Note: Descriptions are shown in the official language in which they were submitted.
1 , MINIATURE CRYOGENIC COOLING SYSTEM WITH SPLIT
I PHASE DURl COMPRESSOR AND PH~SE-~HIFTI~G DEVICE
Background of the Inv ntion
Thi~ inven~ion rela~es in general to cryogenic cooling
~y~tem~ and more par~icularly ~o a miniature dual-split cryogenic
system with the compressor ~ection ~eparated from the cooling
~ ection.
i
: Miniature cryogenic oooling systems are known and
widely u~ed t~ cool cry~tal~ used as radia~ion detectorsO
Cooling to cryogenic temperatures reduces the crystal lattice
vibrations 60 aR ~0 improve the signal to noise ratio. A par-
ticularly important application for such miniature cryogenic
coolers is in the cooling of infrared detectors for use in nigh~
vision or heat seekiny devices. The~e systems are also uæeful
for medical application~ where it i~ de~ired to de~troy ti~ue by
means of freezing. Although the requirement~ for such miniature
cryogenic sys~ems will vary depending on the end use of ~he
, cooler, typical design csnsiderations are ~heir operating effi
ciency, durability~ compactne~s, weight, microphonics charac-
teristics ~generally vibration~ resulting rom the compressor
motor, vibra~ions within the working fluid, or the physical
impact of moving components in the ~ystem3, and thermophonics
(which i~ noi~e resulting from thermo~radients within the
~ystem). For applicationg involving infrared ~ensors for air-
borne devicz6, all of the~e design considerativn6 are importantO
Kn~wn cryvgenic coolin~ ~ystem~ fall into æeveral cate-
gorie~. In one type the compre~sor and expander units form an
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1 integral system opera-ting on stirling cycle. Because of the
proximity of the compressor and expander sections, such integral
systems are partlcularly susceptible to the effects of micro-
phonics or mechanical vibra-tions. The sys-tems also tend to be
relatively heavy and have typical operating lives of only
300-500 hours. Thus, integral sys-tems are not particularly
effective especially for use in airborne systems.
~ s a way of isolating the compressor vibration from
the expander (known as the "cold finger"), split stirling
devices are known which separate the compressor system from
the cold finger by conduits carrying the working fluid. A
typical design problem with such split systems is the acoustic
noise generated by an oscillating displacer which is continually
being accelerated and decelerated as it oscillates within the
cold inger. Such noise is particularly troublesome in
single-split stirling systems in which but a single conduit
extends from a single compressor to the expander.
Dual-split systems have also been developed in an
effort to overcome problems in controlling the motion
2~ of the displacer so as to m;n;m; ze microphonics. In
such systems two conduits extend from two separate compressoxs
to the expander. In general the pressure waves in each of
the conduits are out of phase with one another so that a
push-pull arrangement is effected. That is, in such a system
the d-splacer is moved by alternately introducing high
pressuxe on one side and a lower pressure on the other so as
to reciprocate the displacer element. One such dual split
cooler is disclosed by Durenec in U.S. Patent No. 4,092~833
which issued June 6, 1978. The Durenec system employs a
3~ compressor with pistons opposed at a 180 degree angle. In
1 this system, however, the compressor pistons are of different
areas so that the pressure waves in the individual conduits
are of different magnitudes. In addition, -there is no
suggestion in the Durenec patent of using the split phas~
relationship to decelerate and reverse the direction of
motion of the displacer in a way to prevent contact of the
displacer with the walls of the housing. Such contact with
the housing or "slamming" not only causes additional
microphonics but also audible acoustic noise. Furthermore
1~ this Durenec reference n~ither teach~s nor suggests using
the cooling effects of a second working volume -to precool
the working fluid destined for the main workin~ volume at
the tip o~ the cold finger~ In addition, the known split
phase coolers have short operating lives.
