How to Select a High Vacuum Pump, Part 3: Momentum Transfer Pumps
The momentum transfer pump category is comprised of Turbomolecular Pumps ("Turbopumps") and Diffusion Pumps.
Turbopumps are a workhorse in the high vacuum market. The smaller sizes are compact and quiet, and since they do not have any oil (except in the bearing if it is not a magnetically levitated bearing type) they are generally more forgiving in terms of backstreaming if the wrong valve is opened and the system is vented to atmosphere.
Turbo pumps come in a large variety of sizes, some having magnetic bearings which eliminate the threat of oil vapors moving back into the work chamber. These pumps spin at very high speeds with multiple rings of blades which are used to move the molecules from low pressure to high pressure and then into the positive displacement pump.
Diffusion Pumps were the first high vacuum pump which was used in large quantities for manufacturing. This pump uses oil heated to high temperatures which is then forced through tiny openings to create a waterfall effect which adds the momentum of the oil vapor to the gas being evacuated to carry it to the positive displacement pump. Diffusion pumps are still in use today where very high speeds and high volumes are involved.
||Base Pressure (mTorr)
The fourth post of this series will cover "Entrapment" or "Capture" Pumps.
This article was authored by Michael A. Grandinetti. More information about Mike can be found at: http://www.linkedin.com/in/michaelgrandinetti1
IES Technical Sales is a value added technical sales, distribution, and solutions provider serving the high technology vacuum, plasma/thin film, temperature, fluid handling and metrology markets.