Zeotropic Refrigerant Research Articles

Modelica-based modelling and simulation of dry-expansion shell-and-tube evaporators working with alternative refrigerant mixtures

A new methodology of intermediate complexity level is developed to model the dry-expansion shell and U-tube evaporators. The model has a reasonable level of accuracy and uses fundamental physical principles in a distributed parameters approach capable of detecting the complex circuit of the shell-side flow. This level of details is necessary to simulate accurately the zeotropic refrigerant mixtures evaporation. Using Modelica language gives a heat exchanger model with a generic flow arrangement. The model is experimentally validated using a standard shell-and-tube evaporator working with HFC-134a. Three distinct working fluids, pure HFC-134a, R-407C, and a specially selected glide matching refrigerant mixture are simulated in the same heat source duty with different shell-and-tube configurations. Three different gas superheat values are also taken into account. The total amount of irreversibility is considered by calculating the total exergy losses. It is concluded that the effect of the temperature profile of any refrigerant mixture can be substantial on the relative performance of a particular heat exchanger configuration compared to counter-flow configuration.

Theoretical and experimental study on several refrigerants of moderately high temperature heat pump

The aim of the study is to select working fluids which have excellent performance and are environment friendly for moderately high temperature heat pump. Theoretical study is executed on several HCFCs and HFCs, with ODP of zero or small value firstly. And then research is carried out on a water-to-water vapor compression heat pump system applying the experimental assessment method of specifying working fluid parameters. Based on theoretical analysis, HFC245fa, HC600, HC600a, and a zeotropic refrigerant mixture HC600/HFC245fa (mass fraction: 2.83%/97.17%) are used as refrigerant for the experiment, with the evaporating temperature range of 30–55 °C, and condensing temperature range of 60–100 °C. Theoretical analysis shows that HCFC141b, HCFC123, HFC245ca, HC600 and HFC245fa have large COPh. However, pressure ratio of HFC245ca, HCFC141b and HCFC123 is too large. The experimental results indicate that both the natural working fluids have excellent cycle performances in moderately high temperature heat pump cycle. Their heating coefficients of performance (COPh) have little difference in the conditions of lower temperature cycle, but the cycle performance of HC600 is better than HC600a in higher temperature conditions. When the evaporating temperature and the condensing temperature are 44 °C and 90 °C, COPh of HC600 and HC600a are 3.84 and 3.33 respectively. In higher temperature conditions, the performance of the zeotropic refrigerant mixture HC600/HFC245fa is much better than HFC245fa. When the evaporating temperature is 50 °C, and the cycle temperature lift is 45 °C, COPhs of HC600/HFC245fa are 3.74 and 3.83 respectively.

A new evaluation method for zeotropic refrigerant mixtures based on the variance of the temperature difference between the refrigerant and heat transfer fluid

Compared to the pure refrigerants, the zeotropic refrigerant mixtures have the obvious temperature glide during phase change. Therefore, the Lorenz cycle can be approached with this special attribute. By analysing the heat transfer in the counter flow heat exchanger, a new evaluation method for zeotropic refrigerant mixtures based on the variance of the temperature difference between the refrigerant and heat transfer fluid (HTF) is proposed in this paper. For approaching to the Lorenz cycle and perfect glide matching, the zeotropic mixture which has smaller variance in the heat exchanger should be chosen in the refrigeration cycle. The variance of temperature difference is affected by two factors which are the temperature difference between the inlet and outlet of HTF and the linear relationship between the refrigerant temperature and enthalpy, respectively. The smallest variance of the zeotropic refrigerant can be obtained by setting the temperature difference of HTF to be the optimal temperature difference.

