U.S. patent application number 10/456627 was filed with the patent office on 2003-11-20 for expander in a pulsation tube cooling stage.
Invention is credited to Hofmann, Albert.
Application Number | 20030213251 10/456627 |
Document ID | / |
Family ID | 7666484 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030213251 |
Kind Code |
A1 |
Hofmann, Albert |
November 20, 2003 |
Expander in a pulsation tube cooling stage
Abstract
In an expander in a pulse tube cooling stage of a cooling system
which pulse tube cooling stage comprises a pressure generator, a
pulse tube cooling unit connected to the pressure generator and
including a regenerator and a pulse tube with a heat exchanger
arranged therebetween and a buffer volume in communication with the
pulse tube and an expander arranged functionally between the pulse
tube and the buffer volume, the expander includes capillary flow
passages consisting of a material with high heat conductivity for
dissipating heat from the gas flowing through the capillary flow
passages.
Inventors: |
Hofmann, Albert; (Karlsruhe,
DE) |
Correspondence
Address: |
KLAUS J. BACH & ASSOCIATES
PATENTS AND TRADEMARKS
4407 TWIN OAKS DRIVE
MURRYSVILLE
PA
15668
US
|
Family ID: |
7666484 |
Appl. No.: |
10/456627 |
Filed: |
June 9, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10456627 |
Jun 9, 2003 |
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PCT/EP01/13683 |
Nov 24, 2001 |
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Current U.S.
Class: |
62/6 |
Current CPC
Class: |
F02G 2243/52 20130101;
F25B 2309/1426 20130101; F25B 2309/14241 20130101; F25B 9/10
20130101; F25B 9/145 20130101; F25B 2309/1418 20130101; F25B
2309/1407 20130101; F25B 2309/1424 20130101; F25B 2309/1408
20130101; F25B 2309/1413 20130101; F25B 2309/1421 20130101 |
Class at
Publication: |
62/6 |
International
Class: |
F25B 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 9, 2000 |
DE |
100 61 379.9 |
Claims
What is claimed is:
1. An expander in a pulse tube cooling stage in a cooling system,
said pulse tube cooling stage comprising essentially a pressure
generator, a pulse tube cooling unit connected to said pressure
generator and including a regenerator (4), a pulse tube (6) with a
heat exchanger (5) arranged therebetween and a buffer volume (21)
in communication with said pulse tube (6) and an expander (20)
arranged functionally between the pulse tube (6) and the buffer
volume (21), said expander (20) including capillary flow passages
in a material with high heat conductivity.
2. An expander according to claim 1, wherein said expander
comprises a plurality of capillary tubes of equal length and
diameter.
3. An expander according to claim 1, wherein said expander consists
of a rod of a sintered, porous, heat-conductive material disposed
in a gas and liquid tight housing.
4. An expander according to claim 1, wherein said pressure
generator includes an inlet and an outlet and is connected to said
regenerator (4) by way of a line including a first branch (14)
connected to the inlet and a second branch (15) connected to the
outlet, each branch including a control valve (16a, 16b).
5. An expander according to claim 4, wherein said buffer volume
(21) includes a control volume which is separated from the buffer
volume in a gas and liquid tight manner and which is
non-compressible and by which the buffer volume is adjustable.
6. An expander according to claim 4, wherein said expander (20) is
disposed in a housing connected to a cooling circuit for cooling
the expander (20).
7. An expander according to claim 5, wherein said expander extends
at least partially into said buffer volume 21 and the buffer volume
walls consist of a material with high heat conductivity.
8. An expander according to claim 6, wherein said buffer volume is
connected to the pressure generator (2a) by way of a line (24)
including a control valve (25).
9. An expander according to claim 1, wherein said buffer volume
wall is provided with surface increasing means of heat conductive
material for dissipating heat from said buffer.
Description
[0001] This is a Continuation-In-Part application of international
application PCT/EP01/13683 filed Nov. 24, 2001 and claiming the
priority of German application 100 61 379.9 filed Dec. 9, 2000.
