High Vacuum Conductance: Bigger is Better
To properly design a vacuum pumping system, it helps to have a full understanding of the concept of moving gas through the connecting tubing between the chamber and the vacuum pump. The real issue is, as the pressure in the system is reduced, gas molecules are more difficult to extract due to the principal of conductance.
As the pressure in a system is reduced, the distance a gas molecule travels before hitting another molecule (“mean free path”) increases. That is, instead of flowing in an orderly path to the vacuum pump, the molecules begin to wander in different directions and it becomes more difficult to extract them. The reduction in pressure results in a lower density of gas molecules, and the distance they travel before they interact with another molecule or the walls of the system is increased.
As gas density decreases the flow of gas changes from viscous flow, to transitional flow, to molecular flow. In essence the flow path changes from a direct line towards the pump inlet to a much more of a random motion, and it becomes much harder to move gas through any given space.
One significant implication of this change is that the diameter, length and path of the tubing connecting the vacuum pump to the vacuum chamber become very important. In molecular flow, which is at pressures of approximately 1x10-3 Torr, the controlling equation that relates conductance to the diameter of the tube and its length is:
C = 12 x D3/L
where:
C = conductance in liters per second (l/s)
D = diameter of the tube in centimeters (cm)
L = the length of the tube in cm
For example a 2cm diameter tube that is 80cm long would have a conductance of 12 x (8/80) = 1.2 l/s. If you are using a 25 l/s pump, the flow would be very constricted. To dramatically improve the removal of gas at low pressure, the diameter of the tube could be increased from 2cm to 5cm. This may not seem so dramatic; but the conductance increases to 12 x (125/80) = 18.75 l/m, which is very close to the full pumping speed of the pump.
The bottom line is that when designing a vacuum pumping station, keep your connecting tubing as short as possible and select tubing with as large an inner diameter as possible.
This article was authored by Michael A. Grandinetti, who holds a degree in mechanical engineering from Northeastern University. Mike can be reached by email at mikeg@iestechsales.com
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.