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Another consideration is related to the possible impact on the required refrigerant charge, where higher ambient temperatures can imply greater heat loads, larger system capacity and thus larger refrigerant charge. Therefore, where limits on refrigerant charge apply, those limits may be approached at smaller capacities; in these cases, additional (safety) measures may need to be applied to the equipment as well as a more thorough focus on system charge optimisation; this is being perceived as a consideration seldom applied by many system designers today. Currently, the most widely applied replacements for HCFC-22 in most air conditioning applications are HFC blends, primarily R-410A and R-407C. HCs are also being used in some low refrigerant-charge applications. High-efficiency HFC-32 invertor-driven split wall units are already commercialised since 2015. More than 14,000 units have already been placed on the market. R-410A and R-407C both have lower critical temperatures than HCFC-22 (refer to Chapter 2 for values) because HFC-125 (a component of both R-407C and R-410A) has a comparatively low critical temperature; this is an important parameter since such refrigerants will exhibit a steeper decline in capacity with increased ambient (outdoor) temperatures than refrigerants having higher critical temperatures. This steeper decline in capacity is of particular importance in geographic regions, which have condensing design temperatures approaching the critical temperature of the refrigerant. As well as the use of high efficiency components, the optimum selection of compressor, airflow, condenser design (i.e., tube diameter, fin design, coil circuitry, etc.) and expansion device can reduce the performance losses at high ambient temperatures (Bitzer, 2012). Thus, for most refrigerants, by taking measures to appropriately optimise the system, similar efficiencies to HCFC-22 can be achieved even at higher ambient temperatures. Systems using low-GWP refrigerants are not currently available for large capacity systems in most regions with high ambient temperatures. In HAT countries, special consideration must be taken when designing or selecting components for the air to air systems, including heat pumps. The following considerations may be taken during the design stage: a. The condensing temperature must be reduced in order for the refrigeration cycle not to reach or be close to the refrigerant critical temperature. The low critical temperature for R-410A, requires the system condensing temperature to be lower than 70°C; consequently, special larger or more effective condensers are needed; b. The discharge temperature is also an important factor to be considered when designing for HAT conditions. For air-to-air systems this can be controlled by using electronic type expansion valves and liquid injection systems; c. Two-stage compressors can be used; however, there are obstacles for using them including cost and availability as two-stage compressors are currently available from a limited number of manufacturers. Most of the compressor manufacturers have chosen the invertor technology instead. 13.5.1 Refrigerant charge amount With the use of larger condensers (fin-and-tube type) for HAT conditions to reduce the condensing temperatures, the refrigerant charge amount becomes an issue as it will be increased in this type of heat exchangers. There are technologies that are commercially available, including smaller tube diameters and microchannel condenser coils, which reduce the size of the internal condenser volume while also providing higher efficiency. The need for changing the design and the technology of the air-cooled condensers to meet HAT condition efficiencies requires a major change to the production lines for manufacturing the condenser coils adding a major component to the cost of line conversion. 260 2018 TOC Refrigeration, A/C and Heat Pumps Assessment ReportPDF Image | Heat Pumps Technical Options
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