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consideration should be given to the age of the plant, the cost of replacement with a modern, more efficient system and the risks to continued operation of retrofit. A good approach is to calculate the systems as one would do with the new refrigerants and then see where the main differences are between the old system and the imaginary new plant. From this point one can then properly see what compromises are taken if one would just take out one oil-refrigerant pair and replace it with another. An overseen problem in many retrofit projects is the O-rings and other sealings that absorb the working pair. When one would change just one of the pairs one would also change the composition of the pair in the O-rings and sealings. If they have been in place for an long time, they will have lost their flexibility and one would need to replace them when retrofitting the system, otherwise one would end up with a leaking system within a few days. When calculating pipe sizes one would most likely find the biggest differences in the liquid lines which then will need upsizing or additional pipes in parallel with the old piping. One also has to be aware of chemical reactions if there are residues left in the system. This is most notable when converting systems from HCFC-22 or any HFC system to R-717. Reactions can occur between the refrigerants where a reaction between RHCFC-22 and R-717 will form prussic acid. If the oils are not compatible they can react to form different polymers that will cause them to form non-desirable substances e.g. one charges POE oil into a R-717 system they will form polymers comparable to wine-gums. 5.4.1 Conversion to HFC blends There are numerous blends for the replacement of HCFC-22 in DX (superheat controlled) systems, but there is none that replicates the pressure temperature relationship of HCFC-22 without significant glide, and so these blends are much less common in flooded systems where fractionation of the blend is a major problem. The fractioning of the blend can cause the system to use more energy and in more severe cases it can fail or blow off refrigerant though the safety valves. Where industrial systems are converted to a blend it may also be necessary to change from mineral or alkyl benzene lubricant to a synthetic polyolester oil (POE oil). Some blends are formulated with hydrocarbons in the mix so that, although still non-flammable, the lubricant is more miscible and less likely to accumulate in the evaporator of the system. For a large flooded system it might be appropriate to convert the high pressure side e.g. compressors and condensers to an HFC blend, but convert the low pressure side to a secondary fluid e.g. propylene glycol or even to R-744, as a secondary volatile secondary loop. This can in many cases reduce the high stage part of the system by 90%. This type of system conversion is mainly done when the system is relatively new. Retrofitting of HCFC-22 plants in Article 5 countries is very uncommon but it is a viable solution. 5.4.2 Conversion to R-744 The high operating pressure of R-744 systems makes it highly unlikely that an existing HCFC-22 system could be converted to operate on R-744. Conversion to a cascade system is possible, greatly reducing the inventory of fluorocarbon refrigerant in the system. It may even be possible to reuse the low pressure pipework and evaporators in the system if they are suitably rated and documented. A cold storage or freezing system operating as a cascade on R-744 could be limited to an allowable pressure of 25 bar gauge, however this is a complex retrofit and it may well be more economic to replace the whole plant, especially if it is already more than ten years old. 112 2018 TOC Refrigeration, A/C and Heat Pumps Assessment ReportPDF Image | Heat Pumps Technical Options
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