United States National Risk Management

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-97/058 July 1997

Project Summary

D. T. Ray, M. B. Pate, and H. N. Shapiro

Comparison of CFC-114 and

HFC-236ea Performance in

Shipboard Vapor Compression

Systems

In compliance with the Montreal Protocol and Department of Defense directives, alternatives to the refrigerant

1,2-dichloro-tetrafluoroethane (CFC-114)

are being investigated by the U.S. Navy

and the U.S. EPA for use in shipboard

chillers. The refrigerant 1,1,1,2,3,3-

hexafluoropropane (HFC-236ea) has

emerged as a candidate for drop-in replacement.

A computer model was developed for

comparing these two refrigerants in a

simulated 440-kW centrifugal chiller

system. Equations for modeling each

system component were developed and

solved using the Newton-Raphson

method for multiple equations and unknowns. Correlations were developed

for CFC-114 and HFC-236ea boiling and

condensing coefficients taken at the

Iowa State Heat Transfer Test Facility.

The model was tested for a range of

inlet condenser water temperatures and

evaporator loads. The results are presented and compared with data provided by the Naval Surface Warfare

Center (NSWC) in Annapolis, MD.

The experimental data provided by

the NSWC sufficiently validate the

model, and the simulation model predicts that HFC-236ea would perform favorably as a drop-in substitute for CFC114.

Several recommendations are discussed which may further improve the

performance of HFC-236ea in Navy chillers. Recommendations include adjusting the load of the evaporator to achieve

positive gage pressure, use of a purge

device, use of a variable speed compressor, further testing with azeotropic

mixtures, and use of high performance

tubes in the heat exchangers.

The work represented by the report

was funded by the Department of

Defense’s Strategic Environmental Research and Development Program

(SERDP).

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

Background

Fully halogenated chlorofluorocarbons

(CFCs) are manufactured chemicals with

properties that make them useful for such

applications as aerosol propellants, foam

blowing agents, solvents, and refrigerants

for automotive, residential, and commercial applications. CFCs became popular

in part because they were chemically

stable, nonflammable, and nontoxic.

Ironically, the chemical stability of CFCs

is the cause for their present perceived

threat to the environment. Scientific evidence suggests that the harmful alterations

of the Earth’ s atmosphere occurring from

the use of CFCs are of regional and global proportions. As early as 1974, concerns about the potential harmful environmental effects associated with the use of

CFCs were raised when it was suggested

that the chlorine from these compounds

could efficiently destroy stratospheric

ozone. Additionally, there is a growing consensus among scientists that CFCs may

contribute to global warming.
2

CFC-114 has been in use on Navy ships

since 1969 and has demonstrated excellent reliability. However, design improvements have often lagged behind commercial advancements in compressor technology, advanced heat transfer surface

technology, and intelligent control system

technology. Trichlorofluoromethane (CFC11), used extensively on commercial ships,

was found to be unsatisfactory to the Navy

because of problems unique to surface

craft and submarine applications. For example, CFC-11 decomposes at high temperatures causing toxicity problems on

submarines as the air is recycled in hightemperature air purification equipment. In

contrast, CFC-114 remains stable at high

temperatures.

Other requirements unique to the Navy

include the need for small inventory and

small components due to space constraints. Energy efficiency has been a low

priority in the past, but with shrinking defense budgets, it has become more important. Additionally, surface craft and submarines need to operate silently in tactical

situations, and recycle air in living spaces.

Cooling systems must be able to operate

at as low as 10% of maximum capacity

during normal peacetime operations yet

handle dramatic increases in load when

firing weapons in combat or training situations. Fully halogenated refrigerants, such

as CFC-114, generally exhibit the best

compatibility and impose the least restriction in choice of materials; a suitable replacement must display similar material

compatibility. Other requirements for a suitable replacement include meeting safety

and environmental standards for toxicity,

flammability, ozone depletion potential, and

global warming potential.

A need exists for a suitable near-term

replacement for existing equipment using

CFC-114 . Because industry attention has

been focused on CFC-11 and dichlorodifluoromethane (CFC-12) replacements, the

Navy must devote substantial resources

to address the CFC-114 problem. Current

potential alternatives for CFC-114 are not

well developed, and significant modifications to system equipment will likely be

necessary in order to accommodate them.