BACKGROUND OF THE INVENTION
[0002] The invention relates to an expander for a pulse tube
cooling system comprising a pulse tube cooling stage or more
co-operating pulse tube cooling stages and each pulse tube cooling
stage comprises in principle a compressor, a pulse tube cooler
consisting of a regenerator and a pulse tube with a heat exchanger
disposed therebetween, a heat exchanger at the exit of the pulse
tube and an expansion container connected thereto.
[0003] A pulse tube cooler is based on the known Stirling process
wherein a gas is compressed and expanded in a cycle. The process
has the advantage that there are no moving parts in the cold part
of the cycle. This permits the use of a relatively simple design
and provides for high operational reliability. Furthermore, there
are only small mechanical vibrations. For temperatures down to
20.degree. K, single stage arrangements may be used. Lower
temperatures can be reached by the use of multiple stages.
[0004] Various types of such coolers are known in the art. Each
pulse tube cooler type comprises a compressor capable of generating
a cyclic gas flow, which is supplied to a regenerative heat
exchanger--known as regenerator. From there, the gas flows through
the pulse tube with the heat exchanger-cooling stage at its cool
end and an expander operating at ambient temperature at the other
end. The expander is a device in which acoustic energy associated
with the pulsing gas flow is removed from the pulse tube. The main
part of the cooling energy is provided by the flow of mechanical
energy, which is part of the pulsed gas flow. The expander must
transfer this mechanical energy flow from the pulse tube wherein
the mechanical energy flow is converted to heat, to the
environment.
[0005] It is known that, in order to obtain an efficient cooler,
the expander must operate in such a way, that, at the end of the
pulse tube, the phase angle of the periodic pressure wave is ahead
of the phase angle of the volume flow.
[0006] Preferred phase angles are in the range of 30 to 60.degree.
. To achieve this, various methods are known. The three most
important methods will lead to the invention, which is described
further below. These three methods, their operations and
efficiencies are therefore shortly described.
[0007] FIG. 9 shows schematically a pulse tube cooler based on the
known principle, called double inlet system. The pressure wave
generator 2 may be a piston compressor or any other type of
compressor with separately controlled inlet and outlet valves. The
oscillating gas flow is conducted by way of the connecting line 3
to the refrigeration generator 1, mainly to the regenerator 4,
which is arranged in series with the pulse tube 6. The low
temperature heat exchanger 5 is disposed between the regenerator 4
and the pulse tube 6 and the high-temperature heat exchanger 7 is
disposed at the other end of the pulse tube 6. A second line 10
with a flow restrictor 11, the throttle impedance, provides for
communication between the heat exchanger that is, its warm end, and
the gas container 12. A part of the gas flow generated by the
pressure wave generator 2 is branched off the connecting line and
conducted, by way of the bypass line 8, which includes a flow
restrictor 9, to the second line 10 to which it is connected
between the heat exchanger 7 and the flow restrictor 11. The gas
flow at the warm end of the pulse tube can be considered to be
composed of two components, the so-called bypass flow through the
bypass line 8 and the so-called throttle flow through the
throttling section 11. The two gas flows differ in their
amplitudes; their phase relationship can be adjusted so as to
obtain optimal flow conditions at the warm end of the pulse tube.
The expansion energy is dissipated in the throttling impedance
provided by the restrictor 11.
[0008] This method however suffers from the fact that it is very
difficult to suppress the detrimental time-averaged current, that
is, the so-called DC current in the bypass branch 8. The line to
which the lead line of the reference number 13 extends symbolizes
the vacuum section. The components within that section are at low
temperature whereas all other components are at ambient
temperature.
[0009] FIG. 10 shows schematically another known pulse tube cooler.
In this case, the expander is in the form of a so-called inertia
tube phase shifter. This part consists of the conduit 10a with
circular cross-section, which extends between the pulse tube 6 and
the buffer container 12a. Its function is based on the inertia of
the gas column, which oscillates in the conduit 10a. But, because
of the small mass of the gas, such arrangements must be operated
either at a relatively high frequency or they must have long lines
for low-frequency systems which are needed for obtaining very low
temperatures.
