The design criteria for cascade Joule-Thomson refrigerators for cooling in the temperature range from 300 K to 4.2 K were studied. The systems considered use three or four refrigeration stages with various working gases to achieve the low temperatures. Each stage results in cooling to a progressively lower temperature and provides cooling at intermediate temperatures to remove the substantial amount of parasitic heat load encountered in a typical dewar. With careful dewar design considerable cooling can be achieved with moderate gas flows. For many applications, e.g., in the cooling of sensitive sensors, the fact that the refrigerator contains no moving parts and may be remotely located from the gas source is of considerable advantage. A small compressor suitable for providing the gas flows required was constructed

Steyert, W. A.

Tward, E.

English

NASA Technical Reports Server

CASCADEJOULE-THOMSONREFRIGERATORS*
+E. Twardand W. A. Steyert
Jet Propulsion Laboratory
4800 OakGrove Drive
Pasadena, CA 91109
ABSTRACT
The design criteria for cascade Joule-Thomson refrigerators for cooling
in the temperature range from 300K to 4.2K have been studied. The systems
considered use three or four refrigeration stages with various working gases
to achieve the low temperatures. Each stage results in cooling to a progress-
ively lower temperature and provides cooling at intermediate temperatures to
remove the substantial amount of parasitic heat load encountered in a typical
dewar. With careful dewar design considerable cooling can be achieved with
moderate gas flows. For many applications, e.g., in the cooling of sensitive
sensors, the fact that the refrigerator contains no moving parts and may be
remotely located from the gas source is of considerable advantage. A small
compressor suitable for providing the gas flows required has been constructed
and tested. Performance of the system is described.
* This paper represents one phase of research carried out at the Jet Propul-
sion Laboratory, California Institute of Technology, under NASA Contract
NAS7-918.
+ Present address: Air Products and Chemicals, Inc., Box 538, Allentown, PA
18105
419
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INTRODUCTION
The applications for electronics operating at cryogenic temperatures are
continually expanding, particularly for military applications. Examples in-
clude infrared detectors, Josephson junction computers, communication receiv-
ers, precise navigation devices and magnetic sensors. With the advent of
nuclear magnetic resonance imaging systems, the production of liquid helium
temperatures even in medical instrumentation will soon be routine. Unfortu-
nately, the actual use of cryogenic electronics has been hampered by the
expense, complexity, size, and power requirements of the cryogenic refrigera-
tors. In addition, cryogenic refrigerators, in most circumstances, require
highly trained maintenance personnel to keep them operating.
The Joule-Thomson (J-T) refrigerator is extremely simple and has no
moving parts at cryogenic temperature. In a small J-T refrigerator very high
pressure gas flows through a capillary sized tube to a valve or flow restric-
tion where it expands to a lower pressure and cools. This, now cool, gas is
used both to provide the required refrigeration and to precool the high
pressure gas moving in the capillary tube. This very simple refrigeration
method is based entirely on simple energy conservation and requires no in-
tricate or speculative design. Apparently, two factors have limited the use
of J-T refrigerators for cryogenic applications. First, very high pressure
gas and very clean gas is required, and second, J-T refrigerators are theo-
retically not as efficient as other types of refrigerators. In order to pro-
vide clean high pressure gas researchers have been trying for several years
to develop a compressor to meet the very exacting requirements of J-T refrig-
erators. At the Jet Propulsion Laboratory, a need for long-lived refrigera-
tors (up to i0 years) for use on spacecraft has resulted in a considerable
effort to develop non-mechanical adsorption and absorption compressors [i]
for use with J-T refrigerators. It is anticipated that such systems could
deliver the long lifetime required because of the absence of motion induced
wear which is the chief performance degradation mechanism in conventional
refrigerators.
In the meantime, W. A. Little, at Stanford, has developed a method of
producing tiny J-T refrigerators for electronics applications on small glass
slides using photolithographic techniques. [2] These small refrigerators are
now produced commercially [3] and are very promising for high volume, inex-
