Pump, cryogenic diffusion

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

Cryogenic and turbo pumps are known for their cleanliness and effectiveness. However, these pumping systems are very expensive and require significant training for proper operation. The most common auxiliary pump used in the laboratory is the diffusion (or vapor) pump. 

A drawing of the cross section of a Dewar vessel is shown in Figure 2 [1]. It shows the basic elements of a high performance cryogenic storage vessel. A fill and drain line is provided at the bottom of the vessel to transfer fluid in and out of the tank. Liquid can be removed either by pressurization of the inner vessel with a pressurization gas or by a liquid pump. A vapor vent line is located near the top of the vessel to allow vapor formed from heat leak to escape. This line can also be used to introduce a pressurization gas. If pressurization is used to force liquid from the tank, a diffuser is provided to distribute the pressurization gas in the vapor space away from the surface of the cold liquid. This prevents the unwanted condensation of the warm pressurization gas by the cold liquid surface. 

The costs of cryogenic fluid for pumping air at low pressures are compared with the costs of the electricity for operating a conventional diffusion pump in Table IV. For this comparison the costs are assumed to be 0.10 per liter for liquid nitrogen, 2.00 per liter for liquid hydrogen, 3.00 per liter for liquid helium and 0.01 per kw-hr for electricity. 

The conventional method for evacuating the annular space surrounding a cryogenic container by external pumping requires the use of an oil-immersed rotary and/or diffusion pump. Such pumping systems are capable of inducing a vacuum as low as 1.3 x 10 MPa and are often used in series with themselves or each other to increase their effectiveness. 

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