Analysis of some recharge solutions on varying the R407C composition

In the vapour compression plants possible leaks can vary the composition of a zeotropic refrigerant mixture. The main aim of this paper is to verify experimentally if the plant performances are restored with a proper recharge, when leaks are imposed. As for the R407C mixture the percentage of the less volatile component (R134a) increases, while the percentage of the more volatile components (R32 and R125) decreases when leaks occur. The experimental tests have been realized determining refrigerant leaks from the liquid receiver placed at the condenser outlet and measuring the properties in steady state condition. The recharge is realized in different ways: with R407C, with R134a and R410A or with R32 R125 and R134a. The different solutions are then compared from the economical point of view, in terms of COP, refrigeration power and air temperature at the evaporator outlet, adopting an iteration method. This method determines the real composition of a zeotropic mixture working in a compression plant, when only the temperature and pressure values at the expansion valve inlet and outlet are known.

A Comparative Study on Flow Boiling Heat-Transfer Coefficient of R-134a and R-134a/R-290/R-600a Refrigerant Mixture

The flow boiling heat-transfer coefficients of R-134a and R-134a/R-290/R-600a zeotropic refrigerant mixture were experimentally measured in a smooth horizontal tube of a 9.52-mm diameter. The values were compared for similar working pressure, reduced pressure, and liquid entry saturation temperature. The tests were conducted under varied heat flux conditions for stratified flow patterns; the conditions are found in evaporators of refrigerators and deep freezers. The varied heat flux conditions were imposed on the test fluids using a coaxial counter-current heat exchanger test section. The heat-transfer coefficients of the R-134a/R-290/R-600a refrigerant mixture were found to be better than those of R-134a under the same pressure and reduced pressure. However, under the same liquid entry temperature, the values of R-134a were found to be higher at low vapor qualities and lower at higher vapor qualities.

A comparative study on the performance of a R22 window air conditioner retrofitted with R407C/R290/R600a refrigerant mixture

SYNOPSIS HFCs are the leading options for replacing R22 in air conditioning applications due to the Montreal Protocol. Among the HFCs, one of the more promising refrigerants is R407C, a zeotropic refrigerant mixture with a glide temperature difference of roughly 6°C. However, R407C could not be used as a drop-in substitute since HFCs are immiscible with conventional mineral oils and POE is used as a lubricant. But POE is highly hygroscopic in nature and will cause adverse effects in the system. Previous research reported that M20 (80% R407C and 20% HC blend by weight) could be a viable substitute without changing the mineral oil. But, in that study, the condenser tube length was increased by 19% to limit the discharge pressure within the acceptable limits of an existing R22 compressor. In the present study a test is conducted to assess the performance of M20 as a drop-in substitute for R22 in a window air conditioner, without increasing condenser length but with the capillary length alone changed. The indoor and outdoor air temperatures were varied in accordance with BIS 1391 (1992) standard and the test was carried out in a psychrometric test facility. Experimental results showed that M20 refrigerant had 4.18% to 7.47% lower cooling capacity compared to the R22 system. Hence M20 can be used as a drop-in substitute for R22 in a window air-conditioner but with marginal reduction in cooling capacity.

Contribution to the gas chromatographic analysis for both refrigerants composition and cell gas in insulating foams – Part I: Method

In this paper, an easy low cost chromatographic analysis for both refrigerants blend composition and cell gas in insulating foams is presented. This work firstly deals with the measurement protocols for these substances. The presented protocols are simple, likely to be used by non-specialists of chemical analysis (i.e. by refrigeration engineers). Among other methods, gas chromatography was used for separation and detection of halocarbons. For refrigerants extracted from refrigeration plants, the analysis requires a prior oil separation. As for insulating foams, analysis of cell gas composition is processed with the same protocol. Sampling remains of main importance; two possible sampling methods were validated and compared. Finally, the interest in the analysis method is illustrated with the example of a real application: study of the distillation phenomenon of a zeotropic refrigerant in an industrial refrigeration plant with flooded evaporators. The results are discussed for their consequences on the working performances of the refrigerating system. In part II, we propose original results obtained with that method to study the aging of insulating foams.

