Refrigerant blends

Refrigerant blends: Classification and experiences

Refrigerant blends have been developed for existing as well as for new plants with properties making them comparable alternatives to the previously used substances. It is necessary to distinguish between three categories:

  1. Transitional or service blends,
    which mostly contain HCFC R22 as the main constituent. They are primarily intended as service refrigerants for older plants with view on the use ban of R12, R502 and other CFCs. Corresponding products are offered by various manufacturers, there is practical experience covering the necessary steps of conversion procedure. However, the same legal requirements as for R22 apply to the use and phase-out of these blends (R22 as transitional refrigerant).
     
  2. HFC blends
    These are substitutes for the refrigerants R502, R22, R13B1 and R503. Above all, R404A, R507A, R407C and R410A, are being used to a great extent. One group of these HFC blends also contains hydrocarbon additives. The latter exhibit an improved solubility with lubricants, and under certain conditions they allow the use of conventional oils. In many cases, this permits the conversion of existing (H)CFC plants to chlorine-free refrigerants (ODP = 0) without the need for an oil change.
     
  3. HFO/HFC blends
    as successor generation of HFC refrigerants. It concerns blends of new "Low GWP" refrigerants (e.g. R1234yf) with HFCs. The fundamental target is an additional decrease of the global warming potential (GWP) as compared to established halogenated substances ("Low GWP" HFOs and HFO/HFC blends as alternatives to HFCs).
    ​​​​

Blends of two and three components already have a long history in the refrigeration trade. A difference is made between the so called "azeotropes" (e.g. R502, R507A) with thermodynamic properties similar to single substance refrigerants, and "zeotropes" with "gliding" phase changes (General characteristics of zeotropic blends). The original development of "zeotropes" mainly concentrated on special applications in low temperature and heat pump systems. Actual system construction, however, remained the exception.

A somewhat more common earlier practice was the mixing of R12 to R22 in order to improve the oil return and to reduce the discharge gas temperature with higher pressure ratios. It was also usual to add R22 to R12 systems for improved performance, or to add hydrocarbons in the extra low temperature range for a better oil transport.

This possibility of specific "formulation" of certain characteristics was indeed the basis for the development of a new generation of blends.

At the beginning of this Report (Refrigerant developments and legal situation) it was already explained that no direct single-substance alternatives (on the basis of fluorinated hydrocarbons) exist for the previously used and current refrigerants of higher volumetric refrigeration capacity than R134a. This is why they can only be "formulated" as blends. However, taking into account thermodynamic properties, flammability, toxicity and global warming potential, the list of potential candidates is strongly limited.

For the previously developed CFC and HCFC substitutes, the range of substances was still comparably large, due to the fact that substances of high GWP could also be used. However, for formulating blends with significantly reduced GWP, in addition to R134a, R1234yf and R1234ze(E), primarily refrigerants R32, R125 and R152a can be used. Most of them are flammable. They also exhibit considerable differences with respect to their boiling points, which is why all "Low GWP" blends of high volumetric refrigerating capacity have a substantial temperature glide (General characteristics of zeotropic blends).

BITZER products and experiences with refrigerant blends

BITZER has accumulated extensive experience with refrigerant blends. Laboratory and field testing was commenced at an early stage so that basic information was obtained for the optimizing of the mixing proportions and for testing suitable lubricants. Based on this data, a large supermarket plant – with 4 BITZER semi-hermetics in parallel – could already be commissioned in 1991. The use of these blends in the most varied systems has been state-of-the-art for many years – generally with good experiences.

General characteristics of zeotropic blends

As opposed to azeotropic blends (e.g. R502, R507A), which behave as single substance refrigerants with regard to evaporation and condensing processes, the phase change with zeotropic fluids occurs in a "gliding" form over a certain range of temperature.

This "temperature glide" can be more or less pronounced, it depends mainly on the boiling points and the percentage proportions of the individual components. Certain supplementary definitions are also used, depending on the effective values, such as "near-azeotrope" or "semiazeotrope" for less than 1 K glide.

Essentially, this results in a small temperature increase already in the evaporation phase and a reduction during condensing. In other words: At a certain pressure level, the resulting saturation temperatures differ in the liquid and vapour phases (Evaporating and condensing behavior of zeotropic blends).

