United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/SR-96/129 December 1996
4>EPA Project Summary
A Transient and Steady State
Study of Pure and Mixed
Refrigerants in a Residential
Heat Pump
John Judge and Reinhard Radermacher
The report gives results of an experi-
mental and theoretical investigation of
the transient and steady state perfor-
mance of a residential air-conditioner/
heat pump (AC/HP) operating with dif-
ferent refrigerants. The project was
motivated by environmental concerns
related to the replacement of strato-
spheric ozone depleting refrigerants as
required by international agreement and
U.S. law. Hydrochlorofluorocarbon
(HCFC)-22, a medium pressure refrig-
erant, is scheduled to be phased out of
production and must be replaced. Sig-
nificant empirical data are available on
HCFC-22, but relatively little data exist
on the transient performance of any of
the zeotropic mixtures being consid-
ered as HCFC-22 replacements.
The experimental work, conducted by
testing an AC/HP in environmental
chambers, documented refrigerant per-
formance for steady state, cyclic, and
seasonal performance, evaluated vari-
ous equipment modifications, and mea-
sured changes in the concentrations
of refrigerant mixtures as a function of
time. A computer model capable of
modeling the transient and steady state
performance of an AC/HP was devel-
oped. This model is the first capable of
representing the significant transient
and steady state physics of an AC/HP
operating with pure and mixed refriger-
ants while using minimal empirical data.
The simulation was used to study sev-
eral system configurations transiently
and at steady state with both HCFC-22
and mixtures.
This Project Summary was developed
by EPA's National Risk Management
Research 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).
Introduction
The transient and steady state perfor-
mance of a vapor compression system
operating with pure and mixed refriger-
ants was investigated. This was accom-
plished both experimentally and
theoretically by examining the perfor-
mance of a residential air-conditioner/
heat pump (AC/HP) operating with dif-
ferent refrigerants. The experimental in-
vestigation involved testing an AC/HP in
environmental chambers designed and
built for this purpose. The theoretical
investigation involved a computer simu-
lation which was developed for this work.
In the work, the effects of various sys-
tem configurations were also investi-
gated.
Background
The project was motivated by environ-
mental concerns related to the replace-
ment of stratospheric ozone depleting
refrigerants as required by international
agreement and U.S. law. Hydrochloro-
fluorocarbon (HCFC)-22, a medium pres-
sure refrigerant used in heating, cooling,
and refrigeration applications, is sched-
uled to be phased out of production and
must be replaced. Currently, no pure non-
flammable refrigerant has been identified
as an acceptable replacement for HCFC-
22. However, several suitable zeotropic
mixtures of hydrofluorocarbons (HFCs)
have been identified. Most notable, the
mixture of HFC-32, -125, and -134a in the
proportions of 23/25/52 wt % (industry-
designated as R-407C) is the refrigerant
most likely to replace HCFC-22 in retrofit
applications.
-------
The steady state performance of a va-
por compression system is of interest since
it represents the ideal mode of operation.
As such, it is an upper limit for the perfor-
mance of any vapor compression system.
Transient operation of a vapor compres-
sion system is of interest since an over-
whelming majority of these units control
capacity and temperature by cycling the
system on and off. Hence, to develop
control equipment, it is important to un-
derstand the transient aspects of these
systems. System reliability is also affected
by transient operation. For example, at
start-up, a system will typically pump some
fraction of liquid into the compressor. If
too much liquid is pumped into the com-
pressor, the compressor is likely to fail.
Furthermore, cycling results in capacities
and energy efficiencies which are roughly
75% of their steady state values. Hence,
real world performance is significantly
lower than that predicted by steady state
system evaluation.
Significant empirical data are available
on the steady state performance of HCFC-
22 and its replacements. Relatively little
data are available on the transient perfor-
mance of HCFC-22 and no data are avail-
able on the transient performance of any
of the zeotropic mixtures being consid-
ered as HCFC-22 replacements. It was
the goal of this work to address these
deficits. Specifically, the experimental as-
pects of this work were to (a) measure the
steady state, cyclic, and seasonal perfor-
mance of R-407C relative to HCFC-22,
(b) evaluate the effects of vapor-to-liquid-
line heat exchange and different expan-
sion devices on performance, and (c)
measure the circulated concentration of
refrigerant mixtures. The theoretical as-
pects of the work were to (a) develop a
detailed simulation capable of modeling
the steady state and transient behavior of
an AC/HP using pure and mixed refriger-
ants, and (b) accurately model the effects
of different system configurations.
Results
The experimental work documented the
performance of R-407C relative to HCFC-
22 for steady state, cyclic, and seasonal
performance. The combination of steady
state and cyclic performance showed that
R-407C had a 4.3% lower cooling sea-
sonal performance factor (CSPF) than
HCFC-22 and a 1.5 to 7% lower heating
seasonal performance factor (HSPF) than
HCFC-22. The lower energy efficiency of
R-407C contributes to greater generation
of global warming gases at the power
plant. The performance of a vapor-to-liq-
uid-line heat exchanger was also evalu-
ated with both refrigerants. While this
equipment had no impact on steady state
performance, it did marginally improve the
cyclic performance of both fluids. The cy-
clic and steady state performance of the
heat pump was quantified with both short
tube restrictors (STRs) and thermostatic
expansion valves (TXVs). When the AC/
HP used the STR, it had a 3.6% lower
CSPF and a 0 to 3.9% lower HSPF than
when the TXV was used. Furthermore,
the STR significantly increased the sensi-
tivity of system performance to the amount
of refrigerant charge. The concentrations
of two refrigerant mixtures, R-407C and a
yet-to-be-designated mixture of 30% HFC-
32 and 70% HFC-134a, were measured
in the system as a function of time. Al-
though the concentrations changed with
time as the system was started up, the
steady state circulated concentration was
reached within about 3 minutes. The cir-
culated concentration was also not equal
to the charged concentration, the concen-
tration of both refrigerant mixtures shifting
away from the less volatile component.
This concentration shift was attributed to
the velocity difference between the phases
in the heat exchangers.
On the theoretical side, a fully implicit,
distributed parameter simulation computer
model was developed, capable of model-
ing the transient and steady state perfor-
mance of an AC/HP. This model is the
first capable of representing the signifi-
cant transient and steady state physics of
an AC/HP operating with pure and mixed
refrigerants while using minimal empirical
data. The simulation was used to study
several system configurations transiently
and at steady state with both HCFC-22
and mixtures. The model demonstrated
that R-407C is more sensitive to counter
and parallel flow geometries in the heat
exchangers than is HFC-22. It also pre-
dicted the shift in the circulated concen-
tration shown in the experimental work.
The penalty associated with having long
connecting piping between the outside and
inside components of an AC/HP was stud-
ied with the model and was shown to be
significant due to transient effects. The
major energy losses associated with tran-
sient operation were shown to be due to
reducing the temperature in the evapora-
tor and redistributing the refrigerant, with
little loss due to overcoming the inertia in
the compressor.
-------
John Judge and Reinhard Radermacher are with the University of Maryland,
College Park, MD 20742.
Robert V. Hendriks is the EPA Project Officer (see below).
The complete report, entitled "A Transient and Steady State Study of Pure and
Mixed Refrigerants in a Residential Heat Pump," (Order No. PB97-117741;
Cost: $49.00, 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
National Risk Management Research Laboratory
Air Pollution Prevention and Control Division
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental 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-96/129
------- |