 At one time, 1,1,1-trifluoro-2,2-

dichloroethane (HCFC-123) and 1,1,1,2-

tetrafluoro-2-chloroethane (HCFC-124)

were leading alternatives for CFC-11 and

CFC-114, respectively. When it became

apparent that these HCFCs would also be

phased out as environmentally unsuitable,

the EPA began investigating “backup” alternatives. As a result, a series of propanes

have emerged as candidate replacements

for CFC-114.

HFC-236ea is a promising candidate

for replacing CFC-114 for several reasons.

First, a commercial production route is

available for large quantities through the

use of hexafluoropropylene. Second, initial modeling conditions appear favorable

as a drop-in substitute, with modeled performance being within 1% of CFC-114

and operating capacities, pressures, and

temperatures matching closely. Flammability tests, materials compatibility tests,

and oil miscibility tests appear favorable.

Acute inhalation test results indicate lower

acute toxicity than CFC-114. In addition,

estimates predict that HFC-236ea has a

short atmospheric lifetime.

Objective

The objective of this work is to evaluate

HFC-236ea as a potential near-term replacement for CFC-114 in shipboard chillers using a computer-simulated system

with data from the Iowa State University

Heat Transfer Test Facility and also using

data provided by the NSWC. With the

information gathered, appropriate design

recommendations to accommodate the use

of HFC-236ea in shipboard chillers are

offered.

Scope

A computer program has been developed that simulates the performance of a

440-kW capacity, single-stage, centrifugal,

chilled-water air-conditioning plant. The

design conditions shown in Table 1 are

based on the design of a typical air-conditioning plant in use on Navy surface craft

and submarines.

Given the entering and leaving temperatures of the chilled water, the entering temperature of the condenser water,

and the flowrates of the chilled water and

condenser water, the model predicts the

required compressor power and the refrigerant saturation temperatures in the

heat exchangers. With knowledge of fluid

properties and tube geometries, the performance of the system with different refrigerants and enhanced surface tubes can

be compared under similar operating conditions. For this study, the model is used

to compare refrigerants CFC-114 and

HFC-236ea using 10.23 fins per centimeter (fpc) tubes in the condenser and evaporator. The results are presented with design recommendations.

Model and Experimental

Results Comparison

The NSWC, in cooperation with the U.S.

EPA, has tested CFC-114 and HFC-236ea

in a 440-kW laboratory centrifugal chiller

representative of those used in the U.S.

Navy’s fleet of surface craft and submarines. The laboratory chiller is fully instrumented.

Modeled and measured performance

values were compared for both refrigerants. The measured compressor power

provided by the NSWC was calculated

from measurements of torque and angular velocity of the compressor shaft. The

modeled value of compressor power is

the rate of work performed directly on the

fluid and does not include the mechanical

or heat losses as the power is transferred

from the compressor shaft to the impeller

and ultimately to the working fluid. A linear relationship was found to exist between the shaft power and the power transferred to the working fluid for the data

provided by the NSWC. The correlation

was applied to the results of the model as

an assumed mechanical efficiency.

Even with an efficiency factor applied,

the model underpredicts the amount of

compressor power required to meet the

specified load by 6 to 26%. This could be

related to the use of inlet guide vanes to

the compressor which are not modeled in

this study.

Table 1. Simulated Design Conditions

Component Design Condition Value

Evaporator Chilled Water Flowrate 28.4 L/s

Evaporator Entering Chilled Water Temperature 10.7°C

Evaporator Leaving Chilled Water Temperature 7.0°C

Condenser Water Flowrate 31.5 L/s

Condenser Entering Water Temperature 31.4°C
3

A comparison of the system coefficient

of performance calculated using NSWC’s

measurements and that predicted using

the model developed in this study shows

that the model overpredicts the coefficient

of performance for HFC-236ea by as much

as 38%. The result is consistent with what

one would expect when comparing modeled with measured results. Since models

often make use of simplifying assumptions, the results tend to be idealized.

One would expect to see the actual results to be less favorable than modeled

results.

A comparison of modeled and measured refrigerant temperatures in the condenser shows that the model predictions

compare well with CFC-114 data; however, it underpredicts the condenser temperature for most HFC-236ea data by as

much as 9°C. This difference could be

caused, in part, by poor heat transfer in

the condenser with HFC-236ea. If this were

the case, it would take a higher condenser

temperature to overcome whatever thermal resistance is present. More compressor power would be required to provide

this additional temperature lift, resulting in

lower system performance. Because the

refrigerant temperature in the condenser

is closely tied to the condensing coefficient by use of the log mean temperature

difference equation for heat transfer in the

condenser, one would also expect to see

a difference in a comparison of the measured and modeled condensing coefficients. As the condensing temperature increases while entering and leaving, cooling water temperatures remain constant

and the log mean temperature difference

increases. This would result in a modeled

decrease in the condensing coefficient.