pensive mass production. Currently, these refrigerators are run "open cycle"
from a compressed gas cylinder with the exhaust gas being vented to the atmos-
phere. The development of a small reliable compressor would open up many more
applications for use of small J-T systems. Cross, Lawless, and Steyert have
designed, but not built, a J-T compressor which uses new electrostrictive
materials developed at Pennsylvania State University. In addition, a proto-
type small mechanical compressor which appears to meet the exacting require-
ments of J-T systems has been built and tested at the Jet Propulsion
Laboratory.
420
There are two important features of J-T systems which are of particular
importance for many applications and should be emphasized. The first is that
the J-T refrigerator itself can be made very small. This means, that for the
cooling of devices such as sensors the dewar can also be made small. For
many sensor applications the major load on the cooling system is due to para-
sitic heat leaks which scale with the size of the dewar system. Therefore,
one often would require smaller cooling powers for a J-T system than if
either a mechanical refrigerator system or a stored cryogen system is chosen.
Secondly, for sensors that are vibration sensitive, the J-T cooler itself has
no moving parts and the compressor can be remotely sited. This leads to po-
tentially very low vibration at the sensor for J-T refrigerators with small
gas flows [4].
JOULE-THOMSON REFRIGERATION
J-T refrigerators operate on a simple conservation of energy principle.
This is illustrated with the aid of Figure i. High pressure gas of enthalpy
hh with mass flow rate _ enters the high pressure side of the heat exchanger
where it is precooled before it expands through the J-T valve to a low pres-
sure and cools. The gas then picks up the heat load Q and exits through the
low pressure side of the heat exchanger so that it precools the incoming high
pressure gas. The gas exits the heat exchanger with an enthalpy h I . From
energy conservation,
= _ (hh - hI) (i)
if there are no additional parasitic heat loads.
This simple expression is valid independent of the quality of the heat
exchanger, although, obviously the better the heat exchanger the larger is
(h - h ) and therefore the larger the cooling power Many gases can be usedh
to provide J-T cooling over a wide temperature range The primary physical
constraint on the cooler is that for a particular gas which is to be used,
the inlet temperature to the heat exchanger must below the inversion tempera-
ture of that gas. It is this constraint that limits use of a single working
fluid design for an all J-T cooler operating to liquid helium temperatures
and at room temperature. He 4 has an inversion temperature of 51K and there-
fore to reach temperatures around 4K requires upper stage refrigeration to
well below 51K in order to precool the high pressure He gas. Therefore, in
order to reach _4K with an all J-T cooler design one requires a cascade
process in which one has a few stages of cooling using appropriate working
gases. For example, in order to reach 4.2K starting at room temperature one
could design a three-stage J-T system using the working gases N2, H2 and He.
421
A N 2 gas source provides high pressure gas to a N 2 J-T cooler which
reaches 77K. A H 2 gas source provides high pressure gas which is precooled
by heat exchange with the N 2 J-T cooler before it enters the H 2 J-T cooler
and provides cooling to approximately 2OK. The last cooling sLage uses He
gas precooled by both the N 2 and H 2 refrigerators before entering the He J-T
refrigerator and achieving the 4.2K low temperature. For each cooling stage
one must include additional cooling power in order to handle the inevitable
parasitic heat loads encountered in a real system. However, nature is kind
to the designer in the following way. For a given flow rate, the cooling
power available from a N cooler is far larger than that available from the
H 2 which is in turn far _arger than that available from a He cooler. This
means that per unit of cooling power less is demanded of the compressor for
the higher temprature stages.
In a related paper at this conference [5] we give an example of a cas-
cade J-T refrigerator which is designed for a very long lifetime. In this
system non-mechanical adsorption compressors are used as the high pressure
gas sources. The long lifetime is expected because of the absence of moving
parts in the refrigerator. A feature of these compressors is that they must
also be precooled to their appropriate operating temperatures. Therefore,
for this design an upper stage refrigerator has to provide, in addition,
