Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Page 146
Vacuum Technology
The vessel has the following data:
V = 0.2 m³ volume
A = 1.88 m² surface area
q = 2.7 x 10 desM - 6 mbar . m³ / (s . m²) desorption rate of stainless steel
q = 1.2 x 10 desK - 5 mbar . m³ / (s . m²) desorption rate of FPM
A d = 0.0204 m² surface area of the FPM seal
Q l < 10 - 8 mbar . l / sec leakage rate
The backing pump should evacuate the vessel to 0.1 mbar in t =180 s, and should also be able
1
to achieve this pressure with the gas ballast valve open. The volume flow rate can be obtained
in accordance with Formula 7-8:
V p0 S = S = ln 10.2 l / s = 36.8 m³ / h.
v v
t p 1 1
We select a Penta 35 with a pumping speed S = 35 m³ / h.
v
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
V p 1
t 2 = ln = 2.01 s
S p 2
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 t is approximately 1 h.
0
Using Q = q des des (Formula 1-24), we calculate the time needed to attain the working
. A .
pressure p b3 =10 - 8 mbar:
t 0
t 3
qdesM . A . t0 t = ; t = 2.67 3 3 . 10 6 s = 741 h.
S . pB3
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 p will then be attained after the recipient
b3
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
t0 (Formula 1-25).
t 4
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