Practical Considerations
Laws of fluid dynamics do not apply in high vacuum environments. The maximum compression varies linearly with circumferential rotor speed. In order to obtain extremely low pressures down to 1 micropascal, rotation rates of 20,000 to 90,000 revolutions per minute are often necessary. Unfortunately, the compression ratio varies exponentially with the square root of the molecular weight of the gas. Thus, heavy molecules are pumped much more efficiently than light molecules. Most gases are heavy enough to be well pumped but it is difficult to pump hydrogen and helium efficiently.
An additional drawback stems from the high rotor speed of this type of pump: very high grade bearings are required, which increase the cost.
Because turbomolecular pumps only work in molecular flow conditions, a pure turbomolecular pump will require a very large backing pump to work effectively. Thus, many modern pumps have a molecular drag stage such as a Holweck or Gaede mechanism near the exhaust to reduce the size of backing pump required.
Read more about this topic: Turbomolecular Pump
Famous quotes containing the word practical:
“As for Clothing, to come at once to the practical part of the question, perhaps we are led oftener by the love of novelty and a regard for the opinions of men, in procuring it, than by a true utility.”
—Henry David Thoreau (1817–1862)