Thursday, November 29, 2007

Vacuum Pump Technology

Vacuum pumps can be broadly categorized according to three techniques:
Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere. Examples include: piston pump, diaphragm pump, liquid-ring pump, sliding vane rotary pump, miltiple-vane rotary pump, rotary piston pump, rotary plunger pump and roots pump.
Momentum transfer pumps, also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gaseous molecules out of the chamber. Examples include: turbine pump, turbomolecular pump, liquid jet pump, vapor jet pump, diffusion pump and diffusion ejector pump.
Entrapment pumps capture gases in a solid or absorbed state. These include: absorption pump, sublimation pump, sputter-ion pump amd cryopump.
Positive displacement pumps are the most effective for low vacuums, but their high backstream flows through mechanical seals generally limit their usefulness in high vacuums. Momentum transfer pumps in series with positive displacement pumps are the most common configuration used to achieve high vacuums, but they stall at low vacuums. (Hence the need for a positive displacement pump in series.) Entrapment pumps can be added to reach ultrahigh vacuums, but they have a maximum operational time since they do not exhaust materials. They periodically saturate and require regeneration, which usually means bringing the system back up to higher pressures and temperatures. The available operational time is usually unacceptably short in low and high vacuums, thus limiting their use to ultrahigh vacuums. Pumps also differ in details like manufacturing tolerances, sealing material, pressure, flow, admission or no admission of oil vapor, service intervals, reliability, tolerance to dust, tolerance to chemicals, tolerance to liquids and vibration.

1 comments:

Elizabeth J. Neal said...

The available operational time is usually unacceptably best small canister vacuum short in low and high vacuums, thus limiting their use to ultrahigh vacuums.