Capture Pumps

Authored by: Igor Bello

Vacuum and Ultravacuum

Print publication date:  November  2017
Online publication date:  November  2017

Print ISBN: 9781498782043
eBook ISBN: 9781315155364
Adobe ISBN:

10.1201/9781315155364-15

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Abstract

The pumping mechanism of capture pumps differs from the pumping principles employed in transfer pumps. Unlike in transfer pumps, gases are not transported via the pump and removed from the pump interior, but they are captured on the internal surfaces of pumps. The major pumping effect is based on sorption phenomena. In principle, all solid surfaces free of adsorbed molecules and exposed to a vacuum environment can work as pumps. They capture gases until a dynamic equilibrium occurs between the volume and surface density of a molecule. If an absolutely clean surface of a solid with a size of 1.0 cm 2 captures as many molecules as is in a single monolayer, then the total number of molecules on an area of cm 2 is about 1015. The equal number of molecules (1015) is in cm 3 at a pressure of 3.77 × 10−2 Pa and standard temperature (273.15 K). Hypothetically, the capture of all molecules that are in the volume of 1.0 cm 3 at 3.77 × 10−2 Pa by a material with a surface area of 1.0 cm 2 (initially free of molecules) should yield zero pressure. However, such a state, volume free of molecules, is unattainable. At a given temperature, the surface density of adsorbed molecules is always in equilibrium with the molecular volumetric density, while gases from secondary sources continuously contribute to the molecular volume density. Nevertheless, capturing molecules by sorption is one of the pumping mechanisms employed in vacuum production. These phenomenal physical and chemical sorption processes can be assisted either by thermal activation of material surfaces or cooling surfaces to low temperatures to trap real gases. The size of the sorption surfaces, methods of surface activation, or cooling temperature are vital parameters to obtain practical pumping effects.

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