|
INTRODUCTION
BACKGROUND
HFC-134a has been
introduced by DuPont as a replace-ment for
chlorofluorocarbons (CFCs) in many applications.
CFCs, which were developed over 60 years ago, have
many unique properties. They are Iow in toxicity,
nonflammable, noncorrosive and compatible with
other materials. In addition, they offer the
thermodynamic and physical properties that make
them ideal for a variety of uses. CFCs are used as
refrigerants; as blowing agents in the manufacture
of insulation, packaging and cush-ioning foams; as
cleaning agents for metal and elec-tronic
components; and in many other applications.
However, the stability of these compounds,
coupled with their chlorine content, has linked
them to depletion of the earth's protective ozone
layer. As a result, DuPont plans to phase out
production of CFCs and introduce environ-mentally
acceptable alternatives, such as
hydrofluoro-carbon (HFC) 134a.
HFC-134a-AN ENVIRONMENTALLY ACCEPTABLE
ALTERNATIVE
HFC-134a does not contain
chlorine; therefore, it has an ozone depletion
potential (ODP) of zero. Listed below are all
generic and DuPont trade names:
Hydrofluorocarbon-134a
H FC-
134a
H FA- 134a
SUVA~ 134a
SUVA~ Trans
A/C (automotive market)
SUVA~ Cold MP
(stationary refrigeration/air conditioning
market)
FORMACEL~ Z-4 (foam blowing agent
market)
DYMEL~ 134a/P (aerosol pharmaceutical
market)
DYMEL~ 134a (general aerosol
market)
The chemical properties of
HFC-134a are listed below.
HFC-134a
Chemical Name
1,1,1,2-tetrafluoroethane
Molecular
Formula CH2FCF3
CAS Registry Number
811-97-2
Molecular Weight 102.0
USES
HFC-134a can be used
in many applications that cur-rently use
dichlorodifluoromethane (CFC-12). These include
refrigeration, polymer foam blowing and aerosol
products. However, equipment design changes are
sometimes required to optimize the performance of
HFC-134a in these applications.
The thermodynamic and physical properties of
HFC-134a, coupled with its Iow toxicity, make it a
very efficient and safe replacement refrigerant
for CFC-12 in many segments of the refrigeration
industry, most notably in automotive air
conditioning, appliances, small station-ary
equipment, medium-temperature supermarket cases
and industrial and commercial chillers. Table 1 provides a comparison of
the theoretical performance of CFC-12 and HFC-134a
at medium-temperature conditions.
As a blowing agent in polymer foams, HFC-134a
can be used to replace CFC-12 in many
thermoplastic foam applications. Recent
developments, however, are also providing new
technology that uses HFC-134a as a replacement for
CFC-12 in thermoset foams. HFC-134a features
properties that are advantageous for high
value-in-use products and meets the requirements
of safety/environmental issues. HFC-134a is
nonflammable, has negligible photochemical
reactivity and Iow vapor ther-mai
conductivity.
HFC-134a is also being developed for use in
pharma-ceutical inhalers because of its Iow
toxicity and nonflam-mability. Other aerosol
applications may use HFC-134a where these
properties are critical. See DuPont DYMEL~Bulletin
ATB-30 (H-44691) for additional information on
aerosol applications of HFC-134a.
PHYSICAL PROPERTIES
Physical properties of HFC-134a are given in
Table 2
and Figures 2 to 8.
Additional physical property data may be found
in other DuPont publications. Bulletin ART-1 (H-43855-1)
contains viscosity, thermal conduc-tivity and
heat capacity data for saturated liquid and vapor
in addition to heat capacity data and heat capacity
ratios for both saturated and superheated va-pors.
Thermodynamic tables in English and SI units are
available in Bulletins T-images/134a-enG (H-47751)
and T-134a-SI (H-47752). Liquid and vapor densities
are included in the thermodynamic tables.
