Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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VACUUM PUMP
The invention relates to a vacuum pump and in particular to a vacuum pump for
differentially evacuating a vacuum system.
In a differentially pumped scientific instrument system, such as a mass
spectrometer, a sample and a carrier gas are introduced for analysis. One such
example
is given in Figure 1. With reference to Figure 1, in such a system there
exists a high
vacuum chamber 110 immediately following first, (depending on the type of
system)
second, and third evacuated interface chambers 111, 112, 114. The first
interface
chamber is the highest-pressure chamber in the evacuated spectrometer system
and
may contain an orifice or capillary through which ions are drawn from the ion
source
into the first interface chamber 111. The second, optional interface chamber
112 may
include ion optics for guiding ions from the first interface chamber 11 into
the third
interface chamber 114, and the third chamber 114 may include additional ion
optics for
guiding ions from the second interface chamber into the high vacuum chamber
110. In
this example, in use, the first interface chamber is at a pressure of around 1-
10 mbar,
the second interface chamber (where used) is at a pressure of around 104-1
mbar, the
third interface chamber is at a pressure of around 10-2- 10-3 mbar, and the
high vacuum
chamber is at a pressure of around 10-5-10-6 mbar. Differentially pumped
vacuum
system may have different pressures dependent on requirements.
The high vacuum chamber 110, second interface chamber 112 and third interface
chamber 114 can be evacuated by means of a compound vacuum pump 116. In this
example, the vacuum pump has two pumping sections in the form of two sets 118,
120
of turbo-molecular stages, and a third pumping section in the form of a
Holweck drag
mechanism 122; an alternative form of drag mechanism, such as a Siegbahn or
Gaede
mechanism, could be used instead. Each set 118, 120 of turbo-molecular stages
comprises a number (three shown in Figure 1, although any suitable number
could be
provided) of rotor 119a, 121a and stator 119b, 121b blade pairs of known
angled
construction. The Holweck mechanism 122 includes a number (two shown in Figure
1
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although any suitable number could be provided) of rotating cylinders 123a and
corresponding annular stators 123b and helical channels in a manner known per
se.
In this example, a first pump inlet 124 is connected to the high vacuum
chamber
110, and fluid pumped through the inlet 124 passes through both sets 118, 120
of turbo-
molecular stages in sequence and the Holweck mechanism 122 and exits the pump
via
outlet 130. A second pump inlet 126 is connected to the third interface
chamber 114,
and fluid pumped through the inlet 126 passes through set 120 of turbo-
molecular
stages and the Holweck mechanism 122 and exits the pump via outlet 130. In
this
example, the pump 116 also includes a third inlet 127 which can be selectively
opened
and closed and can, for example, make the use of an internal baffle to guide
fluid into
the pump 116 from the second, optional interface chamber 112. With the third
inlet
open, fluid pumped through the third inlet 127 passes through the Holweck
mechanism
only and exits the pump via outlet 130.
In this example, in order to minimise the number of pumps required to evacuate
the spectrometer, the first interface chamber 111 is connected via a foreline
131 to a
backing pump 132, which also pumps fluid from the outlet 130 of the compound
vacuum pump 116. The backing pump typically pumps a larger mass flow directly
from
the first chamber 111 than that from the outlet 130 of the compound vacuum
pump
116. As fluid entering each pump inlet passes through a respective different
number of
stages before exiting from the pump, the pump 116 is able to provide the
required
vacuum levels in the chambers 110, 112, 114, with the backing pump 132
providing the
required vacuum level in the chamber 111. The vacuum pumping arrangement shown
in
Figure 1 is also described in for example US5,733,104.
