Specifications

Turbo-V Pumps
191
Varian, Inc. Vacuum Technologies
Turbo
Pumps
The pumping speed of the backing pump should be the higher
of the two values calculated as above (roughing and backing).
Finally, it is possible to use a dry pump (scroll or diaphragm)
for hydrocarbon-free operation when pumps of the
MacroTorr® type are used.
Turbomolecular Pump Parameters and
Definitions
Throughput
“Throughput” is the flow rate of pumped gas through the
turbomolecular pump (and foreline pump).
Throughput (Q) is measured in mbar l/s ≅ 1/60 standard cm
3
/min.
The maximum throughput a pumping system can handle is, in
general, dependent upon the size of its foreline pump rather
than the turbomolecular pump.
Pumping Speed
“Pumping speed” (S) (volumetric flow rate) of a turbomolecular
pump is the ratio between throughput and inlet pressure
(foreline pump size must be the recommended one as a
minimum).
S = Q / p
The pumping speed of a turbomolecular pump is constant
over a wide pressure range and depends upon geometric
factors such as diameter and rotational speed. For most
turbomolecular pumps, pumping speed is nearly independent
from gas species (molecular weight).
Compression Ratio
“Compression Ratio” is the ratio between foreline (partial)
pressure and inlet (partial) pressure for a given process gas,
measured in “zero flow” conditions (performed by injecting
the process gas in the pump foreline while the high vacuum
port is blanked off).
Compression ratio is generally indicated with the letter “K”.
In technical specifications of turbomolecular pumps, it is the
maximum attainable value of K (at low foreline pressure).
Compression ratio is, in fact, a function of the foreline
pressure as shown in Figure 1.
Compression ratio decays at high pressure depending on
turbomolecular pump configuration (the number of
molecular stages) and/or power limitations that slow down
the rotor (gas friction increases with pressure).
The maximum compression ratio is strongly influenced by gas
species: it is an exponential function of the molecular weight
of the pumped gas (compression ratio is considerably lower
for light gases).
Pumping Speed and Pressure Ratio
The pressure ratio between foreline and inlet pressures in
each operational situation is indicated by “R
p
”. This is, in
general, equal to pumping speed ratio
R
p
= p
foreline
/ p
inlet
= S
eff
/ S
foreline
where S
eff
is the effective pumping speed, and S
foreline
is the
pumping speed of the foreline pump.
In fact
Q = S
eff
p
inlet
= S
foreline
p
foreline
therefore
S
eff
/ S
foreline
= p
foreline
/ p
inlet
The pumping speed of a turbomolecular pump is minimally
affected by pressure ratio (and foreline pump size) in most
common operational conditions (when pressure ratio is much
smaller than K).
Generally, however, the effective pumping speed “S
eff
” is a
linear function of the pressure ratio “R
p
” as shown in
Figure 2 (and therefore is also dependent upon the size of the
backing pump).
10010
10
10
10
8
10
6
10
4
Compression ratio
10
2
10
-2
10
-1
1
Foreline pressure (mbar)
1
Compression Ratio vs Foreline Pressure
Figure 1
Inlet pressure (mbar)
200
0
10
-10
Pumping speed (l/s)
10
-5
10
-4
10
-3
10
-2
10
-1
50
250
150
100
1
300
350
10
-6
Pumping Speed vs Inlet Pressure
Figure 2
Nitrogen
Helium Hydrogen
Nitrogen
Helium Hydrogen