HFC32 vaporisation inside a Brazed Plate Heat Exchanger (BPHE): Experimental measurements and IR thermography analysis

This paper presents HFC32 average boiling heat transfer coefficients and pressure drops measured inside a small Brazed Plate Heat Exchanger (BPHE): the effects of heat flux, saturation temperature (pressure), and outlet conditions are investigated. The experimental tests were carried out at four different saturation temperatures (5, 10, 15, and 20 °C) and four different evaporator outlet conditions (vapour quality around 0.80 and 1.00, vapour super-heating around 5 and 10 °C). The average heat transfer coefficients show great sensitivity to heat flux and outlet conditions and weak sensitivity to saturation temperature (pressure). The saturated boiling heat transfer coefficients were compared with a new model for refrigerant vaporisation inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 4.7%. The heat transfer and pressure drop measurements are complemented with a IR thermography analysis for a better understanding of the vaporisation process inside a BPHE.     

Comparative performance analysis of the low GWP refrigerants HFO1234yf,HFO1234ze(E) and HC600a inside a roll-bond evaporator

This paper presents the comparative performance analysis of the low GWP refrigerants HFO1234yf, HFO1234ze(E) and HC600a inside a commercial roll-bond evaporator for household refrigerators. The vaporisation performances were evaluated at two evaporation temperatures, −15 and −20 °C, and different refrigerant mass flow rates and compared with those of the traditional refrigerant for domestic refrigeration HFC134a. The performance analysis was carried out using both thermocouples installed on the rear side of the roll-bond evaporator and an IR thermo-camera. Each of the low GWP refrigerants tested can be considered a good substitute for HFC134a, provided that the compressor displacement is adjusted to deliver the proper refrigerant mass flow rate. Only HFO1234yf exhibits performance similar to HFC134a at the same mass flow rate, therefore it can be considered a direct drop-in substitute for HFC134a.     

Exergy analysis of R1234yf and R1234ze as R134a replacements in a two evaporator vapour compression refrigeration system

Exergy analysis is a useful way for determining the real thermodynamic losses and optimising environmental and economic performance in the systems such as vapour compression refrigeration systems. The present study deals with the exergy analysis on a two evaporator vapour compression refrigeration system using R1234yf, R1234ze and R134a as refrigerants. In the calculation of losses occurring in different system components, besides the exergy efficiency of the refrigeration cycle, a computer code was developed by using Engineering Equation Solver (EES-V9.172-3D) software package program. The effects of the evaporator and condenser temperatures on the exergy destruction and exergy efficiency of the system were investigated. R1234yf and R1234ze, which are good alternatives to R134a concerning their environmentally friendly properties and this is the most significant finding emerging from this study.     

R1234ze(E) flow boiling inside a 3.4 mm ID microfin tube

R1234ze(E) has a GWP<1 and a normal boiling temperature approximately 7.3 °C lower than that of R134a; it represents an interesting candidate for its replacement as working fluid in refrigerating machines. The refrigerant charge minimization in refrigerating and air conditioning equipment is a key issue for the new environmental challenges. Mini microfin tubes represent an optimal solution for both heat transfer enhancement and charge minimization tasks. This paper presents an experimental study of R1234ze(E) flow boiling inside a mini microfin tube with internal diameter at the fin tip of 3.4 mm. The experimental measurements were carried out at constant saturation temperature of 30 °C, by varying the refrigerant mass velocity between 190 kg m⁻² s⁻¹ and 940 kg m⁻² s⁻¹, the vapour quality from 0.2 to 0.99 at three different heat fluxes: 10, 25, and 50 kW m⁻². The experimental results are then compared with those obtained for the more traditional R134a.     

Experimental study of an R1234ze(E)/R134a mixture (R450A) as R134a replacement

This work presents an experimental analysis of a non-flammable R1234ze(E)/R134a mixture (R450A) as R134a drop-in replacement. While R134a has a high GWP value (1430), the R450A GWP is only 547. The experimental tests are carried out in a vapour compression plant equipped with a variable-speed compressor. The replacement suitability has been studied combining different operating conditions: evaporation temperature, condensation temperature and the use of an internal heat exchanger (IHX). The drop-in cooling capacity of R450A compared with R134a is 6% lower as average. R450A COP is even higher to those resulting with R134a (approximately 1%). The discharge temperature of R450A is lower than that of R134a, 2K as average. The IHX has a similar positive influence on the energy performance of both fluids. In conclusion, R450A can be considered as a good candidate to replace R134a.     

Theoretical energy performance evaluation of different single stage vapour compression refrigeration configurations using R1234yf and R1234ze(E) as working fluids

R1234yf and R1234ze(E) have been proposed as alternatives for R134a in order to work with low GWP refrigerants, but this replacement results generally in a decrease of the performance. For this reason, it is interesting to explore ways to improve the system performance using these refrigerants. In this paper, a comparative study in terms of energy performance of different single stage vapour compression configurations using R1234yf and R1234ze(E) as working fluids has been carried out. The most efficient configuration is the one which uses an expander or an ejector as expansion device. On the other hand, using an internal heat exchanger in a cycle which replaces the expansion valve by an expander or an ejector could produce a detrimental effect on the COP. However, for all the configurations the introduction of an internal heat exchanger produces a significant increment on the cooling capacity.     