Other known coolers are described in U.S. Patent
Nos. 4,090,859 to Hanson which issued May 23, 1978; 4,078~389
to Bamberg which issued March 14, 1978; 3,523,427 to Simpson
which issued August 11~ 1970; and 4/206~609 to Durenec which
issued June 10, 1980.
~ It is there~ore a principal object of this invention
to pro~ide a miniature cryogenic system which is hiyhly
efficient, compact and characterized by low level o~ micro-
phonics and thermophonics.
' Another principal object is -to provide such a system
operating on a dual split "compound" stirling cycle which has
a comparatively long operating life.
A further object of the invention is to provide a
cryogenic cooler employing a co~pressor assembly with two
isolot~d compressors and producing a split-phase relationship
of 180 degrees.
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;l
A still fur~her object of the invention i~ ~o provide a
6y5tem having all of the foreyolng adv 3.ntag~s and w~ich i~ manu-
~actured of conven~ional ma~erials in a rela~ively simplP a.nd
~traightforward way.
,
Summary c: f ~he Inventioll
'lThe miniature cryogenic ~ystem according ~o the pre~nt
invention includes; two com1?ressor~ with two outlets. ~irst and
second condults containing a working fluid are connected to the
: compressor ou~lets. The compres~or~ pressurize the working fluid
in a ~plit~phase relation~hip. These conduits are connected to a
cryogenic cold finger a~sembly, including an elongate housing
. having a cryoyenically cooled end and an internal abutment
dividing the housin~ in~o irst and second interior compartments.
The f irst compartment extends from the cooled end to the abut-
ment. A diæplacer is disposed slidingly within the first: com-
partment or longitudinal reciprocating motion therein and
crea~es a first (or primary) working volume between the coole~
end and the displacer and create~ a ~cond (or ~econdary) working
I volume between ~he displacer and the abutmen~0
A first regenerative bed is isposed within the
displacer and the housing has a firæt inle~ adapted for providing
fluid communication between ~he first conduit and the fir~
I interior compartment, The displacer has first and ~econd port
mean~ so ~s to provide fl~id communlcation through the first
regensrative bed ke~ween the fir~t inlet and the first working
volume. In addi~ion, a second regenerative bed i~ disposed
within the second interior compartment within the housing. The
housing also has a 6econd inlet w~ich provide~ 1uid com~
4--
. !
munication ~hrough the econd regenexative bed between the ~econd
conduit and the second working volumeO ~ean~ are provided to
develop tran ient cu6hioning overpr~essur~s in he fix~t and
6eeond working volumes as the displacer reciprocates under the
push-pull influence of the spli~-phase working fluid flow from
the compre~or ~o that cooling is effec~ed in the fir~ and
second working volumes and ~lammin~ i6 controlledO
The di~placer i~ guided for longitudinal reciprocating
I motion within the housing by &liding 6eals. In a preferred
embodiment the fir6t conduit i5 coiled around the exterior of the
hvusing adjacent ~o the second workin~ volume to ef~ect a pre-
cooling of ~he working fluid carried in the fir~t conduitO In
thi~ embodiment the abutment compri~es a web extending acro s the
! interior of the hou~ing and includes an opening to provide fluid
communication ~etween the fir~t ~nd 6econd interior compartments.
A plate is provided for 6ecuring the second regenerative bed in a
fixed location within the second interior compartment, and a heat
exchanger i6 disposed b~ween this plate and this abutment. A
preferred heat exchanger includes a plurality of longitudinally-
~paced annular discs.
In order to enhance ~he pneumatic cu6hioning, a phase
! shifting plug is di~posed on the lower end of the displacer adja-
cent to the web a~d located 80 as to eeal t11~ opening in the
abutment when ~he displacer iE closely spaced from the web. This
plug can be a pad of resilliant material or a ~pring disposed
within a coverlng that i8 located nd 6tructured to cover the
hole in the abutmerlt~ By ~ealing the opening in the web, the
phaæe relation~hip of the pressures in the working volume~ i5
73
.
