A Primer For Understanding The Chemistry Of Refrigerant Selection
By James R. Parsnow, Director, Environmental Systems Marketing, Carrier Corporation.
The current migration away from chlorofluorocarbon
(CFC) refrigerants is in direct response to the need to preserve
Earth's delicate, protective ozone layer. In fact, Earth is the
only planet in the solar system with an atmosphere that supports
life. Preserving the ozone is one of the many essential steps
we must take to protect life on our planet for future generations.
Similarly, the spectrum of alternative refrigerant choices is
limited to the basic elements found on Earth.
The Periodic Table of The Elements (fig. 1) is a
compilation of the basic substances that comprise the planet Earth.
Compounds are simply combinations of these basic elements. Not
every elements can be used to create a refrigerant compound, however.
Elements marked by a single cross-hatch solidify at the temperatures
required for refrigeration. The shaded elements are toxic in compound
form. Those marked with a double cross-hatch are very rare, manmade
or radioactive. Elements marked with a plaid hatch form unstable
compounds with boiling points too low for refrigeration applications.
Fig 1.
By process of elimination, eight elements remain
as candidates for refrigerant compounds: carbon, nitrogen, oxygen,
sulfur, hydrogen, fluorine, chlorine and bromine. Focusing on
these eight elements, Thomas Migley, Jr.1 formulated the first
CFCs used for refrigeration in 1930. Prior to Migley's discovery,
the elements used for refrigeration had serious drawbacks. For
example, ammonia (NH3) was (and still is) used for isolated industrial
process refrigeration. Since ammonia is moderately flammable and
toxic, it is not a viable solution for commercial refrigeration.
Sulfur dioxide (SO2) was also used as a refrigerant, but its higher
toxicity makes it a potential liability in commercial applications
(ASHRAE 15 Safety Code for Mechanical Refrigeration2 currently
classifies SO2 as a B2 Class, higher-toxicity refrigerant).
One of the most common compounds, water (H2O) is
used as a refrigerant (R-718) in absorption chillers. These systems
use lithium bromide (salt solution) to quickly absorb and evaporate
the water in a vacuum, creating a refrigeration effect. However,
the application of water as a refrigerant is limited by several
factors: lower efficiency, equipment size, heat sources required,
steam, gas-fired heat and first costs.
The elements that hold potential for refrigerant
compounds are clearly limited; they are further narrowed by the
choices for fluorocarbon refrigerants. Chlorine (Cl) and bromine
(Br) have been eliminated as refrigerant elements as they have
proven to be catalysts that destroy ozone when discomposed from
their compound form.
Clearly, the group of elements considered viable
presents a limited set of options when applied to refrigerant
compounds. The matrix triangle depicted in fig. 2 shows the relationship
of the elements Hydrogen, Chlorine and Fluorine combined with
a base element Carbon (not shown). This triangle - with each element
at an extreme apex - represents all of the compounds that may
be formed. Refrigerants located in the left side of the triangle
contain either too much hydrogen, and are thus flammable, or too
much of both hydrogen and chlorine, and are thus toxic. Today's
common refrigerant compounds are found in the right area of the
triangle. In figs. 3 and 4, the combinations are shown as both
their ASHRAE refrigerant designations (i.e., R-11) and their chemical
makeup (i.e.,CFCl3 - one Carbon, one Fluorine and three Chlorine).
fig. 2
fig. 3
For years, chlorofluorocarbon refrigerants - CFC
11 (R-11), CFC 12 (R-12), CFC 113 (R-113) and CFC 114 (R-114)
- were preferred for their stability, non-toxicity, non-flammability,
thermodynamic properties (ability to cool) and practical use in
designing equipment. Now, a new consideration, ozone protection,
has forced the elimination of chlorine-bearing refrigerant compounds
from the already limited list of viable options. Both international
agreements (i.e., Montreal Protocol) and the legislation of individual
nations (i.e., U.S. Clean Air Act) mandate the phaseout of all
chlorine-bearing or CFC refrigerants.
CFC refrigerants, with their long atmospheric life
and high chlorine content, are no longer choices for new equipment.