An example of when conditions may

exist in the condenser that hamper heat

transfer is when noncondensable gases,

left unpurged, accumulate in the upper

vapor space of the condenser. This is a

plausible explanation for the differences

in condenser saturation temperatures observed. For HFC-236ea, both the measured and modeled evaporator temperatures are near 2°C. The corresponding

saturation pressures for these saturation

temperatures are less than atmospheric

pressure. This could cause noncondensable gases to leak into the evaporator

due to negative gage pressure. These

gases would migrate and collect in the

condenser and could significantly degrade

the performance of the condenser and the

entire system. If air, in fact, was present

in the condenser, it would drive the outside heat transfer coefficient down, resulting in a high condenser saturation temperature.

A comparison of modeled and measured cooling water temperatures leaving

the condenser shows that modeled values are within 0.2°C of measured values.

This is consistent with trends observed for

condenser capacity. This is expected,

since the rate of heat transfer and the

temperature of the water leaving the condenser are the two variables in the waterside heat transfer equation for the condenser.

A comparison of modeled and measured evaporator temperature shows that

modeled and measured boiling coefficients

compare well despite some variance. One

would therefore expect to see a variance

in a comparison of boiling coefficients since

these variables must balance in the log

mean temperature difference equation for

the heat transfer in the evaporator.

A comparison of modeled and measured evaporator capacity shows that the

values match because the capacity is calculated based on three common inputs:

entering and leaving evaporator water temperatures, and evaporator water flowrate.

Thus, the evaporator capacity remains

fixed as long as these three input values

are held constant.

A comparison of modeled and measured refrigerant flowrate shows that the

model consistently predicts the flowrate

for both refrigerants within ±5%. This suggests that the enthalpy differences also

compare favorably since the rate of heat

transfer in the evaporator is constant and

is equal to the refrigerant mass flowrate

times the enthalpy difference across the

evaporator.

Comparison of CFC-114 and

HFC-236ea Performance

The model is used to predict the performance of both refrigerants, CFC-114 and

HFC-236ea, through a range of operating

conditions. This is done by using the fleet

design point as the default and varying

one parameter at a time over an appropriate range to see the effects on the system. The results yield additional insight as

to the possible suitability of HFC-236ea

as a drop-in substitute for CFC-114.

Entering Condenser Water

Temperature

As the Navy operates its fleet around

the world, ships encounter a wide range

of condenser water temperatures because

sea water is used directly in the heat

exchanger to remove heat from the working fluid. Chillers for Navy ships are designed for a condenser water temperature

of 31.4°C; however, temperatures encountered may range from -1.3 to 35.3°C depending on where the ship is operating.

Since heat transfer in the condenser is

driven by the temperature difference between heat transfer fluids, a condenser

water temperature that is too high could

hamper the performance of the condenser

and subsequently the entire refrigeration

cycle. Thus, the entering condenser water

temperature is significant to the performance of the overall system.

The predicted power required to drive

the compressor more than doubles for

both refrigerants as the water temperature entering the condenser increases from

16 to 38°C. The trend is expected since

better heat transfer occurs as the temperature of the cooling water entering the

condenser decreases. The efficiency of

the refrigeration cycle should thereby improve, resulting in less power input required to the compressor. Additionally, the

model predicts that HFC-236ea used as a

drop-in substitute for CFC-114 may result

in energy savings. At the design point of

operation, the predicted power required to

drive the compressor using HFC-236ea is

91.4% of the power required using CFC114. The model predicts that, for any cooling water temperature, the power required

for a refrigeration cycle using HFC-236ea

as a drop-in will be significantly less than

the same cycle using CFC-114 as the

working fluid. The predicted savings in

power consumption by using HFC-236ea

at the design point of operation is 550

kW.

The NSWC data for CFC-114 show

nearly constant compressor power over

the range of entering condenser water

temperatures. The data for HFC-236ea

show significant scatter. Both the CFC114 and HFC-236ea measured values of

required compressor power are above the

predicted values for the range of entering

condenser water temperatures.

