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Pfeiffer <strong>Vacuum</strong><br />
Page 142<br />
<strong>Vacuum</strong> <strong>Technology</strong><br />
The pump-down time will additionally be influenced by the leakage rate of the vacuum system,<br />
the conductivities of the piping and of vaporizing liquids that are present in the recipient, as<br />
well as by degassing of porous materials and contaminated walls. Some of these factors<br />
will be discussed in Sections 7.2.3.1 and 7.3. If any of the above-mentioned influences are<br />
unknown, it will be necessary to provide appropriate reserves in the pumping station.<br />
7.2.2 Condenser mode<br />
In many vacuum processes (drying, distillation), large volumes of vapor are released that have<br />
to be pumped down. Moreover, significant volumes of leakage air will penetrate into large<br />
vessels, and those substances that are being vaporized or dried will release additional air that<br />
is contained in pores or dissolved in liquids.<br />
In drying processes, the vapor can always be displaced against atmospheric pressure by<br />
a vacuum pump having sufficient water vapor capacity and can then be condensed there.<br />
<strong>How</strong>ever, this process has the following disadvantages:<br />
The pump must be very large<br />
A large volume of gas ballast air will be entrained which, together with the vapor,<br />
will carry a great deal of oil mist out of the pump<br />
It will be necessary to dispose of the resulting condensate from the water vapor and oil<br />
mist, which is a costly process<br />
Distillation processes operate with condensers, and the object is to lose as little of the<br />
condensing distillate as possible through the connected vacuum pump.<br />
Let us consider a vacuum chamber containing material to be dried, to which enough energy<br />
will supplied by heat that 10 kg of water will evaporate per hour. In addition, 0.5 kg of air will<br />
be released per hour. The pressure in the chamber should be less than 10 mbar. A pumping<br />
station in accordance with Figure 7.2 is used for drying, enabling the steam to be condensed<br />
cost-effectively through the employment of a condenser.<br />
1<br />
2<br />
3<br />
4<br />
5<br />
Figure 7.2: Drying system (schematic)<br />
S 1<br />
6<br />
S 2<br />
1) Recipient<br />
2) Material to be dried<br />
3) Roots pump<br />
4) Condenser<br />
5) Water cooling<br />
6) Backing vacuum pump<br />
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