United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-95/168 May 1998
Project Summary
Two-Phase Flow of Two HFC
Refrigerant Mixtures Through
Short-Tube Orifices
W. Vance Payne and Dennis L. O'Neal
An experimental investigation was
performed to develop an acceptable
flow model for short-tube orifice expan-
sion devices used in heat pumps. The
refrigerants investigated were two
hydrofluorocarbon (HFC) mixtures con-
sidered hydrochlorofluorocarbon
(HCFC)-22 replacements: HFC-32/HFC-
125/HFC-134a (23/25/52% on a mass
percentage basis) and HFC-32/HFC-125
(50/50%). Tests for both refrigerants
were performed to generate data at
varying operating conditions with 12
short tubes. The tests included both
single- and two-phase flow conditions
at the inlet of the short tube with differ-
ent oil concentrations. Experimental
data were presented as a function of
major operating parameters and short-
tube diameter. Based on test results
and analysis, a mass flow model was
developed. The test results for both re-
frigerants showed that the mass flow
rate was strongly dependent on up-
stream conditions, but slightly depen-
dent on downstream conditions. The
mass flow rate was extremely sensi-
tive to changes in short-tube diameter.
The presence of oil below a concentra-
tion of approximately 2% would appear
to affect the mass flow rate only slightly
(less than 5%). It was found that the
semi-empirical flow model estimates
were in good agreement with labora-
tory results for both single- and two-
phase flow entering the short tubes.
This Project Summary was developed
by EPA's National Risk Management Re-
search Laboratory's Air Pollution Pre-
vention and Control Division, Research
Triangle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Overview
The need for new refrigerants was es-
tablished when scientists first realized the
ozone depleting effects of chlorine-con-
taining refrigerants. The chlorine in these
refrigerants is capable of reaching the
upper atmosphere where one chlorine
atom can destroy more than 100,000
ozone atoms. Section 608 of the Clean
Air Act Amendments of 1990 (CAAA) pro-
hibits the venting of ozone depleting re-
frigerants as of July 1, 1992. In addition,
the CAAA also require the Environmental
Protection Agency to develop regulations
limiting the emissions of ozone depleting
refrigerants. Efforts are currently under-
way to find replacement refrigerants be-
fore the complete phaseout of most of
these refrigerants in January 1996.
Much of the effort to replace chlorofluo-
rocarbon (CFC) and hydrochlorofluo-
rocarbon (HCFC) refrigerants has cen-
tered on development of refrigerant mix-
tures that could replace HCFC-22 used
in domestic heat pumps and air condi-
tioners. An important component in these
systems is the expansion device. Because
of their low cost, short-tube orifices have
been chosen by several manufacturers
as expansion devices in their systems
instead of the more expensive thermal
expansion devices. Designing a system
with an orifice requires knowledge of the
flow characteristics of short-tube orifices.
Recent work on orifices has focused on
-------
CFC-12 and HCFC-22. In addition, unpub-
lished data exist on HFC-134a and the ef-
fect of lubricants on flow characteristics.
To develop an acceptable flow model, an
experimental investigation was performed.
The refrigerants investigated were two HFC
mixtures considered HCFC-22 replace-
ments: HFC-32/HFC-125/HFC-134a (23/257
52% on a mass percentage basis) and
HFC-32/HFC-125 (50/50%). Tests for both
refrigerants were performed to generate
data at varying operating conditions with 12
short tubes. The tests included both single-
and two-phase flow conditions at the inlet
of the short tube with different oil concen-
trations. Experimental data were presented
as a function of major operating param-
eters and short-tube diameter. Based on
test results and analysis, a mass flow model
was developed.
