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Vacuum Pumps - Cryogenic
Sorption roughing pumps or sorption pumps are used for pumping systems from
atmospheric pressure to a pressure of approximately 10-2 Torr. They rely on the
dispersion forces existing between a gas and a surface to bind gas molecules on chilled
surfaces inside the pump. In other words, they pump by cryosorption.
Sorption pumps typically consist of a cylindrical canister or body that is filled with
an adsorbent material. The adsorbent is usually a molecular sieve material, or
zeolite, which consists of pellets made of a calcium or a sodium aluminosilicate crystalline
matrix. The canister is placed in a dewar cooled by liquid nitrogen. Zeolite is a poor
heat conductor, so an array of aluminum fins inside the pump is used to
improve thermal contact with the sieve material. The pump body and internal cooling fins are specially
designed for maximum heat transfer. The pump neck and flange are made of stainless
steel. The pumps are mounted and supported by the flanges and since stainless
steel is a poor thermal conductor frosting of adjacent components is minimized. Sorption
pumps need liquid nitrogen to operate and, as with any capture pump, they have to be
periodically regenerated. Sorption pumps are very clean noncontaminating roughing
pumps and are ideal for low throughput applications. They are used in
conjunction with getter pumps, ion pumps, or mechanical cryopumps.
Dual Sorption Pump Assembly
The adsorbent used is a Type 5A synthetic zeolite molecular sieve material. Zeolite is a
highly porous material with a surface to volume ratio of about 800 square meters per
cubic centimeter. It is supplied in pellets of about 1.58mm in diameter with molecular
sized cavities that are linked by 5 Angstrom size pores. These pores are large enough to
trap nitrogen, oxygen, and argon molecules, the main constituents of air. Zeolite also has
a very high affinity for water vapor.Water vapor accumulated through repeated pump
cycles of a chamber filled with ambient air will eventually saturate the sieve material,
reducing and eventually eliminating its capacity for adsorbing nitrogen and oxygen.
To remove the accumulated water and regenerate the adsorbent material, the pump
must then be baked to 250°C or higher. Under normal operating conditions, the sieve
material can be recycled indefinitely.
Sorption pumps do not include the optional bakeout heater. Heaters must be purchased
separately. During pump operation, do not run the heater while it’s immersed in liquid
Noble gases such as neon and helium are pumped poorly by sorption pumps. If, for
instance, neon is pumped together with air, its capacity will be less because the neon
will be replaced by the active air gases, starting at pressures below 7.5 Torr. For this
reason , sorption pumps are quite often staged. When two pumps are staged, one
pump is used to achieve a pressure of 7.5 Torr and is then valved off.
The second pump is then opened and the pressure is further reduced. By this method, 99% of the air is
removed with the first pump, and noble gases are also swept into this pump and
cannot backstream into the system when pressure is further reduced.
Staged or multiple pump assemblies are fitted with both Bourdon and Thermocouple vacuum
gauge tubes for monitoring vacuum levels. These manifolds are supplied with three
manual UHV angle valves which allow the isolation of each pump and the
manifold from the main vacuum system.
Pump operation is simple and fast. To begin pumping, add liquid nitrogen to the
dewars. No electrical power is required and there are no moving parts and no vibration. A single
sorption pump can evacuate a 100 liter chamber from atmosphere to 10-2 Torr in
approximately 10 minutes. Each sorption pump has a capacity of 60,000
Torr-liters. Multiple pump systems are commonly used as they are faster and more efficient. Double
and triple pump systems are mounted or connected to the chamber via a vacuum
manifold. Single pumps, on the other hand, mount directly to a chamber. Pumpdown
begins as soon as the adsorbent material in the pump is chilled with liquid nitrogen.
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