U.S. patent application number 10/525030 was filed with the patent office on 2006-07-06 for oil carry-over prevention from helium gas compressor.
This patent application is currently assigned to OXFORD MAGNET TECHNOLOGY. Invention is credited to Millind Diwakar Atrey, David Michael Crowley, Peter derek Daniels.
Application Number | 20060147318 10/525030 |
Document ID | / |
Family ID | 31892155 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060147318 |
Kind Code |
A1 |
Atrey; Millind Diwakar ; et
al. |
July 6, 2006 |
Oil carry-over prevention from helium gas compressor
Abstract
The present invention provides a pumped helium circuit
comprising a compressor (14) with a high pressure port (16) and a
low pressure port (18) each connected to a supplied equipment
(61,63,65,67) to respectively supply compressed helium to, and
receive compressed helium from, the supplied equipment; a pressure
relief valve (12) operable to link the high pressure port to the
low pressure port in response to a predetermined pressure
differential; a non-return valve (13) located between a low
pressure side of the pressure relief valve and the supplied
equipment; and means for preventing oil carry-over from the
compressor to the supplied equipment.
Inventors: |
Atrey; Millind Diwakar;
(Witney, GB) ; Crowley; David Michael; (Marlow
Bottom, GB) ; Daniels; Peter derek; (Daventry,
GB) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
OXFORD MAGNET TECHNOLOGY
Eynsham
GB
|
Family ID: |
31892155 |
Appl. No.: |
10/525030 |
Filed: |
June 26, 2003 |
PCT Filed: |
June 26, 2003 |
PCT NO: |
PCT/GB03/02797 |
371 Date: |
October 14, 2005 |
Current U.S.
Class: |
417/310 |
Current CPC
Class: |
F04C 29/026 20130101;
F04C 2210/105 20130101; F04C 23/00 20130101; F04C 2210/10 20130101;
F25B 9/002 20130101; F25B 2500/07 20130101; F04C 18/0215 20130101;
F25B 43/02 20130101; F04C 29/0092 20130101; F25B 2309/14181
20130101; F04C 2220/22 20130101; F25B 9/14 20130101; F25B 9/145
20130101 |
Class at
Publication: |
417/310 |
International
Class: |
F04B 49/00 20060101
F04B049/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 17, 2002 |
GB |
0219210.2 |
Aug 17, 2002 |
GB |
0219211.0 |
Aug 17, 2002 |
GB |
0219209.4 |
Mar 20, 2003 |
GB |
0306364.1 |
Claims
1. A pumped helium circuit comprising a compressor (14) with a high
pressure port (16) and a low pressure port (18) each connected to a
supplied equipment (61,63,65,67) to respectively supply compressed
helium to, and receive compressed helium from, the supplied
equipment; a pressure relief valve (12) operable to link the high
pressure port to the low pressure port in response to a
predetermined pressure differential; a non-return valve (13)
located between a low pressure side of the pressure relief valve
and the supplied equipment; and means for preventing oil carry-over
from the compressor to the supplied equipment, characterised in
that said means comprises means for preventing oil leaving the low
pressure port and travelling towards the supplied equipment.
2. A pumped helium circuit according to claim 1, wherein said means
comprises an oil trap located in the circuit between the low
pressure port and the supplied equipment.
3. A pumped helium circuit according to claim 1, wherein said means
comprises an oil adsorber located in the circuit between the low
pressure port and the supplied equipment.
4. A pumped helium circuit according to claim 1, wherein said means
comprises a gas reservoir located in the circuit between the low
pressure port and the supplied equipment.
5. A pumped helium circuit according to claim 1, wherein said means
comprises a combined gas reservoir and oil adsorber located in the
circuit between the low pressure port and the supplied
equipment.
6. A pumped helium circuit according to claim 1, wherein said means
comprises a pressure actuated switch in the circuit between the low
pressure part and the supplied equipment, said switch being
operable to stop operation of the compressor in response to a gas
pressure at the low pressure port falling below a predetermined
value, the predetermined value being less than the minimum pressure
at the low pressure port during normal operation.
