R134a was the first chlorine free (ODP = 0) HFC refrigerant that was tested comprehensively. It is now used world-wide in many refrigeration and air conditioning units with good results. As well as being used as a pure substance, R134a is also applied as a component of a variety of blends.
Refrigerating capacity, energy demand, temperature properties and pressure levels are comparable, at least in air conditioning and medium temperature refrigeration plants. This refrigerant can therefore be used as an alternative for most former R12 applications.
For some applications R134a is even preferred as a substitute for R22, an important reason being the limitations to the use of R22 in new plants and for service. However, the lower volumetric refrigerating capacity of R134a (R134a/R22 – comparison of performance data) requires a larger compressor displacement than with R22. There are also limitations in the application with low evaporating temperatures to be considered.
Comprehensive tests have demonstrated, however, that the performance of R134a exceeds theoretical predictions over a wide range of compressor operating conditions. Temperature levels (discharge gas, oil) are even lower than with R12 and substantially lower than R22 values. There are thus many potential applications in air conditioning and medium temperature refrigeration plants as well as in heat pumps. Good heat transfer characteristics in evaporators and condensers (unlike zeotropic blends) favour an economical use.
R134a is also characterized by a comparably low GWP (1430). Therefore, in view of future restrictions (for example EU F-Gas Regulation), the use of this refrigerant will still be possible for quite some time. If required, systems can later be converted relatively easily to non-flammable (A1) HFO/HFC alternatives with a GWP of approx. 600 (“Low GWP” alternatives for HFC refrigerants).
The traditional mineral and synthetic oils are not miscible (soluble) with R134a and other HFCs described in the following and are therefore only insufficiently transported around the refrigeration circuit.
Immiscible oil can settle out in the heat exchangers and prevent heat transfer to such an extent that the system can no longer be operated.
New lubricants with appropriate solubility were developed and have been in practical use for many years. These lubricants are based on Polyol Ester (POE) and Polyalkylene Glycol (PAG).
For further explanations on lubricants: Lubricants for compressors.
Suitable compressors are required for R134a with a special oil charge and adapted system components. The normal metallic materials used in CFC plants have also been proven with ester oils; elastomers must sometimes be matched to the changing situation. This is especially valid for flexible hoses where the requirements call for a minimum residual moisture content and low permeability.
The plants must be dehydrated with particular care and the charging or changing of lubricant must also be done carefully. In addition relatively large driers should be provided, which have also to be matched to the smaller molecule size of R134a.
Meanwhile, many years of very positive experience with R134a and ester oils have been accumulated. For this refrigerant, BITZER offers an unequalled wide range of reciprocating, screw, and scroll compressors.
At the beginning this subject was discussed very controversially, several conversion methods were recommended and applied. Today there is a general agreement on technically and economically matching solutions.
The characteristics of ester oils are very favourable here: Under certain conditions they can be used with CFC refrigerants, they can be mixed with mineral oils and tolerate a proportion of chlorine up to a few hundred ppm in an R134a system.
The remaining moisture content has, however, an enormous influence. Very thorough evacuation (removal of remaining chlorine and dehydration) is therefore essential, as well as the installation of generously dimensioned driers. There is doubtful experience with systems where the chemical stability was already insufficient with R12 operation e.g. with bad maintenance, small drier capacity, high thermal loading. Increased deposition of oil decomposition products containing chlorine is found often. These products are released by the influence of the highly polarized mixture of ester oil and R134a and find their way into the compressor and control devices. Conversion should therefore be limited to systems which are in a good condition.
An EU Directive on “emissions from air-conditioning systems in motor vehicles” (2006/40/EC) bans the use of R134a in new systems. Various alternative technologies are already in use. Further explanations: “Low GWP” HFO refrigerant R1234yf and CO₂ in car air conditioning systems.
(see also https://www.bitzer.de)
For mobile air conditioning systems (MAC) with open drive compressors and hose connections in the refrigerant circuit, the risk of leakages is considerably higher than with stationary systems. An EU Directive (2006/40/EC) has been passed to reduce direct emissions in this application area. Within the scope of the Directive, and starting 2011, type approvals for new vehicles will only be granted if they use refrigerants with a global warming potential (GWP) of less than 150. Consequently, this excludes R134a (GWP = 1430) which has been used so far in these systems.
