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Service blends The operating conditions with R502 (includ- ing 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. 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 com- pressors. Economical considerations also arise, espe- cially 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 avail- able for such plants as an alternative to R134a and are offered under the designations R401A/R401B, R409A. The main compo- nents 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 (prefer- ably semi or full synthetic) is also possible due to the major proportion of HCFC. 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 re- stricted 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 super- heat. In addition the application range must also be observed. Different refrigerant types are required for high and low evaporating tem- peratures 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 compres- sor should therefore be checked before converting. The remaining application criteria are simi- lar to those for the substitute substances for R502 which have already been mentioned. * By using R22 containing blends the legal require- ments are to be followed, see chapter R22 as tran- sitional refrigerant, page 8. 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 develop- ment history of service blends, R413A will continue to be covered in this Report. The constituents of R413A consist of the chlor- ine free substances R134a, R218, and R600a. In spite of the high R134a content, the use of conventional lubricants is pos- sible 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 is also chlorine-free (ODP = 0). However, due to the limited miscibility of R413A and R437A with mineral and alkyl- benzene 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 com- pressor manufacturer's view, such a meas- ure 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". 16PDF Image | REFRIGERANT REPORT 21
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