7. A pumped helium circuit comprising a compressor (14) with a high
pressure port (16) and a low pressure port (18) each connected to a
supplied equipment (61,63,65,67) to respectively supply compressed
helium to, and receive compressed helium from, the supplied
equipment; and a pressure relief valve (12) operable to return
compressed helium from the high pressure port to the compressor in
response to a predetermined pressure differential; characterised in
that the pressure relief valve is connected between the high
pressure port and the compressor, independently of the low pressure
port.
8. A method for preventing oil carry-over from a helium compressor
(14) to a supplied equipment (63,67,61,65) comprising the steps of
supplying compressed helium through a high pressure port (16) to
the supplied equipment; receiving compressed helium through a low
pressure port (18) from the supplied equipment; operating a bypass
relief valve (12) in response to a differential pressure exceeding
a predetermined value, thereby allowing oil-laden compressed helium
to flow from the high pressure port to the compressor,
characterised in that the method further comprises the step of
preventing oil from the oil-laden compressed helium from travelling
from the low pressure port to the supplied equipment.
Description
BACKGROUND
[0001] When helium gas is compressed, a relatively large amount of
heat is produced. Helium has one of the highest specific heat
capacity ratios of known gases (.gamma.=Cp/Cv=1.67 for helium).
When helium is compressed, a very effective cooling mechanism must
be provided. In the absence of such a cooling mechanism, it would
be impossible to reach the temperature of liquefaction of helium,
and it would be impossible to produce liquid helium. In
applications such as magneto-resonance imaging (MRI), it is
necessary to achieve very low temperatures, of the order of 4-10 K.
This is currently required to keep superconducting magnets in the
superconducting state. Helium is the only known gas which remains
gaseous at such temperatures, and accordingly the problems
associated with the liquefaction of helium must be tolerated.
[0002] Two alternative methods are known for removing the heat from
compressed helium. In one method, helium is compressed in stages,
and the compressed gas is cooled after each stage by passing over
cooled heat-conductive vanes, for example, water-cooled metallic
vanes. In the second method, oil is mixed in with the helium under
pressure. The heat generated by pressurising the helium gas is
absorbed by the oil. The oil must be removed from the helium before
the helium is used for cooling, since the oil would solidify and
cause problems in the cryogenic application if subjected to a
temperature in the range of interest, that is, of the order of 4-10
K.
[0003] The present invention relates to the second method of
compression and cooling, in which oil is mixed with the helium.
[0004] FIG. 1 shows a schematic diagram of a known helium
compressor with internal bypass relief valve 12. In cryogenic
operations, for example magneto-resonance imaging, it is common to
compress Helium gas using a Helium compressor with internal bypass
relief valve. Such apparatus is manufactured and supplied as a
complete unit, with High Pressure (HP) and Low Pressure (LP) ports
16,18. The internal bypass relief valve 12 is provided to prevent
damage to the compressor capsule 14, which might otherwise occur if
the HP port 16 were blocked, for example. The internal bypass
relief valve 12 reacts to an increase in differential pressure
between the HP and LP ports by effectively connecting the HP port
16 to the LP port 18. This provides a path 11 for the pressurised
helium, and prevents damage to the compressor capsule 14. A
non-return valve (NRV) 13 is also typically provided, between the
LP port 18 and the internal bypass relief valve connection 15. This
is intended to prevent backflow of gas and also to prevent the
gases and any contaminants that pass through the bypass relief
valve 12 from reaching the LP port 18. Oil separator 17 is provided
in the high pressure output line of the compressor capsule 14 to
separate the oil from the compressed helium gas. This oil separator
may not retain 100% of the oil present in the helium, so it is
known to provide an oil adsorber 19, for example of activated
charcoal, either within the compressor upstream from the HP port
16, or externally, downstream from the HP port 16.
[0005] A known type of helium pump is known as a scroll compressor.