[0010] FIG. 11 shows a third arrangement of a conventional pulse
tube cooler. In this case, the oscillating gas flow is generated by
a rotary piston compressor 2a with valves 16a to 16d which are
mounted in the suction line 14 and the supply line 15. The gas
flow, which is supplied to the regenerator 4 by way of the supply
line 3, is controlled by the valves 16a and 16b and the gas flow
through the supply line 10b to the pulse tube 6 is controlled by
the valves 16c and 16d. Also, in this case, it is very difficult to
provide for valve switching times such that optimum conditions for
the cooler are provided.
[0011] As mentioned earlier, the known pulse tube coolers cannot
provide sufficient phase shifting unless they have gas flows
controlled by four valve pulse tube coolers. They require a well
adjusted superimposition of two gas flows (two-inlet pulse tube
cooler) or they do not operate at low frequency (inertia phase
shifter). In addition, they need components, which provide for a
continuous transition from the large flow cross-section present in
the pulse tube to the small cross-section of the attached
connecting lines. This function is included in the heat exchanger
7.
[0012] It is the object of the present invention to provide a
cooling system without these disadvantages of the conventional
systems.
SUMMARY OF THE INVENTION
[0013] In an expander in a pulse tube cooling stage of a cooling
system which pulse tube cooling stage comprises a pressure
generator, a pulse tube cooling unit connected to the pressure
generator and including a regenerator and a pulse tube with a heat
exchanger arranged therebetween and a buffer volume in
communication with the pulse tube and an expander arranged
functionally between the pulse tube and the buffer volume, the
expander includes capillary flow passages consisting of a material
with high heat conductivity for dissipating heat from the gas
flowing through the capillary flow passages.
[0014] The invention is based on the fact that an oscillating gas
flow is maintained at a constant temperature. As a result, heat is
transferred to the surrounding medium during compression and heat
is retrieved from the surrounding medium during expansion of the
gas in the conduit. This process causes a phase shift between the
oscillating pressure and the volume flow.
[0015] The parallel conduits need to be so dimensioned, that is
they must have such a length and open width, that the expansion
energy--the part responsible for the cooling--is converted by way
of friction into the heat flow to be transferred to the surrounding
medium. The flow within each capillary must therefore by isotherm.
As a result, the diameter of the capillaries must be small in
comparison with the thermal depth of penetration into the gas.
[0016] The isothermal pulsating gas flow in narrow conduits can be
described by differential equations of the same type as they are
used for electrical transmission lines subjected to losses.
Therefore, an arrangement of parallel lines can be provided in such
a way that they act like transmission lines, in which the inductive
effect dominates the capacitive effect. With the capillary or the
bundle of capillaries and the gas buffer volume as final impedance,
the entrance impedance of such an arrangement causes the pressure
wave to precede the volume flow as it is required for the
withdrawal of the expansion energy from the pulse tube.
[0017] The impedance adaptation is achieved by the fact that in
each capillary there is a sufficiently large friction resistance
for the gas flow and a sufficiently large heat exchange through the
capillary wall. To this end, the inner and outer surface areas of
the capillaries must be large enough for conducting this heat
away.
[0018] In a modification of the invention, an arrangement using,
instead of a large number of parallel capillaries, a rod of a
porous sintered material or a fleece or a stack of net-like discs,
possibly in a compressed form. The components used should have good
heat conductivity and are surrounded by a wall or a sheet, which
also has good heat conductivity. The structure must have, over its
cross-section and length, a suitable flow resistance and must be
able to transfer the heat flow to the ambient medium as required.
Such a rod can be described for dimensioning with an arrangement of
sufficient quality by an equivalent bundle of capillaries. In order
to have a continuous transition at the pulse tube exit, the
cross-section of the rod should be of equal size, or the transition
must at least be conical.
