Abstract
Unlike other refrigerants, R744 (or carbon dioxide) has a very low critical temperature (30.98 C) which
hampers working in subcritical conditions especially when the environment is used as a heat rejection
sink. Accordingly, CO2 refrigeration plants are designed to work in supercritical conditions. Notwithstanding,
the heat rejection temperature may decrease below the critical one during long periods of time
(for example in winter or autumn), thereby offering the possibility of operating in subcritical conditions.
According with this and in order to give useful information for related system design and operation,
the aim of this paper is to analyse the behaviour of a CO2 refrigeration plant designed for supercritical
conditions when it works in subcritical conditions. To achieve this objective a series of experimental
assays has been performed. The results obtained shown a decrease in the coefficient of performance
(COP) and cooling capacity ðQ_ evÞ if the heat exchanger designed as a gas-cooler operates as a condenser
in some cases. To avoid this, a possible solution is proposed consists on using an inverter drive to reduce
the refrigerant mass flow rate in order to enhance the gas-cooler/condenser thermal effectiveness.
From the experimental results obtained, it could be remarked that if the heat exchanger is designed as
a condenser and operates as a gas-cooler, the energy performance of the refrigeration plant could
improve under certain subcritical and supercritical conditions.