The performance and power consumption of the compound pump 116 is
dependent largely upon its backing pressure, and is therefore dependent upon
the
foreline pressure (and the pressure in the first interface chamber 111)
offered by the
backing pump 132. This in itself is dependent mainly upon two factors, namely
the total
mass flow rate entering the foreline 131 from the scientific instrument and
the pumping
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capacity of the backing pump 132. Many compound pumps having a combination of
turbo-molecular and molecular drag stages are only ideally suited to
relatively low
backing pressures, and so if the pressure in the foreline 131 (and hence in
the first
interface chamber 111) increases as a result of increased mass flow rate or a
smaller
backing pump size, the resulting deterioration in performance and increase in
power
consumption can be rapid. In an effort to increase mass spectrometer
performance,
manufacturers often increase the mass flow rate into the spectrometer, thus
requiring
increased size or number of backing pumps in parallel to accommodate for the
increased
mass flow rate. This increases costs, size (footprint) and power consumption
of the
overall pumping system required to differentially evacuate the mass
spectrometer.
The present invention provides a vacuum pump for differentially pumping a
plurality of chambers, the vacuum pump comprising: a housing which houses a
plurality
of compound pumping arrangements supported for independent rotation one from
another on respective drive shafts by separate motors; a first housing inlet
for receiving
fluid from a first chamber; a second housing inlet for receiving fluid from a
second
chamber; a first of the compound pumping arrangements comprising a first
pumping
section comprising a turbo molecular pumping mechanism and a second pumping
section downstream from the first pumping section, the sections being arranged
such
that fluid entering the compound pump from the first inlet passes through the
first and
second pumping sections and fluid entering the compound pump from the second
inlet
passes through, of said sections, only the second section; the vacuum pump
further
comprising: a third housing inlet for receiving fluid from a third chamber; a
fourth
housing inlet for receiving fluid from a fourth chamber; a second of the
compound
pumping arrangements comprising a third pumping section comprising a turbo
molecular pumping mechanism and a fourth section downstream from the first
pumping
section, the sections being arranged such that fluid entering the second
compound
pumping arrangement from the third inlet passes through the third and fourth
pumping
sections and fluid entering the second compound pump from the fourth inlet
passes
through, of said sections, only the fourth section.
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In another aspect there is provided a vacuum pump for differentially pumping a
plurality of chambers at different pressures, the vacuum pump comprising: a
housing
comprising first and second housing inlets for connection to respective vacuum
chambers, the housing comprising a bore for receiving a cartridge casing of a
compound
vacuum pumping arrangement, the compound vacuum pumping arrangement
comprising a first pumping section and a second pumping section downstream
from the
first pumping section, the sections being arranged such that fluid entering
the
compound pump from the first housing inlet passes through the first and second
pumping sections and fluid entering the compound pumping arrangement from the
second inlet passes through, of said sections, only the second section, the
cartridge
casing comprising a first fluid inlet arrangement exposed to receive fluid
from the first
housing inlet when the cartridge is located in the housing and a second fluid
inlet
arrangement exposed to receive fluid from the second housing inlet when the
cartridge
is located in the housing, the first fluid inlet arrangement comprising at
least one radial
fluid inlet for receiving fluid in a generally radial direction into the
casing and at least
one axial fluid inlet for receiving fluid in a generally axial direction into
the casing.
The second fluid inlet arrangement may be formed in a portion of the cartridge
casing which protrudes into a volume in gas communication with the first
housing inlet
and the volume may encircle an axis of the compound pumping arrangement.
The first fluid inlet arrangement may comprise a plurality of said radial
inlets
spaced about the circumference of the casing and exposed to the volume.
Said at least one axial fluid inlet may be formed at least in part by a
bearing
mount supporting a bearing of the compound pumping arrangement and supported
by
the cartridge casing. The bearing mount, or spider, may be received in the
volume.
At least one turbo molecular stage (or array of rotor blades) may be located
at
the axial fluid inlet for drawing gas through the inlet from the volume.
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The first section of the compound vacuum pumping arrangement may be spaced
from the spider or said at least one turbo molecular stage by an amount
generally equal
to or greater than an axial width of the radial fluid inlets.
Other preferred and/or optional aspects of the invention are defined in the
accompanying dependent claims.