Evaluation of zeotropic refrigerants based on nonlinear relationship between temperature and enthalpy

In order to evaluate cycling characters of zeotropic refrigerants in air-conditioning operation, and to reveal distribution rules of temperature difference between refrigerants and heat transfer fluids in condenser and evaporator, theoretical researches were carried out based on nonlinear relationship between temperature and enthalpy in period of refrigerants’ phase change. Firstly, a phase changing model of refrigerants was built, and refrigerants state parameters were decided in the air-conditioning operation. Secondly, the state equation of refrigerants was applied for computing relationship between temperature and enthalpy, else based on some suppositions, temperature differences between 15 sorts of refrigerants and heat-transfer fluids were gotten too. Through concluding those temperature differences changing in condenser and evaporator, some rules were found. Lastly, after calculating and comparing the additive exergy loss among 15 sorts of refrigerants, which resulted from the changing of temperature difference, their cycling characters evaluation were presented.

Zeotropic Refrigerant Mixtures in Vapour Compression Refrigeration Systems - Issues and Implications

Use of zeotropic refrigerant mixtures introduce a number of novel issues related to vapour compression systems used in refrigeration and air conditioning industry. Certain attributes associated with evaporation and condensation behaviours of zeotropic mixtures are responsible for most of these. This paper discusses how the mixture attributes influence the established practices, design and operation of conventional vapour compression systems.

Zeotropic refrigerants and the special characteristics

Zeotropic refrigerants, compared with conventional refrigerants, have different characteristics during evaporation and condensation. This article summarizes the special characteristics of zeotropic refrigerants in five aspects: fractionation behaviour; non-leakage-related concentration change; leakage-associated concentration change; gliding temperature and the limitation on media flow rate; and degradation in heat transfer. The relevant thermo-dynamic performance has been analysed. Newly developed technology for improving system efficiency and reliability has been incorporated in the discussion.

Local shifts of the fluid composition in a simulated heat pump using R-407C

Because of the potential variation of the fluid composition, lots of designers are concerned by the use of zeotropic refrigerant mixtures in refrigeration and air-conditioning systems. In this paper, a local simulation model of a water-to-water heat pump is proposed by adopting a modular approach. Thus, the overall model consists of the association of an elementary model for each basic component, namely the compressor, condenser, evaporator and receiver. Implemented with a ternary zeotropic blend R-134a/R-125/R-32, the computer simulation, based on the mass and energy conservation laws, allowed to evaluate not only the overall performances of the simulated machine, but also the circulating composition of refrigerant, the local temperature and heat transfer coefficient along heat exchangers and the local composition of refrigerant in each point of refrigerating circuit. The numerical results show that, compared to the nominal composition of R-407C (R-134a/R-125/R-32; 52/25/23%), the circulating composition in the machine is low in the less volatile component R-134a (−3% absolute variation) and rich in the most volatile component R-32 (+3% absolute). In the regions of condensation beginning and evaporation end, the local composition is noticeably different from both the circulating and nominal compositions. In these regions, the local mass composition of R-134a increases (+4/+10% absolute) whereas the local mass composition of R-125 and R-32 are in deficit (−3/−6% absolute).

Considerations about evaporator thermal design in a vapour compression liquid chiller. Experimental analysis with HFC fluids (R134a and R407C)

In this paper, the predicted performance of a shell-and-tube (1-2) evaporator installed in a vapour compression liquid chiller is analysed. The classical thermal design methods are applied to the evaporator performance with two HFC refrigerants, a pure fluid (R134a) and a zeotropic blend with an appreciable glide (R407C). From the experimental results obtained it is possible to discuss the validation of the simplificatory assumptions usually taken, evaluating the resulting error introduced due to the no consideration of pressure drops and temperature glide (in the case of zeotropic blends) at the evaporator. Concluding that is not possible to consider the behaviour of a zeotropic refrigerant as a pure refrigerant, rejecting the glide. Whereas the assumption of no pressure drops in evaporator, leads to an error of about 5% in cooling capacity calculation. Copyright © 2004 John Wiley & Sons, Ltd.

