United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-96/070 August 1996
EPA Project Summary
Heat Transfer Evaluation of
HFC-236ea and CFC-114 in
Condensation and Evaporation
W. W. Huebsch and M. B. Pate
With the mandatory phaseout of chlo-
rofluorocarbons (CFCs), as dictated by
the Montreal Protocol and the Clean
Air Act Amendments, it is imperative
for the Navy to find a replacement for
1,1,2,2-dichloro-tetrafluoroethane(CFC-
114) that is environmentally safe and
possesses similar performance char-
acteristics. Currently, one of the lead-
ing candidates to replace CFC-114 is
hexafluoropropane (HFC-236ea). This
research focuses on comparing the re-
frigerants not only in condensation and
pool boiling, but also with various tube
surfaces.
The test facility used in this study
was initially used for spray evapora-
tion testing; however, it was redesigned
and modified for use with condensa-
tion, pool boiling, or spray evaporation
testing. During condensation, the rig
was capable of producing saturated or
superheated vapor. During pool boiling
or spray evaporation, the test facility
was capable of testing pure refriger-
ants or refrigerant/lubricant mixtures.
The test facility is described in detail in
the full report.
The two refrigerants produced simi-
lar performance characteristics in con-
densing vapor on integral-fin tubes, so
that the transition to HFC-236ea should
be accomplished without major modifi-
cations to existing condensers. The re-
sults also showed that the condensa-
tion of superheated vapor had negli-
gible effects on the shell-side heat
transfer coefficient as compared to con-
densation of saturated vapor results.
The superheated vapor data for the 26
and 40 fpi (fins per inch) tubes were
within 5 and 3%, respectively, of the
saturated vapor results for the same
tube surface.
HFC-236ea produced higher boiling
coefficients than CFC-114 for all tubes
tested. In addition, the 26 fpi tube out-
performed the 40 fpi tube by 18% and
the plain tube by 41% for HFC-236ea.
The maximum increase in boiling with
HFC-236ea was 39% for the 26 fpi tube
and 34% for the 40 fpi tube.
The mineral oil used with CFC-114
showed a general improvement in the
heat transfer performance, while the
polyol-ester oil consistently degraded
the performance of HFC-236ea. Even
then the boiling performance of HFC-
236ea was either equal to or greater
than the performance of CFC-114 for
all tested parameters.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Air Pollution
Prevention and Control Division, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The U.S. Navy presently uses CFC-114
as the working refrigerant in shipboard
and submarine chiller units. With the man-
datory phaseout of CFCs dictated by the
Montreal Protocol, it is imperative for the
Navy to find a replacement that is envi-
ronmentally safe and possesses similar
performance characteristics to CFC-114.
Currently, one of the leading candidates
to replace CFC-114 is HFC-236ea. This
-------
alternative refrigerant is the focus of the
results presented here. There are several
reasons for choosing this refrigerant to
replace CFC-114. First, there is currently
a commercial production route available
for acquiring the refrigerant. Of special
importance, the operating capacities, pres-
sures, and temperatures are very similar
to those of CFC-114, and initial modeling
indicates that the performance is within
1% of that of CFC-114.
This research focuses on comparing the
refrigerants not only in condensation and
pool boiling but also with various tube
surfaces. Horizontal, integral finned tubes
have been in service for over 40 years,
and these tubes are widely used because
of their higher performance compared to
plain tubes.
The scope of this project was:
• Modify an existing spray evaporation
test facility so it can perform conden-
sation and pool boiling tests using a
two-pass single-tube setup.
• Test alternative refrigerant HFC-236ea
and compare its performance to CFC-
114 as the reference fluid.
• Evaluate the plain and 26 and 40 fpi
tubes for condensation.
• Evaluate the plain and 26 and 40 fpi
tubes for flooded evaporation.
• Investigate oil effects in pool boiling
on the shell-side heat transfer perfor-
mance by varying the oil concentra-
tion from 0 to 3%.
• Compare results to published correla-
tions for condensation and pool boil-
ing.
Experimental Apparatus
The test facility used in this study was
initially used for spray evaporation testing;
however, it was redesigned and modified
for use with condensation, pool boiling, or
spray evaporation testing. During conden-
sation, the rig was capable of producing
saturated or superheated vapor. During
pool boiling or spray evaporation, the test
facility was capable of testing pure refrig-
erants or refrigerant/lubricant mixtures. The
test facility is described in detail in the full
report.
