurements are taken at the vapor temperature of 39 °C with the wall subcooling of 3–8 °C. Test results showed that condensation HTCs of NARMs on enhanced tubes were severely degraded from the ideal values showing up to 96% decrease. HTCs of the mixtures on Turbo-C tube were degraded more than those on low fin tube such that HTCs of the mixtures at the same composition were similar regardless of the tube. The mixture with larger gliding temperature differences (GTDs), HFC134a/HCFC123, showed a larger heat transfer reduction from the ideal values than the mixture with smaller GTDs, HFC32/HFC134a. Heat transfer enhancement ratios of the enhanced tubes with NARMs were almost 2 times lower than those with pure refrigerants and they decreased more as the GTDs of the mixtures increased.     

Nucleate boiling heat transfer coefficients of HCFC22, HFC134a, HFC125, and HFC32 on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of HCFC22, HFC134a, HFC125, HFC32 were measured on a low fin, Turbo-B, and Thermoexcel-E tubes. All data were taken at the liquid pool temperature of 7 °C on horizontal tubes of 152 mm length and 18.6–18.8 mm outside diameter at heat fluxes of 10–80 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. For a plain and low fin tubes, refrigerants with higher vapor pressures showed higher nucleate boiling HTCs consistently. This was due to the fact that the wall superheat required to activate given size cavities became smaller as pressure increased. For Turbo-B and Thermoexcel-E tubes, HFC125 showed a peculiar behavior exhibiting much reduced HTCs due to its high reduced pressure. The heat transfer enhancement ratios of the low fin, Turbo-B, and Thermoexcel-E tubes were 1.09–1.68, 1.77–5.41, 1.64–8.77 respectively in the range of heat fluxes tested.     

Condensation heat transfer coefficients of halogenated binary refrigerant mixtures on a smooth tube

In this study, external condensation heat transfer coefficients (HTCs) of nonazeotropic refrigerant mixtures of HFC32/HFC134a and HFC134a/HCFC123 at various compositions were measured on a horizontal smooth tube of a 19 mm outside diameter. All data were taken at the vapor temperature of 39 °C with a wall subcooling of 3–8 °C. Test results showed that HTCs of the tested mixtures were 19.4–85.1% lower than the ideal values calculated by the mole fraction weighting of the HTCs of the pure components. A thermal resistance due to the diffusion vapor film seemed to be partly responsible for the significant reduction of HTCs with these nonazeotropic mixtures.     

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%).     

Heat transfer coefficient during two-phase flow boiling of HFC-134a

This paper reports a study of the evaporation of HFC-134a inside smooth, horizontal tubes. Tests were performed with the pure refrigerant and with oil-refrigerant mixtures. The heat flux was varied from 2 to 10 kW m⁻². The inner diameter of the tubes was 12 mm. Two evaporators were used, 4 and 10 m long, and the oil content was varied from 0 to 2.5 mass percentage (synthetic oil, EXP-0275). Oil-free HFC-134a had a higher heat transfer coefficient than HCFC-22 at the same heat and mass fluxes. The effect of oil in the refrigerant is dependent on the heat flux. At 2 and 4 kW m⁻² the heat transfer coefficient had a maximum value for an oil content of around 0.5 mass percentage; no increase is registered for a heat flux of 6 kW m⁻². The heat transfer coefficients for the pure refrigerant were also compared with two existing correlations. The measured heat transfer coefficients averaged over the evaporator deviate less than 40% from the correlation according to Pierre. The heat transfer coefficients at the short evaporator lie within 20%. The correlation given by Jung overestimates the heat transfer coefficient by approximately 50%.     

CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO₂ flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m⁻² s⁻¹, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO₂ two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.