Synthetic Refrigerants Research Articles

Effect of oxide nanoparticles on the thermal, rheological and tribological behaviours of refrigerant compressor oil: An experimental investigation

Oxide based nanolubricants are prepared by dispersing TiO2, SiO2 and Al2O3 nanoparticles into synthetic refrigerant compressor oil (Polyalkylene glycol) using two-step method. Thermal conductivity of nanolubricants for 0.07 to 0.6% volume fractions in the temperature range 20 °C–50 °C and rheological properties in the range 20 °C–80 °C have been investigated. The tribological characterization is performed with a four ball tribo-tester. The results show that as the volume fraction increases thermal conductivity and viscosity of nanolubricants increase. The maximum increase in thermal conductivity ratio for TiO2, SiO2 and Al2O3 nanolubricants are 1.38, 1.31 and 1.48, respectively and corresponding increase in viscosity ratios are 5.64, 10.34 and 9.71, respectively. Rheological studies show that, unlike pure lubricant, nanolubricants exhibit non-Newtonian shear thinning behaviour. The tribological test results disclose that, lubricant appended with TiO2 and SiO2 have excellent friction reduction and anti-wear properties compared to pure lubricant.

Redesign 3 R Machine as a Refrigerant Waste Treatment Alternative in Environmental Rescue

Cooling machine technologies really affect nowadays’ modern life, not only limited in enhancement of life quality and comfort, but it has also reached the essential things of humans’ life supporter (Arora, 2001). Cooling machine technologies have direct contribution toward environmental damage such as depletion of ozone layer and global warming through synthetic refrigerant waste and leakage (CFC and HFC) to environment. The refrigerant release to the environment is 60% of the service sector. Destructive characteristics of ozone possessed by CFC were first proposed by Rowland and Molina which were then supported by yard measurement. It is estimated that ozone layer damage occurs for about 3% every decade. The ozone layer located in the stratosphere is functioned to prevent ultraviolet-B ray from entering into earth surface. This Ultraviolet-B is suspected to be the cause of health problem for humans and disorder for plants on earth. As for the purpose of this research is to obtain a product design of refrigerant waste processing system (recovery and recycle refrigerant) as well as to acknowledge the work method (COP) of cooling machines that use CFC refrigerant (R-12) as the result of recovery and recycle compared to CFC refrigerant (R-12)/pure R134a. One method that can be used is by redesigning existing equipment namely 3R machine that cannot be used anymore thus it can be reused. This research will be conducted through modifying the existing 3R machine therefore it can be reused and be easily operated as well as doing the maintenance, after that the refrigerant as the result of recovery will be tried on a refrigeration system and a test of refrigeration system work method will be conducted by using the refrigerant recycle product which is obtained and compared with the work method of the one with pure refrigeration.The result has been achieved that the redesign product of refrigerant waste processing equipment can be reused and able to perform the recovery, recycle and richarging process, although using semi-automatic control system. So the use of car air conditioning refrigerant can be more efficient. With the functioning of 3R mesi is expected wastes refrigerant is not wasted which is one of the efforts to save the environment.

Multi-criteria evaluation of several million working fluids for waste heat recovery by means of Organic Rankine Cycle in passenger cars and heavy-duty trucks

Automotive industry is driven by economic and legislative constraints to increase fuel efficiency and reduce CO2-emissions to a certain extend. To reach the required threshold values, manufacturers consider waste heat recovery by means of Organic Rankine Cycle (ORC) in passenger cars and heavy-duty trucks. This work deals with the crucial issue of identifying an optimal working fluid which is flexible in terms of application and condensing temperature and which is applicable in real systems. For this purpose, a large-scale screening based on computational chemistry and thermodynamic process simulation is coupled with a multi-criteria evaluation. In total, about 72 million chemical substances provided by the PubChem database are screened and more than 3000 promising candidates are evaluated considering COSMO-RS based thermodynamic data as well as constructional, regulatory and security aspects. Five promising working fluids are identified and it is shown that these fluids outperform widely discussed candidates like synthetic refrigerants. Even more remarkably is the fact that within the TOP 100 working fluids only twelve have already been reported in ORC literature. However, the optimal set of working varies as it depends on configuration (with and without mass flow splitting) and condensing temperature. In general, the study demonstrates that a large-scale screening of the complete chemical space can reveal unconventional working fluids for thermodynamic cycles.

