Multistage vacuum pump with interstage inlet

A vacuum pump comprising a plurality of vacuum stages and having a first pump inlet through which gas can pass through all the pump stages and a second inlet through which gas can enter the pump at an interstage location and pass only through subsequent stages of the pump, wherein the pump stages prior to the interstage are sized differently to these stages subsequent to the interstages such that the pump overall suits the pressure requirements and pumping capacity of the different systems attached to the first and second inlet, respectively.

TECHNICAL FIELD 
This invention relates to improved vacuum pumps with particular reference 
to those employing a turbo-molecular mode of operation. 
BACKGROUND 
A conventional turbo-molecular stage arrangement of a vacuum pump comprises 
a stack of alternate rotors and stators. Each stage effectively comprises 
a solid disc with a plurality of blades depending (nominally) radially 
therefrom; the blades are evenly spaced around the circumference of the 
disc and angled "about" radial lines out of the plane of the disc in the 
direction of rotation of the rotor stage. 
The rotor and stator blades have positive and negative gradients 
respectively when viewed from the side in a radial line from the disc. 
This arrangement has the effect in molecular flow conditions of causing 
the movement of molecules through the pump. 
There are a number of types of apparatus where a plurality of chambers 
needs to be evacuated down to different levels of vacuum. For example, in 
well known types of mass spectrometer that part of the apparatus known as 
the detector commonly has to be operated at, say 10.sup.-6 mbar whereas 
that part known as the analyser has to be operated at a different level of 
vacuum, say 10.sup.-3 mbar. 
In addition and importantly, the throughput of gas from the different parts 
of the apparatus will generally vary also. For example in a typical mass 
spectrometer of the type discussed above, there may need to be a 60 
l/second capacity for the detector and a 200 l/second capacity for the 
analyser. 
In apparatus of the type including but not restricted to mass 
spectrometers, a number of different vacuum pumps are normally employed. 
For example, in mass spectrometers, the detector and analyser may be 
evacuated by separate turbo-molecular vacuum pumps which themselves need 
to be backed by separate pumps, for example rotary vane pumps. 
There is an ever increasing need to rationalise the use of the various 
vacuum pumps for overall reduced apparatus size and power requirements. A 
single backing pump is relatively common for supporting two (or more) 
turbo-molecular pumps. In addition, it has more recently been proposed to 
employ a single turbo-molecular pump to replace two (or more) individual 
pumps with the single pump having a normal inlet for gas required to pass 
through all the stages of the pump and an intermediate inlet, i.e. between 
the stages, for gas required to pass through only the latter stages of the 
pump. 
However, even these proposals for rationalisation of the apparatus pumping 
system do not overcome all the problems associated with size and power 
consumption in particular. 
There is therefore a need for improved vacuum pumps in which 
rationalisation can be further enhanced. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided a vacuum pump 
comprising a plurality of vacuum stages and having a first pump inlet 
through which gas can pass through all the pump stages and a second inlet 
through which gas can enter the pump at an interstage location and pass 
only through subsequent stages of the pump, wherein the pump stages prior 
to the interstage are sized differently to those stages subsequent to the 
interstage such that the pump overall suits the pressure 
requirement/pumping capacity of the different systems attended to the 
first and second inlets respectively. 
The invention has advantageous application to turbo-molecular pumps in 
particular. 
In terms of suiting the pressure requirements of the different systems, 
that system requiring the lower pressure (higher vacuum) will generally 
need to be attached to the first inlet so that gas being evacuated is 
subject to all the stages of the pump whereas that system requiring the 
higher pressure (lower vacuum) will generally need to be attached to the 
second inlet so that gas being evacuated is subject only to the pump stage 
subsequent to the interstage. 
In those cases, for example, in which the system needing the lower pressure 
(high vacuum) requires a smaller pumping capacity in terms, in particular, 
of speed and compression and, for example, in which the system needing the 
higher pressure requires a higher pumping capacity, the stages prior to 
the interstage can be of a smaller size than those stages subsequent to 
the interstage. 
In the case of a turbo-molecular pump in particular, this means that the 
tip diameter of the rotor is smaller in the stages before the interstage 
than after the interstage. 
In the case of turbo-molecular pumps in particular, it is preferred that 
there are three, four, five, six or more stages (rotor/stator pairs) both 
before and after the pump interstage. 
In preferred embodiments associated with a turbo-molecular pump, one or 
more Holweck pump stages are employed between the final turbo-molecular 
stage and the pump outlet.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawing, there is shown a vacuum pump having a 
multi-component body 1 within which is mounted a shaft 2. Rotation of the 
shaft 2 is effected by means of a motor generally indicated at 3 
positioned about the shaft 2. The position of the shaft 2 is controlled by 
bearings at its base generally indicated at 4 and at its top generally 
indicated at 5, all of design well known in the art. 
The pump possesses two sets of turbo-molecular stages generally indicated 
at 6 and 7 before and after an interstage therebetween, respectively. 
The first set of turbo-molecular stages comprises four rotors (impellers) 
of angled blade construction as described above and of known construction, 
one of which is indicated at 8 and four corresponding stators again of 
angled blade construction and again as described above and of known 
construction, one of which is indicated at 9 in the drawing. 
The tip diameter D.sub.1 of the rotors is indicated in the drawing. 
An inlet 10 to the first set of stages allows gas entry through a 
perforated inlet screen 11 in to the four rotor/stator stages of the first 
set. 
A second set of turbo-molecular stages 7 comprises a further six rotors 
(impellers) of angled blade construction, one of which is indicated at 12 
and six corresponding stators again of angled blade construction, one of 
which is indicated at 13 in the drawing. 
The tip diameter D.sub.2 of these rotors is also indicated in the drawing. 
At an interstage position between the first and second sets of 
turbo-molecular stages is positioned a stator bridge 14 of heavily 
perforated design. 
Gas exiting from the first set 6 of turbo-molecular stages can pass through 
the interstage area and into the second set 7 of turbo-molecular stages. 
A second inlet 16 is formed in the pump body 1 and allows entry of gas 
directly in to the interstage area via the apertures in the stator bridge 
14. 
At the exit of the second set 7 of turbo-molecular stages is a number of 
Holweck stages. These Holweck stages comprise two rotating cylinders 17, 
18 and corresponding annular stators 19, 20 having helical channels formed 
therein (on one side for stator 19, on both sides for stator 20) all in a 
general manner known per se. 
Gas exiting the Holweck stage is urged into a passageway 21 found in the 
pump body 1 and thence to a pump outlet 22. 
In this embodiment, the sets of turbo-molecular pump stages are therefore 
sized to reflect the pressure requirements and pumping capacities of the 
respective vacuum systems to be attached to the inlet 1 and to the inlet 2 
thereby leading to overall pump improvements in terms of lower power 
consumption and smaller size.