Pfeiffer Vacuum A Passion for Perfection

7.2.3.1 Evacuating a vessel to 10-8 mbar by means of a turbopumping station

A vessel made of bright stainless steel is to be evacuated to a pressure pb of 10-8 mbar in 12 hours. As can be seen from Section 1.3, there are other effects to consider in addition to the pure pump-down time for air. Both desorption of water vapor and adsorbed gases as well as outgassing from seals will lengthen the pump-down time. The pump-down times required to attain the desired pressure of 10-8 mbar are comprised of the following:

The desired base pressure pb is comprised of the equilibrium pressure caused by gas flowing into the vessel through leaks and permeation Ql , as well as by gas released from the metal surface QdesM and the seals QdesK:
pb =

The vessel has the following data:

V=0.2 m3 volume
A=1.88 m2 surface area
qdesM=2.7 x 10-6 mbar · m3/(s · m2) desorption rate of stainless steel
qdesK=1.2 x 10-5 mbar · m3/(s · m2) desorption rate of FPM
Ad=0.0204 m2 surface area of the FPM seal
Ql < 10-8 mbar · l/s leakage rate

The backing pump should evacuate the vessel to 0.1 mbar in t1 =180 s, and should also be able to achieve this pressure with the gas ballast valve open. The volume flow rate can be obtained in accordance with Formula 7-8:
Sv = 10.2 l/s = 35.8 m3/s.

We select a Penta 35 with a pumping speed Sv = 35 m3/h.

The turbomolecular pump should have approximately 10 to 100 times the pumping speed of the backing pump in order to pump down the adsorbed vapors and gases from the metal surface. We select a HiPace 700 with a pumping speed S of 685 l/s. Using Formula 7-8 yields
t2 = = 2.01 s

Desorption from the surface of the vessel

Gas molecules (primarily water) adsorb on the interior surfaces of the recipient and gradually vaporize again under vacuum. The desorption rates of metal surfaces decline at a rate of 1/t. Time constant t0 is approximately 1 h.

Using Qdes = (Formula 1-24) we calculate the time needed to attain the working pressure pb3 = 10-8 mbar: t3 = ; t3 = 2.67 · 106 s = 741 h.

This takes too much time! The process must be shortened by baking out the vessel. Increasing the temperature of the vessel from 293 to 370 K, a temperature that the FPM seals can easily withstand, will theoretically increase the desorption speed by more than a factor of 1,000 [6], and the bake-out time will in effect be shortened to several hours.

High desorption rates can also be lowered by approximately a factor of 100 by annealing the vessel under vacuum or by means of certain surface treatments (polishing, pickling). Bakeout, however, is the most effective method.

Since many pre-treatment influences play a role, precise prediction of the pressure curve over time is not possible. However in the case of bake out temperatures of around 150 °C, it will suffice to turn off the heater after attaining a pressure that is a factor of 100 higher than the desired base pressure. The desired pressure pb3 will then be attained after the recipient has cooled down.

Seal desorption

The outgassing rates of plastic are important at operation below 10-6 mbar. Although the surface areas of the seals are relatively small, desorption decreases only at (Formula 1-25).

The reason for this is that the escaping gases are not only bound on the surface, but must also diffuse out of the interior of the seal. With extended pumping times, desorption from plastics can therefore dominate desorption from the metal surfaces. The outgassing rate of plastics is calculated in accordance with:
QdesK = (Formula 1-25).

We use QdesK = S · pdesK and obtain the following for pb4 = 10-8 mbar: t4 = 459 106 s = 1,277 h. In this connection, t0 = 3,600 s and the associated value qdesK is read from the diagram for FPM [19]. It can be seen that the contribution to pump-down time made by desorption of the cold-state seal is on a similar order of magnitude as that of the metal surface.

Since the diffusion of the gases released in the interior of the seal will determine the behavior of the desorption gas flow over time, the dependence of diffusion coefficient D upon temperature will have a crucial influence on pumping time:

Formula 7-10:

Diffusion coefficient (T)

As temperature rises, the diffusion coefficient increases, as well; however it will not rise as much as the desorption rate of the metal surface. We thus see that elastomer seals can have a pronouncedly limiting effect on base pressure due to their desorption rates, which is why they are not suitable for generating ultra high vacuum.

Leakage rate and permeation rate

The gas flow that flows into the vacuum system through leaks is constant and results in pressure pl = . A system is considered to be sufficiently tight when this pressure is less than 10 % of working pressure. Leakage rates of 10-8 mbar · l/s are usually easy to attain and are also required for this system. This results in a pressure proportion of the leakage rate of pl = 1.46 · 10-11 mbar. This value is not disturbing and can be left out of consideration.

Permeation rates through metal walls do not influence the ultimate pressure that is required in this example; however diffusion through elastomer seals can also have a limiting effect on base pressure in the selected example.

Summary

Pressures of up to 10-7 mbar can be attained in approximately one day in clean vessels without the need for any additional measures.

If pressures of up to 10-4 mbar are to be attained, the pump-down times of the backing pump and the turbopump must be added together. In the above-mentioned case, this is approximately 200 s. At pressures of less than 10-6 mbar, a high turbomolecular pump pumping speed will be required, in particular in order to pump down the water adsorbed on the metal walls.

This will only be possible by additionally baking out the vacuum vessel (90 to 400 °C) if the required base pressure pb of 10-8 mbar is to be attained within a few hours. The heater is turned off after 100 times the value of the desired pressure has been attained. The base pressure will then be reached after cool-down of the vacuum vessel.

At pressures of less than 10-8 mbar, only metal seals should be used in order to avoid the high desorption rates of FPM seals.

Leakage and permeation rates can easily be kept sufficiently low in metal vessels at pressures of up to 10-10 mbar

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