During some boiling experiments, lubri-
cant was mixed with the refrigerant. Misci-
bility and solubility testing for CFC-114
and HFC-236ea were performed previous
to this research in another portion of the
project. These results along with other
criteria such as material compatibility de-
termined which lubricants would be used
in the refrigerant/lubricant mixtures. The
mineral oil used with CFC-114 was York
"C" with a viscosity of 315 SUS (68 cSt-
centistoke— at 40°C). The miscibility data
also showed that a synthetic ester refrig-
erant oil was to be used with HFC-236ea.
This lubricant is a polyol-ester oil with a
viscosity of 340 SUS. The trade name is
Castrol Icematic SW-68. The two lubri-
cants were miscible with the correspond-
ing refrigerants over the entire range of
conditions tested in this research.
Results and Discussion
The main objective of this study was to
conduct an experimental heat transfer
evaluation comparing the performance of
CFC-114 and HFC-236ea in the conden-
sation and pool boiling environments. The
condensation testing included an investi-
gation of saturated and superheated va-
por on fin-tube surfaces. The pool boiling
research involved nucleate boiling of pure
refrigerant and refrigerant/lubricant mix-
tures on fin-tube surfaces.
All of the tubes used in this study had a
nominal outside diameter of 19.1 mm (0.75
in.) and a length of 838.2 mm (33 in.).
The shell-side heat transfer coefficients
presented in this study were based on the
outside surface area of a corresponding
smooth tube, with the outer diameter mea-
sured over the surface enhancement.
Therefore, the calculated heat transfer co-
efficient takes into account the area en-
hancement, fin efficiency, and surface en-
hancement of the tubes tested.
Condensation Heat Transfer
The refrigerants CFC-114 and HFC-
236ea were evaluated in the condensa-
tion environment on the plain and 26 and
40 fpi tube surfaces. In addition, the ef-
fects on the heat transfer performance
from condensing superheated vapor were
investigated with CFC-114. During satu-
rated vapor testing, the saturation tem-
perature was held constant at 40°C. For
condensation of superheated vapor, the
saturation temperature was also 40°C, but
the incoming vapor was 3 to 5°C higher
than Tsat.
For condensation of both refrigerants,
the integral-fin tubes yielded heat transfer
coefficients approximately four times those
produced from the plain tube. In addition,
all combinations of the finned tubes and
refrigerants produced similar shell-side
condensation coefficients in the heat flux
range tested, with a maximum deviation
of 9%.
The results also showed that the con-
densation of superheated vapor had neg-
ligible effects on the shell-side heat trans-
fer coefficient with respect to saturated
vapor results. The superheated vapor data
for the 26 and 40 fpi tubes were within 5
and 3%, respectively, of the saturated va-
por results for the same tube surface.
The correlation comparison made with
the plain tube results showed excellent
agreement with the Nusselt correlation.
The CFC-114 and HFC-236ea data were
predicted within +3 and +10%, respec-
tively. The Beatty and Katz correlation was
able to predict the 26 fpi tube data for
both refrigerants with a maximum devia-
tion of 15%. The predictions for the 40 fpi
tube resulted in larger deviations. The
Beatty and Katz correlation predicted the
40 fpi tube data within 18 and 21% for
CFC-114 and HFC-236ea, respectively.
The two refrigerants produced similar
performance characteristics in condens-
ing vapor on integral-fin tubes, so the tran-
sition to HFC-236ea should be accom-
plished without major modifications to ex-
isting condensers. Overall, the above in-
formation shows that HFC-236ea is a valid
replacement for CFC-114 in the conden-
sation environment.
Pool Boiling Heat Transfer
CFC-114 and HFC-236ea were evalu-
ated in the pool boiling environment on
plain and 26 and 40 fpi tube surfaces. In
addition, this study investigated the ef-
fects of small concentrations of oil on the
heat transfer performance. The concen-
trations tested were 1 and 3% by mass
using a 68 cSt mineral oil for CFC-114
and a 340 SUS polyol-ester oil for HFC-
236ea. During pool boiling, data were
taken at a constant saturation tempera-
ture of 2°C for both pure refrigerant and
refrigerant/lubricant mixtures.
