Bitzer refrigerant report: Part 2

Bitzer refrigerant report: Part 3

Published with permission of Bitzer

Stratospheric ozone depletion as well as atmospheric greenhouse effect due to refrigerant emissions have led to drastic changes in refrigeration and air conditioning technologies since the beginning of the 1990s. For reference to earlier information, please see Part 1 and 2 published in the previous issues of Cold Link Africa.

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Substitutes for R22/R502 in refrigeration systems: R407A/407B/407F/407H

As an alternative to the earlier described substitutes, additional mixtures have been developed based on R32 which is chlorine free (ODP = 0) and flammable like R143a. The refrigerant R32 is also of the HFC type and initially was regarded as a main candidate for R22 alternatives.

However, due to extent of blend variations comparable thermodynamic characteristics to R404A/R507A can also be obtained. These kinds of refrigerant were marketed at first under the trade name KLEA® 60/61 (ICI) and are listed as R407A/R407B* in the ASHRAE nomenclature.

Honeywell has developed another blend with the trade name Performax® LT (R407F according to ASHRAE nomenclature) and introduced it into the market, similar Daikin Chemical with R407H. For both blends, the R32 proportion is higher than for R407A, while the R125 proportion is lower. With R407H, this results in certain restrictions for low temperature applications.

However, the necessary conditions for alternatives containing R32 are not quite as favourable compared to the R143a based substitutes discussed earlier. The boiling point of R32 is very low at -52°C, in addition the isentropic compression exponent is even higher than with R22. Rather high proportions of R125 and R134a are necessary to match the characteristics at the level of R404A and R507A.

The flammability of R32 is thus effectively suppressed, but the large differences in boiling points with a high proportion of R134a lead to a larger temperature glide. The main advantage of R32 is the extraordinarily low global warming potential (GWP = 675), so that even in combination with R125 and R134a it is significantly lower than with the R143a based alternatives mentioned above (R407A: GWP = 2107, R407F: GWP = 1825, R407H: GWP = 1490).

Thus, they also comply with the requirement of the new EU F-Gas Regulation which from 2020 will only allow refrigerants of GWP < 2500. Measurements made with R32 containing blends do show certain capacity reductions compared to R404A and R507A, with low evaporating temperatures.

The COP however shows less deviation and is even higher in medium temperature applications (Figure 8).

Whether these favourable conditions are confirmed in real applications is subject to the system design. An important factor is the significant temperature glide, which can have a negative influence upon the capacity/temperature difference of the evaporator and condenser.

With regard to the material compatibility, R32 blends can be assessed similarly to R404A and R507A; the same applies to the lubricants. Despite the relatively high proportion of R125 and R134a in the R32 blends, the discharge gas temperature is higher than with the R143a based alternatives (especially for R407F and even to a higher degree with R407H).

This results in certain limitations in the application range as well as the requirement for additional cooling of compressors when operating at high pressure ratios. 2-stage compressors can be applied very efficiently where especially large lift conditions are found. An important advantage in this case is the use of a liquid sub-cooler.

The experience with R404A/R507A and R22 can be used for plant technology in many respects, although the temperature glide as well as the difference in the thermodynamic properties have to be considered. This especially concerns the design and construction of heat exchangers and expansion valves.

Converting existing R22 plants to R407A/407B/407F/407H

Practical experiences show that qualified conversions are possible. Compared to R22 the volumetric refrigeration capacity is nearly similar while the refrigerant mass flow is only slightly higher. These are relatively favourable conditions for the conversion of medium and low temperature R22 systems. The main components can remain in the system provided that they are compatible with HFC refrigerants and ester oils.

However, special requirements placed on the heat exchanger with regard to the significant temperature glide must be considered. A conversion to ester oil is also necessary, which leads to increased dissolving of decomposition products and dirt in the pipework. Therefore, generously dimensioned suction clean-up filters must be provided.

Conversion of existing R404A/R507A systems to R407A/407B/407F/407H

Larger differences in thermodynamic properties (e.g. mass flow, discharge gas temperature) and the temperature glide of R407A/F/H may require the replacement of control components and if necessary retrofitting of additional compressor cooling when existing systems are converted. For newly built systems, a specific design of components and system is necessary.

R422A as substitute for R22 and R502 Amongst other aims, R422A (ISCEON® MO79 – Chemours) was developed in order to obtain a chlorine-free refrigerant (ODP = 0) for the simple conversion of existing medium and low temperature refrigeration systems using R22 and R502. For this, it was necessary to formulate a refrigerant with comparable performance and energy efficiency to that of R404A, R507A, and R22, which also permits the use of conventional lubricants.

R422A is a zeotropic blend of the basic components R125 and R134a with a small addition of R600a. Due to its relatively high R134a percentage, the temperature glide lies higher than for R404A, but lower than other refrigerants with the same component blends – such as R417A and R422D. The adiabatic exponent, and therefore also the discharge gas and oil temperatures of the compressor, are lower than for R404A and R507A.

At extremely low temperatures, this can be advantageous. However, in cases of low pressure ratio and suction gas superheat, this can be a disadvantage due to increased refrigerant solution if ester oil is used. The material compatibility is comparable to the blends mentioned previously, the same applies to the lubricants. On account of the good solubility of R600a, conventional lubricants can also be used under favourable circumstances. In particular, advantages result during the conversion of existing R22 and R502 systems as mentioned above. However, for plants with high oil circulation rates and/or large liquid charge in the receiver, oil migration might occur – 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.

Under the influence of the highly polarised 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’.

From a thermodynamic point of view, a heat exchanger between suction and liquid line is recommended, improving the refrigerating capacity and coefficient of performance. Besides this the resulting increase in operating temperatures leads to more favourable lubricating conditions (lower solubility). Due to the high global warming potential (GWP ≥ 2500), R422A will no longer be allowed for new installations in the EU from 2020 onwards. The requirements and restrictions are specified in the F-Gas Regulation 517/2014.

 * Meanwhile, R407B is no longer available in the market. Due to the historical development of HFC blends this refrigerant will, however, still be considered in this Report.

 

** General proposal for screw compressors and liquid chillers when used with DX evaporators with internally structured heat exchanger tubes. Furthermore, an individual check regarding possible additional measures will be necessary. 

 

To be continued in the next issue of Cold Link Africa

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