PROPERTIES OF CO2 AS A REFRIGERANT - Centro Studi Galileo

In all cases, not only with reference to an ideal simple cycle, the trend of the cycle
coefficient of performance as a function of the gas cooler pressure for p>p opt is rather flat.
Therefore a slight overpressure in the gas cooler with respect to the optimum, does not
penalise too much the cycle efficiency. On the other hand, a gas cooler pressure below the
optimum value can sometimes drastically penalise the cycle efficiency. A proper control of
the high-pressure level is indispensable to take best advantage of CO 2 refrigeration
technology.
A further important difference of CO 2 transcritical cycles, as compared to traditional
refrigerating cycles, is given by the much higher pressure levels (see data reported in Tab. 1),
at equivalent working conditions as far as temperatures of the external source and sink are
concerned.
It is no doubt that high pressure levels penalise the structural design of components of a
refrigerating circuit, and in particular of the compressor; nevertheless their reduced sizes
counterbalance this drawback. As compared to traditional refrigerants, the reduced volume
flow rate of CO 2 necessary to yield a certain refrigerating power is evidenced by the value of
the volumetric latent heat reported in Tab. 1. This is six times larger for CO 2 than for R-22,
even if one must also take into account the high vapour quality of CO 2 at the evaporator inlet,
as compared to that of R-22 in comparable situations, proper to the transcritical cycle.
The high pressure level with CO 2 also brings about a reduced penalty due to the fluid
pressure loss when designing the refrigerating circuit appropriately.
From what said above it can be concluded that equipment with CO 2 as the refrigerant, in
spite of the higher working pressure, is not necessarily heavier or bulkier or potentially more
dangerous as compared to equivalent equipment operated with the traditional working fluids.
This is due to the smaller flow cross sections of the components, consequent on the reduced
refrigerant volume flow rate.
Moreover, at equivalent working conditions, the compressor pressure ratio with CO 2 is
notably lower than with traditional refrigerants. Together with a higher pressure level, this
fact makes it possible to get higher compression isoentropic efficiencies, with advantages in
energy consumption. On the other hand, CO 2 compressors work with much higher differential
pressures than traditional refrigerating compressors, and this fact may enhance backflow
losses.
Another peculiarity of carbon dioxide is evident in the diagram of Fig. 1: the triple point
pressure (p t =5.18 bar) is above that of the natural environment. At atmospheric pressure the
transition from solid to gas (sublimation process) takes place at temperature t t =-78.9 °C, and
the cooling effect is exploited in the so-called dry ice. In mechanical refrigeration this means
that, in the event of collapse of a circuit, residual CO 2 solidifies in the plant at around –79 °C,
sublimating successively into the atmosphere.
3. THE CO 2 TRANSCRITICAL REFRIGERATING CYCLE
Energy performance of a CO 2 transcritical cycle is now discussed and compared with that
of a traditional refrigerating cycle operated with conventional working fluids. To make the
discussion concrete and to be able to argue about definite numerical values, two refrigerating
cycles are directly analysed: a transcritical one operating with CO 2 , and a traditional one
operated with R-22; this last refrigerant is in fact often taken as a bench-mark. The operative
conditions for the two cycles, illustrated in Tab. 2, are fixed in such a way that they can be
considered approximately equivalent with respect to external constraints.
The situation considered refers to an outside air temperature t a =28 °C, with an air
temperature increase in the R-22 condenser equal to 10 °C, and condensation of the
refrigerant 5 °C above the condenser air exit temperature. For the CO 2 transcritical cycle, a 3