The coefficient of performance is predicted to decrease as the inlet condenser

water temperature increases. Additionally,

at the design point of 31.4°C, the predicted coefficient of performance for HFC236ea is 4.25 compared to 3.91 for CFC114. The model predicts better performance using HFC-236ea over the range

of condenser water temperatures simulated.

The measured values of coefficient of

performance for both CFC-114 and HFC236ea are less than predicted values. As

the entering condenser water temperature

increases, the measured and predicted

values of the coefficient of performance

move toward better agreement.
4

Modeled trends predict that both performance indicators—compressor power requirement and coefficient of performance—

improve as the inlet condenser water temperature decreases from 38 to 16°C since

the required power consumption decreases

and the coefficient of performance increases. These are expected trends since

lower condenser water temperatures provide a higher temperature difference between the heat transfer fluids, resulting in

increased cooling potential in the condenser. HFC-236ea is also predicted to

outperform CFC-114 over a range of inlet

condenser water temperatures, partly because measured heat transfer coefficients

for HFC-236ea were found to be greater

than those of CFC-114.

The predicted values for CFC-114 and

HFC-236ea saturation temperature in the

condenser for a given temperature of the

cooling water entering the condenser are

nearly identical. The measured values of

condenser saturation temperature for CFC114 agree with the predicted values, while

the HFC-236ea data show the same trend

but are generally higher than predicted.

The predicted saturation temperature for

HFC-236ea in the evaporator as a function of the entering condenser water temperature is higher than predicted for CFC114 over the range of entering condenser

water temperatures. The HFC-236ea data

compare well with predicted values, while

there appears to be less agreement between measured and modeled values for

CFC-114.

The evaporator capacity is compared

with the temperature of the water entering

the condenser. Since the evaporator capacity is fixed by holding the water-side

conditions constant, the predicted capacity values for HFC-236ea and CFC-114

are identical. Scatter is shown in the measured values of HFC-236ea, while the

measured values of CFC-114 agree well

with predicted values.

The condenser capacity is also compared with the entering condenser water

temperature. Predicted values for CFC114 and HFC-236ea are nearly equal. For

both CFC-114 and HFC-236ea, measured

capacity values are higher than predicted,

with significant scatter observed in the

HFC-236ea data.

The refrigerant mass flowrate is compared with the temperature of the water

entering the condenser. The measured

and predicted values for CFC-114 are in

close agreement, while there is significant

scatter in the data for the flowrate of HFC236ea. As condensing water temperature

increases, an increasing trend is predicted

in refrigerant mass flowrate.

Entering Evaporator Water

Temperature

In this situation, the evaporator load is

defined by a constant chilled water flowrate

of 28.4 L/s, a chilled water inlet temperature of 7°C, and a chilled water exit temperature ranging from 9.2 to 12.6°C. Additionally, the temperature of the water entering the condenser is held constant at

31.4°C, and the flowrate of the condenser

water is held constant at 31.5 L/s. As the

cooling load is systematically varied, the

effect on various performance indicators

may be observed.

As the temperature of the chilled water

returning from the load and entering the

evaporator increases while other design

operating conditions remain constant, increasing power is required to drive the

compressor. This is an expected trend

because, as the water temperature entering the evaporator increases, the load is

increased in the evaporator. In order to

accommodate this increased load, either

the refrigerant mass flowrate must increase

or the enthalpy difference across the

evaporator must increase in order to provide enough heat transfer to maintain a

constant chilled water exit temperature.

The result is the need for more power to

drive the compressor. A comparison of

HFC-236ea and CFC-114 shows that, for

the compared range of chilled water temperatures entering the evaporator, HFC236ea always required less compressor

power than CFC-114 when modeled as

the working fluid.

A comparison of the coefficient of performance as a function of chilled water

temperature entering the evaporator shows

that as the temperature increases the coefficient of performance decreases. This

means that as temperature increases, the

rate of increase of power required by the

compressor is greater than the increase

in cooling capacity. Additionally, the coefficient of performance for HFC-236ea is

higher than that for CFC-114 for the range

of temperatures modeled.

Design Recommendations

One way to improve the performance of

the fleet’s 440-kW chiller and allow the

use of HFC-236ea as an alternative working fluid is to reduce the load on the

evaporator by increasing the temperatures

of the chilled water entering and leaving

the evaporator by a few degrees. This

would allow the refrigerant temperature in

the evaporator to rise slightly, which, in

turn, would result in an evaporator pressure that is above atmospheric pressure.