Identifying lubricant options for compressor bearing designs

Today’s refrigeration and air conditioning market is not only driven by the environmental aspects of the refrigerants, but also by the energy efficiency and reliability of system operation. Numerous types of compressor designs are used in refrigeration and air conditioning applications which means that different bearings are used; and in some cases, multiple bearing types within a single compressor. Since only one lubricant is used, it is important to try to optimize the lubricant to meet the various demands and requirements for operation. This optimization entails investigating different types of lubricant chemistries, viscosities, and various formulation options.What makes evaluating these options more challenging is the refrigerant which changes the properties of the lubricant delivered to the bearing. Once the lubricant and refrigerant interaction are understood, through various test methods, then work can start on collaborating with compressor engineers on identifying the lubricant chemistry and formulation options. These interaction properties are important to the design engineer to make decisions on the adequacy of the lubricant before compressor tests are started.This paper will discuss the process to evaluate lubricants for various types of compressors and bearing design with focus on what’s needed for current refrigerant trends. In addition, the paper will show how the lubricant chemistry choice can be manipulated through understanding of the bearing design and knowledge of interaction with the refrigerant to maximize performance. Emphasis will be placed on evaluation of synthetic lubricants for both natural and synthetic low GWP refrigerants.

Comparison of refrigerated warehouse energy demand with R-717 and R-507 using eQUEST model

ABSTRACTNatural refrigerant ammonia R-717 and synthetic azeotropic refrigerant R-507 (a blend containing 50% R-143a and 5% R-125 by weight) are used in a wide range of refrigeration systems especially in low-temperature applications. R-717 and R-507 are ozone friendly refrigerants, which have no Ozone Depletion Potential (ODP). Global Warming Potential (GWP) of R-717 and R-507 is equal to zero and 3300, respectively. The high amount of R-507 GWP demonstrates its negative effect on the Earth’s climate change. In this study, a refrigerated warehouse located in Cincinnati, Ohio was modeled and the total energy demand and Coefficient of Performance (COP) was evaluated by eQUEST using two scenarios. The R-717 and R-507 were used as refrigerant in the first and second scenarios, respectively. The results showed that using R-717 in the refrigeration system leads to a 15% energy saving and a higher COP compared to R-507 in all working conditions. The only exception is that at an evaporating temperature below −35°C which COP values of both refrigerants are approximately equal.

Near-isothermal-isobaric compressed gas energy storage

In this paper, the effectiveness of storing energy by compressing and expanding a condensable gas is evaluated. A high efficiency energy storage system, which stores energy by compressing/expanding gas (air) using a liquid (water) piston has been recently introduced and extensively studied. With the use of the liquid piston, the inefficient gas turbomachines used in conventional gas compression/expansion systems are replaced with high efficiency hydraulic machines. Utilizing heat transfer techniques and replacing the non-condensable air with a condensable gas (i.e. CO2, synthetic refrigerants, hydrocarbon refrigerants, etc.) have been proposed as methods to improve energy density and roundtrip efficiency of such systems, leading to near isothermal and near isobaric charge/discharge processes. In order to investigate the effectiveness of the proposed concept, a miniature lab-scale experimental setup was designed and built to investigate the compression/expansion characteristics and energy storage efficiency of a system utilizing R134a as the energy storage (primary) working fluid, and mineral refrigerant oil as the liquid piston (secondary) working fluid. Several tests are carried out to quantify energy storage efficiency and energy density. It is found that improving heat transfer rates from/into the storage fluid results in increased efficiency and energy density. A heat-transfer enhancement strategy to achieve near isothermal, isobaric expansion and compression is proposed and investigated experimentally. Some results are generalized and presented in non-dimensional form which can be applied to describe and/or design scaled-up systems. Lastly, a potential working fluid for a scaled-up system is discussed.