~hif~ed ~o as ~o provide a cu hioning effect which deceler te~
the displacer in a controlled fa~hionO
In another embodiment the ,abu~ment comprises a soiid
member which ex~ends ~ub~tan~ially the leng~h of the ~econd
interior compartment, and the second regenera~ive bed is located
between ~he ~olid member and the hou~ingO ~ heat exchanger,
including longitudinally spaced-apart di~cs 1anking the 6econd
worXiny volume i5 adap~ed for æecuring the 6econd regenerative
bed in a fixed loeati~n within ~he 6econd interior compartment.
In this embodiment a hea~ radiation 6hield i8 provided for
surrounding the hou~ing in a 6paced relation~hip and extending
from the cooled end of the housing toward the 6econd worXing
volumeO The radiation shield reduces the heat load falling on~o
the fir~t working volume. The bottom of ~he di~placer functions
as its own phas~-shiting device. W~en the displacer moves down-
ward, it gradually closes off ~he fluid passage~ between th~ heat
exchanger discs in the second working volume.
Brief Description of the Drawing
The invention disclosed herein will be better
u~der~ood wi~h reference to th~ following figure~ of ~he drawing
of which:
I Fig. 1 is a vertical 6ectional Yiew of a cold fin~er
a~sembly according to this invention ~uitable for operation in a
dual æplit "compound" 6tirling cycle.
Fig. 2 is a cross-seetional view of another embodiment
of such a cold finyer~
~ig. 3 is a top pl~n vi~w in partial horizontal ~ec~ion
of a compre~ or ~ui~ed for u~e with the cold fingers of Figs, 1
1 and 2.
Fig. 4 is a side elevational view in partial section
of the compressor.
Fig~ S is a pressure versus time diagram o~ the
pressure waves produced by the compressor.
Fig. 6 is a side eleva~ional view in partial vertical
cross section of a displacer disclosed herein.
Fig. 7 is a side elevational view in partial vertical
cross section of the displacer disclosed herein.
13 Fig. 8 i~ a side elevational view in partial cross
section showing an alternate embodiment of a phase shifting plug
for use with the displacer7
De'scr'iption'o'f'the Preferred Embodiments
The same reference numerals will be used to identify
like components in the various figures of the drawing.
With reference first to Fig, 1, shown is a cold finger
assembly lQ, including a housing 12 made of a material having a
low therm~l conductivity such as stainless steel. Disposed
within the housing 12 is an abu~ment 14 which divides the
in~erior of the housing 12 into two interior compar~ments 16 and
18. The compartment 16 has a substantially circularr cylindrical
cross section. A displacer 20 is adapted for reciprocating
longitudinal motion within the compartment 16. The displacer 20
is formed of a material such as nylon~
The displacer ~0 is hollow and contains a first bed of
regenerative material 22 comprising, for example, copper screens
or copper spheres. The displacer 20 also includes entrance ports
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5i~
1 24 which can be seen more clearly with re~erence to Figs. 6 and
7~ Exi-~ por-ts 26 are also provided in the upper end of the
displacer 20. The displacer 20 has a length which is shorter
than the length of the interior compartment 16 so tha-t the dis-
placer 20 crea-tes within the interior compartment 16 a first (or
primary) wor~ing volume 28 and a second (or secondary~ working
volume 30 in a working fluid ~o be described hereinafter~ As
will become clearl the working volume 28 effects a cooling to
cryogenic temperatures of the cold end 32 o~ the housing 12. The
cold end 32 may be made of a material having a very high thermal
conductivity such as copper for more efective heat transfer from
a thermal load (e.g. - an infraxed detector) in contact with the
cold end 32O
A second bed of regenerative material 34 is included
within the interior compartment 18 and held in place by a plate
36. The plate 36 is preferably made of a material having high
thermal conductivity such as copper. A series of longitudinally
spaced-apart discs 38 are located between the plate 36 and the
abutment 14 and are made of a material having a high thermal
2~ conductivity so as to serve as an effective heat exchanger.
