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Pfeiffer <strong>Vacuum</strong><br />
Page 138<br />
7<br />
Formula 7-1<br />
Roots pump<br />
gas load<br />
<strong>Vacuum</strong> <strong>Technology</strong><br />
Configuration<br />
7.1 General<br />
This section will discuss simple dimensioning questions:<br />
What size pump should I select in order to attain a specific pressure in a vacuum<br />
vessel within a given period of time?<br />
<strong>How</strong> large should the backing pump be for a high vacuum pump?<br />
What do I need to be aware of when pumping high gas loads?<br />
What is the influence of piping on the volume flow rate of a vacuum pump?<br />
All of these questions can naturally not be discussed exhaustively in this chapter. Simple<br />
examples will be used and the anticipated results estimated. The technical data of the pumps<br />
and components that are used must be taken into consideration for the concrete application<br />
in question. And special literature can also be useful for dimensioning.<br />
Units<br />
Every physical technical parameter consists of a numeric value and a unit. The SI system has<br />
been adopted worldwide and standards have been defined for the basic values of length (m),<br />
mass (kg), time (s), temperature (K), substance volume (mol), electric current (A) and luminous<br />
intensity (cd); these values are used for calibration purposes in the individual countries. All<br />
other values are derived from these basic values. With few exceptions, the formulas that are<br />
used in this discussion contain only the physical technical values and no conversion factors<br />
whatever, such as Pa to mbar. This means that after employing the values in SI units, the results<br />
will also be in SI units. Examples of SI units are 1 Pa = 1 N / m2 = 0.01 mbar for pressure<br />
and 1 m3 / s = 3,600 m3 / h. Used largely throughout the following sections are popular non-SI<br />
units; however SI units will be used wherever it would appear to be appropriate due to the<br />
required conversion.<br />
Exclusive use of SI units would avoid many errors and much conversion effort. Unfortunately,<br />
this advantage is only very slowly gaining acceptance throughout the world.<br />
7.2 Calculations<br />
7.2.1 Dimensioning a Roots pumping station<br />
Various preliminary considerations are first required in dimensioning a Roots pumping station.<br />
Compression ratio<br />
The compression ratio K of a Roots pump is typically between 5 and 70. To determine this ratio,<br />
0<br />
we first consider the volume of gas pumped and the backflow by means of conductivity L , as R<br />
well as the return flow of gas from the discharge chamber at volume flow rate S : R<br />
p a<br />
. S = pa . S 0 - L R (p v - p a ) - S R<br />
. pv<br />
where p a = intake pressure and p v = backing vacuum pressure.<br />
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