Flow boiling heat transfer on a Carbon/Carbon surface

Abstract Carbon/Carbon (C/C) is one of the most high-tech materials developed by the aerospace industry and then applied to critical components in several different applications. This composite material appears to be a viable option for future thermal management devices because it exploits interesting properties having a low density and a relatively high thermal conductivity as compared to copper; moreover, it has already been used in many industrial applications where it is shaped in various forms even complex. This study explores C/C heat transfer capabilities during the boiling process of a synthetic refrigerant. In particular, this paper presents the experimental measurements carried out during flow boiling heat transfer of R134a on a C/C surface. The sample was tested in a new experimental facility especially designed for studying the flow boiling heat transfer process on innovative materials and enhanced micro- and nano-structured surfaces. The tests were run at constant mean saturation temperature of 30 °C, by varying the heat flux from 50 kW m −2 to 100 kW m −2 , and the refrigerant mass velocity from 50 to 200 kg m −2 s −1 . The heat transfer measurements were compared with flow visualizations of boiling heat transfer at different operating conditions. Finally, a new model was developed and validated to estimate the refrigerant flow boiling heat transfer coefficients on C/C surface.

Comparative Study of R134a and R744 Driven Solar Assisted Heat Pump Systems for Different Applications

A solar assisted heat pump (SAHP) system is considered in this study where space cooling, water heating, drying and desalination are carried out using solar energy, ambient energy and the waste heat available from air conditioning system. The integration of different applications with SAHP system leads to many benefits, but the most significant is the energy conservation issue where the prices of fossil fuels are increasing. In recent years, researches have been conducted worldwide to improve the performance of these systems. These systems usually use synthetic refrigerant, such as R134a, which contributes to many atmospheric problems. Currently, embracing natural refrigerants, such as, carbon dioxide in SAHP has become a trend specifically after Kyoto agreement to reduce global warming and improve the performance of SAHP systems. In this paper, comparative study using R134a and R744 in SAHP systems have been considered. The comparison includes the most significant results of previous work relevant to this issue, and the thermo physical properties of the two refrigerants when used in SAHP.

Extending the Capillary Tube of a Propane Air-conditioner to Reduce the Refrigerant Charge

Abstract Hydrocarbon refrigerants are employed to replace synthetic refrigerants due totheir low global warming and ozone depletion potentials; however, these refrigerants are flammable; therefore, their mass in the system should be minimized to reduce the associated risks. This study deals with the novelidea of extending the capillary tube of a portable air-conditioner in orderto decrease the amount of refrigerant charge in the system. Extending the length of the capillary tube will shorten thelength of the liquid line in applications in which the distance between theoutlet of the condenser and the inlet of the evaporator cannot be reduced. A script was developed and its accuracywas experimentally assessed. It was then used to estimate the amount of charge for the existing design and the extended capillary tube. The results show a significant reduction (63.9%) in the amount of propane in the capillary tubeand liquid line, and a reasonable decrease (8.3%) in the maximumspeed of refrigerant for the airconditioner used in this work.

Effect of Inter-stage Pressure on the Performance of a Two Stage Refrigeration Cycle Using Inter Cooler

Abstract Effect of inter-stage pressure on the performance of a two stage refrigeration cycle using inter cooler is studied based on thermodynamic modelling. Six common refrigerants are selected for the analyses. These are R134a, R22 and R143a as synthetic refrigerants and propane, carbon dioxide and nitrous oxide as natural refrigerants. The designed input range is -50oC to -30oC for evaporator temperature and 40oC to 60oC for heat rejection temperature. Three classical relations for estimating inter-stage pressure in a two stage refrigeration system are selected and the performance is evaluated against optimized inter-stage pressure corresponding to the best COP. Results show that for trans-critical cycles, the deviation of optimized inter-stage pressure with the classical ones is considerable, while for sub-critical cycles, COP is very less sensitive to inter-stage pressure. Further, comprehensive analysis of CO2 based trans-critical refrigeration system is presented for a wider range covering -50oC to 10oC evaporation temperature and 35oC to 60oC gas cooler outlet temperature. A correlation for estimating optimum inter-stage and gas cooler pressure for the same is also formulated. With gradual increase in focus towards adoption of natural refrigerants like CO2, the effect of inter stage pressure is expected to be an important consideration in warmer climatic condition.

