Alternatives for R13 and R503

For these substances, the situation is still quite favorable from a purely technical point of view; they can be replaced by R23 and R508A/R508B. R170 (ethane) is also suitable if the safety regulations allow a flammable substance (safety group A3).

Due to the partly steeper pressure curve of the alternative refrigerants and the higher discharge gas temperature of R23 compared to R13, differences in performance and application ranges for the compressors must be considered. Heat exchangers and controls have to be adapted individually.

As lubricants for R23 and R508A/B, polyol ester oils are suitable, but must be matched for the special requirements at extremely low temperatures.

R170 is also well soluble with conventional oils, but an adaptation to the temperature will be necessary.

Applications with these refrigerants are purely for cooling products below -50°C. Hence, the exemption described in the previous chapter in the EU F-Gas Regulation applies in particular.

For R23 and R508A/B, however, the effects of "phase-down" are particularly serious. The GWP values range from 13200 to 14800 (AR4). Even relatively small quantities are therefore very much at the expense of the available quotas.

Apart from R170 (ethane) with the special safety precautions required for A3 refrigerants, there are no directly comparable alternatives for R23 and R508A/B within the group of HFOs or HFO/HFC mixtures (safety groups A1 or A2L). In many cases, however, the use of A3 refrigerants is not possible or would involve unjustifiable expenses and high costs in the relevant special applications.

Following these challenges, the company Weiss Technik in cooperation with the TU Dresden has developed a non-flammable (A1) refrigerant mixture of R32, R125 and CO2, which has proven to be a well suited alternative to R23, e.g. when used in so-called climatic stress chambers. It is marketed under the trade name WT69 and sold by TEGA Technische Gase. The refrigerant is now also listed in the ASHRAE nomenclature under R469A.

A major advantage over R23 is that the GWP is reduced by more than 90% (1398). This ensures at least medium to longer-term availability.

From a thermodynamic point of view, there are major differences to R23, therefore the suitability in the respective application must be checked individually. Although the differences in boiling points are not large (-78.5°C vs. -82°C for R23), the mixture features a distinct temperature glide. Apart from the required specific design of the heat exchangers, this may affect the operating process and compressor size.

Furthermore, research projects have been initiated that examine the use of N2O (nitrous oxide) and mixtures of N2O and CO2 in more detail. Extensive examinations and tests at the Karlsruhe University of Applied Sciences and the Institute of Air Handling and Refrigeration (ILK) in Dresden show revealing results.

N2O (R744A) has similar thermodynamic properties and pressures as CO2, identical molecular weight, a very low triple point (-90.8°C) and a critical temperature of 36.4°C. The GWP is 298, which is a fraction of the R23 and R508A/B values. In sum, an ideal alternative for special applications up to about -80°C evaporating temperature?

At first glance, these are very positive features. Unfortunately, there are also negative aspects that virtually preclude the use of N2O as a pure substance. Pure N2O as a refrigerant is a safety risk: It has a narcotic effect and promotes fire. N2O can oxidize other substances. In addition, under specific conditions (pressure, temperature or ignition source), exothermic decomposition reactions can occur, which fundamentally call into question the permanently safe operation of a refrigeration system with pure N2O.

By adding CO2 in higher percentages (over approx. 15%), the triple point is slightly shifted towards higher temperatures, but at the same time a positive effect on oxidation and chemical decomposition is achieved ("phlegmatization"). The safety risk is reduced significantly, and material compatibility is considerably improved. Nevertheless, there are special challenges i.a. for lubricants with a high resistance to oxidation which must also be suitable for the special requirements at low temperatures.

Investigations are ongoing. A final assessment is not yet possible, which is why no guidelines can currently be drawn up for the design and implementation of such systems.