For the past decade, military IR systems have preferred to see cryogenic coolers provided as split units; separating the functions of compressor and cold-end for system packaging and vibration isolation reasons. A family of split-cycle coolers designed for long MTBF and in the final stages of development is the focus of the discussion. Their technological evolution, from multi-year-MTBF satellite system Stirling coolers developed in the U.S., and the UA 7011 cooler (the first all-linear, military, production cooler) developed in Holland, is explained. Two new split-cycle machines are discussed. They provided 1/4 watt and 1 watt (nominal capacity) at 80 K and 85 K respectively. These linear-resonant, free-displacer Stirling coolers are designed for thousands of hours of service-free operation. They are designed to be compatible with standard U.S. 60 element and 120/180 element detector/dewars, respectively

Lehrfeld, D.

English

NASA Technical Reports Server

SPLIT-STIRLING,LINEAR-RESONANT,CRYOGENICREFRIGERATORS
FORDETECTORCOOLING
Daniel Lehrfeld
MagnavoxGovernmentand Industrial Electronics Company
Electro-Optical Systems
46 Industrial Avenue, Mahwah,N. J. 07430
ABSTRACT
Unfortunately, for user and manufacturer both, the closed-cycle cryo-
genic cooler to date has deserved its reputation as the "weak-link" in IR
systems. When the cooler requires service at intervals of a few hundred
hours at best, the quality of the system it serves is unfairly diminished.
This paper addresses technological advances in the art of Stirling-
cycle coolers which will increasingly cause that image of military cryo-
coolers to change for the better.
For the past decade, military IR systems have preferred to see cryogenic
coolers provided as "split" units; separating the functions of compressor
and cold-end for system packaging and vibration isolation reasons.
A family of split-cycle coolers designed for long MTBF and in the final
stages of development is the focus of the discussion. Their technological
evolution, from multi-year-MTBF satellite system Stirling coolers developed
in the U.S., and the UA 7011 cooler (the first all-linear, military, produc-
tion cooler) developed in Holland, is explained.
Two new split-cycle machines are discussed. They provide i/4 watt and
I watt (nominal capacity) at 80°K and 85°K respectively. These linear-
resonant, free-displacer Stirling coolers are designed for thousands of
hours of service-free operation. They are designed to be compatible with
standard U.S. 60 element and 120/180 element detector/dewars,respectively.
The technologies of linear-resonant compressor and free-displacer
expanders as embodied in these machines is discussed in sufficient detail
that the reasons for their superior performance (i.e., service-free life,
low acoustic noise) will be clear.
INTRODUCTION
The Stirling refrigeration cycle, in its fifth decade of development,
is by now well known to the technical community. One will recall that in
215
https://ntrs.nasa.gov/search.jsp?R=19840007274 2020-03-22T08:11:56+00:00Z
its simplest embodiment, two pistons (the compressor piston and the gas
displacer) operating on a fixed charge of non-condenslble gas (helium),
cause the gas to be compressed, cooled, expanded and reheated repeatedly so
that in a certain portion of the machine, net refrigeration at very low
temperature is achieved.
The ideal Stlrling cycle is "Carnot-limlted", that is, its efficiency for
cold production is the highest possible. It is not the intent of this paper
to review the theory of operation; for that the reader is referred to the
original references, Reference 1 and 2, or a number of later papers.
Rather, we shall discuss new embodiments which have been developed recently
from innovations introduced by the same research laboratories which
"discovered" Stlrllng cycle technology in this century, the Philips Research
Laboratories of N.Y. Philips in Holland.
LIFE LIMITING CONSIDERATIONS IN STIRLING COOLERS
The most significant llfe-llmitlng mechanisms in conventional miniature
Stirllng refrigerators are wear and contamination.
Figure 1 is a schematic illustrating a conventional embodiment of the
Stirllng refrigerator, and useful for pointing out its weaknesses. The t_
pistons must reciprocate with a fixed phase relationship for the refrigera-
tor to function; as shown, this is accomplished by using a kinematic mecha-
nism to convert rotary motion from a motor crankshaft into the required
linear motion of the pistons. The mechanism requires bearings which in turn
require lubricants. Since the dynamic seal around the piston is not herme-
tic, lubricant vapors can migrate and be "gettered" in the low temperature
region of the machine. Alternatively, without adequate lubrication or
lubricant containment in the bearing, bearing failure can result.
The kinematic mechanism inparts a slde-load which must be carried by a
dry-lubrlcated guide on the pistons. This load causes accelerated wear of
the guide bearings and seals, which both decreases compression and produces
particulate debris over time.