R744 ejector technology future perspectives

Carbon Dioxide, CO2 (R744) was one of the first commonly applied working fluids in the infancy of refrigeration more than 100 years ago. In contrast to ammonia it mainly disappeared after the first generation of synthetic refrigerants have been introduced to the market after 1930. One reason was that the transition from low-rpm belt driven compressors towards the direct electrical motor driven compressors (50-60 Hz) was not performed for CO2 compressors before the revival introduced by Gustav Lorentzen in the 90is of last century. Since 1988 an enormous R & D effort has been made to further develop CO2 refrigeration technology in spite of the opposition from the chemical industry.Today CO2 refrigeration and heat pumping technologies are accepted as viable and sustainable alternatives for several applications like commercial refrigeration, transport refrigeration, vehicle air conditioning & heat pumping, domestic hot water heat pumps and industrial applications. For some applications, the current threshold to introduce R744 technology can be overcome when the system design takes into account the advantage of the thermo dynamical- and fluid properties of CO2. I.e. the system is designed for transcritical operation with all it pros and cons and takes into consideration how to minimize the losses, and to apply the normally lost expansion work. Shortcut-designs, i.e. drop in solutions, just replacing the H(C)FC refrigeration unit with an CO2 systems adapted for higher system pressures will not result in energy efficient products. CO2 systems do offer the advantage of enabling flooded evaporators supported with adapted ejector technology. These units offer high system performances at low temperature differences and show low temperature air mal-distributions across evaporators.This work gives an overview for the development possibilities for several applications during the next years. Resulting in a further market share increase of CO2 refrigeration and heat pump systems, as energy efficient alternatives to current systems not applying natural working fluids.

Modeling of In-Tube Condensation of Zeotropic Mixtures

Studies in the literature have shown that zeotropic mixture condensation rates are lower than those predicted using a pure-fluid approach. This has been attributed to the decrease in fluid temperature that occurs with zeotropic mixtures and to the development of concentration gradients in the vapor-phase that limit the condensation heat transfer. The decrease in the apparent heat transfer coefficient is not consistent across mass fluxes, tube diameters, fluid combinations, saturation pressures, and concentrations. Several modeling techniques exist, which allow engineers to model the decrease in heat transfer rates. This study provides guidelines on when the mass transfer effects can be neglected and when it is appropriate to apply established models in the literature. A condensation database containing fluid combinations of pairs of hydrocarbons, ammonia and water, and synthetic refrigerants across large changes in operating conditions, tube diameters, and concentrations is used to validate the approach. The proposed framework predicts that the Bell and Ghaly (1973, “An Approximate Generalized Design Method for Multicomponent/Partial Condensers,” AIChE Symp. Ser., 69, pp. 72–79) approach is valid for mid- and high-reduced pressures, i.e., above 0.40, while explicitly accounting for mass transfer is necessary at lower reduced pressures, i.e., below 0.40, where the influence of the temperature glide in the Bell and Ghaly method is weighted too strongly.

A Novel Low-Energy Ventilative Cooling System for Sustainable Built Environment

A numerical investigation into determining the thermal and ventilation capability of wind towers integrated with the heat pipe technology was carried out in this work. The water-charged copper heat pipes were systematically arranged in a horizontal orientation and integrated inside a modern roof-mounted wind tower. Water was used as the working fluid instead of synthetic refrigerants in order to make the system carbon-neutral alongside maintaining the indoor air quality of the built environment. The three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations along with the momentum, continuity and energy equations were solved using the commercial Computational Fluid Dynamics (CFD) ANSYS code for velocity and pressure field simulations. Using the inlet wind speeds ranging from 1m/s to 5m/s, the results of the study showed that the proposed cooling system was capable of meeting the regulatory fresh air intake requirements per occupant of 10L/s. In addition, the findings determined that a passive cooling capacity of up to 11K was achievable when the system was subjected to inlet temperatures of 310K or 37°C. The work characterised the sustainable operation of wind tower in delivering energy-free ventilative cooling in regions encompassing hot and dry climatic conditions. The technology presented in this work is currently under an Intellectual Property (IP) protection (GB1321709.6).

