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 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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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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