Meanwhile, alternative refrigerants and new technologies were developed and tested. This also involved a closer examination of the use of R152a.
For quite some time the automotive industry has agreed on so-called “Low GWP” refrigerants. The latter is dealt with as follows.
R152a is very similar to R134a with regard to volumetric refrigerating capacity (approx. -5%), pressure levels (approx. -10%) and energy efficiency. Mass flow, vapour density and thus also the pressure drop are even more favourable (approx. -40%).
R152a has been used for many years as a component in blends, but not as a single substance refrigerant till now. Especially advantageous is the very low global warming potential (GWP = 124).
R152a is flammable – due to its low fluorine content – and classified in safety group A2. As a result, increased safety requirements demand individual design solutions and safety measures along with the corresponding risk analysis.
For this reason, the use of R152a in mobile air conditioning systems for passenger cars (MAC) has not been implemented yet.
The ban on the use of R134a in mobile air conditioning systems within the EU has triggered a series of research projects. In addition to CO2 technology (CO₂ in car air conditioning systems), refrigerants with very low GWP values and similar thermodynamic properties as R134a have been developed.
In early 2006, two refrigerant mixtures were introduced under the names “Blend H” (Honeywell) and “DP-1” (DuPont). INEOS Fluor followed with another version under the trade name AC-1. In the broadest sense, all of these refrigerants were blends of various fluorinated molecules.
During the development and test phase it became obvious that not all acceptance criteria could be met, and thus further examinations with these blends were discontinued.
Consequently, DuPont (meanwhile Chemours) and Honeywell bundled their research and development activities in a joint venture which focused on 2,3,3,3- tetrafluoropropene (CF3CF=CH2). This refrigerant, designated R1234yf, belongs to the group of hydro fluoro olefins (HFO). These refrigerants are unsaturated HFCs with a chemical double bond.
The global warming potential is extremely low (GWP = 4). When released to the atmosphere, the molecule rapidly disintegrates within a few days, resulting in a very low GWP. This raises certain concerns regarding the long-term stability in refrigeration circuits under real conditions.
However, extensive testing has demonstrated the required stability for mobile air conditioning systems.
R1234yf has lower flammability as measured by ASTM 681, but requires significantly more ignition energy than R152a, for instance. Due to its low burning velocity and the high ignition force, it received a classification of the new safety group “A2L” according to ISO 817. In extensive test series, it has been shown that a potentially increased risk of the refrigerant flammability in MAC systems can be avoided by implementing suitable constructive measures. However, some investigations (e.g. by Daimler) also showed an increased risk. This is why various manufacturers have again intensified the development of alternative technologies.
Toxicity investigations have shown very positive results, as well as compatibility tests of the plastic and elastomer materials used in the refrigeration circuit. Some lubricants show increased chemical reactivity which, however, can be suppressed by a suitable formulation and/or addition of “stabilizers”.
Operating experiences gained from laboratory and field trials to date allow a positive assessment, particularly with regard to performance and efficiency behaviour. For the usual range of mobile air conditioning operation, refrigerating capacity and coefficient of performance (COP) are within a range of 5% compared with that of R134a. Therefore, it is expected that simple system modifications will provide the same performance and efficiency as with R134a.
The critical temperature and pressure levels are also similar, while the vapour densities and mass flows are approximately 20% higher. The discharge gas temperature with this application is up to 10 K lower.
With a view to the relatively simple conversion of mobile air conditioning systems, this technology prevailed up to now over the competing CO2 systems.
However, as already explained before, due to the flammability of R1234yf, investigations focus on other technical solutions. This includes active fire-extinguishing devices (e.g. with argon), but also enhancements of CO2 systems.
For detailed information on “Low GWP” HFOs and blends: “Low GWP” HFOs and HFO/HFC blends as alternatives to HFCs.