ТЕПЛОНАСОСНІ ТЕХНОЛОГІЇ ТА ЇХ ПОТЕНЦІЙНИЙ ВПЛИВ НА ПРОБЛЕМИ ЕНЕРГОЕФЕКТИВНОСТІ ТА ЗАХИСТУ НАВКОЛИШНЬОГО СЕРЕДОВИЩА В ЕНЕРГЕТИЧНОМУ СЕКТОРІ УКРАЇНИ

The increasing demand of energy sources for urban, household and industrial facilities requires strategies development for seeking new energy sources. In recent years an important problem is to have energy storage, energy production and energy consumption which fulfill the environment friendly expectations. A lot of attention is devoted to renewable energy sources. One of the most attracting among them is energy production form geothermal sources. At a few meters below the earth’s surface the underground maintains a constant temperature in an approximation through the year allowing to withdraw heat in winter for heating needs and to surrender heat during summer for air-conditioning purposes. Heat pump is a rapidly developing technology for heating and domestic hot water production. Using ground as a heat source, heat exchange is carried out with heat pumps compound to vertical ground heat exchanger tubes that allows the heating and cooling of the buildings utilizing a single unit installation. Heat pump unit provides a high degree of productivity with moderate electric power consumption. In this paper a theoretical performance study of a vapor compression heat pump system with various natural and synthetic refrigerants (HFCs) is presented. Operation mode of the heat pump unit was chosen according to European Standard EN14511-2:2007 and EN255-2. An influence of discharge temperature on system performance was evaluated at different boiling temperatures. The comparison of mass flow rate and coefficient of performance for considered refrigerants at constant cooling capacity and condensation temperature was performed.

Experimental Validation of Critical Heat Flux (CHF) Predictive Methods for a New Synthetic fluid with Low Environmental Impact

This work presents experimental critical heat flux (CHF) values for a low environmental impact synthetic refrigerant and their comparison with well-known correlations from scientific literature. Tests were performed with HFO-1234yf in an aluminum heat sink made up of seven mini-channels, each of them 2mm wide and 1mm high. The heated length was 25mm. Experiments have been obtained in a variety of thermodynamic condition: the R1234yf saturation temperatures Tsat ranged from 25 up to 65°C (corresponding to medium-to-high reduced pressures), whilst the mass fluxes G had been fixed to 150 up to 300kg/m2 s. The tests have been carried out by increasing the heat dissipated by the boiling refrigerant until the thermal crisis occurred and the corresponding heat flux value was recorded as CHF. The experimental results were finally compared to the well-known correlations of Wojtan et al. [1], Kuan [2], Katto-Ohno [3], Zhang et al. [4] and Anwar et al. [5] to investigate their effectiveness with the present data. The latter correlation was found to better predict the experimental values.

Transient simulation of heat pumps using low global warming potential refrigerants

Due to the relatively high global warming potential value of R410A, a lot of effort has been devoted to the exploration of potential refrigerants to replace R410A in heat pump applications. Natural refrigerants with single-digit global warming potential values have not yet shown readiness. Therefore, some synthetic refrigerants with global warming potential values significantly lower than R410A, such as R32 and the R32-based blend D2Y60, are getting more attention. To evaluate the transient performance of these two drop-in low-global warming potential refrigerants, two dynamic heat pump models, based on different platforms, have been developed to simulate a heat pump cycle during both steady state and transient operations. Both models include an efficiency-based compressor model, two control volume-based heat exchanger models, a control volume-based valve model, a segmented pipe model and a lumped-capacitance accumulator model. In order to speed up the simulations, accelerated refrigerant property routines were developed for R32 and D2Y60 based on National Institute of Standards and Technology's (NIST's) REFPROP database (2013). The simulation results have been compared with the published experimental data. The data includes steady-state operation data based on ASHRAE steady-state tests, and transient data from ASHRAE's cyclic operation D-test. The validation of R410A, R32 and D2Y60 cycles under steady-state operating conditions (ASRHAE A and B tests and one in-house ‘extended’ condition) shows that most of the simulations are within 5% measured values. The transient simulation results demonstrate that both models capture the dynamic performance of vapor compression cycle during start-up and shut-down.

Dryout characteristics of natural and synthetic refrigerants in single vertical mini-channels

Experimental results on dryout of seven refrigerants (R134a, R1234yf, R152a, R22, R245fa, R290 and R600a) in small, single vertical tubes under upward flow conditions are reported in this study. The experiments were conducted under a wide range of operating conditions in stainless steel tubes (0.64–1.70mm and 213–245mm heated length). The effects of operating parameters like mass flux, vapor quality, saturation pressure and channel size are discussed in detail. In general, dryout heat flux increased with increasing mass flux, and with increasing tube diameter. No effect of varying saturation temperature was observed. The experimental findings were compared with well-known macro and micro-scale correlations from the literature and it was found that Wu’s correlation (in modified form) quite satisfactorily predicted the whole database. A new correlation for prediction of heat flux at dryout conditions is also proposed.

