Specifications

190
Turbo-V Pump Technical Notes
Turbopump Operating Principles
Turbomolecular pumps consist of a series of bladed impellers
rotating at high speed and fixed bladed stators. These
impellers and stators are alternately spaced and are inclined
in opposite directions.
The pumping action is based on momentum transfer from the
fast moving impeller surface to the gas molecules.
The speed of the moving surface must be as high as possible
to achieve optimum pumping efficiency in terms of pumping
speed and compression ratio.
When this mechanism takes place several times in a pump a
pumping action is created. The sequence of alternating rotors
and stators typical of a conventional turbomolecular pump
develops the compression ratio.
Turbomolecular drag pumps operate according to the same
principle, but with a different geometry in the pumping stages.
Gas molecules collide against a fast moving wall and are
dragged into a channel toward the high pressure region.
Conventional turbomolecular pumps have high pumping
speed but low compression ratio at foreline pressures higher
than 10
-1
mbar. Molecular drag pumps have low pumping
speed but high compression ratios up to foreline pressures of
more than 20 mbar.
When the two types of stages are combined together in one
pump, as in the Varian MacroTorr® pumps, extended
operational pressure ranges can be achieved. (See page 195
for further description of the MacroTorr® principle).
Pump Selection
How to Select a Turbo-V Pump
The right choice of a turbomolecular pump depends on the
application; as a general rule we can reduce the choice to two
types of use:
UHV (no gas flow) operations and Process Gas flow operations.
– UHV (no gas flow) operations.
The former case includes most cases in which the
turbomolecular pump is employed to create vacuum in
systems where the gas load is mainly produced by
outgassing. In this application the choice is typically based
on the desired base pressure within a desired time as a
function of the foreseen outgassing rate, i.e.
S
eff
= Q / p
where:
p is the desired base pressure (mbar)
Q is the total outgassing rate at the desired time (mbar l/s)
S
eff
is the effective pumping speed
– Process Gas flow operations.
The second case relates to all operations where process
gases must be used. The main parameters are therefore the
desired operation pressure and the process gas flow
S
eff
= Q' / p'
where Q' is the total gas flow and p' is the operating
pressure.
How to Select the Backing Pump of a Turbomolecular Pump
The selection of a backing pump should be based analyzing
two requirements of the vacuum system:
a. the roughing time
b. the minimum recommended backing pump of the turbo
a. Roughing: once the desired roughing time is established,
the size of the forepump can be determined through the
following formula:
S
foreline
= (V / t) ln (p
0
/ p
1
)
where
S
foreline
is the pumping speed of the roughing pump (l/min)
V is the volume of the chamber to be evacuated (l)
t is the desired roughing time (min)
p
0
is the starting pressure (mbar)
p
1
is the end pressure (mbar)
When using a foreline pump much larger than the
recommended size, a by-pass line might be necessary to
achieve calculated roughing time.
b. Backing: the backing pump must be big enough to achieve
an effective pumping speed as close as possible to the
nominal speed.
p
foreline
= Q / S
foreline
where
S
foreline
is the pumping speed of the foreline pump
Q is the gas load
p is the operating foreline pressure
It should be noted that Q is the total gas load on the pump
and includes process gases and turbo purge gases when used.
The size of the backing pump can be calculated according
to the following rule:
S
foreline
≥ 20S / K
where
S is the pumping speed of the turbopump
S
foreline
is the pumping speed of the backing pump
K is the maximum compression ratio of the
turbopump for a given gas (i.e.: process gas) at the
operating foreline pressure.