CHEMICAL/THERMAL STABILITY
THERMAL DECOMPOSITION
HFC-134a
vapors will decompose when exposed to high
temperatures from flames or electric resistance
heaters. Decomposition may produce toxic and
irritating com-pounds, such as hydrogen fluoride.
The pungent odors released will irritate the nose
and throat and generally force people to evacuate
the area. Therefore, it is impor-tant to prevent
decomposition by avoiding exposure to high
temperatures.
STABILITY WITH METALS AND REFRIGERATION
LUBRICANTS
Stability tests for
refrigerants with metals are typically performed
in the presence of refrigeration oils. The results
of sealed tube stability tests are available for
CFC-12/mineral oil combinations, which have shown
long-term stability in contact with copper, steel
and aluminum in actual refrigeration systems.
Polyalkylene glycol (PAG) and polyol ester (POE)
lubricants will most likely be used with HFC-134a.
Sealed tube tests were, therefore, run to
determine the relative long-term stability of
HFC-134a/metals in the presence of these
lubricants.
The method followed was generally the same as
ASHRAE 97 with several minor modifications. A 3-mL
volume of refrigerant/lubricant solution was
heated in the presence of copper, steel and
aluminum strips in an oven for 14 days at 175~C
(347~F). Both the neat lubricant and a mixture of
lubricant and refrigerant (50/50 volume ratio)
were tested. Visual ratings were obtained on both
the liquid solutions and the metal coupons after
the designated exposure time. The visual ratings
ranged from 0 to 5, with 0 being the best and 5
being the worst.
After the visual ratings were obtained, sample
tubes were opened and the lubricant and refrigerant
(if present) were analyzed. The lubricant was
typically checked for halide content and viscosity,
while the refrigerant was examined for the presence
of decomposition products. Table 3 summarizes typical
data for both HFC-134a and CFC-12. Visual ratings
are listed for the neat lubricant, the lubricant/refrigerant
solution and the three metals that were present
in the lubricant/refrigerant solutions. Viscosity
was determined on the unused lubricant, the tested
neat lubricant and the lubricant tested in the
presence of refrigerant. A percent change was
calculated for the two tested lubricants. The
de-composition products listed are HFC-143a (the
pre-dominant decomposition product for HFC-134a)
and fluoride ion. Both species are typically measured
in the Iow parts per million (ppm) range.
As the CFC-12/mineral oil combinations have
been proven in actual service, these tests
indicate that HFC-134a/PAG and HFC-134a/POE
solutions have accept-able chemical stability. In
several other tests, results have confirmed that
the HFC-134a molecule is as chemically stable as
CFC-12.
STABILITY WITH FOAM CHEMICALS
As
with other alternative blowing agents, the
stability of HFC-134a in foam chemicals (B-side
systems) is being studied. The first tests
evaluated HFC-134a stability in a sucrose-amine
polyether polyol with either an amine catalyst, a
potassium catalyst, a tin catalyst or an amine
catalyst neutralized with an organic acid. The
initial tests, which included analysis of the
volatile compo-nents, showed no degradation of H
FC-134a in any of the systems, even at elevated
temperatures. The results are summarized in Table 4.
COMPATIBILITY CONCERNS IF HFC-134a AND
CFC-12 ARE MIXED
HFC-134a and CFC-12 are
chemically compatible with each other; this means
that they do NOT react with each other and form
other compounds. However, when the two materials
are mixed together, they form what is known as an
"azeotrope." An azeotrope is a mixture of two
components that acts like a single compound, but
has physical and chemical properties different
than either of the two components. An example of
this is FREON~ 502, which is an azeotrope of
HCFC-22 and CFC-115. When HFC-134a and CFC-12 are
mixed in certain concentrations, they form a
high-pressure (Iow boiling) azeotrope. This means
that the vapor pressure of the azeotrope is higher
than that of either of the two components by
themselves. At 109 psia (752 kPa absolute) the
azeotrope contains 46 weight percent HFC-134a. In
general, compressor discharge pres-sures will be
undesirably high if refrigeration equipment is
operated with a mixture of HFC-134a and
CFC-12.
