United States National Risk Management

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-96/070 August 1996

E AP

Heat Transfer Evaluation of

HFC-236ea and CFC-114 in

Condensation and Evaporation

Project Summary

W. W. Huebsch and M. B. Pate

With the mandatory phaseout of chlorofluorocarbons (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 (CFC114) that is environmentally safe and

possesses similar performance characteristics. Currently, one of the leading candidates to replace CFC-114 is

hexafluoropropane (HFC-236ea). This

research focuses on comparing the refrigerants 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 evaporation testing; however, it was redesigned

and modified for use with condensation, 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 refrigerants or refrigerant/lubricant mixtures.

The test facility is described in detail in

the full report.

The two refrigerants produced similar performance characteristics in condensing vapor on integral-fin tubes, so

that the transition to HFC-236ea should

be accomplished without major modifications to existing condensers. The results also showed that the condensation of superheated vapor had negligible effects on the shell-side heat

transfer coefficient as compared to condensation 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 outperformed 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 HFC236ea 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, Research 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 mandatory phaseout of CFCs dictated by the

Montreal Protocol, it is imperative for the

Navy to find a replacement that is environmentally safe and possesses similar

performance characteristics to CFC-114.

Currently, one of the leading candidates

to replace CFC-114 is HFC-236ea. This
2

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, pressures, 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 condensation and pool boiling tests using a

two-pass single-tube setup.

• Test alternative refrigerant HFC-236ea

and compare its performance to CFC114 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 performance by varying the oil concentration from 0 to 3%.

• Compare results to published correlations for condensation and pool boiling.

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 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 refrigerants or refrigerant/lubricant mixtures. The

test facility is described in detail in the full

report.

During some boiling experiments, lubricant was mixed with the refrigerant. Miscibility 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 determined 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 refrigerant 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 lubricants were miscible with the corresponding 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 condensation and pool boiling environments. The

condensation testing included an investigation of saturated and superheated vapor on fin-tube surfaces. The pool boiling

research involved nucleate boiling of pure

refrigerant and refrigerant/lubricant mixtures 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 measured over the surface enhancement.

Therefore, the calculated heat transfer coefficient takes into account the area enhancement, fin efficiency, and surface enhancement of the tubes tested.

Condensation Heat Transfer

The refrigerants CFC-114 and HFC236ea were evaluated in the condensation environment on the plain and 26 and

40 fpi tube surfaces. In addition, the effects on the heat transfer performance

from condensing superheated vapor were

investigated with CFC-114. During saturated vapor testing, the saturation temperature was held constant at 40o

C. For

condensation of superheated vapor, the

saturation temperature was also 40o

C, but

the incoming vapor was 3 to 5o

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 condensation of superheated vapor had negligible effects on the shell-side heat transfer 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 vapor 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%, respectively. The Beatty and Katz correlation was

able to predict the 26 fpi tube data for

both refrigerants with a maximum deviation 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 condensing vapor on integral-fin tubes, so the transition to HFC-236ea should be accomplished without major modifications to existing condensers. Overall, the above information shows that HFC-236ea is a valid

replacement for CFC-114 in the condensation environment.

Pool Boiling Heat Transfer

CFC-114 and HFC-236ea were evaluated in the pool boiling environment on

plain and 26 and 40 fpi tube surfaces. In

addition, this study investigated the effects of small concentrations of oil on the

heat transfer performance. The concentrations tested were 1 and 3% by mass

using a 68 cSt mineral oil for CFC-114

and a 340 SUS polyol-ester oil for HFC236ea. During pool boiling, data were

taken at a constant saturation temperature of 2o

C for both pure refrigerant and

refrigerant/lubricant mixtures.

The pool boiling results for the pure

refrigerants show that the tube performance 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, respectively. For HFC-236ea, the 26 fpi tube

outperformed the 40 fpi tube by 18% and

the plain tube by 41%. In addition, HFC236ea produced higher boiling coefficients

than CFC-114 for all tubes tested. The

maximum increase in boiling with HFC236ea was 39% for the 26 fpi tube and

34% for the 40 fpi tube.

The lubricant addition with CFC-114 produced enhancements in the boiling coefficients 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 improved the heat transfer coefficients for

the 26 fpi tube by 27%, while the 3% oil
3

Pool boiling of HFC-236ea with the

polyol-ester oil produced consistent decreases 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 foaming 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-ester oil consistently degraded the performance 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 coefficients 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 environment. The boiling performance of HFC236ea was either equal to or greater than

the performance of CFC-114 for all testing

parameters. With the similar boiling characteristics, transition to HFC-236ea in a

flooded evaporator would be relatively

simple.

Summary

The two refrigerants produced similar

performance characteristics in condensing vapor on integral-fin tubes, so that the

transition to HFC-236ea should be accomplished without major modifications to

existing condensers. The results also

showed that the condensation of superheated vapor had negligible effects on the

shell-side heat transfer coefficient with respect to the saturated vapor results. The

superheated vapor data for the 26 and 40

fpi 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 outperformed

the 40 fpi tube by 18% and the plain tube

by 41% for HFC-236ea. The maximum

increase in the boiling heat transfer coefficient 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 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 CFC114 for all testing parameters.

concentration only showed minor improvement over the 1% results. The 40 fpi tube

produced similar trends to the 26 fpi tube

at both oil concentrations.
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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

EPA/600/SR-96/070

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT NO. G-35