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Transcritical Co2 units: Rising efficiency thanks to parallel compression, mechanical subcooling and ejector technologies 

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Experimental tests were carried out on a multi-configuration carbon dioxide refrigeration unit working at medium temperature. Four different configurations were tested:
1. Standard configuration (STD): this configuration includes two single-stage semi-hermetic reciprocating compressors, a round tube finned coil gas cooler equipped with two fans, an high pressure backpressure valve and a flash gas bypass valve, an evaporator plate heat exchanger. This refrigeration unit is also equipped with an oil separator, an intermediate pressure liquid receiver, a low pressure vessel and the pack controller.
2. Parallel compression configuration (PRL): in addition to the standard unit the parallel compression configuration carries out one single-stage semi-hermetic reciprocating compressors and a plate internal heat exchanger;
3. Mechanical subcooling configuration (SBCL): this lay-out includes a subcooler plate heat exchanger and a R290 refrigeration unit. The propane unit carries out one single-stage semi-hermetic reciprocating compressor, a round tube finned coil condenser equipped with one fan;
4. Ejector vapour pre-compression configuration (EJC): in addition to the parallel compression configuration this unit carries out one Danfoss multi-ejector system equipped with the based ejector controller. During experimental tests the multi-ejector system worked in parallel with the Danfoss high pressure back-pressure valve.  
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The tests were carried out in compliance with the EN 14511:2013 “Air conditioners, liquid chilling packages and heat pumps with electrically driven compressors for space heating and cooling. Both parallel compression and mechanical subcooling are particularly suited to southern Europe weather conditions. Energy efficiency improvements range from +12.9 to +18.9% between 32 and 38°C, available swept-volume increments with the same main compressors total displacement is equal ranges from 52.5% to 54.6% at 38°C and from 17.8%to 25.8 at 32°C. The ejector system over perform the standard and parallel system by +24.7 and +10.7%, in terms of energy efficiency and by 63.2 and 18.9% in terms of available low-pressure side swept-volume at 38°C air temperature conditions. temperature conditions.
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Source: www.scmfrigo.com