A first conduit 40 encircles the housing 12 adjacent
to the second working volume 30 and communicates with the interior
compartment 16 at its terminal end 42. A second conduit 44
communicates with the interior compartment 18.
Referring now to Figs. l, 2, 3, and 4, a compressor
assembly 50 includes a structural housing 52 with a circular
cylindrical cavity 54 therein. A piston assembly 56 includes
longitudinally spaced~apart and mutually isolated pistons 58 and
3~
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1 ,' 60 which are riyidly connected to one ano~her by connecting
member 6- The conneeting member 62 i~cludes a 310t 64 which
engages a pin 66. The pin 66 is eccen~rically mounted on a disc
68 driven by a m~or 70u As the disc 68 rQtates, the pin 66
follow~ ~he circular path indicated by the dot~ed circle 72.
Thu6, ~s the pin 66 traveræe~ the circ~lar pa~h ~2, the piston
assembly 66 will reciprocate longitudinally within the cavity 54~
As will be understood by ~hose 6killed in the art, w~en a working
, ~luid such as helium i8 introduced into the cavi~y 54, the
l pressure of the working fluid will al~ernately increase and
decrea~e as the piston assembly 56 reciprocate~. Furthermore, as
shown in ~igO 5, the pres~ure varia~ions in the conduits 40 and
44 will be 180 degrees out of phase, ~ince the pis~ons 58 and ~0
are rigidly linked together. Thus, w~en the pis~on assembly 56
i in the position as 6hown in Fig. 3, the pressure in conduit 40
will be relatively high and the pressure 44 will be relatively
. l~w. This relationship reverses as the pi~ton a~sembly 56 moves
toward the right.
The compres or ~ ~embly 50 disclosed herein is par-
ticularly well adapted for use with ~ cryogenic cooler. Because
the pistons 58 and 60 are of the same diameter and mass, they are
balanced about the drive point ~o that mechanical vibration is
minimized. In addi~ion, the " cotch yoke" design with its short
moment arm and the linear mDtion of the pi~ton assembly 56 result
in low ~eal wear and hence long operating life~ Furthermore, the
prior art did not recognize the desirability or possibility of
using a 180 oppo~ed compressor in which t~e pi6ton8 were of the
~ame ~ize~ This i8 the case becau~e the prior art did not con-
ceive o the phase shifting ~rrangement disclosed herein whi~h
_g_
1 provides pneuma~ic cushioning fsr contr~lla~ly d eelerating the
di~placer so a~ ~o decrea~e ~lamming. ~his d~ign re6ult~ in a
ompre~Ror which is very comRack and one whlch is ~imple to
fabricate.
,' The operation of the compressor a~sembly 50 in onjunc-
¦ tion with the cold finger 10 will now he described~ As sta~e~
! above, a~ the pis~on assembly 56 reciprocates, pres~ure waves are
created in ~he conduits 40 and 44, w~ich are 180 degre~s out of
~ phase with one another. The working fluid in the conduit 40 such
1 as helium circula~es around ~he extexior of the housing 12 and
enters the first interior compartment 16 at the end 42 of the
~onduit 40. RecauRe of the arr~ngement of sliding ~eal~ 80 and
8~ which guide the motion of the displacer 20, th~ working 1uid
which enters the interior compar~ment 16 passes in~o the interior
o~ the displacer 20 through the por~ 24 and travels through the
regenerative bed 22 and finally through the ports 26 into the
first working volume 280 Similarly, the working fluid in the
conduit 44 enters the Eecond interior compartment 18, passes
through the second regenerative bed 34 past the plate 36 and the
heat exchanger discs 38 and from there into interior compartmen~
16 through an opening 84 in the abu~ment 14 to crPate the working
volume 30. ~ecau6e of the ~eals 80 and 82, the working volumes
28 and 30 with their working fluid remain separated and isolated
from one another.
