United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-97/070 August 1997
&EPA Project Summary
Development of a Linear
Compressor for Air
Conditioners and Heat Pumps
Warren D. Waldron
The objective of this work was to
design, build, test, and deliver to the
Environmental Protection Agency (EPA)
a linear compressor for operation in a
3.0-ton (10.5 kW) residential air-condi-
tioning and heat pumping system. The
compressor design evolved from a lin-
ear resonant piston compressor (RPC)
developed previously by Mechanical
Technology Inc. (MTI) for air-condition-
ing applications. During the design ef-
fort, the RPC was modified to extend
its range into the heating mode, and a
voltage controller was developed that
could sense the proximity of the com-
pressor plunger to the top dead center
(TDC) and bottom dead center (BDC)
stops and limit the plunger stroke. Fol-
lowing the design and construction of
the RPC, a test program was conducted.
This effort was successful, except for
several minor difficulties, including a
failure in the epoxy bond of the com-
pressor plunger. The causes of the
problems encountered are well under-
stood, and the modifications required
to correct them are known and simple
to implement. The test results proved
the performance advantage of the RPC
in terms of a high Seasonal Energy
Efficiency Rating (SEER) potential and
also demonstrated that the compres-
sor can be controlled in a stable man-
ner using low cost, commercially avail-
able motor control devices.
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).
Overview
This report describes the work per-
formed to design, build, test, and deliver a
linear compressor intended for operation
in a 3.0-ton (10.5 kW) (nominal) residen-
tial air-conditioning and heat pumping sys-
tem.
The primary advantage of a linear-mo-
tor-driven compressor for a high-efficiency
air-conditioning/heat pumping application
is that it is easily modulated; i.e., it has
variable capacity capability in that the dis-
placement can be varied by varying the
compressor stroke. Conventional rotating
compressors can be modulated by vary-
ing the compressor speed by means of a
variable frequency power source. The lin-
ear compressor, on the other hand,
achieves modulation by means of variable
voltage. Variable voltage power devices
are considerably less expensive than vari-
able frequency devices, thus the potential
advantage of the linear compressor in
modulated (variable capacity) applications.
The subject compressor design evolved
from a linear resonant piston compressor
(RPC) developed previously by MTI for
air-conditioning applications from 1987
through 1991. This compressor design was
modified under the EPA program to ex-
tend its range into the heating mode. A
voltage controller was developed that can
sense the proximity of the compressor
plunger to the top dead center (TDC) and
bottom dead center (BDC) stops and limit
the plunger stroke. This controller uses a
manually operated potentiometer to simu-
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late the input normally derived from a nor-
mal system temperature controller to
modulate the compressor output (mass
flow rate).
A conventional, commercially available,
high-efficiency heat pump compressor pre-
viously tested by its manufacturer (Bristol
Compressor Co.) was used as a qualifica-
tion test system for the linear RPC. The
results showed that the RPC had essen-
tially the same performance as the con-
ventional compressor under normally en-
countered cooling conditions in the U.S.
Combined with the RPC's excellent modu-
lation performance, these results will lead
to very high Seasonal Energy Efficiency
Ratings (SEERs) because the modulation
capability of the RPC eliminates the need
for excessive cycling at moderate outdoor
temperatures.
The test program was prematurely ter-
minated due to structural failure of the
epoxy bond that secures the motor plunger
permanent magnets to the plunger frame.
This failure was due to the use of a plunger
built for another program that was not a
hydrochlorofluorocarbon (HCFC) refriger-
ant application. It was later determined
that the epoxy used was incompatible with
the HCFC-22 refrigerant.
The test program was successful, ex-
cept for several minor development is-
sues. Unfortunately, the plunger epoxy
bond failure prevented conclusion of the
test program and a complete demonstra-
tion of the efficiency and modulation ad-
vantages of the RPC. The problem causes
are well understood, and the needed modi-
fications known and simple to implement.
Such developmental issues are normal in
prototype projects and in no way indicate
that the RPC is not a viable device for a
high-efficiency residential air-conditioning
and heat pumping system. On the con-
trary, the data included in the report prove
its performance advantage in terms of a
very high SEER potential. Further, the test
program demonstrated that the RPC can
be controlled in a stable manner using
low-cost, commercially available motor
control devices.
Warren D. Waldron is with Mechanical Technology Inc., Latham, NY 12110.
James J. Jetter is the EPA Project Officer (see below).
The complete report, entitled "Development of a Linear Compressor for Air Condition-
ers and Heat Pumps," (Order No. PB97-189690; Cost: $21.50, subjectto 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-97/070
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