Turbo Vacuum! Part 1/2

Turbomolecular Pumps (Math and Destruction Lie Ahead)

Diagram of Effects on Vacuum in Outer Space

Can you conceptualize ANYTHING spinning at 600 Hertz? That is 600 revolutions per second, 36000 revolutions per minute and 216000 revolutions per hour. The turbo molecular pumps supporting some of the cleanroom equipment at Georgia Tech spin anywhere between 350 Hz and 600 Hz. They are used to keep our chambers at critically low pressures perfect for processes that have to be free of any contamination. A good performing system using a turbo molecular pump can get as low as 1 x 10e-7. As a reference, Outer Space, which is our closest approximation to perfect vacuum, ranges from 1 x 10e-6 to <1 x 10e-17. This means our chambers are lower pressures than some portions of Outer Space!

The turbo molecular pump (TMP for short) is easily one of the most interesting parts of our equipment packages. Due to their high rotation speed and their fragility to shock there is some fascinating math to consider as well as some astounding disasters. (Think explosive and catastrophic!) Before we can discuss explosions any further I should explain how a TMP works.

This is a cutout of a new turbo

The TMP consists of several rotor/stator pairs. The rotor is essentially a fan blade, while the stator is a stationary slotted plate between each rotor. The rotor collides with molecules forcing them downward from the inlet at the top to the exhaust near the bottom. A special controller, communicating with an encoder, controls the acceleration and speed of the rotor blades. The image to the right shows a cut-out of a small TMP.

TMPs are only appropriate for low vacuum ranges. We actually need a secondary pump, known as a rough pump, to bring the system down to an acceptable pressure before even revving up the TMP. This is because rather than creating vacuum through a typical suction, or viscous flow, the TMP relies on molecular flow to create high vacuum. Molecular flow is created by actual collision of the fan (rotor) blade with each molecule we are trying to remove through the system.

So Now Some Math

How fast are the rotors in a TMP actually moving? I am going to throw some rough numbers around using our largest turbo pump as an example. This turbo pump has a rotor diameter of around 12 inches and rotates at 480 Hz. Based off that we get the following numbers:

Circumference:  12 inches * pi = 37.6991 inches

Rotational velocity: 480 Hz * 60 seconds/min = 28800 rpm

Fan blade velocity at the edge: 37.6991 inches * 28800 rpm = 1085734.08 inches/min

(1085734.08 inches/min) / (60 min/h * 12 inches/foot * 5280 feet/mile) = 

1028.1573 miles per hour

or

1654.66 kilometer per hour

BAM! That's fast. This is comparable to some of the lower speeds of a bullet as it exits the muzzle of a gun, but it is a sustained speed held for sometimes months or years without stopping. We actually have no idea what would happen if this turbo were to be suddenly stopped, so we keep this puppy bolted to the floor as you can see in the picture. I doubt the rotors have enough mass to actually throw the entire tool over, but you can never be too safe when dealing with objects moving in excess of 1000 miles per hour.

STS HRM ICP bolted to the floor for safety

In Part 2 we get to the actual DESTRUCTION. I have a much smaller turbo pump that did catastrophically stop and I am going to document it as I take it apart and look through all the pandemonium that comes with a crashing turbomolecular pump. Look forward to it!