The search for alternatives to CFC refrigerants has led to a new
focus on the refrigerants found toward the lower center and bottom
of the matrix tri-angle (fig. 2). Hydrochlorofluorocarbons (HCFC)
refrigerants such as HCFC 123 (R-123) and HCFC 22 (R-22) may be
used as interim refrigerant solutions. However, HCFCs are also
subject to a phaseout schedule because they, too, contain Chlorine,
though in smaller amounts than the CFCs. Global concern over all
chlorine-bearing refrigerants has produced legislation mandating
the phaseout of all HCFCs through gradual, stepped-down levels
of production.3
HCFC 123 is a close replacement for CFC 11 in centrifugal chillers. However, HCFC borders on the toxi-city range, and so must be used with extreme precaution. This compound has undergone numerous toxicity tests in order to qualify for use (it is rated as B1 Class in ASHRAE 152). Far from new, HCFC 123 is more than 20 years old, but until now has remained in the shadows of CFC 11.
HCFC 22 is also more than 20 years old, and is currently
the most widely used fluorocarbon refrigerant in commercial refrigeration.
It also used in most comfort cooling applications, from room air
conditioners to the largest chillers. The large volume of installed
HCFC 22 combined with legislation prohibiting HCFC 22 venting
should ensure adequate amounts of this refrigerant for extended
interim usage.
Given the realities of an accelerated CFC phaseout
and a somewhat less accelerated HCFC phaseout, the search for
long-term refrigerant solutions must focus on the bottom of the
matrix triangle (fig. 2). HFC 134a is the leading choice for a
variety of reasons. A close replacement for CFC 12, HFC 134a will
grow in demand at a rate that will easily exceed even the quantities
of HCFC 22. Given its chlorine-free, phaseout-free status and
its extreme stability, HFC 134a is probably the next safest refrigerant
to water. The lack of chlorine in HFC 134a did lead to the study
of its performance apart from thermodynamic applications. For
example, HFC 134a does not mix with mineral oils, a feature that
is essential in refrigeration equipment so that lubricating oil
can be reclaimed and returned to compression devices. This hurdle
has been overcome by the introduction of synthetic refrigerant
oils. Today, HFC 134a is being applied in foam blowing, pharmaceuticals,
auto air conditioning, refrigeration chillers, residential air
conditioning, water fountains, domestic refrigerators and supermarkets.
fig. 4
fig. 5
Although HFC 134a will play a leading role in many
applications, other HFC refrigerants found at the bottom of the
matrix triangle are also good replacements for CFCs and interim
HCFCs. Excluding HFCs that are highly flammable, very high pressure
or lack good application thermodynamics narrows the field to five
compounds. These compounds have recently been commercialized as
azeotropes and zeotropes. Azeotropes are near-compounds - unique
mixtures of two or more compounds which have nearly or precisely
the same composition in both liquid and vapor states. ASHRAE Standard
344 defines azeotropes as "blends comprising multiple components
of different volatilities (evaporating quickly) that, when used
in refrigeration cycles, do not change volumetric composition
(size) or saturation temperatures as they evaporate (boil) or
condense at constant pressure."
Only five base compounds are being used to form azeotropes
and zeotropes: HFC 32, 23, 152a, 143a and 125. HFC 134a is also
included in these blends, which are being marketed under several
different brand names. Understanding that there are only six compounds
being used is helpful in sorting through the various refrigerant
brands for the future. A partial list of these newly-created mixtures
of existing compounds is shown in fig. 5. Allied Signal's AZ20
is a proposed replacement for HCFC 22, although it may not be
used as a drop-in due to its nominal 40% higher condensing pressure.
AZ50, an azeotrope of HCFC 22 and HCFC 115 (fig. 5), is the replacement
for R-502 which was used in the past for low-temperature applications
and is itself an azeotrope of CFC 12 and HFC 152. Both AZ20 and
AZ50 are azeotropes that match a true compound very closely.
The remaining refrigerants on the chart in fig. 5
are either zeotropes or blends. Zeotropes are defined by ASHRAE
34 as "blends comprising multiple components of different
volatilities that, when used in refrigeration cycles, change volumetric
composition and saturation temperatures as they evaporate (boil)
or condense at constant pressure. The word is derived from the
Greek words zein (to boil) and tropos (to change)." Blends
are defined by ASHRAE 34 as "refrigerants consisting of mixtures
of two or more different chemical compounds, often used individually
as refrigerants for other applications."