FIGS. 2A-D schematically represent the operative part of a scroll
compressor. The scroll compressor comprises two similar, concentric
spirals 21, 23, one inserted within the other. Spiral 23 remains
stationary as spiral 21 orbits within it. As shown in FIG. 2A, gas
is drawn into compression chambers 25, 25' when the outer openings
27, 27' are open. As the spiral 21 orbits, and as shown in FIG. 2B,
the outer openings 27, 27' close and the compression chambers 25,
25' are drawn within the spiral 23. As the spiral 21 continues its
orbit, and as shown in FIG. 2C, the compression chambers 25, 25'
are drawn further into the spiral, and its volume reduces,
compressing the gas within the chambers 25, 25'. The outer openings
27, 27' reopen, to expose further compression chambers 29, 29' to
the ambient gas. Chambers 25, 25' move towards the centre of the
scroll, becoming increasingly compressed until the gas within the
chambers reaches maximum pressure at the centre of the compressor,
illustrated in FIG. 2D. There, the high-pressure gas is released
through a discharge port 22 in the fixed scroll 23. The various
compression chambers 25, 25', 29, 29' etc. arrive sequentially at
discharge port 22, while new compression chambers are created by
the opening and closing of the outer opening 27.
[0006] While described above as acting to compress gas, in the
present application, the scroll compressor will be acting upon a
mixture of helium with oil, referred the hereinafter as
"gas+oil".
Introduction
[0007] A typical use for the compressed helium produced by the
helium compressor of FIG. 1 is in supplying a pulse tube
refrigerator 61 for the cooling of superconductive MRI magnets. A
pulse tube refrigerator of known type may be supplied with high
pressure pumped helium gas through an HP line 63 the HP port 16,
while a return flow of helium gas at relatively low pressure
returns through an HP line 65 to LP port 18. In this context, the
HP port typically provides helium gas at a pressure of around 2.4
MPa (24 bar), while the LP port typically receives gas at a
pressure of around 0.6 MPa (6 bar). Present pulse tube
refrigerators typically employ a rotary valve (RV) mechanism 67. A
number of mutually rotating discs define valve opening and closing
times, and valve orifice dimension. Such arrangements ensure
correct and unchanging timing and dimension relationship between
the various valves embodied in the rotary valve mechanism 67. In
the present context, both the LP and HP ports would be connected to
at least one valve of the rotary valve mechanism.
[0008] The HP and LP ports are typically connected to the pulse
tube refrigerator with a relatively long flexible hose 63, 65.
During development trials of the applicant's pulse tube
refrigerator, it was noticed that some pulse tube refrigerator cold
heads with rotary valve and flex lines were flooded with compressor
oil over a period of time. As this occurred on four systems, it
could not be considered a random event. Experiments were performed
in order to understand the mechanism of oil carry over. The present
invention provides means and methods to overcome or at least
alleviate the problems with the prior art compressor/pulse tube
refrigerator assembly, and the present invention may be applied to
any system in which a helium compressor with internal bypass relief
valve has its HP and LP ports connected to a valve mechanism.
[0009] Prior to the present invention, it had been considered that
the most likely cause for the presence of oil in the flex tubes was
the inefficiency of the adsorber 19 connected to the HP port
16.
[0010] In an initial investigation, as shown in FIG. 1, flex line
65 to the PTR was twenty metres in length. The pressure in the HP
line 63 was increased from 2.4 MPa (24 bar) to 2.9 MPa (29 bar) in
steps of 0.1 MPa (1 bar), being run for 4-6 hours for each step.
After each step, the two metres of LP line 65 was subjected to
residual gas analysis (RGA) to trace any oil in the line. The flex
line under examination line was heated to approximately 200.degree.
C. In a line containing oil, very high traces of CO and CO.sub.2
were detected, indicating the breakdown of oil within the tube
under examination. The PTR was run for each trial and showed 10 K
no load temperature on its second stage. The PTR was then subjected
to heater loads of 40 W and 6 W at its first and second stages,
respectively. However no oil could be traced under any of these
conditions. The gas was always able to flow around the gas circuit
63, 67, 65 from the HP port 16 to the LP port 18.
[0011] It is known that several fault conditions may cause the
rotary valve (RV) 67 to stop, while the helium compressor continues
to operate. In these conditions, the helium pressure inside the HP
line rises to a relatively very high value, such as 2.9 MPa (29
bar), while the helium pressure in the low pressure line falls
rapidly to a relatively very low pressure, such as 0.15 MPa (1.5
bar).
