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required additional cooling or two compression stages for freezer applications. In HCFC-22 systems, one would often see copper plating on the bearings if the system had increased air and water levels in the circuit, causing the formation of acids. Saturated HFCs: saturated hydrofluorocarbons include fluids such as HFC-134a, HFC-143a and HFC-125 and blends of fluids, mixed to provide specific advantages for particular applications. HFC-134a is used in small high temperature systems; it is at atmospheric pressure at -26oC, and it requires larger compressors than R-717. Sub-atmospheric operation is less common with HFCs because traces of moisture are liable to freeze and block the expansion valve. Also HFC refrigerants form acids in the presence of water and air in the system. Here the dominating acid is HF. HFC-134a is also widely used in centrifugal compressors, including some very large systems used for district heating. A trial system using HFO-1234ze(E) as an alternative to HFC-134a in district heating has been tested in Norway (Nørstebø, 2013). There is no single fluid alternative to HCFC-22 for use in industrial systems. HFC-125 has approximately the right pressure temperature relationship, but has an extremely low critical temperature of 66oC, and would therefore be extremely inefficient if used in industrial systems, unless the condensing temperature would be very low. It is used as a component of several of the most popular blended refrigerants, where the deficiencies in its physical properties can be offset by careful selection of the other components of the blend. The most common blends used in the industrial sector are R-404A and R-507A, which are primarily mixtures of HFC-125 and HFC- 143a; with the latter providing a higher critical temperature and hence improved efficiency. Many industrial systems use flooded evaporators, where the refrigerant boils in a pool. Zeotropic blends (with a temperature glide during evaporation) are not suitable in these systems because the blend components may fractionate, so R-407C and service replacement blends such as R-417A have not been much used in the industrial sector. It is surprising that R-410A has not been more widely used in industrial systems because it has a low boiling point at atmospheric pressure (-51.4oC), very low glide (less than 0.2K at -40oC) and the critical temperature is almost the same as R-404A. The compressor swept volume required for R-410A is about 30% less than for R-404A, so equipment costs, including installed pipework should be less, although operating pressures are higher. This can be because of the high pressure, which has limited the use of R-410A and not the price of the refrigerant. The main barrier to its use is probably the high price of the refrigerant, particularly compared to R-717 and R-744. When the refrigerant inventory in a system is in tonnes the cost of the charge may be a significant part of the total cost of the installation. Typical installations are therefore low capacity, low temperature, for example blood freezing and small pharmaceutical systems. The introduction of rules and guidance for the use of “lower flammability” refrigerants (those that have a burning velocity less than 10 cm s-1 and therefore do not explode) might result in an increase in the use of HFC-32 as an alternative to R-717 in industrial systems. Unsaturated HFCs, HFOs: unsaturated hydrofluorocarbons such as HFO-1234yf and HFO- 1234ze(E) have not to date been used in industrial systems. The low global warming potential suggests that they may be a suitable alternative to R-717 and R-744, but it is very likely that they will be even more expensive than R-410A, with the further disadvantage of being flammable. It is therefore likely that none of this family of chemicals will achieve any significant market penetration in the industrial sector, even if blended with other compounds to reduce price or flammability. An exception may be found with centrifugal compressors for industrial chillers and heat pumps where unsaturated HFCs, i.e., HFO-1234yf or HFO-1234ze(E), might provide an alternative to HFC-134a (Nørstebø, 2013). HCFO-1233zd(E) might be a better solution but is seeing opposition to application in some countries due to its chlorine content. 2018 TOC Refrigeration, A/C and Heat Pumps Assessment Report 109PDF Image | Heat Pumps Technical Options
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