As the compres~or assembly 50 operates, t.he pressure in
wvrking volume 2~ is rela~ively high when the pressure in the
working volume 30 is relatively l~wer and vice versa in a cycli-
cal manner~ Becau~ of this cyclically varying pressure dif-
.~ O -
7~
1 ferential, the displacer 20 reciprocates within the first
interior compartmen~ 16. As is understood by those skilled in
the thermodynamic art, as ~he displacer 20 reciprocates in a
cyclical fashion, temperatures within both oE the working volumes
28 and 30 decrease. Tha~ is, the energy expended results in
cooling by the same means as in the well known ~hermodynamic
stirling cycle.
CooIing in both working volumes 28 and 30 is advanta-
geous for two reasons. First, because the conduit 40 encircles
the housing 12 adjacent to the wor~ing volume 30, cooling in the
working volume 30 will cool the working fluid in the conduit 40
before it enters the interior compar~ment 16. Thus, when the
working fluid reaches the ~orking volume 28~ i~ is in a precooled
condltion so that the overall cooling capability of the cold
finger 10 is improvedr Second, the thermal gradient across the
longitudinal length of the displacer 20 is reduced. Such a
reduction in the thermal gradient ameliorates thermophonics.
A significant aspect of the present invention is that
the cold finger structure, in combination with the split phase
pressure waves applied to the inlets provides for pneumatic
cushioning of the displacer so that physical contact between the
displacer and the cold end 32 and the abutme~t 14 is eliminated
or controlled. An important factor producing this cushioning is
the fluid flow impedances through the regenerative beds 22 and
34. That is, because o~ the nature of the flow paths, as the
displacer rapidly approachas the end 32, for example, pressure
builds up in the working volume 28 so as to slow and cushion
the displacer before it comes into contact with the end 32.
3~
3~
1 , This ef~et i~ enhanced by ~he la~eral loca~ion o the por~ 26
unlike an end location a~ taught irl the prior art. The lateral
location also improv~ the heat tr~n~fer characteristics a~ the
cold end 32.
The important embodiment ill~strated in ~ig. 6 and 7
enhanc~ thi~ pneumatic cu~hioning effec ~ A p~ase ~hifting plug
90 of a re~ilian material is disposed on the lower surface of
the displacer 20. As the di~plac~r 20 moves downwardly, the plug
90 cover the op~ning 84 sealing off the working volume 30
, thereby enhancing the transient pressure buildup BO a~ ~o
de~elerate the displacer 20~ By ~ealing the opening 84, the
pha~e relation~hip of the pressures in the two working volumes
has thus been ~hifted from the ~ubstantially 180D out-of-phase
condition. Fig. 8 illustrates yet another embodiment adapted for
providing enhanc~d pneumatic cushionîng. In ~his embodiment a
~prill9 92 i8 at~ached to th~ lower surface of the displacer 20
and covered by a metallic material 94~ A~ with the embodiment of
Fig. 6 and 7~ when the material 94 reache6 the abutment 14, the
opening 84 is ~ealed off so a~ to enhance the buildup of
pressure. The Rpring ~2 restore6 the covering to 6ubstantially
the same po~ition with each cycle.
Yet another em~odiment of a cold finger assembly 100
adapted for operation with the compressor a sembly 50 ~f Fig. 3
i5 ~h~wn in Fig. 2~ In thi embodiment a housing including 6ec-
tion~ 102 and 103 i5 6eparated into interior compartmen~ 104 and
106 by ~ solid abu~nent lOB. The di~placer 110 i8 adapted ~or
reciprocal movement wi~hin the in~erior compartment 104. As in
the embodiment of Fig~ he di~pla2er 110 i8 hollow and filled
1 with a bed of regenerative material such as ~mall copper æpheres.
. Likewise the annul~r region between ~he housing ~sction 103 and
the ~olid abutment 108 is also filled with a bed of regener~tive
material~ ~lso lncluded in this embodiment i5 a heat radia~ion
shield 111 which surrounds the housing 6ec~.ion 102 from ~he cold
end 112 t~ a 1uid inlet 114. The radiation ~hield 111 i8 made
of a highly thermally conductive material such as copper ~nd is
effective in keeping radiant energy away from the cooled end 112.
