The objective of this work is to define the main geometrical characteristics of a hermetic reciprocating compressor working with CO{sub}2 under trans-critical conditions. The working conditions considered are: a refrigerating capacity of 800 W, an evaporation temperature of 7.2°C and a super-heating temperature of 35°C, with a gas cooler temperature of 46°C. Firstly, a brief presentation of the refrigerating cycle based on pure thermodynamic analysis is presented, with the idea to analyse the differences between a conventional sub-critical R134a cycle and a trans-critical CO{sub}2 cycle. After the thermodynamic analysis, a numerical comparative study of both compressor models is presented for the same conditions. The present paper finishes with a CO{sub}2 prototype compressor description, experimentally tested in a companion paper.
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