jr. · a molecular sieve trap for use at torr david j. goerz jr. microwave laboratory stanford...
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A MOLECULAR SIEVE TRAP FOR USE AT TORR
David J. Goerz Jr.
Microwave Laboratory
Stanford University Stanford, Cal i fornia
W. W. Hansen Laboratory o f Physics
ABSTRACT
A molecular s ieve t r a p can be made which w i l l operates i n range of 10 -8
t o
easy and economical t o use with an i n t e r n a l heater and self-contained
water b a f f l e .
Torr over a period of severa l months without processing. It i s
A MOLECULAR SIEVE TRAP FOR USE AT 10-9 TORR
To provide f o r a long l i f e t r a p which does not remre t h e use o_f liquid
nitrogen, a molecular s ieve b a f f l e has been designed. The molecular
sieve behaves as a clean s t ick ing surface. Oil molecules sorb when
they come i n conbact with the surface. The design developed includes a
water cooled chevron b a f f l e which forms t h e lower flange assembly shown
i n Figure 1.
r ing which i n turn i s brazed i n t o the s t a i n l e s s s t e e l f lange.
cooling on t h e chevron b a f f l e a l s o cools the seal a rea , hence e i t h e r a
copper gasket or organic O-ring may be used between t h e b a f f l e and t h e
diffusion pump. The molecular s ieve consisting of 1/4" x 1/8" p a l l e t s
i s held t o t h e walls of t h e t r a p by a 3/32 inch mesh stainless s t e e l
screen formed i n the shape of a basket.
system through t h e annular r ing i n t h e upper flange shown i n Figure 2.
A s t a i n l e s s s t e e l stamping i s shaped t o prevent the p e l l e t s from f a l l i n g
out. To prevent o i l creepage i n t o t h e system, no contact i s made between
t h e water b a f f l e and the upper w a l l .
s ieve i s supported by ceremic d i s c s on t h e cal-rod heater i n t h e center
of the chamber.
assembly. The heater, ra ted a t 1250 watts, i s wound t o provide uniform
and correct heat f o r processing a t 350 C.
PROCESSING
The copper chevrons are preformed and brazed i n t o a copper
The water
The p e l l e t s are poured i n t o t h e
A Recond basket containing molecular
A port i n the top of t h e basket permits f i l l i n g a f t e r
0
The ul t imate pressure depends on the past h i s t o r y of t h e molecular
s ieve. With a new charge of zeolyte X-13 molecular sieve, a processing
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Figure 1 Lower section molecular sieve trap.
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0
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3
0 time of 3 hours at forepressure and 350 C. has given an ultimate pressure
of 2 to 3 x 10 Tom when the diffusion is turned on.
of 8 hours with the diffusion pump operating will produce a pressure of
1 to 5 x lo-' Torr in a test dome.
to depend on the processing.
system pressure did not increase above 2 x 10
reactivated after contamination by oil vapors, teflon and glyptol Many
contaminants were pumped through the pump without an increase in base
-8 Further processing
The life of the trap does not seem
In tests up to three months the ultimate
Torr. The trap may be -8
pressure above lo-'' Torr.
PUMPING SPEED
The pumping speed curves were taken both before and after contamina-
tion.
reactivation
A s can be seen from the graph in Figure 3,
from 150 liters/sec at Torr to 250 liters/sec at Torr. The
diffusion pmp used for the testing was a PMC 720 made by Consolidated
Vacuum Corporation.
ionization gauge and Veeco gauge amplifier.
a tube extending to the center of the test dome.
CHOICE OF MOLECULAR SIEVE
After contamination the pumping speed was decreased; however, after
the pumping speed was increased to the original level.
the pumping speed ranged
These values were taken using a Vacuum Tube Products
The guag;e was connected to
Several molecular sieves were used in the experimentation; these
included Linde zeolyte molecular sieve X-13, Alcoa alumina oxide and
barium oxide. The most effective and easy to process was Linde X-13.
Chipping, flaking and break-up were also less.
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a - w 3 v) v) w QI e
ACKNOWLEDGEMENT
T h i s w o r k was done under the sponsership of Contract AT(04-3)-21,
Project Agreement No.-1, U. S. Atomic Enera Commission.
The writer wishes t o acknowledge the able assistance of Professor
Simon ankin, D r . R. B. Neal, and M r . W. Coates in many phases of this
work.
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