cooling for the lower stage compressor. This differs from the example given
later in this paper, where room temperature gas sources are assumed. There-
fore, for adsorption J-T systems, the upper refrigeration stages would re-
quire larger cooling power than a comparable cascade J-T refrigerator using a
room temperature compressor. In some applications, however, such as for a
long space mission where the cooler is inaccessible for maintenance, cascade
J-T refrigerators using adsorption compressors are very promising, especially
given the history of lifetime problems with mechanical cryocoolers.
CASCADE J-T REFRIGERATOR
To illustrate the performance requirements of a cascade J-T refrigerator,
we consider for simplicity a two-stage design operating between 300K and 84K
rather than a four-stage design operating down to 4.5K. A schematic of the
cascaded J-T system is shown in Fig. 2. The upper temperature CF_ stage is
used for precooling the gas to the lower temperature N 2 stage. In addition,
the CF_ stage is used to trap parasitic heat leaks into the dewar system. If
required, sufficient additional cooling capacity may be included for a spe-
cific cooled instrument. In Table I we show some performance characteristics
for a two-stage system. The available cooling power is calculated conserva-
tively, assuming that the heat exchangers are not perfect resulting in 85% of
ideal efficiency for each J-T heat exchanger system. As well, the heat load
422
on the CF_ refrigerator for precooling the N 2 gas has been included. There-
fore, the available cooling power represents the cooling which can be used
for both dewar parasitics and instrument heat loads. Note that even with
these modest flow rates we have quite reasonable cooling powers available.
Table I
Two-stage Refrigerator Performance
Input pressure (atm)
Output pressure (atm)
Flow rate STP cm3/sec
Precool temperature (K)
J-T temperature (K)
Total cooling power (W)
Precool power (W)
Available cooling power (W)
CF 4 stage
68
3
15
300
163
2.0
0.8
1.2
N 2 stage
68
2
15
165
84
1.0
1.0
Such a refrigerator can be run either open or closed cycle and additional
cooling stages to lower temperatures could be added. The addition of H 2 and
He stages would provide temperatures down to <5K with cooling powers at the
lowest temperature approximately 25mW.
A real closed cycle refrigerator requires a small high pressure com-
pressor for the gas source. We have built such a small mechanical compressor
which is oilless and provides almost isothermal compression. This single-
stage compressor is capable of providing compression ratios of 40:1 with flow
rates larger than those used in the previous example. We have operated this
compressor for periods of weeks as a gas source for a small J-T refrigerator
using argon as the working fluid. In that period we had no failure or block-
age in the small refrigerator. From data taken with the cooler we can make
the following estimates on its performance. Because of the low flow rates a
simple parallel capillary tube heat exchanger can easily be made to give
refrigerator efficiency of at least 85% relative to ideal values. The com-
pressor and drive combined are 60% efficient and a motor of at least 70%
efficiency can be used. Therefore, the refrigerator of the example for an
85% efficient J-T and heat exchanger would have available cooling powers of
1.2W at 165K and 1.0W at 84K with an input power of 19W.
423
hh h
JT
VALVE (_
Fig. 1
JT
HEATEXCHANGER
CF4
JT
VALVE
CF,
N
CF4
OUT
N2
IN
N2
OUT
165K(_L
84K QL
.r.h_.Z
MULTIPLETUBE
HEATEXCHANGER
PRECOOLER
N2 HEAT
EXCHANGER
424
QUESTIONS
K. B. Sigurdson, JPL
i. For a 4.2K, .01W, 4-stage cascaded J-T refrigerator, is 18W input
power the total required for all 4 compressors?
Author 's reply :
Yes. The calculation assumes the same efflclencles as given in the two-
stage example of the text, a particular dewar design whose small parasitic
heat loads are handled by the refrigerator, and no excess cooling capacity at
the upper stages.
REFERENCES
i. Chan, C. K.; Tward, E.; and Elleman, D. D.: Miniature J-T Refrigerators
Using Adsorption Compressors, Adv. in Cryogenic Engineering 27, 735,
1982.
2. Little, W. A. : Scaling of Miniature Cryocoolers to Microminlature Size.
NBS Special Publication 508, 75, 1978.
3. NMR Technologies, Mountain View, CA 94043.
4. Elleman, D. D.; Petrac, D.; Chan, C.K.; and E. Tward: Thermal Stability
and Noise of a Miniature J-T Heat Exchanger Refrigerator System. Pro-
ceedings of the 9th International Cryogenic Engineering Conference,
p. 385, Butterworth 1982.
5. Chan, C. K. : Optimal Design of Gas Adsorption Refrigerators for Cryogenic
Cooling, Proceedings of this conference.
425


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