Page 445 - Mechanical Engineers' Handbook (Volume 4)

434 Refrigeration

class I, including CFCs, halons, and other major ozone-depleting chemicals; and class II,
HCFCs.
Two ratings are used to classify the harmful effects of a refrigerant on the environment. 15
The first, the ozone depletion potential (ODP), quantifies the potential damage that the re-
frigerant molecule has in destroying ozone in the stratosphere. When a CFC molecule is
struck by ultraviolet light in the stratosphere, a chlorine atom breaks off and reacts with
ozone to form oxygen and a chlorine/oxygen molecule. This molecule can then react with
a free oxygen atom to form an oxygen molecule and a free chlorine. The chlorine can then
react with another ozone molecule to repeat the process. The estimated atmospheric life of
a given CFC or HCFC is an important factor in determining the value of the ODP. The ODP
for CFC-11 is 1.0. All other ODP values for substances are normalized to that of CFC-11.
The second rating is known as the global warming potential (GWP), which represents
how much a given mass of a chemical contributes to global warming over a given time
period compared to the same mass of carbon dioxide. 16 Carbon dioxide’s GWP is defined
as 1.0. The GWP of all other substances is normalized to that of carbon dioxide. Refrigerants
such as CFCs, HCFCs, and HFCs can block energy from the earth from radiating back into
space. One molecule of R-12 can absorb as much energy as 10,000 molecules of CO .
2
Table 4 shows the ODP and GWP for a variety of refrigerants. As a class of refrigerants,
the CFCs have the highest ODP and GWP. Because HCFCs tend to be more unstable com-
pounds and, therefore, have much shorter atmospheric lifetimes, their ODP and GWP values
are much smaller than those of the CFCs. All HFCs and their mixtures have zero ODP
because fluorine does not react with ozone. However, some of the HFCs, such as R-125, R-
134a, and R-143a, do have GWP values that are as large or larger than some of the HCFCs.
From the standpoint of ozone depletion and global warming, hydrocarbons provide zero
ODP and GWP. However, hydrocarbons are flammable, which makes them unsuitable in
many applications.

4.2 Refrigerant Selection for the Closed Cycle
In any closed cycle, the choice of the operating fluid is based on the refrigerant with prop-
erties best suited to the operating conditions. The choice depends on a variety of factors,
some of which may not be directly related to the refrigerant’s ability to remove heat. For
example, flammability, toxicity, density, viscosity, availability, and similar characteristics are
often deciding factors. The suitability of a refrigerant also depends on factors such as the
kind of compressor to be used (i.e., centrifugal, rotary, or reciprocating), safety in application,
heat-exchanger design, application of codes, size of the job, and temperature ranges. The
factors below should be taken into account when selecting a refrigerant.
Discharge (condensing) pressure should be low enough to suit the design pressure of
commercially available pressure vessels, compressor casings, etc. However, discharge pres-
sure, that is, condenser liquid pressure, should be high enough to feed liquid refrigerant to
all the parts of the system that require it.
Suction (evaporating) pressure should be above approximately 3.45 kPa (0.5 psia) for
a practical compressor selection. When possible, it is preferable to have the suction pressure
above atmospheric to prevent leakage of air and moisture into the system. Positive pressure
normally is considered a necessity when dealing with hydrocarbons, because of the explosion
hazard presented by any air leakage into the system.
Standby pressure (saturation at ambient temperature) should be low enough to suit
equipment design pressure, unless there are other provisions in the system for handling the
refrigerant during shutdown—for example, inclusion of expansion tanks.