[0019] The pulse tube cooler may be further optimized if the
pressure generator or compressor is connected to the pulse tube
cooler by way of a conduit which originates from the inlet opening
of the regenerator and includes two branches ending in the pressure
generator and each provided with a control valve.
[0020] For fine-tuning a small control volume is arranged in the
buffer volume, which small control volume is adjustable and is not
compressible, whereby the buffer volume can be continuously
adjusted within certain limits (FIG. 3). The small control volume
may be a hydraulically operated piston or a solid material piston,
which is insertable into, and removable from, the buffer
volume.
[0021] If the surface is not sufficiently large for the transfer of
heat to the ambient medium, the heat exchanger may be surrounded by
a gas and liquid-tight housing with an inlet and an outlet
connected to a cooling circuit (FIG. 4).
[0022] If the capillary flow channel or the enclosed and sintered
rod extend into the buffer volume, the wall of the buffer volume
must consist of a material with good heat conductivity in order to
permit a sufficient heat flow to the ambient medium (FIG. 5). The
final impedance, the expander, then consists in principle of the
unit, which forms the heat exchanger at the pulse tube outlet, and
at the same time, the connection to the buffer volume and also of
the buffer volume.
[0023] The buffer volume can be connected to the pressure generator
by way of a line, which includes a dosing valve (FIG. 7). This
permits to provide a time-averaged net mass flow-through the pulse
tube.
[0024] The unit providing for the heat dissipation to the ambient
medium and also for the conduction of gas between the pulse tube
and the buffer volume consists of parallel capillaries or sintered
sponge-like or wool or felt-like materials with good heat
conductivity. If the capillaries are evenly distributed over the
cross-section at the warm end of the pulse tube or the materials
have there the same cross-section, an additional flow controller as
used in the state of the art is not needed. In this case, the flow
controller is the device, which ensures a uniform flow with uniform
flow speed over the whole cross-section. The heat exchanger at the
warm end provides for the flow direction.
[0025] Such arrangements are very compact even with frequencies in
the range of 2 Hz. In spite of the relatively small size, the
arrangements have a large area for the heat transfer. In addition,
the time-averaged net mass flow through the pulse tube can be
interrupted or controlled if desired.
[0026] The invention will be described below in greater detail on
the basis of the accompanying drawings comprising FIGS. 1-8. FIGS.
9-11 are provided for an explanation of the principle involved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows a pulse tube cooler with capillary flow
channels installed therein,
[0028] FIG. 2 shows the valve-controlled supply line from the
pressure generator to the regenerator,
[0029] FIG. 3 shows the buffer volume with a controllable expander
installed therein,
[0030] FIG. 4 shows an arrangement for a forced heat removal at the
expander,
[0031] FIG. 5 shows the expander arranged so as to extend directly
into the buffer volume,
[0032] FIG. 6 shows a sintered rod as an expander,
[0033] FIG. 7 shows an arrangement in which the buffer volume can
be selectively connected to the pressure generator,
[0034] FIG. 8 shows a cooling arrangement with a two-stage setup of
pulse tube coolers,
[0035] FIG. 9 shows the general known arrangement of a pulse tube
cooler with a flow resistance in the connecting line to the buffer
volume and in the bypass line,
[0036] FIG. 10 shows a conventional pulse tube cooler in its most
simple form for an explanation of the principle, and
[0037] FIG. 11 shows a conventional pulse tube cooler with
connecting lines between the pressure generator and the regenerator
and a connecting line between the pressure generator and the heat
exchanger at the warm end of the pulse tube each provided with two
control valves.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0038] The expander 20 shown in FIG. 1 is so designed that a phase
shift between the pressure and the volume flow at the warm end of
the pulse tube 6c can be established which is optimal for the
cyclic process. In this way, a pulse tube cooler of high efficiency
and simple design can be provided. New herein is the arrangement of
narrow flow channels or capillary passages 20 between the outlet of
the pulse tube 6, that is, the warm end thereof, and the gas
container, that is, the buffer volume 21, which is dimensioned in
accordance with the output, that is, the size, of the pulse tube
cooler. It is important that the flow channels 20 consist of a
material with good heat conductivity to the ambient medium, which
is the cooling medium.
