EVALUATING ALTERNATIVE REFRIGERANTS
A GUIDE TO THE CRITERIA FOR REFRIGERANT SELECTION
INTRODUCTION
The mandated phase-out of chlorofluorocarbon (CFC)
refrigerants by January 1, 1996 has brought many building owners
and managers face-to-face with a difficult question: Which new
refrigerant is the best choice for the future? Because the information
surrounding refrigerant selection is complex and dynamic, there
is a tendency to think in terms of automatic replacements (i.e.,
"I have CFC 11, so my only choice is to convert to HCFC 123").
This "knee-jerk" kind of thinking is limiting, and unlikely
to lead to the optimum solution. A broad look at the criteria
for refrigerant selection leads to a better understanding of the
optimum solution.
ENVIRONMENTAL CRITERIA
Environmentalists tend to consider two criteria when
evaluating refrigerants: ozone depletion and global warming. However,
ozone depletion is not a concern so long as refrigerants are not
vented to the atmosphere, a practice that is now prohibited by
law. Moreover, new equipment must contain non-chlorine containing
refrigerants. As for global warming, the more scientists study
the issue, the clearer it becomes that refrigerants con-tribute
less than 2% to this problem.
The environmentalists' criteria shed limited light
on today's refrigerant selection issue. In fact, if we consider
these two criteria alone, the refrigerants R-718 (water) and R-717
(ammonia) appear to be viable options. Of course, other considerations
which limit these options - economics, energy, safety and so on
- immediately come to mind.
BROADENING THE SELECTION CRITERIA
Expanding the criteria for refrigerant selection
helps clarify priorities and ultimately, produce the optimum refrigerant
choice. A broader list of criteria for alternative refrigerant
selection would include:
ALTERNATIVE REFRIGERANTS: A SAMPLE ANALYSIS
Before beginning the analysis, it is a good idea
to prioritize the above criteria for your specific situation.
The sample analysis below compares the three primary alter-native
refrigerants for the future - HFC 134a, HCFC 22 and HCFC 123 -
based on the above criteria:
Accelerated phase-out due to ozone depletion
When vented to the atmosphere, the high chlorine
con-tent of CFCs is a catalyst that destroys ozone, allowing harmful
ultraviolet rays to reach the earth's surface. Refrigerants like
HCFCs (i.e., 22 and 123) also contain chlorine, but in much smaller
amounts. HCFCs are not currently subject to the same accelerated
phase-out underway for CFCs. However, HCFC production has been
capped for 1996 and a stepped phase-out has begun. The industry
is clearly moving toward a chlorine- free future. In light of
this, HFC 134a has the edge as the most viable long-term solution.
Both CFCs and HCFCs can not be vented by law under
the U.S. Clean Air Act. There is some argument as to the long-term
vs. short-term life of these refrigerants in the atmosphere, however
this must be balanced against the quantity used and the move by
environmentalists to ban all chlorine-containing refrigerants.
Already, several nations are moving to ban not only CFCs but
also HCFCs. It is noteworthy that the Montreal Protocol Copenhagen
Agreement has a stepped down phase-out of HCFCs that resembles
its original stepped phase-out schedule of CFCs in 1987. The CFC
schedule under the Copenhagen Agreement has moved CFC phase-out
up to the end of 1995. It is clear that the HFCs and HFC 134a
have the edge as a viable long-term solution.
Global Warming
How does replacing CFCs with HCFCs and HFCs affect
the energy we consume and thus contribute to global warming?
fig. 1
fig. 2
The U.S. Department of Energy's new Total Equivalent
Global Warming Impact (TEWI) measure defines the contribution
of systems. TEWI is a measure of the total amount of gas released
to the atmosphere as a result of burning fossil fuels (fig. 1).
TEWI is the sum of direct releases (refrigerant emissions)
and indirect releases (from fuels that emit global warming gases).
Once CFCs are replaced, direct releases are immaterial (2%), and
good containment vessels will reduce emissions even further. Indirect
releases are the key (98%), and they are defined by system efficiency,
not refrigerant type. In fact, the Department of Energy has stated
that where HCFCs and HFCs replace CFCs, the difference in TEWI
due to refrigerant is minor.
The U.S. National Action Plan for climate change
will have an impact on global warming reduction. It con-tains
50 options to reduce U.S. greenhouse gas emissions to 1990 levels
by the year 2000. As this plan is enacted, greater emphasis will
be placed on substituting HFCs for CFCs and HCFCs. Containment
will be critical in meeting the plan's goals for non-venting CFCs,
HCFCs and even HFCs.
Efficiency
There is no question that converting a chiller's
lifeblood, its refrigerant, impacts system efficiency. The good
news is that manufacturers have done an excellent job of designing
new equipment and modifying existing designs to make up for efficiency
losses. In most cases, the efficiencies of the new alternative
refrigerant-based systems match or exceed those of existing CFC-based
systems.
Comparing refrigerants as thermodynamic fluids in
simple cycles is very misleading (fig. 2). These ratings are strictly
theoretical and reflect the fluid's performance under ideal conditions.
In the real world of a chiller, actual refrigerant
efficiency is affected by a host of factors related to normal
chiller operation, including mechanical, thermal and aerodynamic
factors. In the end, superior chiller design is far more important
than a superior theoretical rating. And in fact, the positive
pressure refrigerants offer an advantage for improved efficiency
by utilizing the larger pressure differential between condensing
and cooling pressures for advanced sub-cooling technology. Negative
pressure refrigerants such as HCFC 123 do not offer this advantage.
