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HVAC Fundamentals: Refrigeration
were recharged rather than repaired. CFCs were used for
cleaning and insulation manufacturing. These stable
compounds diffused into the atmosphere where conditions
allow them to break down. The released chlorine
then reacts with and begins to deplete the atmosphere’s
protective ozone layer. HCFCs also contribute somewhat
to ozone depletion, but they are less stable and
tend to break down before reaching the stratosphere.
The Montreal Protocol, originally signed in 1987 and
continuously amended, is an international agreement to
phase out the production of CFCs by 1996 and essentially
phase out HCFCs by 2020 (UNEP 2003).
Currently a variety of hydrofluorocarbons (HFCs)
and mixtures of HCFCs and hydrocarbons are being
used as replacement refrigerants. There are advantages
and disadvantages to all options that cannot be adequately
addressed in this document. References are
available for additional information (ASHRAE 2005,
chapters 19, 20; ASHRAE 2002, chapter 5; McLindon
et al. 2000). However, some general characteristics are
listed in the following sections.
Refrigerant Numbering System
• For methane, ethane, and propane derivatives, the
refrigerant number is related to the number of
atoms in the compound structure by
R- [carbon atoms −1][hydrogen atoms +1]
[fluorine atoms] .
For example,
chlorodifluoromethane (CHClF 2 ) is
R- [1−1][1+1][2] = R-022 or R-22.
Propane (C 3 H 8 ) is R-[3−1][8+1][0] = R-290.
1,1,1,2-tetrafluoroethane (CH 2 FCF 3 ) =
R-[2−1][2+1][4]a = R134a with the “a” used
to designate the molecule arrangement in the
chain.
• Zeotropic blends of methane, ethane, and propane
derivatives: R-4xx
• Azeotropic blends of methane, ethane, and propane
derivatives: R-5xx
• Miscellaneous organic compounds: R-6xx
• Inorganic compounds: R-7xx
Thermophysical Properties
Higher theoretical vapor compression cycle efficiency
is achieved by fluids with low vapor heat capacity.
This is characteristic of fluids with simple molecular
structure and low molecular weight.
High thermal conductivity is desirable to minimize
heat exchanger size and low viscosity is desirable to
minimize friction losses.
Many alternative refrigerants (i.e., R-410, a potential
R-22 replacement) operate at higher pressure, which
provides higher capacity but greater power consumption.
This typically results in high compression ratios,
greater loads on the compressor, shortened compressor
life, and higher required rating pressure for heat
exchangers and related components (Hughes 2003).
There are a variety of potential replacements for R-22
applications that demonstrate a variety of benefits and
limitations.
Azeotropic refrigerant mixtures exhibit characteristics
similar to pure substances in that they evaporate and
condense at a constant temperature when pressure is
held constant. However, zeotropic mixtures do not
exhibit constant temperature during evaporation and
condensation. This undesirable trait (glide) results in a
reduction in efficiency unless heat exchangers are
enlarged to compensate. A more practical problem
results from the fact that they also tend to have different
component fractions in the gas phase at various temperatures.
When leaks occur, the individual component
losses are not in proportion to the original refrigerant
components. Recharging is complicated because the
makeup of the remaining refrigerant is not easily determined.
Safety and Environmental Issues
ASHRAE Standard 34 (ASHRAE 2004b) classifies
refrigerants into six safety groups (A1, A2, A3, B1, B2,
B3). Letter designations refer to toxicity, with class A
being no identifiable toxicity and B being toxic. Numerical
values indicate flammability, with 1 being no flame
propagation in standard air, 2 having a low, lower flammability
limit (LFL) and low heat of combustion (<8174
Btu/lb), and 3 having high LFL or a high heat of combustion.
Three factors are used to express the environmental
impact of refrigerants. The ozone depletion potential
(ODP) is a measure of a refrigerant’s ability to destroy
stratospheric ozone relative to R-11 (CCl3F). Other
CFCs are less than 1.0 (ODP for R-12 is 0.82), HCFCs
are much lower (ODP for R-22 = 0.034), and HFCs have
0 ODP. The second measure is global warming potential
(GWP), which is a measure of the material’s ability to
trap radiant energy (greenhouse effect) relative to carbon
dioxide. The third measure is also connected to global
warming in that refrigerant thermal efficiency
impacts the amount of energy and associated generation
pollution and greenhouse gas emission required to drive
vapor compression appliances. GWP and refrigerant
efficiency are combined into a total equivalent warming
impact (TEWI).
Material Compatibility
Refrigerants can react with tubing materials (i.e.,
ammonia with copper), sealant materials, and insulating
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