Vacuum pump, compression diffusion

The ejector is widely used as a vacuum pump, where it is staged when required to achieve deeper vacuum levels. If the motive fluid pressure is sufficiently high, the ejector can compress gas to a slightly positive pressure. Ejectors are used both as subsonic and supersonic devices. The design must incorporate the appropriate nozzle and diffuser compatible with the gas velocity. The ejector is one of the ( to liquid carryover in the suction gas. 

Mechanical pumps are used directly to produce a low and medium vacuum, as well as extensively to back Roots vacuum, turbomolecular and diffusion pumps. These pumps are also called oil-sealed rotary vane pumps as they rely on the use of vanes or blades to compress gases. 

Steam ejectors Operability of steam ejectors is very sensitive to the stability in the motive fluid (steam) pressure. Prefer vacuum pumps to steam ejectors (H). Keep diameter of pipes = diameter of inlet and discharge flanges of ejectors. As the column overhead mass flowrate increases above design, so will the column overhead pressure and vice versa. Compression ratios per ejector 6 1 to 15 1. If inlet gas temperature < 0 °C or below the triple point of water (0.61 Pa) then add steam jacketing to cope with ice formation. Seal for the hot well submerge > 30 cm. The volume in the hotwell between the pipe and the overflow weir should be 1.5 times the volume in the down spout sealed. Replace any nozzles or diffusers where the area is > 7 % larger than design. 

Diffusion pump (DP) (vacuum technology) A compression-type vacuum pump that operates by the collision of heavy vapor molecules with the gas molecules to be pumped, giving the gas molecules a preferential velocity toward the high pressure stages of the pump. See also Vacuum pump. 

Backflow (also called back diffusion or back migration) occurs when the pressures at the outlet and inlet have established a constant ratio (this is analogous to the compression ratio found in mechanical pumps). At that point, gases can drift either way in the vacuum system. Proper trapping or baffling is an easy mechanism to prevent this problem. 

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. 

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