The refrigerants from DuPont, HC62 and AC9000, are
referred to as zeotropes, as is ICI's refrigerant KLEA 66. As
shown in fig. 4, all of the azeotropes and zeotropes are mixtures
of the refrigerants from the bottom of matrix triangles in figs.
2, 3 and 4. Since azeotropes perform without volumetric composition
or saturated temperature change, they are promising as replacements
for new designs.
Zeotropes may fit some limited drop-in applications
as well as several replacement design applications. Each application
must be reviewed in light of the potential for different boiling
and condensing temperature ranges of the zeotropes, commonly referred
to as "guide." Guide is defined by ASHRAE 34 as "the
absolute value of the difference between the starting and ending
temperatures of a phase change process by a refrigerant within
a component of a refrigerating system, exclusive of any sub-cooling
or superheating. This term usually describes condensation or evaporation
of a zeotrope." Simply put, the components of the blend each
evaporate and con-dense at different rates. For this reason, zeotropes
will not be used in flooded chiller applications where the design
requires a constant evaporation/condensation rate.
In addition, as a zeotrope refrigerant leaves a system,
its composition may change, leaving behind a mixture that is different
from the original virgin blend. This change is called "fractionalization"
and is defined by ASHRAE 34 as "a change in composition of
a blend by preferential evaporation of the more volatile component(s)
or condensation of the less volatile components." The following
is an example of fractionalization:
HFC 134a + HFC 125 + HFC 32 ------> LEAK
Nominal: -15.1° F -55.3° F -61.1°F
Boiling Point: -26.2° C -48.5° C -51.7°C
Note: One component of the blend would not leak out
singularly, but as a mixture of its next evaporating components,
as noted by the plus (+) marks.
Testing on azeotropes and zeotropes continues, and
these refrigerants will ultimately supersede the HCFC refrigerants
now used as interim solutions. A broad range of these substances
will be available (see charts at the end of this paper) and brand
marketing to target applications will continue.
By taking the mystery out of the subject, a basic
under-standing of refrigerant chemistry can help decision-makers
sort through complex and often confusing claims. The migration
away from CFC and HCFC refrigerants makes it important to understand
that current options are limited by both the chemistry and the
application. Could a "miracle" refrigerant be developed
in the future? Perhaps, but the substance will require elements
beyond those found here on Earth.
Today's choices are limited, but the HFCs and specifically,
HFC 134a, are excellent choices for a future of chlorine-free,
non-ozone depleting refrigerant compounds. As HFCs are applied
to new technology and continuously improving efficiencies, it
is unlikely that they will present other environmental concerns,
such as global warming. At this point, any refrigerant selected
for long-term use must support an overall system that is safe,
environmentally sound, reflecting considerations such as new technologies,
waste minimization, containment and energy efficiency. It is clear
that today's refrigerant choices must be made with an eye toward
protecting and preserving the future of our delicate planet Earth.