The processes of wear and contamination are inter-related. Particulate
contamination is an obvious wear product, and can obstruct internal heat
exchanger flow passages within the machine. Not as obvious are gases, which
are evolved from freshly exposed wear surfaces, and vapors evolved from the
lubricant under high shear stress and temperature. These gaseous contami-
nants freeze within the cold region of the machine and degrade cooling
performance as well.
HISTORY OF THE PHILIPS LINEAR-RESONANT COMPRESSOR, FREE DISPLACER MACHINE
In 1968, recognizing that the weak-llnk in conventlonally-drlven embod-
iments of Stirling cryogenic coolers developed to date was the compressor
(bearing, sealing, and contamination problems, as well as high cost to
manufacture), work was begun in the Philips Research Laboratories in Holland
on the concept of a llnear-resonant compressor, driven by a linear motor,
216
for application to the Stirllng cooler. This type of "free-piston,
free-displacer" machine concept has far fewer moving parts, has eliminated
crankshaft, bearings, and side-loads on dynamic seals, embodies clearance
seals, eliminates lubricants, and can preserve the inherent efficiency of
the Stlrling cycle (the purpose of the "resonant" portion of its
nomenclature), thus lending itself to long life and low cost in production.
This new construction is discussed in a later paragraph.
The first generation of linear-resonant Stlrling cooler product devel-
oped in Holland was the non-military MC-80 (Miniature Cooler, 800,K), IW @
80°K cooler, brought into limited commerlcal production in 1975 by the
Science and Industry Division of N.V. Philips.
In 1976, a second generation of linear-resonant Stirllng coolers began
in parallel in Holland and the U.S. In Holland, Philips Usfa B.V., a
manufacturer of military infrared systems and products, undertook the devel-
opment of a new cooler, in effect, a militarized version of the MC-80
machine. Initially dubbed the MMC-80 (Militarized Miniature Cooler, 800K),
and later officially designated the UA-7011 (see figure 2), this cooler was
designed for and met full specifications for field, vehicle and aircraft
use. Utilizing rare-earth cobalt magnets in its linear motor, and hermetic
sealing of the helium, that cooler was brought into production and remains
in production through the present day. Of particular significance is the
fact that the cooler is sold with a 2500 hour guarantee on servlce-free
MTBF.
The UA-701! is distributed as the Magnavox MX7011 cooler in the U.S.;
it is produced for Magnavox by Phillps Usfa B.V. in Holland. While not
readily compatible with standard U.S. military FLIR system components, there
is interest in this highly reliable machine for use as a satellite-borne
unit for cryogenic cooling in space.
In the U.S., Philips Laboratories Division of North American Philips
Corporation, a Magnavox affiliate, began R&D for NASA and DARPA, applying
the design concepts of the linear-resonant construction to satellite system
coolers intended for three to five years of malntenance-free operation
(reference 3.)
Figure 3 is a photograph of the NASA machine, designed for 5 W @ 65°K
of refrigeration capacity. Its refrigeration performance has been proven in
recent tests.
Currently at Magnavox, there is ongoing a program to design, fabricate,
and prepare for test the first prototype operational cyrogenlc cooler of
this type for future use in multl-year satellite systems.
The third generation of linear-resonant cooler product development
activities is ongoing at present and is the subject of this paper. The
design experience and production know-how established in the development of
the UA7011 cooler has been brought to bear on development of a family of
spilt-cycle linear resonant coolers. This family consists of a standardized
217
compressor section and two cold fingers which are designed expressly to
interface with U.S. 60 element and 120/180 element detector/dewar
assemblies; the resulting coolers having i/4 watt and 1 watt refrigeration
capacities, respectively.
Magnavox/EOS is the U.S. distributor for these linear-resonant cool-
ers. Currently, these coolers are produced in Holland.
DISCUSSION
The Stirling cycle requires the interphased reciprocating motion of its
two major elements, the piston and the displacer. Traditionally, this
motion was generated by either conventional crank-type mechanisms or by the
special Philips rhombic drive. In both instances, the rotary motion
supplied by an electric motor had to be translated into the required linear
motions of the piston and displacer.
In the linear-resonant cooler, the piston is directly reciprocated by
supplying an ac waveform to the coil of an electric motor. The piston, in
turn, by fluidic coupling, drives the free-displacer.
Elimination of the direct-drive coupling to moving piston and displacer
made the problem of analysis, optimization, and design of the coolers far
more difficult. While formerly the analysis task was fundamentally a prob-
lem in thermodynamic modelling and analysis, the step to free piston-free
displacer required the integration of equations of the dynamics of motion of
the moving masses, the electrodynamic equations governing the performance of
the linear-motor,and more careful analysis of the flow-losses in the
system. Several years of research, experimentation, and development were
spent in validating the design methods and principles applied to this new
class of machines. References 4 and 5 discuss the thermodynamic/dynamic and
electrodynamic theory and principle of operation of the machine in detail,
and should be consulted by those desiring a detailed theoretical understand-
ing of the linear-resonant, free displacer, free-piston Stirling cooler.