[0012] Further investigation was made into the effect of stopping
the rotary valve 67 while the compressor was still in operation,
after cooling the PTR cold head. As soon as the rotary valve stops,
the helium pressure in the HP line 63 and within the connected
parts of the compressor increases. The rate and magnitude of this
increase depends on the stop position of the rotary valve 67. If
the HP port 16 is connected to PTR in the stop position, the
pressure increase in the HP line is not very high. This is due to
the fact that the complete PTR volume is in line with the
compressor. However, if the LP port is connected to the compressor
during the rotary valve stop position, the pressure increase in the
HP line is very high. As the LP port is connected to the
compressor, the gas pressure in the whole LP line is reduced by the
compressor to a very low value.
[0013] During the investigation rotary valve 67 was stopped in a
position which increased the compressor pressure and the pressure
in the HP line to 2.8-2.9 MPa (28-29 bar) and the compressor was
run in this condition for 1-2 days. At this point, a small trace of
oil could be observed in the two-metre line 33.
[0014] However, it was noted that the HP line showed a trace of oil
in the line only after a lengthy heating time, while the LP line
showed a trace oil almost instantaneously when heated. This
unexpected and surprising result led to the conclusion that the oil
arriving in the pulse tube refrigerator 61 and the flexible hoses
63, 65 was transferred from the compressor to the LP line first
overcoming the NRV (non return valve) resistance and then went to
HP line during operation via PTR cold head. This conclusion was
tested and led to the present invention, which provides various
methods and apparatus for preventing oil from travelling past the
NRV and through the LP port.
[0015] A further investigation was performed to trace the mechanism
of the oil carry-over. A pressure gauge was connected at position
31, in place of the further adsorber, at the distal end of the two
metre LP flex line 33, while the other end was connected to the LP
port 18 of the compressor. The HP port 16 of the compressor was
kept unattached, and therefore, blocked. The initial pressure in
the LP line was 0.15 MPa (1.5 bar). The compressor was run at high
HP line pressure of 2.8-2.9 MPa (28-29bar) for two to three days.
This essentially ran the compressor in an internal bypass
condition, with the only gas flow being from the HP line through
the internal bypass valve 12 to the LP line. It was found that the
pressure in the LP line increased to 0.4 MPa (4 bar) over a period
of time, due to the mixture of gas+oil which travelled through the
internal bypass valve 12 without going through the adsorber 19. The
gas+oil enters the junction 15. The LP port 18 is at a relatively
very low pressure. If the pressure at the junction 15 rises
sufficiently, due to the entry of high-pressure gas+oil from the HP
line through internal bypass valve 12, it may be possible for some
of that gas+oil to travel through the NRV towards and through the
LP port 18 into the LP line 65. The two-metre line 33 showed traces
of oil when subjected to RGA. This was considered to confirm the
hypothesis that gas+oil could cross the NRV. Over a period of time,
an appreciable quantity of oil could travel in this way to the LP
flex line 65 and then to PTR 61 cold head.
[0016] To confirm this result, the experiment was repeated with the
HP and LP lines 63,65 connected to the PTR 61 and the compressor
was started. The rotary valve 67 was then stopped, simulating a
fault condition. As soon as the rotary valve 67 stopped, the
pressure in the LP line reduced to 0.15-0.2 MPa (1.5-2 bar) and the
pressure in the compressor and the HP line increased to 2.8-2.9 MPa
(28-29 bar). These conditions were similar to those assumed in the
earlier experiment, confirming the validity of that experiment.
[0017] The present invention resides in part in the finding that
oil migration from the compressor to the PTR may be prevented, or
at least substantially reduced, by preventing oil carry over from
the LP side of the compressor, particularly during stoppage of the
rotary valve 67 when the compressor is still in running. In these
circumstances, gas+oil travels from the compressor towards the PTR
61 across the NRV 13 due to high pressure difference between the
compressor pressure and the low pressure in the LP line 65 of the
PTR. This condition should accordingly be avoided wherever
possible. According to a further aspect of the present invention,
methods and apparatus are provided to reduce the effect of this
condition should it occur.
[0018] Accordingly, the present invention provides methods and
apparatus as set out in the appended claims.
[0019] The above, and further, objects, advantages and
characteristics of the present invention will become apparent from
consideration of the following description of certain specific
embodiments of the invention, given by way of non-limiting examples
only, in conjunction with the accompanying drawings, wherein:
[0020] FIG. 1 shows a known helium compressor supplying compressed
helium to a pulse tube refrigerator, according to the prior
art;
[0021] FIG. 2 shows the action of a scroll compressor, according to
the prior art;
[0022] FIG. 3 shows the system of FIG. 1 adapted according to an
embodiment of the present invention;
[0023] FIG. 4 shows the system of FIG. 1 adapted according to a
further embodiment of the present invention; and
[0024] FIG. 5 shows the system of FIG. 1 adapted according to a yet
further embodiment of the present invention.