To enable a comparison with single substance refrigerants, the evaporating and condensing temperatures have been often defined as mean values. As a consequence the measured subcooling and superheating conditions (based on mean values) are unrealistic. The effective difference – based on dew and bubble temperature – is less in each case. These factors are very important when assessing the minimum superheat at the compressor inlet (usually 5 to 7 K) and the quality of the refrigerant after the liquid receiver.

With regard to a uniform and easily comprehensible definition of the rated compressor capacity, the revised standards EN 12900 and AHRI540 are applied. Evaporating and condensing temperatures refer to saturated conditions (dew points).

In this case the assessment of the effective superheat and subcooling temperatures will be simplified.

It must however be considered that the actual refrigerating capacity of the system can be higher than the rated compressor capacity. This is partly due to an effectively lower temperature at the evaporator inlet.

A further characteristic of zeotropic refrigerants is the potential concentration shift when leakage occurs. Refrigerant loss in the pure gas and liquid phases is mainly non-critical. Leaks in the phase change areas, e.g. after the expansion valve, within the evaporator and condenser/receiver are considered more significant. It is therefore recommended that soldered or welded joints should be used in these sections.

Extended investigations have shown in the meantime that leakage leads to less serious changes in concentration than initially thought. In any case it is certain that the following substances of safety group A1 (Refrigerant Properties) which are dealt with here cannot develop any flammable mixtures, either inside or outside the circuit. Essentially similar operating conditions and temperatures as before can be obtained by supplementary charging with the original refrigerant in the case of a small temperature glide.

Further conditions/recommendations concerning the practical handling of blends must also be considered:

  • The plant always has to be charged with liquid refrigerant. When vapour is taken from the charging cylinder, concentration shifts may occur.
  • Since all blends contain at least one flammable component, the entry of air into the system must be avoided. If the proportion of air is too high, a critical shift of the ignition point can occur under high pressure and while evacuating.
  • The use of blends with a significant temperature glide is not recommended for plants with flooded evaporators. A large concentration shift is to be expected in this type of evaporator, and as a result also in the circulating refrigerant mass flow.

Service blends

Service blends with the basic component R22* as substitutes for R502

As a result of the continued refurbishment of older installations, the importance of these refrigerants is clearly on the decline. For some of them, production has already been discontinued. However, because of the development history of service blends, these refrigerants will continue to be covered in this Report.

These refrigerants belong to the group of "Service blends" and have been offered under the designations R402A/R402B* (HP80/ HP81 – DuPont), R403A/R403B* (formerly ISCEON® 69S/69L) and R408A* (“Forane®” FX10 – Arkema).

The basic component is in each case R22, the high discharge gas temperature of which is significantly reduced by the addition of chlorine free substances with low isentropic compression exponent (e.g. R125, R143a, R218). A characteristic feature of these additives is an extraordinarily high mass flow, which enables the mixture to achieve a great similarity to R502.

R290 (Propane) is added as the third component to R402A/B and R403A/B to improve miscibility with traditional lubricants as hydrocarbons have especially good solubility characteristics.

For these blends two variations are offered in each case. When optimizing the blend variations with regard to identical refrigerating capacity as for R502 the laboratory measurements showed a significantly increased discharge gas temperature (Effect of the mixture variation upon the discharge gas temperature (example: R22/R218/R290)), which above all, with higher suction gas superheat (e.g. supermarket use) leads to limitations in the application range.

On the other hand a higher proportion of R125 or R218, which has the effect of reducing the discharge gas temperature to the level of R502, results in somewhat higher refrigerating capacity (Comparison of the performance data of a semi-hermetic compressor).

With regard to material compatibility the blends can be judged similarly to (H)CFC refrigerants. The use of conventional refrigeration oil (preferably semi or full synthetic) is also possible due to the R22 and R290 proportions.

Apart from the positive aspects there are also some disadvantages. These substances are alternatives only for a limited time. The R22 proportion has (although low) an ozone depletion potential. Furthermore, the additional components R125, R143a and R218 have a high global warming potential (GWP).

* When using blends containing R22 legal regulations are to be observed, R22 as transitional refrigerant.