With a positive gage pressure in the evaporator, there is less possibility of

noncondensable gases and contaminants

leaking into the system where they can

accumulate in the condenser and reduce

performance. The low evaporator temperatures and high condenser temperatures

reported by the NSWC suggest the possibility of this occurrence. This solution

avoids the cost of redesigning system components.

A purge device at the high point of the

condenser would allow purging of

noncondensable gases that might accumulate there. If noncondensables are a

persistent problem, the purge unit may be

malfunctioning or the system may have

an air leak larger than the purge unit can

handle.

A variable-speed compressor would

eliminate the need for hot gas by-pass or

the extensive use of inlet guide vanes in

the compressor to control the refrigerant

flow. A variable-speed chiller would allow

maximum system performance to be realized over a broad range of operating conditions, resulting in maximum energy savings.

Another possible improvement might be

to mix HFC-236ea with other non-CFC

refrigerants to form an azeotropic mixture

with properties that allow the saturation

point in the evaporator to stay above atmospheric pressure. The mixture could

be chosen so as to maintain the desired

properties of HFC-236ea.

Additionally, better performance in the

Navy’s fleet air-conditioning units could

be realized by investing in commercially

available high performance heat exchanger

tubes. While not reported in this study,

Turbo B tubes were simulated with CFC114 and HFC-236ea under fleet design

conditions and were predicted to perform

significantly better than 10.23 fpc tubes in

both the evaporator and the condenser.

Finally, the model predicts that HFC236ea used as a drop-in substitute for

CFC-114 without any design modifications

may result in energy savings. The model

predicts that, for any set of conditions, the

power required for a refrigeration cycle

using HFC-236ea as a drop-in will be significantly less than the same cycle using

CFC-114 as the working fluid. The predicted savings in power consumption by

using HFC-236ea at the design point of

operation is 8.6%. If HFC-236ea is to be

used only as a near-term replacement, it

may be appropriate to use it without making any significant design changes to the

system.
5

Conclusion

The Montreal Protocol began a worldwide drive to eliminate the production of

CFCs and other chemicals which are

thought to be harmful to the environment.

As a result of the restrictive legislation

that followed, there is an immediate need

to replace CFC-114 which is used extensively in U.S. Navy surface craft and submarines. Preliminary research conducted

by the EPA suggested that HFC-236ea

might perform suitably as a near-term dropin replacement for CFC-114. However, at

the time of this study, heat transfer data

for HFC-236ea were not available.

For this reason, a computer model was

developed for comparing CFC-114 and

HFC-236ea in a simulated 440-kW laboratory centrifugal chiller system representative of those found in the U.S. fleet. The

model is semi-empirical, combining thermodynamic and heat transfer theory, as

well as boiling and condensing heat transfer coefficient data measured at the Iowa

State University Heat Transfer Test Facility.

The NSWC in Annapolis, MD, also provided data for this study. A 440-kW laboratory centrifugal air-conditioning plant and

HFC-236ea were used for the data collection. The experimental data provided by

the NSWC were compared with the modeled results.

The model was tested for a range of

inlet condenser water temperatures, entering and leaving chilled water temperatures, and evaporator and condenser water flowrates. The simulation model predicts that HFC-236ea would perform favorably as a drop-in substitute for CFC114 .

Additionally, several recommendations

were provided for improved performance

using HFC-236ea in centrifugal chiller systems. Design recommendations discussed

in this study include manipulating the

evaporator load to achieve positive gage

pressure in the evaporator, ensuring the

absence of noncondensable gases in the

system, using a variable-speed compressor with a fixed inlet guide vane angle to

the impeller, conducting further research

using azeotropic mixtures with HFC-236ea

as the major component, and installing

high-performance enhanced surface tubes

in both the evaporator and the condenser.

In summary, the simulation developed

in this study provides results that are consistent with the expected behavior of a

440-kW refrigeration system. The results

provided by the NSWC sufficiently validate the model. Finally, the results suggest that HFC-236ea would perform well

in existing CFC-114 centrifugal chillers,

although design modifications should be

considered for optimal performance.
6

D. T. Ray, M. B. Pate, and H. N. Shapiro are with Iowa State University, Ames, IA

50011.

Theodore G. Brna is the EPA Project Officer (see below).

The complete report, entitled "Comparison of CFC-114 and HFC-236ea Performance in Shipboard Vapor Compression Systems," (Order No. PB97-178735;

Cost: $25.00, 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-97/058

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POSTAGE & FEES PAID

EPA

PERMIT NO. G-35