Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
1998-11-06
2001-04-17
McDermott, Corrine (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
Reexamination Certificate
active
06216467
ABSTRACT:
BACKGROUND OF THE INVENTION
Cryogenic refrigerators, such as those incorporated in cryogenic vacuum pumps (cryopumps), commonly are of a “Gifford-McMahon” design. Under standard operation, a two-stage cryogenic refrigerator of this design can typically cool to extremely low temperatures—typically, 4 to 25K.
A refrigerator that performs a Gifford-McMahon cooling cycle is illustrated in FIG.
1
. The refrigerator includes a displacer
12
including a first stage
14
and a second stage
16
. Both stages of the displacer
12
are filled with regenerative heat-exchange media in the form, for example, of tiny lead balls
18
′ and/or a bronze or copper screen
18
″. The displacer
12
reciprocates linearly within a shell
20
under the force of a motor-driven shaft
22
. The shell
20
includes a first-stage cylinder
24
and a second-stage cylinder
26
conforming to and coaxial with the displacer
12
while accommodating a range of axial reciprocation of the displacer
12
.
Cooling is predicated upon a reversing flow of helium gas through the shell
20
and expansion of the gas. Compressed helium gas is supplied by a compressor through a supply line
28
connected via an inlet valve
30
to the warm end
32
of the first-stage cylinder
24
. With the displacer
12
at a cold end
34
of the shell
20
(remote from the inlet
35
of the supply line
28
), the inlet valve
30
is opened, allowing the shell
20
to fill with compressed gas. As the compressed helium flows through the shell
20
, the displacer
12
is drawn from the cold end
34
to the warm end
32
of the shell
20
, forcing helium gas through passages
64
,
64
′,
64
″, and
64
′″ of the displacer
12
. The helium gas flows through the passages between the regenerative media
18
′,
18
″ filling the displacer
12
, and the helium gas transfers heat to the regenerative media
18
′,
18
″, which have been precooled in previous refrigeration cycles.
When the shell
20
is filled with compressed helium and the displacer
12
is fully withdrawn to the warm end
32
of the shell
20
, the inlet valve
30
is closed and the outlet valve
36
leading to a return line
38
connected to the inlet of the compressor is opened. The compressed helium gas thereby flows back through the displacer
12
and out of the shell
20
, expanding into the return line
38
. The helium cools with expansion, and heat is extracted from heat sinks
40
,
42
(e.g., cryopanels in cryopumps) with which the refrigerator is in thermal contact. As the cooled helium flows through the displacer
12
, heat is also transferred from the regenerative media (e.g., a bronze or copper screen
18
″ in the first stage
14
and lead balls
18
′ in the second stage
16
) to the helium gas.
After the pressure has equilibrated between the shell
20
and the return line
38
, the outlet valve
36
is closed. With the displacer
12
at the cold end
34
of the shell
20
, the inlet valve
30
is reopened and the cycle is repeated.
One application for cryogenic refrigerators is in cryogenic vacuum pumps (cryopumps). Currently available cryopumps generally follow a common design. A low-temperature array, cooled to 4 to 25K (most commonly to 10 to 20K), serves as the second-stage heat sink
42
and the primary pumping surface. This array is surrounded by a higher-temperature radiation shield, usually operated in the temperature range of 40 to 130K. The radiation shield serves as the first-stage heat sink
40
to the refrigerator, and it protects the low-temperature array from radiated heat. The radiation shield generally includes a housing that is closed except at an opening where a frontal array is positioned between the primary pumping surface and a work chamber to be evacuated.
During operation, high-boiling-point gases such as water vapor are condensed on the frontal array. Lower-boiling-point gases pass through that array and into the volume within the radiation shield and condense on the low-temperature array. A surface coated with an adsorbent, such as charcoal or a molecular sieve, operating at or below the temperature of the colder array may also be provided in this volume to remove the very-low-boiling-point gases such as hydrogen. With the gases thus condensed or adsorbed on the pumping surfaces, a vacuum is created in the work chamber. Such a cryogenic refrigerator is described in U.S. Pat. No. 5,775,109, which is hereby incorporated by reference in its entirety.
Plural cryopumps, all fed by a common compressor supplying compressed helium to a common flow circuit, are often incorporated into a cluster tool for processing semiconductor wafers. Within a cluster tool, the vacuum pumps create the vacuums that are needed to perform sensitive processing steps, such as chemical vapor deposition. An embodiment of a representative cluster tool is likewise described in U.S. Pat. No. 5,775,109.
SUMMARY OF THE INVENTION
Though the compressed helium supply for cryogenic refrigerators is often of fairly high purity, some degree of vapor contamination in the helium circuit is typical. While helium will not condense in significant amounts anywhere in the system, common contaminants, such as nitrogen, will often begin to condense in significant quantities at temperatures below 60K. The operation of a cryogenic refrigerator can be improved by reducing the amount of nitrogen and other contaminants that condense within the shell.
As noted, above, the shell of the refrigerator has a temperature profile extending down to 4 to 25K at its cold end. As the temperature drops, the vapor pressure of nitrogen saturation drops. At temperatures where nitrogen has a saturation pressure lower than the partial pressure of nitrogen in the system, nitrogen will condense to lower the partial pressure of nitrogen vapor to the saturation limit at that temperature. As a result, nitrogen will selectively condense toward the cold end of the shell of the refrigerator producing an accumulation of condensed solids that will block the flow of helium gas. This blockage increases the torque needed to drive the displacer and eventually leads to ratcheting, in certain motors, or stalling in the operation of the refrigerator. Besides compromising operating efficiency, ratcheting can be damaging to the refrigerator and may also cause damage to the broader system that depends on the refrigerator for cooling.
Apparatus and methods of this invention remedy this problem with an adsorbent for adsorbing contaminants before they condense. Within a highly-porous adsorbent, such as charcoal, contaminants can safely be adsorbed within pores at temperatures higher than the condensation temperature with reduced risk of blocking the flow of compressed helium gas in the shell.
A cryogenic refrigerator of this invention includes a reciprocative displacer and an adsorbent within a shell. The adsorbent is positioned to adsorb contaminant gases within the shell in accordance with a method of this invention.
In accordance with one aspect of the invention, the adsorbent and a regenerative media are both contained in the displacer, and the regenerative media is positioned on both sides of the adsorbent such that it is both between the displacer cold end and the adsorbent and between the displacer warm end and the adsorbent.
Preferably, the adsorbent has a surface-to-volume ratio greater than 50 m
2
/cm
3
and a mean pore size not greater than 10 times the molecular size of the adsorbent material.
The adsorbent can include carbon, crystalline aluminosilicate, crystalline aluminophosphate or silica gel. Preferably, the adsorbent is charcoal, and the regenerative media is a metal, such as lead.
The refrigerator is preferably a Gifford-McMahon refrigerator, wherein the shell includes an inlet and outlet for helium gas flow, with both positioned at a warm end of the shell. Further, the displacer preferably includes a first stage and a second stage. The adsorbent is positioned in the first stage, with the second stage positioned remotely from the warm end of the
Cheng Gordon C.
Morris Ronald N.
O'Neil James A.
Drake Malik N.
Hamilton Brook Smith & Reynolds P.C.
Helix Technology Corporation
McDermott Corrine
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