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Formula 2-1<br />
Compression ratio<br />
www.pfeiffer-vacuum.net<br />
Gas-displacement pumps, which are also referred to as gas transfer pumps, are classified<br />
either as positive displacement pumps or kinetic vacuum pumps. Positive displacement<br />
pumps displace gas from sealed areas to the atmosphere or to a downstream pump stage.<br />
Kinetic pumps displace gas by accelerating it in the pumping direction, either via a mechanical<br />
drive system or through an aligned vapor stream that is condensed at the end of the pumping<br />
section. Gas-binding vacuum pumps either bind the gas to an especially active substrate<br />
through gettering or condense the gas at a suitable temperature.<br />
2.1.2 Pumping speed and throughput<br />
Pumping speed (Formula 1-12) is the mean volume flow through the cross section<br />
of the inlet port of a vacuum pump. In the volume flow rate diagram, it is applied as a factor<br />
of the inlet pressure of the pump. The pump’s maximum achievable pumping speed is always<br />
referred to as its rated pumping speed. Determination of the pumping speed is described in<br />
base standard ISO 21360-1. Pumping speed is indicated in m3 / s. The units of m3 S =<br />
dV<br />
/ h, l / s and<br />
l / min are also customary.<br />
dt<br />
Throughput (Formula 1-13) denotes the gas throughput in a vacuum<br />
pump as a function of inlet pressure. It is indicated in Pa . l / s or mbar . q<br />
dV<br />
= S pV<br />
l / s. In the case of<br />
pumping stations that consist of gas-displacement pumps, the throughput of all pumps will<br />
be the same.<br />
. p = . p<br />
dt<br />
2.1.3 Ultimate pressure and base pressure<br />
Ultimate pressure pe is the lowest pressure that is asymptotically approached by the pressure<br />
of a blank-flanged vacuum pump under defined basic conditions without gas inlet. If a pump<br />
is operated at ultimate pressure, the usable pumping speed will be zero, as only its own backflow<br />
losses will be displaced. Ultimate pressure is a theoretical value. Today, base pressure<br />
is specified instead of ultimate pressure. The conditions for achieving base pressure are<br />
specified in standard ISO 21360-1. As the base pressure must be attained within a specified<br />
period of time, it is usually higher than the ultimate pressure.<br />
2.1.4 Compression ratio<br />
The maximum pressure ratio between discharge pressure p and intake pressure p is<br />
2 1<br />
referred to as the compression ratio:<br />
K 0 =<br />
p2 p1 In the case of blank-flanged inlet ports, the compression ratio is measured through gas inlet<br />
on the discharge side.<br />
2.1.5 Pumping speed of pumping stages connected in series<br />
Let us consider a vacuum pump having a pumping speed S and a compression ratio K .<br />
0 0<br />
The pump has backflow losses through gaps having conductivity L . Let inlet pressure be p R 1<br />
and discharge pressure p . An additional pump having a pumping speed S is connected on<br />
2 v<br />
the outlet side.<br />
Page 27<br />
<strong>Vacuum</strong><br />
<strong>Technology</strong>
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