[0039] FIG. 2 shows schematically a preferred arrangement of a
single-stage pulse tube cooler. The physical process steps are
described below in detail on the basis of this arrangement:
[0040] The single stage pulse tube cooler is operated by a rotary
piston gas compressor 2a with the intake connection 14 and the
supply connection 15. The two tubular connections 14 and 15 include
each a valve 16a and respectively, 16b for closing and opening the
connection 14 and 15 respectively. By way of a T member 16c, the
connections 14 and 15 are joined and connected by way of line 3 to
the refrigeration head 1 or the cold end of the cooling
arrangement, that is, more accurately, to the inlet of the
regenerator 4.
[0041] The refrigeration head 1 of the pulse tube refrigeration
apparatus comprises the regenerator 4, a heat exchanger 5 at the
cold end, a pulse tube 6c and a final impedance connected at the
exit of the pulse tube and extending into the surrounding medium.
The final impedance comprises a large number of capillaries 20
connected with one of their ends to the exit of the pulse tube 6
and, with their opposite ends to a buffer volume in the form of a
gas container 21.
[0042] The regenerator 4 comprises a stack of porous materials
having a high specific heat capacity, preferably of formed
stainless steel lattice discs stacked on top of one another in a
cylindrical housing. The pulse tube 6 is a cylindrical tube filled
with a refrigeration medium, typically helium, which is maintained
at a pressure oscillating by several bars about an average pressure
of about 20 bar. The heat exchanger 5 is the component by which the
low temperature generated by the oscillating internal gas flow is
transferred to the outside user by way of a heat transfer medium
which is not shown herein. In addition, the heat exchanger 5 acts
as a flow controller or distributor such that the flow is evenly
distributed over the cross-section of the entrance area of the
pulse tube 6.
[0043] The refrigeration effect is achieved by the cyclical process
with the following steps for the gas within the pulse tube:
[0044] A. Compression
[0045] By opening the discharge valve 16a, pressurized hot gas is
conducted through the regenerator where it is cooled and then flows
through the heat exchanger 5 into the pulse tube 6. Another gas
stream from the buffer volume 21 enters the pulse tube 6 through
the capillaries 20 at the warm end of the pulse tube 6. For this to
occur, the buffer volume 21 and the capillary 20 must be so
dimensioned that a resonance with a corresponding phase shift
between the two gas flows entering and then again leaving the pulse
tube occurs.
[0046] B. Shift Toward the Warm End
[0047] After a certain time corresponding to the gas flow entering
at the cold end, the pressure in the pulse tube 6 becomes higher
than the pressure in the buffer volume 21 and the gas having a
temperature higher than the ambient medium at the warm end of the
pulse tube 6 flows back through the capillaries 20 into the buffer
volume 21. During this step, heat is dissipated from the
capillaries to the ambient medium.
[0048] C. Expansion
[0049] At this point the discharge valve 16a is closed and the
intake valve 16b is open. At the beginning of this step, gas is
discharged through both ends of the pulse tube 6. The pressure and
temperature in the pulse tube 6 drop.
[0050] D. Shift Toward the Cold End.
[0051] Finally, a gas stream from the buffer volume 21 enters the
warm end of the pulse tube 6 and, at the same time, cold gas flows
back to the regenerator 4. During this step, heat is absorbed by
the cold gas flow in the heat exchanger 5.
[0052] Expressed differently, the continuous cycle can be described
as a process wherein the gas in the pulse tube acts like a piston
which, in a mechanical way, transfers the expansion energy from the
cold end of the pulse tube to the warm end where it is dissipated
in a heat flow transferred d to the ambient medium around the
capillary bundle. To this end, the warm end of the pulse tube must
be closed by a well-adopted flow impedance that is a component
which must fulfill certain main conditions:
[0053] controlled resonance
[0054] adapted flow resistance
[0055] to convert energy to heat, and
[0056] establishing resilient flux in the pulse tube.