Reasons for drop in shell-and-tube condenser performance when replacing R22 with zeotropic mixtures. Part 2: investigation of mass transfer resistance effects

Detailed calculations of condensation outside a column of horizontal smooth tubes have been carried out in order to investigate the influence of mass transfer resistance for a zeotropic refrigerant mixture. Diffusive transport is calculated locally in both phases. Calculation results show that mass transfer resistance in the gas phase reduces heat transfer by 10–20% for a binary mixture with a glide similar to that of R407C. The decrease in heat transfer due to poor mixing in the condensate is from 15 to 65% for the conditions investigated; the lower the duty, the greater the decrease. Results show that if assuming mixing in the condensate by diffusion only, the degrees of mixing and the duty dependency are similar to results in earlier work where calculations were matched to experimental data. Diffusion is likely to be the dominant mixing mechanism in the condensate, and the mixing is poor under certain conditions, which might explain the drop in condenser performance for some heat pump applications.

Reasons for drop in shell-and-tube condenser performance when replacing R22 with zeotropic mixtures. Part 2: investigation of mass transfer resistance effects

Detailed calculations of condensation outside a column of horizontal smooth tubes have been carried out in order to investigate the influence of mass transfer resistance for a zeotropic refrigerant mixture. Diffusive transport is calculated locally in both phases. Calculation results show that mass transfer resistance in the gas phase reduces heat transfer by 10–20% for a binary mixture with a glide similar to that of R407C. The decrease in heat transfer due to poor mixing in the condensate is from 15 to 65% for the conditions investigated; the lower the duty, the greater the decrease. Results show that if assuming mixing in the condensate by diffusion only, the degrees of mixing and the duty dependency are similar to results in earlier work where calculations were matched to experimental data. Diffusion is likely to be the dominant mixing mechanism in the condensate, and the mixing is poor under certain conditions, which might explain the drop in condenser performance for some heat pump applications.

Effect of Fin Geometry on Condensation of R407C in a Staggered Bundle of Horizontal Finned Tubes

Experimental results are presented that show the effect of fin geometry on condensation of downward flowing zeotropic refrigerant mixture R407C in a staggered bundle of horizontal finned tubes. Two types of conventional low-fin tubes and three types of three-dimensional-fin tubes were tested. The refrigerant mass velocity ranged from 4 to 23 kg/m2 s and the condensation temperature difference from 3 to 12 K. The measured condensation heat transfer coefficient was lower than the previous results for R134a, with the difference being more significant for smaller mass velocity. The effect of fin geometry on the condensation heat transfer coefficient was less significant for R407C than for R134a. The effect of condensate inundation was more significant for the three-dimensional-fin tubes than for the low-fin tubes. By using the dimensionless heat transfer correlation for the condensate film that was based on the experimental data for R134a, a superficial vapor-phase heat transfer coefficient was obtained for condensation of R407C. The vapor-phase heat transfer coefficient showed characteristics similar to the vapor-phase mass transfer coefficient that was obtained in the previous study for R123/R134a.

Prediction of in-tube condensation heat transfer characteristics of binary refrigerant mixtures

This study presents a prediction model for the condensation heat transfer characteristics of binary zeotropic refrigerant mixtures inside horizontal smooth tubes. In this model, both the vapor-side and liquid-side mass transfers are considered, and the high flux mass transfer correction factor is used to evaluate mass transfer coefficients. The model was applied to the binary zeotropic refrigerant mixture R134a/R123, which has a large temperature glide. Calculation results showed that the heat transfer degradation of R134a/R123 due to gradients in the mass fraction and temperature is considerable, and depends on the mass fraction of the more volatile component and the vapor mass quality of the refrigerant mixture. By comparison with experimental data, incorporating the present finite mass transfer model for the liquid film side into the calculation algorithm was shown to reasonably well predict the condensation heat transfer coefficients of binary refrigerant mixtures with the mean deviation of about 10.3%. In the present calculations, however, it was also found that the high flux mass transfer correction factor had only a slight effect on the condensation heat transfer.