Another characteristic of an azeotrope is that
it is very difficult to separate the components
once they are mixed together. Therefore, a mixture
of HFC-134a and CFC-12 cannot be separated in an
on-site recycle machine or in the typical
facilities of an off-site reclaimer. Mixtures of
HFC-134a and CFC-12 will usually have to be
disposed of by incineration.
MATERIALS COMPATIBILITY
Because HFC-134a is used in many applications,
it is important to review materials of
construction for com-patibility when designing new
equipment, retrofitting existing equipment or
preparing storage and handling facilities.
PLASTICS
Customary industry
screening tests, in which twenty-three typical
plastic materials were exposed to liquid HFC-134a
in sealed glass tubes at room temperature, are
summarized in Table 5.
Observations of weight gain and physical change
were used to separate materials meriting further
laboratory and/or field testing from ma-terials
which appeared unacceptable. The majority of the
materials tested merit further evaluation.
Since the performance of plastic materials is
affected by polymer variations, compounding
agents, fillers, and molding processes, verifying
compatibility using actual fabricated parts under
end-use conditions is advised.
ELASTOMERS
Compatibility results for
HFC-134a and CFC-12 are com-pared for 11 typical
elastomers in Tables 6 through
17. It should be recognized, however, that
effects on spe-cific elastomers depend on the
nature of the polymer, the compounding formulation
used and the curing or vulca-nizing conditions.
Actual samples should be tested under end-use
conditions before specifying elastomers for
critical components.
Recommendations, based on the detailed data in
Tables 7through 17, are given
in Table 6. Data on "temporary"
elastomer swell and hardness changes were used as
the prime determinants of compatibility. The
subse-quent "final" data were used as a guide to
indicate if the seals in a refrigeration system
should be replaced after equipment tear down.
Most polymeric materials used in refrigeration
equip-ment are exposed to a mixture of refrigerant
and refrig-eration oil. Data on the compatibility
of elastomers and motor materials with HFC-134a in
combination with mineral oils and a PAG lubricant
are available in Bulle-tins ARTD-18 (H-26845) and
ARTD-30 (H-32123). Data for nylon and for
graphite-filled TEFLON~ fluorocarbon resin are
included in ARTD-30. Additional data are being
developed by equipment manufacturers.
HOSE PERMEATION
Elastomeric hoses
are used in mobile air conditioning systems and
for transferring HFC-134a in other applica-tions.
The permeation rates of HFC-134a and CFC-12
through several automotive NC hoses were measured
as a guide to hose selection.
The studies were run at 80'C (176'F) with an
initial 80 volume percent liquid loading of HFC-134a
in 76-cm (30-in.) lengths of 15.9-mm (5/8-in.)
inside diameter automotive air conditioning hose.
Hose construction and permeation rates are summarized
in Table 18. Based on these
tests, hoses lined with nylon, as well as those
made of HYPALON~ 48, appear to be suitable for
use with HFC-134a. Note, however, that these rate
measurements provide a comparison of the various
hoses at a single temperature and should not be
used as an indication of actual permeation losses
from an oper-ating system.
DESICCANTS
Dryers filled with
desiccant are typically used in refrig-eration
systems and bulk storage facilities. A common
molecular sieve desiccant used with CFC-12, UOP's
(formerly Union Carbide Molecular Sieve) 4A-XH-5,
is not compatible with HFC-134a; however, UOP has
developed other molecular sieve desiccants, such
as XH-7 and XH-9, which perform well in HFC-134a
ser-vice. In addition, several other manufacturers
offer loose-fill and molded-core desiccants that
are compatible with HFC-134a and lubricants. Be
sure to indicate your specific HFC-134a
application when ordering a dryer or
desiccant.
REFRIGERATION LUBRICANTS
Most
compressors require a lubricant to protect
internal moving parts. The compressor manufacturer
usually recommends the type of lubricant and
viscosity that should be used to ensure proper
operation and equipment durability.
