Dynamic Cryogenic Seals to Support
Fueling of Fusion Tokomaks
U. Naranjo and J. W. Leachman
School of Mechanical and Materials Engineering
Introduction / Motivation
Sealing failures cost billions of dollars in damaged products
every year. Sealing at cryogenic temperatures is a
substantially more difficult task than sealing at room
temperature since materials tend to shrink, become brittle,
and crack.
I have constructed a prototype dynamic seal made of
Polychlorotrifluoroethylene (PCTFE) and a system to test the
seal immersed in liquid nitrogen, along with plans and
drawings to incorporate the seal into the prototype solid
hydrogen extruder in development at the WSU HYPER
laboratory. The purpose of a cryogenic dynamic polymer seal
is to move a gate back and forth which will vary the area of a
nozzle opening. This will be attached to the solid hydrogen
extruder using the mechanism shown in figure 3B.
Knowledge from this extruder will be used to develop fueling
systems for fusion reactors such as the ITER tokomak.
Experimental Technique
2A
2B
Uriel Naranjo
uriel.naranjo@email.wsu.edu
hydrogen.wsu.edu
2C
A
B
Figure 3: A) Cross section of the placement of
PCTFE interference seal on model. The seal will be
placed around a rod to see if the seal can be
maintained while the rod is turned. B) the
mechanism which will be used to turn the rod placed
through the seal.
A
Summary
2D
Steel Bolt
Sealing
points
Sealing
points
B
Epoxy
Brass Vacuum
Hydrogen
Figure 1: Cross sections of A) PCTFE interference seal at
room temperature and B) A PCTFE interference seal at
cryogenic temperature
Acceptable leak rates
Product/System
Chemical Process Equipment
Torque Converter
Beverage Can End
IC Package
Pacemaker
*PCTFE seal estimated leak
rate at cryogenic temperature
jacob.leachman@wsu.edu
509-335-7711
Application to Extruder
How a polymer seal works
PCTFE
SEAL
Jacob Leachman
Leak Rate Specification
(atm cm3) / s
10-1 to
1
10-3 to
10-4
10-5 to
10-7
10-7 to
10-8
10-9 to
10-10
1.6*10-1
Table 1: The acceptable leak rates for various products
(Hablanian 362)
2E
Figures 2A-2E:
2A. Conflat chamber at room temperature
2B. Outgassing the conflat chamber by heating outside to 50°C
2C. Dipping conflat chamber in liquid nitrogen for 3-5 minutes
2D. Heating conflat chamber to 50 °C once again
2E. Checking for leaks in the seal by pressurizing chamber with nitrogen
gas and dipping the seal in water
Observations
Procedure
Heating and pulling vacuum
Immersing seal in Nitrogen for 3-5 Minutes
Heating conflat chamber for 3-5 minutes
Bolt removed and vacuum grease added
Bolt removed and Apiezon cryogenic vacuum
grease added
Rotation
No rotation
Trial 10 Trial 11 Trial 12
Vacuum (in Hg)
22.8
22.8
22.8
25.2
22.8
X
24.2
21
X
24.5
21.2
X
X
X
X
The dynamic polymer seal was not able to maintain a
vacuum after being heated, immersed in liquid
nitrogen, and heated once again. The most promising
experiment, where vacuum grease was used, was able
to hold a seal at room temperature and cryogenic
temperatures but not while being rotated at cryogenic
temperatures. The estimated leak rate agrees with the
bubble test as there came a point where very few
bubbles escaped from the pressurized chamber and
leaks larger than 10-1 atm*cm3/s can be spotted
visually (Hablanian 368). The leak rate of the
dynamic polymer seal is currently too high to use in a
high vacuum cryogenic chamber.
Future Recommendations
I recommend applying a cryogenic lubricant such as
spray on graphite (Ekin 533) between the dynamic
polymer seal and bolt, and running the experiment
once more to ensure an adequate seal.
In the future this test will be run once again using a
seal with minimum epoxy to ensure the PCTFE is
shrinking around the bolt as intended. A helium leak
detector will be used to accurately measure the leak
rate up to 10-12 atm*cm3/s /
Table 2: The vacuum inside the conflat chamber at different steps and for
different trials
REFERENCES
Ekin, J. W. Experimental Techniques for Low-temperature Measurements: Cryostat Design, Material
Properties, and Superconductor Critical-current Testing. Oxford: Oxford UP, 2006. Print
Hablanian, M. H. High-vacuum Technology: A Practical Guide. New York: M. Dekker, 1990. Print
ACKNOWLEDGEMENTS
This work was supported by the National
Science Foundation’s REU program under
grant number EEC 1157094
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H Y drogen
P roperties for
E nergy
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