Heat Transfer Coefficients During Condensation of the Zeotropic Refrigerant Mixture HCFC-22/HCFC-142b

Heat transfer coefficients during condensation of zeotropic refrigerant mixtures were obtained at mass fractions of 90 percent/10 percent, 80 percent/20 percent, 70 percent/30 percent, 60 percent/40 percent, and 50 percent/50 percent for HCFC-22/HCFC-142b and for pure HCFC-22 in a horizontal smooth tube at a high saturation pressure of 2.43 MPa. The measurements were taken in a series of eight 8.11 mm inner diameter smooth tubes with lengths of 1 603 mm. At low mass fluxes, from 40 kg/m2s to 350 kg/m2s where the flow regime is predominately stratified wavy, the refrigerant mass fraction influenced the heat transfer coefficient by up to a factor of two, decreasing as the mass fraction of HCFC-142b is increased. At high mass fluxes of 350 kg/m2s and more, the flow regime was predominately annular and the heat transfer coefficients were not strongly influenced by the refrigerant mass fraction, decreasing only by 7 percent as the refrigerant mass fraction changed from 100 percent HCFC-22 to 50 percent/50 percent HCFC-22/HCFC-142b. The results also indicated that of three methods tested to predict heat transfer coefficients, the flow pattern correlation of Dobson and Chato (1998) gave the best results for pure HCFC-22 and for the mixtures utilizing the Silver-Bell-Ghaly method (1964).

Performance evaluation of heat exchanger using alternative refrigerant R407C

AbstractR22 (HCFC22) has been widely used as the refrigerant in air conditioners. According to the Montreal protocol for ozone layer protection, the total production of HCFCs has been capped since the beginning of 1996. Zeotropic refrigerant mixture R407C and nearly azeotropic refrigerant mixture R410A have been selected as alternatives to R22.We examined refrigerant passages in heat exchangers used in heat pump‐type room air conditioners using zeotropic refrigerant R407C through simulation, and obtained the following conclusions. In an indoor heat exchanger, a counter flow configuration when operating as a condenser has higher temperature efficiency. When an outdoor heat exchanger operates as an evaporator, a configuration that suppresses the temperature glide by partially reducing the refrigerant passage not only produces high efficiency, but also reduces the frost formation on fins. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 626–638, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10064

Detailed simulation of fluted tube water heating condensers

Fluted tube-in-tube condensers are key components in advanced energy efficient water heating heat pumps. Therefore, there exists a need for a computer design tool that incorporates all the essential features of these heat exchangers. This paper describes the development of a detailed model to simulate fluted tube refrigerant-to-water condensers. The model allows the surface area to be divided into any number of sections for which all the refrigerant and water properties can be evaluated. This allows for the extension of the model to simulate heat exchangers for cycles employing zeotropic refrigerant mixtures. For the waterside existing empirical equations are used for both friction and heat transfer. However, on the refrigerant side no correlations are available in the literature to calculate friction and heat transfer coefficients. The approach followed in this paper is therefore to use existing smooth tube correlations combined with enhancement ratios based on correlations available for helical coils as well as enhancement factors based on empirical data for fluted tube condensers. The model is validated with the aid of results from independent tests on two commercial fluted tube heat exchangers. The average difference between the simulated and measured pressure drops is 7.27% and the average difference for the log mean temperature difference (LMTD) is 4.41%.

Heat transfer and pressure drop in a plate-type evaporator

A plate-type evaporator, working with natural refrigerant circulation, has been investigated both experimentally and theoretically. Motivated by the phase-out of ozone-depleting substances, HCFC22 was compared to HFC134a and two zeotropic refrigerant mixtures. The effect of different separator liquid levels, i.e. refrigerant flows, and its influence on heat transfer was also studied. The investigated plate-type evaporator consists of thirteen vertical flow channels and its size is 3.0 m × 0.5 m. The heat source for the evaporator is a falling water film on the outside of the plate. Experimental studies have been carried out using a test facility that enabled detailed measurements of heat transfer and pressure drop. Experiments were compared to results from a calculation method that simultaneously calculates heat transfer and pressure drop in a variable number of steps along the evaporator. The calculation method is based on a pressure drop correlation proposed by the VDI-Wärmeatlas and a heat transfer correlation for vertical tubes proposed by Steiner and Taborek. For different evaporator duties, heat transfer was over predicted by 12% for pure fluids by 15% for mixtures. Calculated pressure drops were well within ±5% of the measured values. Changes in heat transfer due to different flows were closely predicted by the proposed calculation method.