Computational analysis of a wind tower assisted passive cooling technology for the built environment

This work investigates a novel closed-loop thermal cycle containing cylindrical heat pipes integrated within a roof-mounted circular wind tower to achieve internal comfort. The water-filled copper heat pipes having an outer diameter of 20mm were systematically arranged in a horizontal orientation. Water was used as the working fluid instead of synthetic refrigerants in order to make the system carbon-neutral alongside maintaining the indoor air quality of the built environment. The three-dimensional Reynolds-Averaged Navier–Stokes (RANS) equations along with the momentum, continuity and energy equations were solved using the commercial FLUENT code for velocity and pressure field simulations. Using the inlet wind speeds varying from 1m/s to 5m/s, the results of the study showed that the proposed cooling system was capable of meeting the regulatory fresh air intake requirements per occupant of 10L/s. In addition, the results showed that a passive cooling capacity ranging between 6K and 15K depending on the operating configuration. The findings of the study were validated by comparing the results with similarly analysed wind tower structures found in previous literature. The present work successfully classified the sustainable operation of natural ventilation systems in delivering energy-free cooling in regions encompassing hot and moderately humid or humid climatic conditions. The technology presented here is subject to an international patent application (PCT/GB2014/052263).

Efficiency analysis of alternative refrigerants for ejector cooling cycles

Computer software, basing on the theoretical model of Huang et al. with thermodynamic properties of selected refrigerants, was prepared. Investigation was focused on alternative refrigerants that belong to two groups of substances: common solvents (acetone, benzene, cyclopentane, cyclohexane and toluene) and non-flammable synthetic refrigerants applied in Organic Rankine Cycle (ORC) (R236ea, R236fa, R245ca, R245fa, R365mfc and RC318). Refrigerants were selected to detect a possibility to use them in ejector cooling system powered by a high-temperature heat source. A series of calculations were carried out for the generator temperature between 70 and 200°C, with assumed temperatures of evaporation 10°C and condensation 40°C.Investigation revealed that there is no single refrigerant that ensures efficient operation of the system in the investigated temperature range of primary vapor. Each substance has its own maximum entrainment ratio and COP at its individual temperature of the optimum. The use of non-flammable synthetic refrigerants allows obtaining higher COP in the low primary vapor temperature range. R236fa was the most beneficial among the non-flammable synthetic refrigerants tested. The use of organic solvents can be justified only for high values of motive steam temperature. Among the solvents, the highest values of entrainment ratio and COP throughout the range of motive temperature were noted for cyclopentane. Toluene was found to be an unattractive refrigerant from the ejector cooling point of view.

EXPERIMENTAL STUDY OF LPG AND R134a REFRIGERANTS IN VAPOR COMPRESSION REFRIGERATION

Natural refrigerants, such as hydrocarbons, have been renewed in recent years as a result of the environmental problems associated with chlorofluorocarbon (CFC) and hydro-chlorofluorocarbon (HCFC) refrigerants. Due to the depletion of the ozone layer and global warming effects, synthetic refrigerants are being gradually phased out in accordance with the international protocols that aim to protect the environment. A refrigerator designed to work with R134a was used for this experiment, liquefied petroleum gas (LPG) which consists of 60% propane (R290) and 40% butane (R600) was compared with R134a refrigerant in a vapor compression refrigerator with a total volume of 62 L. The experiments were carried out using different charges of 40, 60, 80, and 100 g for R134a and LPG refrigerants, the charges were measured with a digital charging scale. The K-type thermocouples were used to measure the temperatures at the inlet and outlet of the four major components (evaporator, compressor, condenser, and expansion device) of the refrigeration system. The system was instrumented with two pressure gauges at the inlet and outlet of the compressor for measuring the suction and discharge pressures. The results obtained were used to determine the thermodynamic properties of the refrigerants using Refprop, version 9. The results obtained showed that the design temperature and pull-down time set by the International Standard Organisation (ISO) for a refrigerator were achieved with LPG earlier than with R134a. The coefficient of performance (COP) of the system increases by 9.5% and the power consumption is reduced by 12%, when compared with R134a. Therefore, LPG can successfully substitute R134a in domestic refrigerators.