Safety/Risk of Use
All three of the new refrigerants can be applied
at different levels of precaution, and users assume different
levels and types of risks with each refrigerant. The American
Society of Heating, Refrigeration and Air Conditioning Engineers
(ASHRAE) provides an excel-lent Safety Code for Mechanical Refrigeration
in its new standard 15-1992. However, this standard does not review
the risks and liabilities associated with refrigerant exposure
in light of the exposure limits recommended in documents such
as the Program for Alternate Fluorocarbon Toxicity Testing (PAFTT).
The refrigerants of the past (i.e., CFC 11) had very high safe
exposure levels. The higher the exposure level, the lower the
potential for risk and liability (fig. 3).
The PAFTT analysis led to ASHRAE's safety classification
for refrigerants (fig. 4).
Containment
Which new refrigerants require no add-on containment
systems? This is a particularly important consideration in light
of the first cost and the ongoing maintenance/ service costs associated
with containment devices.
Both HFC 134a and HCFC 22 operate in positive pressure
systems, which typically have built-in containment. No add-on
containment devices (or the additional maintenance associated
with them) are necessary.
fig. 3
fig. 4
In addition, the pressure vessels in positive pressure
chillers are constructed to meet the high performance standards
of the American Society of Mechanical Engineers (ASME) - seven
times normal operating pressures. The production process is scrutinized
at each succeeding step before the vessel may leave the factory
bearing an ASME-certified stamp. These vessels are so reliable
that refrigerant can even be stored and transported to the job
site in the chiller.
ASME construction also ensures a quality containment
product, not only from the quality of the steel used but also
fabrication requirements, quality checks, proof testing and national
registration approval by an independent inspector. The final result
is a guaranteed pressure vessel for maximum containment. This
is demonstrated by the new leak specifications for the latest
chiller designs, which are as low as .05 ounces per year. The
performance history of installed positive pressure chillers demonstrates
that even 20- to 30-year-old chillers typically run with their
original refrigerant charge.
HCFC 123 operates in negative pressure systems, as
did most of the centrifugal chillers of the past when containment
was not a concern. To safely operate a negative pressure system
today, add-on containment devices are a must. These include high-efficiency
purges, which can reduce emissions by as much as 99% and range
in price from $4,400 to $6,000, and refrigerant sensors, which
are required under ASHRAE 15-1992 and range from $3,000 to $15,000.
Other containment devices that bring negative pressure chillers
up to positive pressure standards include: refrigerant management
systems, pressurizing systems, back-up pressure relief valves,
along with other containment factors (see fig. 5 on page 4). All
add to the up-front cost and the servicing costs of the system.
Size
Is your machine room already bursting at the seams?
Chiller size is important, particularly since one that is too
large may require the removal of a wall for its installation.
Larger systems also have large parts, which, of course, cost more
to replace.
Since they require no add-on containment devices,
positive pressure systems tend to be significantly smaller than
their negative pressure counterparts (i.e., storage is built in).
The smaller size is also due to the smaller molecular size of
the positive pressure refrigerants (HFC 134a and HCFC 22), which
allows more mass of refrigerant circulation in a smaller space
per capacity of cooling (fig. 6).
Smaller chillers are typically easier and less costly
to install, and their smaller parts (i.e., valves) are significantly
less expensive initially.
Potential for Corrosion
Another advantage of HFC 134a and HCFC 22, the positive
pressure refrigerants, is that these systems historically have
required fewer tube and tube joint replacements (even when mechanically
expanded into these joints) due to corrosion. By nature, negative
pressure systems tend to take in air, which contains moisture
that mixes with the refrigerant (specifically, its chlorine and
hydrogen) to form acids that eventually cause corrosion problems.
(For further reading on this subject, see the March 1976 ASHRAE
Journal.)
fig. 5
fig. 6
Availability
No matter how terrific your refrigerant of choice
may be, if there's not enough of it to go around in 5, 10 or 20
years, your system will be in jeopardy. HFC 134a has the edge
here because this stable refrigerant is being adopted by many
other industries, including automotive, pharmaceutical and industrial
processing. By the end of 1995, HFC 134a will have the highest
production demand in the global market, at 116,000 tons per year
(fig. 7). Conversely, as industries move away from CFCs and later,
HCFCs, their production will fall off sharply. And because of
its ample supply, the price of HFC 134a is projected to drop to
less than $4/lb in 1994. HCFCs may rise in price due to excise
taxes (the U.S. government is currently reviewing this tax proposal).
HCFC 22 will have a good supply as a result of the broad usage.
On the other hand, HCFC 123 is just now being applied, by just
two manufacturers and only in centrifugal chillers (fig. 8). No
chlorine-free commercial replacement is planned for HCFC 123.
HCFC 22 will be replaced in new designs with HFC combinations.
Fig. 8
THE REFRIGERANT OF THE FUTURE
Based on the above analysis, HFC 134a emerges as
the refrigerant of the future, with HCFC 22 a strong, viable transition
solution. Not surprisingly, a review of the product lines currently
offered by the major chiller manufacturers reveals that most manufacturers
are planning for a CFC/HCFC-free future as well. Some are moving
quicker than others, which reflects the cost and time involved
in converting manufacturing facilities and redesigning base products.
CONCLUSION
Environmental concerns about ozone depletion and global warming are only two of the many criteria to consider when comparing alternative refrigerants. A systematic evaluation of all the criteria based on your priorities will lead to the best choice for your facility, both today and in the future. As with any environmental issue, corporations that choose to move ahead of legislation and ahead of the industry demonstrate environmental responsibility. If your company does not have a strategic refrigerant plan, start one now. By the end of 1995, virgin CFC production will stop. Good solutions exist today if you choose wisely.