Vacuum forepump

Caution. The vacuum forepump is protectedfrom the corrosive, reactive fluorine gas by pumping all gases from the line through a trap containing granules of activated alumina or soda lime. 

A diffusion pump needs a forepump (usually a rotary pump). Oil can also migrate from the forepump into the vacuum chamber through the diffusion pump, when the rotary pump works in the molecular regime ( 10 2torr). 

For the production of pressures in the ultrahigh vacuum region, sputter-ion, and sublimation pumps, as well as turbomolecular pumps and cryopumps, are used in combination with suitable forepumps. The pump best suited to a particular UHV process depends on various conditions (for further details, see Section 2.5). 

Fig. 4. Schematic vacuum system for metal atom reactions. X represents the stopcock or Teflon-in-glass valve. Satisfactory components (for a general discussion of vacuum line design see References 1 and 4) forepump, 25 L/min free air capacity diffusion pump, 2 L/sec main trap is removable and measures about 300 mm deep main manifold has a diameter of about 25 mm, stopcock or valve in manifold should be at least 10 mm substrate container is removable container with 1-2 mm Teflon-in-glass needle valve connected to bottom of container. Connection between this needle valve and the reactor may be 1/8 in. od. Teflon tubing is used. Alternatively, the substrate may be added as shown in Fig. 3.

The vacuum line used in the following preparations is similar to that described by Shriver.14 It consists of a pump station, a main reaction manifold with six reaction stations, a fractionation manifold with four U-traps and a reaction station at each end, a McLeod Guage, and a Topler pump. The pump station employs a two-stage mechanical forepump and a two-stage mercury diffusion pump. Operating vacuum is 1.0 X 10"S torr. Teflon valves are employed throughout. 

The annular space between the two tubes is evacuated by a mercury diffusion pump and a Hyvac forepump, and the inner tube is connected to an all glass vacuum system. 

The main vacuum pumping system consists of a high-speed, singlejet, Illinois-type mercury diffusion pump, a two-jet Princeton-type mercury diffusion pump, and a Hyvac forepump. 

For Dewar flasks, metal evaporation apparatus, and most research apparatus, a vacuum of 10 ° to 10 ° Torr is sufficient this is in the high vacuum range, while 10 ° Torr would be termed ultrahigh vacuum. However, for many routine purposes a utility vacuum or forepump vacuum of about 10 ° Torr will suffice, and for vacuum distillations only a partial vacuum of the order of 1 to 50 Torr is needed. 

The principal types of vacuum pumps will be discussed in this section. The water aspirator is a crude but useful pump for many routine operations. Rotary oil pumps are used for pumping on refrigerant baths and as the forepump for backing low-pressure pumps. For most high-vacuum work, diffusion pumps are utilized to achieve pressures... 

Mechanical Pumps. Perhaps the most common form of vacuum pump is a mechanical pump that operates with some sort of rotary action, with moving parts immersed in oil to seal them against back-streaming of exhaust as well as to provide lubrication. These pumps are used as forepumps for diffusion pumps. Other common laboratory applications are the evacuation of desiccators and transfer lines and distillation under reduced pressure. These pumps have ultimate pressures ranging from 10 to 0.05 Torr, and pumping speeds from 0.16 to 150 L s or more, depending on type and intended application. 

A mechanical pump providing even lower vacuum levels is the turbomolecular pump, in which one or more balanced rotors (turbine blades) spin at 20,000 to 50,000 rpm. At these rotation rates, the periphery moves at a speed that exceeds the mean molecular speeds of most molecules, and gas-rotor collisions impart a momentum component to the gas in the direction of the exhaust. Compression ratios up to 10 can be achieved as long as the outlet pressure is kept below about 0.1 Torr by a forepump. 

Balzers TPU 170 and the Welch 3134 are good-quality turbomolecular pumps. They have an ultimate vacuum limit of 10 Torr and pumping speeds of 300 L s from 10 to 10 Torr. Other pumps with speeds up to 10,000 L s are available, although these rates are reduced somewhat for light gases such as H2 and He, which have high molecular velocities. The turbomolecular pump is a very clean pump, requires no trap between itself and the system, and is not sensitive to contamination (although in some cases the forepump should be protected by a cold trap). It is not widely used in routine laboratory applications however, as it is quite expensive and demands careful maintenance. 

Typical all-glass air-cooled oil diffusion pump, shown with appropriate connections to the vacuum manifold and to the mechanical forepump. Cooling of the vapor condensation area can be achieved with a small electric fan or a compressed air line (not shown). The use of a liquid nitrogen trap is optional. 

Attachment of the distillation apparatus through a joint to a glass high-vacuum line with forepump and mercury diffusion pump is recommended. Distillation may be carried out in ordinary apparatus with a mechanical pump only, but even minute traces of oxygen leaking into the apparatus will react at once with the hot diphenylphosphine vapors, causing smoke to appear in the still head and condenser. 

The vacuum line is isolated from the forepump, and a liquid-nitrogen bath is put around trap D. The fluorine metering system is partially evacuated with the water aspirator, as discussed above. Then the vacuum line is completely evacuated with the forepump. Fluorine is allowed to expand slowly into the metal system by means of the needle valve 3. As the pressure increases above atmospheric, some fluorine is allowed to bleed into the vacuum line and storage bulbs until a pressure of 650-700 mm. is reached. First needle valve 1 on the metal system and then the fluorine control valve 3 are quickly closed. (Some fluorine may escape from the blowout manometer during this operation.) Approximately a 10% excess of fluorine (0.023 mole in this case) is condensed into the metal pressure reactor containing the thionyl fluoride. The amount of fluorine used is measured by the pressure drop in a calibrated volume in the pressure range of approximately 700-400 mm., since fluorine has a vapor pressure of approximately 400 mm. at —196°. 

In the following procedures a standard glass vacuum line with high-vacuum stopcocks (greased with Kel-F grease) and mercury manometerf is used. A —195 cold trap is placed between the vacuum line and the forepump, which has an operating vacuum of about 10 mm. 

The large vacuum chamber is situated between the pole faces of the electromagnet. Forepumps are used to begin pressure reduction vacuum is maintained primarily by one or two high-capacity diffusion pumps with pipe-throat diameters of 15-20 inches. 

The vacuum line used in this synthesis is a standard, calibrated, high vacuum system equipped with a two stage mechanical forepump and a dual stage mercury diffusion pump. The operation pressure in this system is 10 torr or lower. ... 

General forepump requirements are quite simple. The pump should be capable of attaining a minimum ultimate vacuum of 10 torr with a minimum pumping speed of 60 1/min. Higher pumping speeds are... 

Two types of vacuum pumps commonly are found in a laboratory. These are the mechanical pumps, often called forepumps, and diffusion pumps. There are several more types of pumps, but they are used for specialized high vacuum work. 

Note that this list is not exhaustive. In all cases, a forepump (oil-sealed rotary vane) is needed in addition to the specific high-vacuum pump. 

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