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booster systems, the possibility of combining comfort cooling and providing refrigeration for food was mentioned. Such a system integration may contribute to reduce the overall power consumption and also the overall cost of the supermarket energy system. Mechanical subcooling Mechanical subcooling is the process of cooling the refrigerant liquid out of the condenser or flash tank in order to increase the realised cooling capacity in the evaporator coil and improve the performance of the expansion device at the inlet to the evaporator. This is a commonly used method of improving system efficiency and is often overlooked, especially the part about the expansion device performance improvement, and can be applied to all refrigerants, but is often used with transcritical R-744 systems. Ejector An ejector can recover some of the energy released during expansion instead of only dissipating it as is the case when using a throttling valve. The most common use is to compress gas (and liquid) exiting from the evaporator. Two important effects are then: that the evaporators can be operated flooded, without superheat, thus increasing the evaporation temperature, and secondly: that the pressure at the inlet of the compressor can be increased, reducing the power input for the compression due to lower pressure lift. System efficiency may often be increased by 20% (Hafner, 2014b). This is a concept also being implemented for R-744 to achieve high efficiency for systems used in high ambient temperature conditions (Gullo, 2018). Vapour injection This is another method to lower the temperature of the liquid out of the condenser, but instead of using an external heat sink, the refrigerant itself is used to lower the liquid temperature. This method of efficiency improvement in systems has become quite common in developed countries and the flash tank described in the booster system is one example of this technology. Heat exchangers are also commonly used and the vapour from the flash tank or the heat exchanger is compressed from an intermediate pressure to discharge by injecting the vapour into the sytem’s compressor (common in scroll and screw compressors). Parallel compression Parallel compression is a method to reduce the power consumed in taking the vapour from the flash tank (described above) and compressing this to the higher pressure in the condenser, and is discussed in section 4.3.3. This method of improving efficiency is most commonly used with R- 744 transcritical systems. Suction line heat exchanger A suction line heat exchanger may improve the system efficiency for certain refrigerants if the benefit of subcooling exceeds the extra compression power resulting from superheat of the suction gas. Heat is removed from the warm liquid before entering the expansion device by the cool vapour leaving evaporator coil through a simple heat exchanger. This has the dual effect of improving efficiency and preventing liquid from entering the compressor which can help improve reliability as well. 4.4 Options for existing equipment This section covers the retrofit options for the installed base of equipment. Many of the new lower GWP refrigerant options are higher pressure, higher flammability and as such, cannot be 90 2018 TOC Refrigeration, A/C and Heat Pumps Assessment ReportPDF Image | Heat Pumps Technical Options
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