LINEAR-RESONANT COMPRESSOR
Figure 4 illustrates the construction of the linear-resonant compres-
sor. The compressor contains a piston which is fixed to the moving coil of
a rare-earth-cobalt permanent-magnet linear motor. The linear motor is an
alternating current device, i.e., when a voltage of a given polarity is
impressed across its coil windings it delivers a force proportional to the
current in one direction, when the polarity is reversed the direction of the
force is reversed.
A mechanical spring, connected between the base of the piston/coil
subassembly and the housing, maintains the midposition of the piston.
A single piston seal/guide is needed to separate the compression space
above the piston from the motor compartment below the piston.
218
All of the forces acting on the piston are in the direction of its
motion; i.e., no side loads are imposedon the piston guide and seal.
Hence, a clearance-type of seal design may be used, resulting in extremely
low wear of the seal/guide surface. No lubricants are needed or used.
A spring/mass vibration absorber is incorporated in the compressor to
automatically attenuate the momentum imbalance resulting from the recipro-
cating motion of the piston/coil assembly.
The helium transfer tube connects the compressor to the displacer.
Through it, the pressure wave created by the piston can act upon the free-
displacer inside the cold finger assembly.
The compressor unit is sealed by a closure weld in order to ensure
helium containment.
FREE-DISPLACER COLD FINGER
Figure 5 illustrates the construction of the cold-finger and the free-
displacer. The function of the displacer is to alternately move the gas
from the expansion to the compression space. When the piston goes down
(expansion) the displacer must be near its lower position, so that the
expansion process occurs at the end of the cold-finger. When the piston
goes up (compression) the displacer must be near its upper position.
A mechanical spring affixed to the displacer and the housing forms part
of a spring-mass system which is tuned to help provide the proper phase
relationship between the movement of the piston and that of the displacer.
Gas from the helium transfer tube flows alternately from the compressor
space towards the expansion space through the body of the displacer, which
is filled with a porous-metal regenerative heat exchanger.
Since the forces on the displacer, like that of the piston, act only in
the directon of motion, the necessary seal at the base of the displacer is a
simple clearance seal.
PERFORMANCE OF LINEAR-RESONANT, FREE-DISPLACER COOLERS
a) MX7011: 1W @ 80K
The MX 7011 cooler currently in production (figure 2) as part of a
complete product test program, was subjected to an endurance test. Five
coolers were each tested for 5000 hours, preceded by a 400 hour "run-ln"
period. The purpose of this test was to demonstrate confidence in the 2500
hour MTBF warranted for the product.
The test was automatically controlled and each 24 hour cycle included a
one hour warm-up to room temperature. Periodically, performance at tempera-
ture extremes was checked against specification.
219
None of the coolers exhibited mechanical problems during the test,
impressively demonstrating the inherent reliability of the linear drive and
the dry-runnlng clearance seals. The worst degradation in cold-finger
temperature observed during test was less than 5°K. At the end of the 5000
hour test, all coolers were still performing to specification, and the test
was discontinued. Inspection of the units upon disassembly revealed negli-
gible wear and tear.
Figure 6 is a comparison of the acoustic noise from a model HDI033, IW
@ 80°K Standard Common Module Cooler and the noise from an MX7011, as
measured in an anechoic chamber. At a frequency of I000 Hz, its noise level
is 20 dB lower.
b) Magnavox MX7040 and MX7043 Coolers
The Magnavox MX7040 and MX7043 (figures 7 and 8) are the first split-
cycle linear resonant coolers in exlstance. They are currently in the final
stages of development testing, with initial production scheduled for 1983.
These coolers are being produced for Magnavox by Philips Usfa B.V. for sale
in U.S. markets. As with the MX7011, they are intended to deliver an MTBF
of 2500 hours without service.
The MX7040 (figure 7) is a unit designed for I/4 W of cold production
at 85°K with less than 40 watts of input power required over full military
ambient temperature extremes. It weighs 4.2 ibs.
The MX7043 (figure 8) is a unit designed for I W of cold production at
80=K with less than 55 watts of input power over full military ambient
temperature extremes. It weighs 4.2 Ibs.
Both coolers have a vibration absorber (passive countermass) contained
within their common compressor.
