Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Page 48
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Vacuum Technology
Backing pumps
Rotary vane pumps, rotary piston pumps or screw pumps can be used as backing pumps:
These kinds of pump combinations can be employed for all applications in the low and
medium vacuum ranges involving high pumping speeds. Liquid ring pumps can also be
used as backing pumps.
Gas-cooled Roots pumps
To allow Roots vacuum pumps to work against atmospheric pressure, some models do not
have overflow valves with gas cooling (Figure 2.13). In this case, the gas that flows from the
outlet flange (6) is re-admitted into the middle of the suction chamber (4) through a cooler (7).
This artificially generated gas flow cools the pump, enabling it to compress against atmospheric
pressure. Gas entry is controlled by the Roots pistons, thus eliminating the need for
any additional valves. There is no possibility of thermal overload, even when operating at
ultimate pressure.
p1
4 4
4 4
4 4
4 4
1 2
1 2
1 2
1 2
1 2
p2
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Phase I Phase II Phase III Phase IV Phase V
Figure 2.13: Operating principle of a gas-cooled Roots pump
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p1
p2
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Figure 2.13 shows a cross section of a Roots vacuum pump. The direction of gas flow is ver-
tical from top to bottom, enabling the liquid or solid particles entrained in the inlet flow to flow
off downward. In phase I, the chamber (3) is opened by the rotation of the pistons (1) and (2).
Gas flows into the chamber through the inlet flange at pressure p . In phase II, the chamber
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(3) is sealed off against both the inlet flange and the pressure flange. The inlet opening (4) for
the cooling gas is opened by the rotation of the pistons. In Phase III, the chamber (3) is filled
at the outlet pressure p , and the gas is advanced toward the pressure flange. Initially, the
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suction volume does not change with the rotary movement of the Roots pistons. The gas is
compressed by the inflowing cooling gas. The Roots piston now continues to rotate (phase IV),
and this movement pushes the now compressed gas over the cooler (7) to the discharge side
(Phase V) at pressure p . 2
Gas-cooled Roots pumps can be used in the inlet pressure range of 130 to 1,013 mbar.
Because there is no lubricant in the suction chamber, they do not discharge any mist or contaminate
the medium that is being pumped. Connecting two of these pumps in series
enables the ultimate pressure to be reduced to 20 to 30 mbar. In combination with additional
Roots vacuum pumps, the ultimate pressure can be reduced to the medium vacuum range.
p1
p2
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p1
p2
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p1
p2
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