As in the embodiment of Fig. 1, ~wo worXing volume~ 116 and 118
are crea~ed within the housing 102. ~lso a5 in the case of the
embodiment of FigO 1, longitudinally ~paced-apar~ circular di~cs
120 are provided ~o enhance heat ~ransfer. A second inlet 122
communicates wi~h the interior compartment 106. The inle s 114
and 122 are connected to a compressor such a~ the compressor
assembly 5~ illu6tr~ed in Fig. 30 It will thus be appreciated
that the displacer 110 will reciprocate in ~he internal compart-
. ment 104 u~der the influence of "push-pull" pressure waves from
the compres~or assembly 50. Such reciprocativn causes cooling
both in ~he primary working volume 116 and in the ~econdary
' working vol~m~ 118.
; Disposed ad3acen~ ~o the working volume 118 and in a
hea~ exchange relation with the discs 120 are thermally c~nduc-
tive masses 130 and 132. A suitable material is copper. As the
working volume 118 i~ cooled, the cooliny effect i~ transmit~ed
through the therm211y conductive material 130 and 132 ~o as to
e~fect a precooling of th~ working fluid coming into the int~rior
compartment 104 through ~he inlet 114. As di~cussed with the
earlier ~mbodim~n~, this arrangemen not ~nly effects a pre-
cooling of ~he working fluid bu~ al~o substantially reduces the
~`
1 thermal gradient acro ~ ~he length o~ ~he di~placer llO which i~
effective to control unwan~ed thermophoni~s. ThusO becaue~ of
the thermally conducting ma~erial 130 and 132, i~ i~ unnece~ary
that the inle~ 113 be coiled around the ou~side of the assembly
lQ0.
A signifieant aspect of the embodiment of Fig. 2 i~ ~
that as ~h~ di~placer travels downwardly, it sequentially comes
abreast of the di6cs 120 ~ Because of the close t:alerance between
these di~cs and the di6pl~cer 110, the fluid flow pa~h i~
progressively reduced a~ each of the di6cs is encountered. When
the displacer xeaches the lowermost disc, the 1OW i8 completely
cut off<, Thi8 progres~ive diminution of ~low thus provides for
the controlled deceleration of the displacer by way of pneumatic
cushioning. In ~his way, slamming is eff~ctiv~ly controlled by
this phase shifting means.
It is thus ~een that the object~ of this invention have
been achieved in ~hat there has been di~closed a miniature
cryogenic cooling ~ystem which has the very desirable charac-
teristics of low microphonics and low thermophonics. These
attributes are accomplished because the cold finger employ5
pneumatic cushioning to prevent contact between the reciprocating
displacer and it~ enclo~ure. In addi~ion, precooling of the
I working fluid in a second working volume reduces thermal gradi-
ent~ which ~ub~tan~ially ameliorate the problem of thermophonics.
The two compre~sors of the compre ~or a~semb~y which drive the
reciprocating displacer are of a ~imple construction becau e ~he
two pi~ton~ reciproc~te colinearly. This results in a compressor
assembly which is much ~impler to manufac~ure than known
~14~
~o~
compres~ors which have pi8ton8 deployed at angles other ~han 180
degree~ with respect o one another or havlng piston6 of dif-
ferent ~ize~ In addition becau~e ~:he com~r*~sor as~embly -
disclo~ed herein use~ pi~ong of the ~ame ~ize, ~he compre sor
assembly i8 self-balancing.
.
j While this invention ha~ been describad wi~h reference
to its preferred embodimen~ hould be under~tood that
variations and modification~ will occur to tho~e ~killed in the
art in view of the foregoing description. It i~ intended that
all ~uch variation~ and modification~ be included within the
scope of th~ appended claim~.
What i~ claimed i~:
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