[0025] FIG. 3 shows apparatus, according to an embodiment of the
present invention, for preventing oil carry-over from the helium
compressor through the low pressure line, comprising an oil trap,
known in itself, in a novel and inventive placement, at position 31
within the LP line 65 between the compressor and the rotary
valve.
[0026] The oil trap is connected to the compressor on the LP line
using a two metre flex line 33 on one side and twenty metre flex
line 32 on the other end. The initial pressure in flex lines 32, 33
was kept to 0.15 MPa (1.5 bar). This embodiment was tested by
running the compressor to very high pressure of 2.8-2.9 MPa (28-29
bar) in internal bypass mode. It was noticed that the pressure on
the gauge increased over a period of time. The compressor was run
at a high pressure of about 2.8 MPa for several days. The RGA of
two-metre line 33 after three days of operation showed
contamination with oil, while the twenty metre line 32 beyond the
oil trap at position 31 did not show any trace of oil. This test
accordingly confirms the satisfactory usage of the oil trap over
the given period of time for preventing oil carry over from the
helium pump, according to an embodiment of the present
invention.
[0027] According to a second embodiment of the present invention, a
further oil adsorber, similar to oil adsorber 19, is placed in
position 31, in substitution for the oil trap discussed above.
[0028] According to a third embodiment of the present invention,
oil travel from the compressor to the PTR is reduced by placing a
gas reservoir in position 31 in the LP line 65 in substitution for
the oil adsorber or oil trap discussed above. This reservoir serves
to reduce the pressure difference across the NRV 13 in case of the
rotary valve stopping. The magnitude of the reduction in pressure
difference depends on the volume of the reservoir.
[0029] Certain known helium compressors such as the SHI and
Cryomech compressors are provided with an internal gas reservoir
with an adsorber/filter in the LP line. Others, such as the Leybold
and APD compressors do not have this feature.
[0030] According to a fourth embodiment of the present invention, a
combined gas reservoir and oil adsorber is placed in position 31 in
the LP line 65. This serves to both prevent and manage the oil
carry-over problem. The gas reservoir feature serves to reduce the
pressure differential across the NRV, thereby reducing the
probability of gas+oil passing through the NRV. The adsorber
feature prevents any oil which may pass the NRV from travelling
further along the LP line towards the PTR.
[0031] According to a fifth embodiment of the present invention, as
illustrated in FIG. 4, a low pressure switch 51 is provided in the
LP line after the NRV. If the RV 67 stops for any reason, the
pressure in the LP line will rapidly drop from its usual 0.5-0.6
MPa (5-6 bar) level. The switch 51 responds to the lowering of the
LP line pressure, and stops the compressor as soon as the lowered
pressure is detected. This prevents the build up of a large
pressure differential across the NRV 13, and reduces the likelihood
of gas+oil travelling through the NRV 13. Since the switch 51
should be designed to react as soon as possible, the switch is
preferably designed to react to a relatively small reduction in LP
line pressure. For example, the switch may be activated, causing
the compressor capsule 14 to stop by a LP line pressure of 0.5 MPa
(5 bar).
[0032] The switch 51 may be any pressure sensor capable of
operating at the temperatures and pressures likely to be
encountered in a helium compressor. In a preferred embodiment, the
pressure switch 51 is an electrical switch, and when activated by
an unusually low pressure in the LP line, causes a power supply to
the compressor capsule to be interrupted, thereby stopping the
operation of the compressor.