[0057] In accordance with such different functions, different
designations for the component may be selected such as pulse tube
expander, thermal phase shifter, or inductively effective
pulse-tube end-impedance.
[0058] The numeric examination of such pulse tube end systems show
that this type of phase shifter is most advantageous for coolers,
which operate at low frequencies. As an example, a pulse tube
cooler is analyzed to increase 50W from 50K to 300K. The respective
pulse tube has a diameter of about 45 mm and is 200 mm long. The
respective expander comprises about 40 capillaries each with an
inner diameter of 0.3 mm and a length of 150 mm. The gas container
of the buffer volume must have about 200 cm.sup.3.
[0059] Further possible components are:
[0060] The gas container or buffer volume 21 in FIG. 3 is provided
with the volume 33, which, within limits, is continuously
adjustable. It is for example in the form of a hydraulic or solid
piston provided with a fluid supply line 23 for changing the buffer
volume, that is for finely adjusting the impedance volume to the
system.
[0061] FIG. 4 shows an expander 20 with forced cooling which for
that purpose is enclosed in a tube 25 with connections 26a and 26b
for conducting a coolant past the expander capillaries 20.
[0062] In FIG. 5, the capillaries 20 are shown arranged within the
buffer volume 21. In order to provide in this case a sufficiently
good heat flow path to the ambient medium the wall of the buffer
volume at least must consist of a material with good heat
conductivity.
[0063] An alternative solution which is considered to be less
effective than a bundle of parallel capillaries is a sintered rod
of heat conducting material with a suitable pore size or
metal/stainless steel wool, possibly with a felt-like structure.
Since the pores or passages are uniformly distributed over the
whole surface of this material and statistically uniformly
distributed flow passages are provided, the rod must be enveloped
between the exit of the pulse tube and the entrance of the buffer
volume in a gas-tight manner by an envelope with good heat
conductivity so that only flow channels are formed leading from the
entrance area at the pulse tube exit to the exit area at the buffer
volume. As in the capillary bundle, the entrance area should be the
same as the exit area and the entrance area should be connected
directly to the pulse tube and have the same diameter (FIG. 6).
[0064] If a net mass flow is to be established which is different
from zero, a conduit 24 can be installed between the buffer volume
21 and the pressure generator with a control valve 25 arranged in
the conduit 24 for continuously adjusting the flow cross-section.
(FIG. 7) If the conduit 24 is not directly connected to the
pressure generator, it should be connected to the suction line 15
between the pressure generator 20 and the respective valve 16b.
[0065] A two-stage pulse tube cooler is shown for example in FIG.
8. The pre-stage comprises a vacuum area 13 including the
regenerator 4a, the pulse tube 6a and the heat exchanger 5a
disposed therebetween. The respective expander 20a consisting of
heat exchanger, capillary bundle and buffer volume extends into the
ambient medium and is connected to the pressure generator 2a by way
of the conduit 24 with the control valve 25a for providing a
specific net volume flow adjustment. The second stage comprises, in
the vacuum container 13, the regenerator 4b, the pulse tube 6b and
the associated heat exchanger 5b. The associated expander 20b is in
principle the same as the expander 20a of the pre-stage and also
extends into the ambient medium. The housing of the expander may be
provided with heat transfer means such as ribs to improve the
transfer of heat to the ambient medium. It too is connected, by way
of the control valve 25b for a specific volume flow adjustment for
the second stage, to the pressure generator 2a. The pressure
generator of the second stage is formed by the connecting conduit
from the regenerator 4a of the first stage to the pulse tube 6a of
the first stage. This connecting conduit and the corresponding one
of the second stage together with the heat exchangers 5a and 5b
form in the vacuum container 13 the heat sinks used in the
system.
* * * * *