ENERGETICAL ANALYSIS OF A REFRIGERATION CYCLE COMBINED WITH CO2, PROPANE AND BRINE

Resumo

The destruction of the ozone layer was mainly caused by chlorofluorocarbons (CFCs) used
as refrigerants, which were widely utilized during the twentieth century. Another problem produced by
the use of these substances was, and still is, the Global Warming, since these fluids have high "Global
Warming Potential" (GWP). Due to these consequences, this category of refrigerant was banned and
gradually replaced by natural refrigerants such as carbon dioxide (CO2 – R744) and propane (C3H8 –
R290). In this work a cascade refrigeration system was analyzed according to the First Law of
Thermodynamics (energy analysis). The cascade cycle consists of three subsystems: a vapor
compression cycle using R744 as a working fluid, a system operating with 40% ethylene glycol and a
vapor compression cycle which uses R290 as a refrigerant. The propane cycle receives heat that has
been absorbed in cycles of R744 and 40% ethylene glycol and rejects it into the atmosphere in the
condenser of the R290 cycle. For the cycle analysis, the system was subdivided into 18 control volumes
and 23 state points. Equations of mass and energy balances were applied, along with complementary
equations, which they were used to complete the system of equations. The solution of the system of
equations was obtained by applying Newton-Raphson method. The model was implemented in EES®
and simulated under different operating conditions. Finally, the thermal efficiency of the R744 cycle
was 8.784; for the R290 cycle, it was 4.09 and 3.393 was the result of the combined cycle.