ACR Journal

CMYK / .ai CMYK / .ai CMYK / .ai 15 REFRIGERANTS acrjournal.uk flammable HFC refrigerants, they are still significantly higher than the highly flammable A3 refrigerants (Figure 2). As shown in figure 2, in this type of application, relatively large maximum charge sizes up to a charge size of 84kg can be used. This opens up the opportunity to consider A2L refrigerants for many applications where a remote condensing unit is used, and the compliant charge sizes for A3 refrigerants are insu cient. There are three options with a GWP below 300 that are good candidates to replace R404A, namely R454A (238 GWP), R454C (148 GWP) and R-455A (148 GWP). Theoretical cycle calculations for all these options compared to R404A, giving the relative capacities and coe cient of performance (COPs), are shown in figures 3 and 4. Another low GWP option that might be considered is, of course, carbon dioxide (R744). R744 has the advantage of being safety class A1 but still has charge limitations (figure 1) and typically has low COP. Applying theoretical cycle calculations, with compressor e ciencies based on OEM data, we can consider a hypothetical application that delivers 10 kW MT and 4kW LT refrigeration at a 32°C ambient design condition, a R744 booster system is compared to two individual refrigeration systems (MT & LT) using R454C at the design condition (figure 5) and a more typical operating condition of 15°C ambient temperature (figure 6). As can be seen, the di erences in COP and therefore energy consumption are not trivial, leading to higher operating costs and higher total system emissions (which include emissions from power generation) when using R744 compared to systems operating with the low GWP A2L options. Therefore, selecting a technology with high total emissions completely undermines the whole point of using a low GWP option. This example highlights the importance of considering all the sources of emissions before selecting a low emissions technology. DT for transcritical =2K, DT for subcritical =10K, evaporator superheat =5K, suction line superheat= 8K, compressor isentropic efficiencies - R-744 MT=68%, LT=48% - R-454A/R454C/R-455A MT=68%, LT= 67%, R-454A/R454C/R-455A liquid subcooling = 2K. Figure3–CapacitycomparisonvsR-404AatMTandLTconditions. Figure 4 – COP comparison vs R-404A at MT and LT conditions.