[0033] In a tested embodiment, a pressure switch 51 (a Barksdale
Control Products GmbH, UDS 7 type) was fixed on the LP side before
LP port 18 of a Leybold helium compressor. The helium compressor
had its LP 16 and HP 18 ports connected to a pulse tube
refrigerator 61, in this case a 10 K OMT PTR 1030207. In order to
establish a suitable switching pressure for the pressure switch 51,
the low pressure cut off value for the system, which occurs when
the PTR is warm, was determined. It was found that with the static
charging pressure of 14 bar on the compressor dial gauge, a minimum
dynamic pressure of 0.51 MPa (5.1 bar) and maximum dynamic pressure
of 2.4 MPa (24 bar) were obtained. The pressures changed to 0.63
MPa (6.3 bar) minimum and 2.2 MPa (22 bar) maximum in dynamic
conditions at lower temperatures with heat loads of 50 W at the
first stage of the PTR and 6 W at the second stage of the PTR. A
pressure switch setting of 0.51 MPa (5.1 bar) was accordingly
considered appropriate.
[0034] Once the low-pressure switch setting was established,
repeated tests were performed to determine the repeatability of the
switching of pressure switch 51, and to obtain a suitable turn off
delay for the compressor capsule 14. In each test cycle, after the
PTR 61 has started to operate, the RV 67 was stopped by turning off
the power supply to the RV drive. The pressure switch 51 was set to
operate at 0.51 MPa (5.1 bar). The pressure increase in the HP line
and pressure decrease in the LP line were recorded. The time delay
from the RV stopping to the compressor stopping was measured. This
cycle was repeated five times. In all cases, the compressor stopped
within five seconds of the RV stopping. The pressure in the HP line
increased to 2.55 MPa (25.5 bar) maximum. This was insufficient to
cause the internal by-pass valve 12 to operate, and any oil to
cross the NRV 13. After these tests, the compressor LP port 18 was
checked for oil. By visual inspection no oil could be seen. The
system further showed no trace of oil or deterioration in
performance of the PTR. The test results show that the pressure
switch 51 had stopped the compressor almost immediately preventing
any possibility of oil carry over from the compressor LP line to
the PTR cold head. The switch operating pressure of 0.51 MPa (5.1
bar) was found suitable in the tested embodiment. The pressure
switch 51 was accordingly demonstrated to operate
satisfactorily.
[0035] The switch operating pressure should be selected carefully,
however. The charging or filling pressure of the PTR should be
correct, to maintain correct operation of the pressure switch at
the selected switch operating pressure. If the filling static
pressure is less than the recommended standard value, or more
precisely the value used in determining the pressure switch
operating pressure, the compressor may stop during the start up
period due to unwanted activation of the pressure switch 51. Also,
if the filling static pressure is too high, the time delay required
to stop the compressor could be lengthened, and the compressor may
go in to bypass mode of operation when the RV stops. This would
entail the activation of the internal bypass valve 12, and the
possible contamination of the LP line by gas+oil travelling through
NRV 13.
[0036] According to a sixth embodiment of the present invention, as
illustrated in FIG. 6, the internal bypass valve 12 is provided
with its own return channel 61 to the compressor capsule 14. In
this way, any gas+oil which passes through the internal bypass
valve due to excess pressure in the HP line 63, for example, in the
case of a stopped rotary 67 valve on an attached equipment 61, will
pass directly to the compressor capsule 14, and will not be able to
reach the NRV 13 or the LP line 65. Any gas+oil passing through the
internal bypass valve 12 will be at a relatively high pressure,
much higher than the pressure inside the LP line 65. To prevent the
gas+oil from flowing through the compressor capsule 14 into the LP
line 65, the return channel 61 is connected to the compressor pump,
such as the scroll pump illustrated in FIGS. 2A-2D at a relatively
high pressure location, closer to the centre of the scrolls than
the openings 27, 27' which will receive gas from the LP port 18.
The return channel 61 is preferably connected to the compressor by
its own manifold, deep in the core of the compressor. Since the
helium gas is mixed with oil in the compressor, the fact that the
return channel 61 provides gas+oil raises no problems. A
disadvantage to this particular embodiment lies in that
modifications are required to the compressor capsule.
[0037] While the present invention has been explained with
reference to a limited number of particular embodiments, numerous
alterations and variations may be made to the invention within the
scope of the appended claims. Certain of the embodiments may be
combined. For example, an oil trap or gas reservoir/absorber may be
placed in the LP line upstream from the pressure switch. The
present invention maybe usefully applied to any situation in which
a helium compressor supplies compressed helium to an equipment
through a system of valves. Although the invention has been
particularly described with reference to pulse tube refrigerators
operated though a rotary valve, it may be usefully applied to any
valve controlled equipment.
* * * * *