Recommendations are based on several criteria,
such as lubricity, compatibility with ma-terials
of construction, thermal stability and
refrigerant/oil miscibility. To ensure efficient
operation and long equipment life, it is important
to follow the manufacturer's recommendations.
Current lubricants used with CFC-12 are fully
miscible over the range of expected operating
conditions, easing the problem of getting the
lubricant to flow back to the compressor.
Refrigeration systems using CFC-12 take advantage
of this full miscibility whe considering
lubri-cant return. Refrigerants such as HFC-134a,
with little or no chlorine, may exhibit less
solubility with many existing mineral oil or
alkylbenzene lubricants.
The search for lubricants for use with HFC-134a
started with commercially available products.
Table 19 shows solubilities
of various refrigerant/lubricant combinations.
Current naphthenic, paraffinic and alkylbenzene
lubri-cants have very poor solubility with HFC-134a.
PAGs with Iow viscosity show good solubility but,
as viscosity increases, they become less soluble.
Ester lubricants, of which there are many types,
generally show good solu-bility with HFC-134a.
When compared to PAGs, ester lubricants are more
compatible with hermetic motor components and
are less sensitive to mineral oil and CFC-12 remaining
in a refrigeration system.
Although HFC-134a and CFC-12 are chemically
com-patible with each other, such is not the case
with CFC-12 and PAG lubricants. Specifically, the
chlorine contained in CFC-12 or other chlorinated
compounds can react with the PAG and cause
lubricant degradation. CFC-11, which is often used
as a cleaning or flushing agent, is also
incompatible with PAGs. At contaminant levels of 1
percent CFC-11 or 2 to 10 percent residual mineral
oil (saturated with CFC-12), the stability of the
system is affected enough to cause possible
degrada-tion. Lubricant degradation can result in
poor lubrication and premature failure. In
addition, sludge will be formed that can plug
orifice tubes and other small openings.
SAFETY
INHALATION TOXICITY
HFC-134a poses
no acute or chronic hazard when it is handled in
accordance with DuPont recommendations and when
exposures are maintained at or below the DuPont
Acceptable Exposure Limit (AEL) of 1,000 ppm (8-
and 12-hour Time-Weighted Average or TWA).
An AEL is an airborne exposure limit
established by DuPont scientists that specifies
time-weighted average (TWA) airborne
concentrations to which nearly all work-ers may be
repeatedly exposed without adverse effects. The
AEL for HFC-134a has the same value as the
Threshold Limit Values (TLVs) established for
CFC-12 and HCFC-22. TLVs are established by the
American Conference of Governmental and Industrial
Hygienists (ACGIH).
However, inhaling high concentrations of
HFC-134a vapor may cause temporary central nervous
system depression with narcosis, lethargy and
anesthetic ef-fects. Other effects that may occur
include dizziness, a feeling of intoxication and a
loss of coordination. Con- tinued breathing of
high concentrations of HFC-134a vapors may produce
cardiac irregularities (cardiac sensitization),
unconsciousness and, with gross over- exposure,
death. Intentional misuse or deliberate
inha-lation of HFC-134a may cause death without
warning. This practice is extremely dangerous.
If you experience any of the initial symptoms,
move to fresh air and seek medical attention.
CARDIAC SENSITIZATION
If vapors are
inhaled at a concentration of 75,000 ppm, which is
well above the AEL, the heart may become
sensitized to adrenaline, leading to cardiac
irregulari-ties and, possibly, to cardiac arrest.
The likelihood of these cardiac problems increases
if you are under physical or emotional stress.
Medical attention must be given immediately if
exposed to high concentrations of HFC-134a. Do not treat with
adrenaline (epinephrine) or similar drugs. These
drugs may increase the risk of cardiac arrhythmias
and car-diac arrest. If the person is having
difficulty breathing, administer oxygen. If
breathing has stopped, give arti-ficial
respiration.
| 