Experimental two-phase heat transfer coefficient and frictional pressure drop inside mini-channels during condensation with R1234yf and R134a

The present paper reports condensing two-phase flow pressure drop gradient and heat transfer coefficient (HTC) inside a mini-channel multiport tube with R1234yf and R134a. Several models available in the literature are used to compare predictions of these two fluids. Experimental data are analysed to get the influence of saturation temperature, mass flux, vapour quality and fluid properties. HTC values of R1234yf seem to be lower than R134a under similar conditions. Two-phase flow pressure drops are also lower in the case of the new refrigerant R1234yf.     

Condensation in two phase and desuperheating zone for R1234ze(E), R134a and R32 in horizontal smooth tubes

Condensation is usually assumed to begin when the bulk enthalpy reaches the saturated vapor enthalpy, which leads to discontinuity of heat transfer coefficient calculation in modeling. This paper addresses the discontinuity by showing the presence of condensation in desuperheating region when the wall temperature decreases below the saturation temperature at any operating condition. The experiments have been conducted with R134a, R1234ze(E) and R32 for mass fluxes of 100–300 kgm⁻² s⁻¹, saturation temperatures of 30^°C–50 °C and from x = 0.05 to superheat of 50 °C in a horizontal smooth tube with 6.1 mm inner diameter. R134a is observed to have approximately 10% higher and 20% lower HTC compared to R1234ze(E) and R32 respectively. Cavallini correlation predicted the data within an accuracy of 12% while Kondo-Hrnjak correlation predicted HTC for condensation in de-superheating zone within accuracy of 23%.     

New Fundamental Equations of State with a Common Functional Form for 2,3,3,3-Tetrafluoropropene (R-1234yf) and <Emphasis Type="Italic">trans</Emphasis>-1,3,3,3-Tetrafluoropropene (R-1234ze(E))

New fundamental equations of state explicit in the Helmholtz energy with a common functional form are presented for 2,3,3,3-tetrafluoropropene (R-1234yf) and trans-1,3,3,3-tetrafluoropropene (R-1234ze(E)). The independent variables of the equations of state are the temperature and density. The equations of state are based on reliable experimental data for the vapor pressure, density, heat capacities, and speed of sound. The equation for R-1234yf covers temperatures between 240 K and 400 K for pressures up to 40 MPa with uncertainties of 0.1 % in liquid density, 0.3 % in vapor density, 2 % in liquid heat capacities, 0.05 % in the vapor-phase speed of sound, and 0.1 % in vapor pressure. The equation for R-1234ze(E) is valid for temperatures from 240 K to 420 K and for pressures up to 15 MPa with uncertainties of 0.1 % in liquid density, 0.2 % in vapor density, 3 % in liquid heat capacities, 0.05 % in the vapor-phase speed of sound, and 0.1 % in vapor pressure. Both equations exhibit reasonable behavior in extrapolated regions outside the range of the experimental data.     

Condensation heat transfer and two-phase frictional pressure drop in a single minichannel with R1234ze(E) and other refrigerants

R1234ze(E), trans-1, 3, 3, 3-tetrafluoropropene, is a fluorinated propene isomer which may be a substitute of R134a for refrigeration applications. R1234ze(E) has a much lower GWP_100-years than that of R134a. In this paper, the local heat transfer coefficient during condensation of R1234ze(E) is investigated in a single minichannel, horizontally arranged, with hydraulic diameter equal to 0.96 mm. Since the saturation temperature drop directly affects the heat transfer rate, the pressure drop during adiabatic two phase flow of R1234ze(E) is also measured. Predictive models are assessed both for condensation heat transfer and pressure drop. A comparative analysis is carried out among several fluids (R1234ze(E), R32, R134a and R1234yf) starting from experimental data collected at the same conditions and using the Performance Evaluation Criteria (PEC) named Penalty Factor (PF) and Total Temperature Penalization (TTP) to rank the tested refrigerants in forced convective condensation.     

Experimental investigation of the effect of condenser subcooling in R134a and R1234yf air-conditioning systems with and without internal heat exchanger

This paper presents an experimental study about the effect of condenser subcooling on the performance of an air conditioning system operating with R134a and R1234yf, under the same operating conditions. For both refrigerants, it has been shown that the COP undergoes a maximum as a consequence of the trade-off between increasing refrigerating effect and increasing specific compression work. At a given operating condition, the system COP increased up to 18% for R1234yf and 9% for R134a. These results confirmed the trends obtained from a previous theoretical analysis, demonstrating that a system operating with R1234yf can benefit more from the condenser subcooling than that with R134a. The experimental results also showed that the presence of an internal heat exchanger significantly reduces the COP increase due to condenser subcooling, since both improvements compete towards reducing the throttling losses. Besides the interference between IHX and condenser subcooling, the use of both simultaneously still yields a more efficient air conditioning system, especially for R1234yf.     

