USA: A new refrigerant blend based around HFO1234ze could be a viable, less flammable alternative to 1234yf in car air conditioning systems.

The new refrigerant is one of two being developed by Mexichem and the subject of extensive testing by SAE International. One of the new refrigerants in particular, AC6 (R445A), displayed lower flammability risks compared to R1234yf.

Although R1234yf has been cleared as safe by the SAE, controversy still exists amongst German car manufacturers, particularly Daimler, regarding the potential safety of this gas in the event of head-on vehicle collisions.

Mexichem’s AC6 is a mixture of 1234ze(E), 134a and CO2 (85%/9%/6%). Its sister product AC5 (R444A) is a blend of 1234ze(E), R32 and R152a (83%/12%/5%).

Both new refrigerants meet the requirements of the EU MAC Directive and exhibit risk profiles said to be equal to or better than R1234yf, the popular choice for the replacement of R134a in car air conditioning systems.

The main thermophysical properties of the fluids are summarised in the table (above). Both AC5 and AC6 have molecular weight, liquid density, latent heat of vaporisation and critical points that are close to those of R134a. This is by design as the fluids were formulated to give operating pressures and cooling capacities close to those typically found using R134a

The main thermophysical properties of the fluids are summarised in the table (above). Both AC5 and AC6 have molecular weight, liquid density, latent heat of vaporisation and critical points that are close to those of R134a. This is by design as the fluids were formulated to give operating pressures and cooling capacities close to those typically found using R134a

AC5 is said to closely match the performance of R134a whilst having similar flammability characteristics to R1234yf. AC6 has a slightly higher refrigeration capacity than R134a but with reduced flammability compared to R1234yf. In fact, according to the report, AC6 is not flammable at normal ambient temperature, unlike R1234yf and AC5.

In general, the electrical properties of both refrigerants mixed with oil were found to be better than R1234yf. Material compatibility was similar to R1234yf. Miscibility and solubility of the two Mexichem refrigerants are said to provide properties that are more conducive to compressor durability than R1234yf.

Drop-in performance of AC5 was found to be similar to R134a at most test conditions, with capacity and COP also close to R134a values.

However, while drop-in performance of AC6 was similar to 134a at high load conditions, it exhibited a 5-10% reduction in COP at part load, with an attendant increase in discharge pressure. According to the report, this could be overcome and match the performance of 1234yf by the selection of an appropriately sized internal heat exchanger and separate receiver dryer

JpegBeing zeotropic, glide is an issue but not significant with AC5. It was found to be slightly more of a problem with AC6, resulting in a loss of the CO2 component. However, the SAE team considered that this could be be corrected during routine maintenance/recharging.

Probably due to its CO2 component, it was in comparative flammability tests that AC6 scored higher marks. Both Mexichem refrigerants are ASHRAE classified A2L (“mildly flammable”, like 1234yf) but while the flammability characteristics of AC5 were found to be similar to R1234yf overall, AC6 was less flammable than either fluid. It is said to be non-flammable at normal ambient temperatures, exhibiting flammability onset (by the ASHRAE test method) between 50 and 60°C. Its flammability range is very small with an LFL higher than either of the other fluids. 

In addition, in tests involving spraying refrigerant/oil mixtures onto hot surfaces it was found that a significantly hotter metal surface was required to ignite an aerosol of AC6 with PAG oil. The temperature of metal required was typically 80-100K higher than that needed to ignite R-1234yf/PAG mixtures in the same test. The gas auto-ignition temperature of AC6 refrigerant in its worst case formulation was determined to be about 80K higher than that of R-1234yf in the same apparatus.

www.sae.org/standardsdev/tsb/cooperative/mrb-ac6_white_paper.pdf

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