Life tests of two prototype MX7040 coolers have each demonstrated over
6500 hours of operation without failure.
c) Magnavox MX7045 Cooler
The MX7045 Cooler was developed by Magnavox to provide a "form-flt and
function" compatible, improved-reliability (true I000 hour) cooler meeting
the U.S. Government B2-Speclflcatlon (#2104070122) for a "I/4 W-Spilt". It
was developed for use in systems which could not employ the superior
linear-resonant coolers. The design approach taken was to incorporate the
same proven design principles embodied in the free-displacer portion of the
linear-resonant coolers (mechanical restoring spring, clearance seal) into
the MX7045 cold finger while utilizing the "U.S. Standard" brushless dc
motor driven compressor design (with a few product improvements devised by
Magnavox engineers).
Magnavox/EOS has demonstrated the life and reliability of this "hybrid"
i/4 W spllt-Stlrling cryogenic cooler under a U.S. Army program. The unit
220
delivers I/4 W of cooling at 85°K with less than 25 watts of input power
over full military ambient temperature extremes.
CONCLUSIONS
The llnear-resonant embodiment of the Stirllng cooler has proven itself
as synonymous with long-llfe and high reliability through two generations of
production hardware. The newest generation of product, the spllt-Stirllng
coolers discussed herein, promises to retain all of the positive
characteristics of its antecedents while providing as well the advantages of
split-cooler construction.
The challenge remaining is for the system designer to exercise imagina-
tion and skill in utilizing this reliable, cost-effectlve component to best
advantage in the IR systems of the future.
REFERENCES
i. J.W.L. Kohler and C.O. Jonkers, Fundamentals of the Gas Refrigerating
Machine, Phillps Technical Review, 16, p. 69, 1954.
2. R. J. Meyer, The Phillps Stlrllng Engine, Engineer, Vol. 81, May 1969,
p. W69-WSI.
3. A. Danlels, et al, Magnetically Suspended Stlrling Cryogenic Space
Refrigerator Status Report, Cryogenic Engineering Conference, San
Diego, CA., August 1981.
4. A. K. DeJonge, A Small Free-Piston Stirllng Refrigerator, Proceedings of
the 14th IECEC, Boston, August 1979.
5. A. Sereny, A. K. DeJonge, Analysis and Optimization of a Linear Motor for
the Compressor of a Cryogenic Refrigerator, 1981 Cryogenic Engineering
Conference, San Diego, CA., August, 1981.
221
GAS
DISPLACER
PISTON
VERY LOW
TEMP REGION
_WORKING GAS
(HELIUM)
SEALS
BEARINGS
LUBRICANT
FIGURE i. SCHEMATIC REPRESENTATION OF A CONVENTIONAL STIRLINC
REFRIGERATOR
222
ii!!iiiiii!!
FIGURE 2. UA 7011/MX7011 LINEAR RESONANT COOLER
223
%FIGURE 3. ULTRA-LONG LIFE PHILIPS LABS/NASA LINEAR-RESONANT,
MAGNETIC BEARING STIRLING COOLER
224
PIN (2)
VIBRATION
ABSORBER
SPRING
/ VIBRATIONABSORBER MASS
MOVING
COIL
PERMANENT
MAGNET RING
CENTERING
SPRING
REJECTION SURFACE
LEAD WIRE (2)
CLEARANCE
SEAL. GUIDE
SURFACE
HELIUM
TRANSFER IUBE
COMPRESSION
SPACE
FIGURE 4. SCHEMATIC REPRESENTATION OF MAGNAVOX LINEAR-RESONANT
COMPRESSOR
225
HELIUMTRANSFERTUBE
HEAT
REJECTION
SURFACE
SPRING
CLEARANCE
SEAL
EXPANSION
SPACE
COMPRESSION /
SPACE
L REGENERAIOR/
GAS DISPLACER
ANNULAR
HEAT EXCHANGER
GUIDE RING
FIGURE 5. SCHEMATIC REPRESENTATION OF MAGNAVOX
FREE-DISPLACER/COLD-FINGER CONSTRUCTION
226
,,?,
,.-.,
--J
_-J
66J
,.,,.,.
_._
_-
_..
Z
G_
50
qO
3O
20
10
0
ACOUSTIC NOISE COMPARISON OF COOLERS
mm_o_ I
!
!
I
!
Lnmwl
L ........ _ [ ......... L ..........
i
NOTES: ^
I) 0 DB " 2.10-5 N/M2
2) INTEGRATION TIME g SEC
_MX 7011
.....BACKGROUND NOISE
.....US COMMON MODULE
| !
I,. .... a
FREQUENCY.-,.HZ
COOLER
HI
FIGURE 6. ACOUSTIC _,TOISE MEASUREMENTS
227
FICURE 7. PROTOTYPE MX7040 SPLIT-STIRLING, 1/4 WATT
LINEAR-RESONANT COOLER
228
FIGURE8. PROTOTYPEMX7043SPLIT-STIRLING,1 WATTLINEAR-RESONANT
COOLER
229


Split-Stirling, linear-resonant, cryogenic refrigerators for detector cooling

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server