Surface tension of low GWP refrigerants R1243zf,R1234ze(Z), and R1233zd(E)

The surface tension of R1243zf, R1234ze(Z), and R1233zd(E) were measured at temperatures from 270 K to 360 K by an experimental apparatus based on the differential capillary rise method. The deviation between the measured surface tension of R134a and R245fa and the calculated surface tension with REFPROP 9.1 (Lemmon et al., 2013) was ±0.13 mN m⁻¹, which is less than the estimated propagated uncertainty in surface tension of ±0.2 mN m⁻¹. Eleven points, thirteen points, and ten points of surface tension data were provided for R1243zf, R1234ze(Z), and R1233zd(E), respectively, in this paper. The measured data and the estimated surface tension using the methods of Miller, 1963, Miqueu et al., 2000, and Di Nicola et al. (2011) agree within the standard deviation of ±0.43 mN m⁻¹. The empirical correlations that represent the measured data within ±0.14 mN m⁻¹ were proposed for each refrigerant.     

Thermodynamic properties of 1,1,1,2-tetrafluoroethane (R-134a) + 2,3,3,3-tetrafluoropropene (R-1234yf) mixtures: Measurements of the critical parameters and a mixture model based on the multi-fluid approximation

Thermodynamic properties are discussed for 1,1,1,2-tetrafluoroethane (R-134a) + 2,3,3,3-tetrafluoropropene (R-1234yf) mixtures. The critical temperatures, densities, and pressures experimentally determined are first presented with their uncertainties. Subsequently a mixture model for calculations of thermodynamic properties is formulated using the multi-fluid approximation. Comparisons to experimental data show that the mixture model calculates the vapor–liquid equilibrium and densities of the mixtures with reasonable accuracies. The critical parameters are also well represented by the mixture model.     

HFC404A condensation inside a small brazed plate heat exchanger: Comparison with the low GWP substitutes propane and propylene

This paper compares HFC404A and two possible hydrocarbon substitutes HC290 (propane) and HC1270 (propylene) in condensation inside a commercial brazed plate heat exchanger (BPHE) at saturation temperatures between 25 and 40 °C. The effects of saturation temperature (pressure), refrigerant mass flux, and vapour super-heating on heat transfer and hydraulic performances are investigated. The transition between gravity dominated and forced convection condensation occurs at a mass flux around 20 kg m⁻² s⁻¹. In forced convection condensation a doubling of the refrigerant mass flux produces a 30% enhancement of the heat transfer coefficient. The condensation heat transfer coefficients of super-heated vapour are 5–10% higher than those of saturated vapour. Propane and propylene exhibit condensation heat transfer coefficients 25–45% higher and frictional pressure drops similar to those of HFC404A under comparable operating conditions. Therefore both propane and propylene are very promising as low GWP definitive substitutes for HFC404A in industrial and commercial refrigeration.     

铜翅片换热器开发应用的分析Ⅰ--铜、铝翅片换热器性能对比

从翅片效率、单个换热单元的传热系数和整个换热器的换热量三个方面对不同翅片结构、不同管子结构和不同工况下的翅片管换热器由铝翅片换为铜翅片前后的换热特性和成本进行了分析比较：详细给出了各个结构和工况参数单独变化对换铜前后的翅片效率、单位制冷剂侧换热面积上的总传热系数和换热器换热量的影响规律。分析结果发现，翅片越薄、越高，管外径越小，风速越大时，换铜后的翅片管的换热能力增强越大，成本增加越小。在所选择的结构和工况范围内，铜管铜片换热器比铜管铝片换热器的翅片效率约提高0．938％～29．86％、总传热系数约提高9.88％～23．276％、总换热量约提高0．112％～22．3％；对于典型的1.1kW空调器的蒸发器，材料成本约增加8～42元，体积可最多缩小18％。     

Isochoric Heat Capacity Measurements for Binary Refrigerant Mixtures Containing Difluoromethane (R32), Pentafluoroethane (R125), 1,1,1,2-Tetrafluoroethane (R134a), and Trifluoroethane (R143a) from 200 to 345 K at Pressures to 35 MPa

Molar heat capacities at constant volume C _v were measured for binary refrigerant mixtures with an adiabatic calorimeter with gravimetric determinations of the amount of substance. Temperatures ranged from 200 to 345 K, while pressures extended up to 35 MPa. Measurements were conducted on liquid samples with equimolar compositions for the following binary systems: R32/R134a, R32/R125, R125/R134a, and R125/R143a. The uncertainty is 0.002 K for the temperature rise and is 0.2% for the change-of-volume work, which is the principal source of uncertainty. The expanded relative uncertainty (with a coverage factor k=2 and thus a two-standard deviation estimate) for C _v is estimated to be 0.7%.