34 Number |
|
| |||||
22 | Allied Signal, Dupont, Others | True Compound |
CHF2CL | A1 | -- | Light Green | 11 |
123 | Dupont | True Compound |
C2HF3CL2 | B2 | -- | Light Blue-Gray | 1 |
134a | Dupont, Allied Signal, ICI, Others | True Compound |
C2H2F4 | A1 | -- | Light Blue | 11 |
401a | Dupont MP 39 | R-22 R-152a R-124 | 53/13/3 | A1/A1 | R-12 | Light Purple | 11 |
401b | Dupont MP66 | R-22 R-152a R-124 | 61/11/28 | A1/A1 | R-12 | Yellow -Brown | 11 |
401c | Dupont MP52 | R-22 R-152a R-124 | 33/15/52 | A1/A1 | R-12 | Blue-Green | 11 |
402A | Dupont HP80 | R-125 R-290 R-22 | 60/2/38 | A1/A1 | R-502 | Light Brown | 111 |
402b | Dupont HP81 | R-125 R-290 R-22 | 38/2/60 | A1/A1 | R-502 | Green -Brown | 111 |
403a | Rone-Poulenc 69S | R-290 R-22 R-218 | 5/75/20 | A1/A1 | R-502 | -- | -- |
404a | Dupont HP62 | R-290 R-22 R-218 | 5/56/39 | A1/A1 | R-502 | Orange | 111 |
405 | Greencool G2015 | R-22 R-152a R-142b R-C318 |
5/7/5.5/42.5 | A1/A1 | R-12 | -- | -- |
406a | Monroe Air Tech
GHG-12 | R-22 R-600a R-142b |
55/4/41 | A1/A2 | R-12 | -- | -- |
407a | ICI Klea 60 | R-32 R-125 R-134a | 20/40/40 | A1/A1 | R-22 | Lime-Green | 111 |
407b | ICI Klea 61 | R-32 R-125 R-134a | 10/70/20 | A1/A1 | R-22 | Cream | 111 |
407c | Dupont AC9000/ICI Klea 66 | R-32 R-125 R-134a |
23/25/52 | A1/A1 | R-22 | Medium -Brown | 111 |
408a | Atochem FX 10 | R-125 R-143a R-22 | 7/46/47 | A1/A1 | R-502 | Medium- Purple | 111 |
409a | Atochem FX 56 | R-22 R-124 R-142b | 60/25/15 | A1/A1 | R-12 | Medium -Brown | 11 |
410a | Allied Signal AZ 20 | R-32 R-125 | 50/50 | A1/A1 | R-22 | Rose | 111 |
410b | Dupont AC9100 | R-32 R-125 | 45/55 | A1/A1 | R-22 | Maroon | 111 |
411a | Greencool G2018A | R-127 R-0 R-22 R-152a | 1.5/87.5/11 | A1/A2 | R-22 | -- | -- |
411b | Greencool G2018B | R-127 R-0 R-22 R-152a | 3/94/3 | A1/A1 | R-502 | -- | -- |
412a | ICI Arcton TP5R | R-22 R-218 R-142b | 70/5/25 | A1/A2 | -- | -- | -- |
413a | Rhone-Poulenc | R-218 R-134a R-600a | 9/88/3 | A1/A2 | R-12 | -- | -- |
507 | Allied Signal AZ50 | R-125 R-143a | 50/50 | A1 | R502/22 | Blue-Green | 111 |
508 | ICI Klea 5R3 | R-23 R-116 | 39/61 | A1 | R-503 | -- | -- |
509 | ICI Arcton TP5R2 | R-22 R-218 | 44/56 | A1 | -- | -- | -- |
*Safety Group per ASHRAE-34 **Class 1: boiling point .68F. Packaged in drums
A1 = Low toxicity, Non-flammable Class II: low pressure Title 49 CRF, Normally in
A2 = Low toxicity, Flammable in cylinders <260 psig
B2 = Higher toxicity, Non flammable Class III: high pressure, compressed gas per
Title 49 CFR min. press. cylinder <260 psig
Class IV: flammable refrigerants
1. Thomas Migley, Jr. directed the research team that
developed the first CFC refrigerants in the General Motors research
laboratory at the request of Frigidaire Corporation of Dayton,
Ohio. In 1937, Migley published From the Periodic Table
to Production: Industrial and Engineering Chemistry.
2. ASHRAE 15-1992 Safety Code For Mechanical Refrigeration,
American Society of Heating, Refrigerating and Air-Conditioning
Engineers, Inc., 1791 Tullie Circle NE, Atlanta, GA, 30329.
3. Summary of Montreal Protocol as amended at the
fourth meeting of the Parties in Copenhagen, Denmark, November,
1992:
Jan. 1, 2004 Cap reduced by 35%
Jan. 1, 2010 Cap reduced by 65%
Jan. 1, 2015 Cap reduced by 90%
Jan. 1, 2020 Cap reduced by 99.5%
Jan. 1, 2030 Cap reduced by 100%
4. ASHRAE 34 (currently under change review for new
refrigerants), Number Designation and Safety Classification of
Refrigerants, American Society of Heating, Refrigerating and Air-Conditioning
Engineers, Inc., 1791 Tullie Circle NE, Atlanta, GA 30329.
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