Environmental Engineering Reference
In-Depth Information
Critical point
Discharge
pressure
Condensation
Suction
pressure
Evaporation
Enthalpy
Figure 8.4 Refrigeration cycle for R-134a, a typical subcritical refrigeration cycle. The cycle takes place
well below the critical point of the refrigerant.
a “cooler” medium than just ambient air. The common practice is to use another refrigeration
unit running, for instance R-404A or R-717, which provides temperatures low enough to
transform the compressed carbon-dioxide gas into liquid. This type of arrangement is called a
“cascade system” (Fig. 8.5). For this particular system, liquid-carbon dioxide is pumped from
the receiver to the evaporator where it partially evaporates before returning to the receiver.
Carbon-dioxide-gas is then taken from the upper part of the receiver, compressed, and sent to
the heat exchanger/evaporator where the carbon dioxide is condensed into liquid. The heat
exchanger acts as a condenser for the carbon dioxide loop and as an evaporator for the high
temperature loop.
So, what is the advantage of using a cascade system instead of traditional refrigeration
units? A cascade system with carbon dioxide as the working fluid on the low temperature loop
reduces the chances of furtive emissions of hydrofluorocarbons. This is especially important
in distributed refrigeration systems as in supermarkets. A typical supermarket uses between
1500 and 2500 kg of hydrofluorocarbon refrigerant that is circulated between the compressor/
condenser located outside the building (normally on the rooftop) and the chilled/frozen food
displays (Sand et al., 1997). Long pipes that contain many joints connect the inside and out-
side units thus creating many opportunity for leaks. According to historical records approxi-
mately 30 percent of the total refrigerant load in supermarkets is lost to leaks every year
(DelVentura et al., 2007). Therefore, by replacing hydrofluorocarbons with carbon dioxide in
the big loop and then having a compact-tight unit with short pipes and few joints running a
hydrofluorocarbon minimizes the chances of leaks.
An alternative to a cascade system is the secondary loop. In this case, a direct expansion
chiller is used to cool a heat transfer fluid that distributes the “cold” throughout all refrig-
eration equipment. This is called a secondary loop system, which uses liquids such as pro-
pylene glycol or other heat transfer fluids. It is estimated that the leaks of hydrofluorocarbons
are reduced from 30 to just 5 percent when using secondary loops (DelVentura et al., 2007).
The disadvantage however, is that nontoxic nonflammable heat transfer fluids are expensive
to implement and operate due to higher viscosity and consequent higher pumping energy,
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