Short-tube orifices 0.5-1.0 in. (12.7-25.4
mm) long with diameters of 0.0431-
0.0763 in. (1.09-1.94 mm) were tested for
the two refrigerant mixtures at selected
testing conditions found in heat pump or
air-conditioner applications. The general
trends observed in both refrigerants were
consistent with the previous results for
HCFC-22. At the same condensing tem-
perature conditions, the mass flow rate of
the ternary mixture varied by approxi-
mately ±5% (compared to HCFC-22) while
the binary refrigerant flow rate averaged
15% higher than that for HCFC-22 due to
its higher operating pressures. The maxi-
mum percent difference occurred at high
levels of subcooling and high qualities
(i.e., a high mass percentage of the re-
frigerant is in vapor form). Generally, flow
trends of both refrigerants were also quite
similar to each other even though mass
flow rate for the binary mixture was ap-
proximately 6-15% higher than that for the
ternary mixture. The test results for both
refrigerants showed that the mass flow rate
was strongly dependent on upstream con-
ditions, but slightly dependent on down-
stream conditions.
The major factor affecting the flow rate
was upstream conditions. For both
subcooled liquid and two-phase flow en-
tering a short tube, the mass flow rate
was directly proportional to upstream pres-
sure. The increase in mass flow rate with
upstream pressure was accelerated for
high levels of upstream subcooling. The
refrigerant flow rate increased in a poly-
nomial fashion with increases in upstream
subcooling. The mass flow rate contin-
ued dropping inside the saturation region
as the quality increased.
The mass flow rate was extremely sen-
sitive to changes in short-tube diameter.
The binary mixture showed more effects
of short-tube diameter on flow rate than
the ternary mixture. While the ternary re-
frigerant mass flow rate in the subcooling
region varied approximately with the
square of the orifice diameter, the binary
refrigerant mass flow rate tended to vary
more closely with diameter raised to the
2.6 power. The effects of diameter varied
as a function of upstream subcooling and
quality.
The effects of oil contamination on the
flow through short tubes were studied by
comparing test results for oil contaminated
refrigerants with pure refrigerants. The
presence of oil below a concentration of
approximately 2% would appear to affect
the mass flow rate only slightly (less than
5%). For both refrigerants at high levels
of subcooling [beyond 10°F (5.6°C)], the
addition of oil varied flow rate from the pure
case by ±5%. As subcooling decreased,
the decrease in mass flow rate (compared
to the pure case) followed a linear trend.
To predict the mass flow rate, the semi-
empirical models for both single- and two-
phase flow at the inlet of the short tubes
were developed by empirically correcting
the modified orifice equation as a func-
tion of normalized forms of operating con-
ditions. Due to the limited range of oil
concentrations tested, new coefficients
were calculated for each oil concentra-
tion tested. It was found that the semi-
empirical flow model estimates were in
good agreement with laboratory results
for both single- and two-phase flow en-
tering the short tubes.
The tests for the effects of oil concen-
tration were performed over a limited
range of test conditions and short-tube
diameters with one lubricant: RL 32S
POE. The polyol ester lubricant was of a
single viscosity, 32 centistokes. Oils of
higher viscosity could produce different
results from that seen here. Also, the mis-
cibility of the oil and refrigerant was not
factored into model development. Al-
though this oil was reported to be mis-
cible with the refrigerants under the test
conditions, other oils may not show this
same behavior. Further study would be
required to characterize the effects of oil
concentration with short-tube geometry
and test conditions.
It was noted earlier that the limitations
on the application of the semi-empirical
flow model were imposed by the range of
the experimental data. Therefore, a more
comprehensive semi-empirical model may
need to be developed to obtain wider appli-
cability.
-------
I/I/ Vance Payne and Dennis L O'Neal are with Texas A& M University, College
Station, TX 77843.
Robert V. Hendriks is the EPA Project Officer (see below).
The complete report, entitled "Two-Phase Flow of Two HFC Refrigerant Mixtures
Through Short-Tube Orifices," (Order No. PB98-142 045; Cost: $36, subject to
change) will be available only from
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
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
United States
Environmental Protection Agency
CenterforEnvironmental Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use
$300
EPA/600/SR-95/168
------- |