In place of R114 and R12B1 previously used in high-temperature heat pumps and crane air conditioning systems, HCFCs R124 and R142b can still be used as alternatives in most regions outside of the EU.
With these gases it is also possible to use long proven lubricants, preferably mineral oils and alkyl benzenes with high viscosity.
Because of their ozone depleting potential, these refrigerants will only be an interim solution. In the EU member states, the application of HCFCs is no longer allowed. For R124 and R142b the same restrictions are valid as for R22 (R22 as transitional refrigerant). The flammability of R142b and the resulting safety implications should also be considered (safety group A2).
Compared to R114, the alternatives have lower boiling temperatures (approx. -10°C), which results in larger differences in the pressure levels and volumetric refrigerating capacities. This leads to stronger limitations in the application range at high evaporating and condensing temperatures.
Converting an existing installation will in most cases necessitate the exchange of the compressor and control devices. Owing to the lower volume flow (higher volumetric refrigerating capacity), adjustments to the evaporator and the suction line may be required.
Over the previous years BITZER compressors have been found to be well suited with R124 and R142b in actual installations. Depending on the application range and compressor type modifications are necessary, however. Performance data including further design instructions are available on request.
Due to the limited market for systems with extra high and low temperature applications, the development of alternative refrigerants and system components for these has been pursued less intensely.
In the meantime, a group of alternatives for the CFC R114 and Halon R12B1 (high temperature), R13B1, R13 and R503 (extra low temperature) have been offered as replacements. On closer examination, however, the thermodynamic properties of the alternatives differ considerably from the previously used substances. This can cause costly changes especially with the conversion of existing systems.
R227ea and R236fa are considered suitable substitutes even though they may no longer be used in new installations in the EU from 2020, due to their high GWP.
R227ea cannot be seen as a full replacement. Although tests and experience in real plants show favorable results, a critical temperature of 102°C limits the condensing temperature to 85..90°C with conventional plant technology.
R236fa provides the more favourable conditions at least in this regard – the critical temperature is above 120°C. A disadvantage, however, is the lower volumetric refrigerating capacity. It is similar to R114 and 40% below the performance of R124, which is widely used for extra high temperature applications today.
R600a (Isobutane) will be an interesting alternative where safety regulations allow the use of hydrocarbons (safety group A3). With a critical temperature of 135°C, condensing temperatures of 100°C and more are within reach. The volumetric refrigerating capacity is almost identical to R124.
The "Low GWP" refrigerant R1234ze(E) ("Low GWP" HFOs and HFO/HFC blends as alternatives to HFCs) can also be regarded as a potential candidate for extra high temperature applications. Compared to R124, its refrigerating capacity is 10 to 20% higher, its pressure level about 25% higher. At identical refrigerating capacities, the mass flow differs only slightly. Its critical temperature is 107°C, which would enable an economical operation up to a condensing temperature of about 90°C. Though, like R1234yf, R1234ze(E) has lower flammability and therefore classified into the new safety group A2L. The corresponding safety regulations must be observed.
As no sufficient operating experience is available so far, the suitability of this refrigerant for long-term use cannot be assessed yet.
For high temperature heat pumps in process technology and special applications at high temperatures, Chemours has presented an HFO-based refrigerant called OpteonTM MZ (R1336mzz(Z)). Its critical temperature is 171°C, the boiling temperature 33.1°C. This enables an operation at condensing temperatures far above 100°C for which only purpose-built compressors and system components can be used.
R1336mzz(Z) has a GWP < 10 but is not flammable according to tests. This means a classification in safety group A1.
A more detailed evaluation is not yet possible with respect to the chemical stability of the refrigerant and of the lubricants at the very high temperatures and the usually very long operating cycles of such systems. The special applications also include cogeneration systems – the so-called "Organic Rankine Cycle" (ORC) – which become increasingly important. In addition to R1336mzz(Z) as a potentially suitable operating fluid, a series of other substances are also possible, depending on the temperature level of the heat source and the heat sink. They include R245fa (GWP = 1050) having a critical temperature of 154°C, which like R1336mzz(Z) is also suitable as refrigerant for chillers with large centrifugal compressors.
Solvay offers further refrigerants for ORC applications, containing the base component R365mfc. A product with the trade name Solkatherm® SES36 already presented several years ago contains perfluoropolyether as a blend component. It is an azeotropic blend with a critical temperature of 178°C. Meanwhile two zeotropic blends containing R365mfc and R227ea have been developed whose critical temperatures are 177°C and 182°C, due to different mixing ratios. They are available under the trade names Solkatherm® SES24 and SES30.
In ORC systems zeotropic behavior may be advantageous. In the case of single-phase heat sources and heat sinks, the temperature difference at the so-called "pitch point" can be raised by the gliding evaporation and condensation. This leads to improved heat transmission due to the higher driving average temperature difference.
As an expander for ORC systems screw and scroll compressors can be adapted in their construction accordingly. For several years BITZER has been involved in various projects and has already gained important knowledge with this technology and experience in design and application. A comprehensive description of ORC systems would go beyond the scope of this Refrigerant Report. Further information is available upon request.
Besides R410A, ISCEON® MO89 (DuPont) can be regarded as potential R13B1 substitute. For R410A, a substantially higher discharge gas temperature than for R13B1 is to be considered, which restricts the application range even in 2-stage compression systems to a greater extent.
ISCEON® MO89 has been used for many years, preferably in freeze-drying plants. Meanwhile, production has ceased. However, for historical reasons the refrigerant will continue to be included in this Report. It is a mixture of R125 and R218 with a small proportion of R290. Due to the properties of the two main components, density and mass flow are relatively high, and discharge gas temperature is very low. Liquid subcooling is of particular advantage.
Both of the mentioned refrigerants have fairly high pressure levels and are therefore limited to 40 .. 45°C condensing temperature with the usually applied 2-stage compressors. They also show less capacity than R13B1 at evaporating temperatures below -60°C.
In addition to this, the steep fall of pressure limits the application at very low temperatures and may require a change to a cascade system with e.g. R23, R508A/B or R170 (ethane) in the low temperature stage.
Lubrication and material compatibility are similar to other HFC blends.
The EU F-Gas Regulation (Annex III) provides an exemption "for applications designed to cool products below -50°C". This means that even after 2020, refrigerants with GWP > 2500 can be used in new plants. Due to the "phase-down" however, quantities will be limited, resulting in a considerable increase in price and very limited availability.
It is therefore imperative to develop alternative solutions for which, however, no overall recommendation is possible. Two-stage compressors may be operated e.g. with R448A/449A (safety group A1) or R1270 (A3) down to an evaporation temperature of -60 ..- 65°C. Although R404A/R507A alternatives with GWP < approx. 250 (safety group A2L) are potentially possible, so far only limited experience has been gained even for typical low temperature refrigeration.
At evaporating temperatures of down to -50..- 52°C, operation with CO2 is also possible − either in a two-stage or a cascade system.
However, each variant generally requires a specific design and laboratory tests.
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 extreme 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, research projects have been initiated, in which the use of N2O (nitrous oxide) and mixtures of N2O and CO2 are examined 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 ("phlegmatization") on oxidation and chemical decomposition is achieved. 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 temperature conditions.
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.
BITZER has carried out investigations and also collected experiences with several of the substitutes mentioned (Refrigerants for special applications). Performance data and instructions are available on request. Due to the individual system technology for these special installations, consultation with BITZER is necessary.