Resulting design criteria / Converting existing R502 plants

The compressor and the components which are matched to R502 can remain in the system in most cases. The limitations in the application range must however be considered: Higher discharge gas temperature than R502 with R402B**, R403A** and R408A** or higher pressure levels with R402A** and R403B**.

The good solubility characteristics of R22 and R290 increase the risk that, after conversion of the plant, possible deposits of oil decomposition products containing chlorine are dissolved and find their way into the compressor and control devices. Systems where chemical stability was already insufficient with R502 operation (bad maintenance, low drier capacity, high thermal loading) are particularly at risk.

Thus, generously dimensioned suction gas filters and liquid line driers should be installed for cleaning before conversion, and an oil change should be made after approximately 100 hours operation. Further checks are recommended.

The operating conditions with R502 (including discharge gas temperature and suction gas superheat) should be noted so that a comparison can be made with the values after conversion. Depending upon the results, control devices should possibly be reset and other additional measures should be taken as required.

** Classification according to ASHRAE nomenclature.

Service blends as substitutes for R12 (R500)

Although (as experience already shows) R134a is also well suited for the conversion of existing R12 plants, the general use for such a "retrofit" procedure is not always possible. Not all compressors which have previously been installed are designed for the application with R134a. In addition a conversion to R134a requires the possibility to make an oil change, which is for example not the case with most hermetic type compressors.

Economical considerations also arise, especially with older plants where the effort of converting to R134a is relatively high. The chemical stability of such plants is also often insufficient and thus the chance of success is very questionable.

Therefore "Service blends" are also available for such plants as an alternative to R134a and are offered under the designations R401A/R401B, R409A. The main components are the HCFC refrigerants R22, R124 and/or R142b. Either HFC R152a or R600a (Isobutane) is used as the third component. Operation with traditional lubricants (preferably semi or full synthetic) is also possible due to the major proportion of HCFC.

A further service blend was offered under the designation R413A (ISCEON® 49 – DuPont), but replaced by R437A by the end of 2008. However, because of the development history of service blends, R413A will continue to be covered in this Report. The constituents of R413A consist of the chlorine free substances R134a, R218, and R600a. In spite of the high R134a content, the use of conventional lubricants is possible because of the relatively low polarity of R218 and the favourable solubility of R600a.

R437A is a blend of R125, R134a, R600 and R601 with similar performance and properties as R413A. This refrigerant also has zero ODP.

However, due to the limited miscibility of R413A and R437A with mineral and alkylbenzene oils, oil migration may result in systems with a high oil circulation rate and/or a large liquid volume in the receiver – for example if no oil separator is installed.

If insufficient oil return to the compressor is observed, the refrigerant manufacturer recommends replacing part of the original oil charge with ester oil. But from the compressor manufacturer's view, such a measure requires a very careful examination of the lubrication conditions. For example, if increased foam formation in the compressor crankcase is observed, a complete change to ester oil will be necessary. Moreover, under the influence of the highly polarized blend of ester oil and HFC, the admixture of or conversion to ester oil leads to increased dissolving of decomposition products and dirt in the pipework. Therefore, generously dimensioned suction clean-up filters must be provided. For further details, see the refrigerant manufacturer's "Guidelines".

* By using R22 containing blends the legal requirements are to be followed, R22 as transitional refrigerant.

Resulting design criteria / Converting existing R12 plants

Compressors and components can mostly remain in the system. However, when using R413A and R437A the suitability must be checked against HFC refrigerants. The actual "retrofit" measures are mainly restricted to changing the refrigerant (possibly oil) and a careful check of the superheat setting of the expansion valve.

A significant temperature glide is present due to the relatively large differences in the boiling points of the individual substances, which requires an exact knowledge of the saturation conditions (can be found from vapour tables of refrigerant manufacturer and in the BITZER Refrigerant App) in order to assess the effective suction gas superheat.

In addition the application range must also be observed. Different refrigerant types are required for high and low evaporating temperatures or distinct capacity differences must be considered. This is due to the steeper capacity characteristic, compared to R12.

Due to the partially high proportion of R22 especially with the low temperature blends, the discharge gas temperature with some refrigerants is significantly higher than with R12. The application limits of the compressor should therefore be checked before converting.

The remaining application criteria are similar to those for the substitute substances for R502 which have already been mentioned.