The pool boiling results for the pure
refrigerants show that the tube perfor-
mance for CFC-114 and HFC-236ea fall
in the following order from high to low: 26
fpi, 40 fpi, and plain tube. The 26 fpi tube
produced boiling coefficients for CFC-114
that were 12 and 30% higher than for the
40 fpi tube and the plain tube, respec-
tively. For HFC-236ea, the 26 fpi tube
outperformed the 40 fpi tube by 18% and
the plain tube by 41%. In addition, HFC-
236ea produced higher boiling coefficients
than CFC-114 for all tubes tested. The
maximum increase in boiling with HFC-
236ea was 39% for the 26 fpi tube and
34% for the 40 fpi tube.
The lubricant addition with CFC-114 pro-
duced enhancements in the boiling coeffi-
cients for the three tubes tested with oil.
The maximum enhancement occurred at
a 3% oil concentration for each tube. The
addition of oil at a 1% concentration im-
proved the heat transfer coefficients for
the 26 fpi tube by 27%, while the 3% oil
-------
concentration only showed minor improve-
ment over the 1% results. The 40 fpi tube
produced similar trends to the 26 fpi tube
at both oil concentrations.
Pool boiling of HFC-236ea with the
polyol-ester oil produced consistent de-
creases in the heat transfer performance
at both concentrations. The 26 fpi tube
showed a decrease in performance of 6
and 17% at oil concentrations of 1 and
3%, respectively. The 40 fpi tube had only
a 10% decrease in the boiling coefficients
at a 3% concentration with respect to the
pure refrigerant. At an oil concentration of
1%, the 40 fpi tube showed negligible oil
effects in the low heat flux range. It is
evident that the oil enhancement gained
from the turbulent mixing within the foam-
ing layer is dependent upon the type of
oil. The mineral oil used with CFC-114
showed a general improvement in the heat
transfer performance, while the polyol-es-
ter oil consistently degraded the perfor-
mance of HFC-236ea.
It is also worth noting that, even though
the pure HFC-236ea results are higher
than those for CFC-114, the oil effects on
both refrigerants cause the boiling coeffi-
cients to be within 12% for the 26 fpi tube
at an oil concentration of 3%. Therefore,
the addition of oil decreased the deviation
in the heat transfer coefficients between
the two refrigerants. CFC-114 consistently
produced higher boiling coefficients than
HFC-236ea for both finned tubes at a 3%
oil concentration.
A review of the above information shows
that HFC-236ea is a valid replacement for
CFC-114 in the nucleate boiling environ-
ment. The boiling performance of HFC-
236ea was either equal to or greater than
the performance of CFC-114 for all testing
parameters. With the similar boiling char-
acteristics, transition to HFC-236ea in a
flooded evaporator would be relatively
simple.
Summary
The two refrigerants produced similar
performance characteristics in condens-
ing vapor on integral-fin tubes, so that the
transition to HFC-236ea should be ac-
complished without major modifications to
existing condensers. The results also
showed that the condensation of super-
heated vapor had negligible effects on the
shell-side heat transfer coefficient with re-
spect to the saturated vapor results. The
superheated vapor data for the 26 and 40
fpi tubes were within 5 and 3%, respec-
tively, of the saturated vapor results for
the same tube surface.
HFC-236ea produced higher boiling co-
efficients than CFC-114 for all tubes tested.
In addition, the 26 fpi tube outperformed
the 40 fpi tube by 18% and the plain tube
by 41% for HFC-236ea. The maximum
increase in the boiling heat transfer coeffi-
cient with HFC-236ea was 39% for the 26
fpi tube and 34% for the 40 fpi tube. The
mineral oil used with CFC-114 produced a
general improvement in the heat transfer
performance, while the polyol-ester oil con-
sistently degraded the performance of
HFC-236ea. Even then, the boiling perfor-
mance of HFC-236ea was either equal to
or greater than the performance of CFC-
114 for all testing parameters.
-------
W. W. Huebsch and M. B. Pate are with Iowa State University, Ames, IA 50011.
Theodore G. Brna is the EPA Project Officer (see below).
The complete report, entitled "Heat Transfer Evaluation of HFC-236ea and CFC-114 in
Condensation and Evaporation," (Order No. PB96-183900; Cost: $31.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT NO. G-35
EPA/600/SR-96/070
------- |