In order that the present invention may be well understood, an embodiment
thereof, which is given by way of example only, will now be described in more
detail,
with reference to the accompanying drawings, in which:
Figure 1 is a simplified view of a prior art vacuum pumping arrangement and
differentially pumped vacuum system;
Figure 2 is a perspective view of a vacuum pump;
Figure 3 is a further perspective view of the vacuum pump shown in Figure 2;
Figure 4 shows a section of the vacuum pump taken along a central longitudinal
and vertical plane;
Figure 5 is a perspective view of the vacuum pump prior to full assembly; and
Figure 6 is a part section for showing a cartridge casing in the vacuum pump.
Referring to Figure 2, a vacuum pump 10 is shown for differentially pumping a
plurality of chambers. As described above in relation to the prior art, the
chambers may
form part of a mass spectrometer system and comprise a high vacuum chamber 110
immediately following first, (depending on the type of system) second, and
third
evacuated interface chambers 111, 112, 114. Alternatively, the vacuum system
may
comprise a plurality of chambers as disclosed in more detail in
US2015/0056060.
The vacuum pump 10 comprises a housing 12. The housing 12 may be cast or
machined from a suitable metallic material, such as steel or iron, and has a
one-piece
construction for housing the various pumping mechanisms of the pump. In this
example
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the housing is aluminium and machined from a solid block, but could be
extruded or
cast. Aluminium is preferred because it is light-weight. The housing comprises
a
plurality of housing inlets 14, 16, 18, 20, 22, 24 and can be fixed to the
vacuum system
by suitable fastening members and sealed to allow fluid communication between
the
different pressure chambers of the system and selected pumping mechanisms of
the
pump. The fluid inlets are surrounded by grooves 26 for receiving sealing
members (not
shown), such as o-rings, for sealing against the vacuum system to avoid or
resist the
leakage of ambient air into the pump. Inlets 18, 20, 22, 24 are surrounded by
a single
groove and sealing member.
The housing comprises a plurality of flow paths which guide fluid from the
housing inlets towards the inlets of the different pumping mechanisms, as
shown in
more detail in Figure 4. These flow paths are formed by the internal structure
of the
housing and may be cast in the manufacturing process or machined into the
housing.
The vacuum pump comprises two vacuum pumping arrangements 28, 30. In this
example, vacuum pumping arrangement 30 is arranged to evacuate pressure
chambers
through housing inlets 18, 20, 22, 24 at a relatively higher vacuum (lower
pressure)
compared to vacuum pumping arrangement 28 which is arranged to evacuate
pressure
chambers through housing inlets 14, 16 at a relatively lower vacuum (higher
pressure).
The vacuum pumping mechanisms for the two arrangements are housed within
the housing 12. In each case, part of the vacuum pumping arrangements extends
externally to the housing and these external parts may include the motor and
electrical
connections for connecting the arrangements to a source of electrical power.
Locating
the motors at partially external to the housing allows heat generated during
use to
escape from the pump more easily.
Figure 3 shows the vacuum pump 10 from a different perspective, underneath
the pump. The first vacuum pumping arrangement 30 comprises an inlet port 32
and
the second vacuum pumping arrangement 28 comprises an exhaust port 34. A
foreline
pipe 36 connects the exhaust port with the inlet port for fluid communication.
As
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described in more detail below gas is exhausted from the second vacuum pumping
arrangement through the foreline to the first vacuum pumping arrangement,
where it is
pumped by at least one pumping mechanism. The first vacuum pumping arrangement
comprises an exhaust port (not shown in this Figure) for exhausting gas
through another
foreline pipe to a separate backing, or primary, pump. In this way, the second
vacuum
pumping arrangement is backed in series by the first vacuum pumping
arrangement and
a primary pump, and the first vacuum pumping arrangement is backed by the
primary
pump. This can reduce the overall power consumption of the system or of the
first
vacuum pumping arrangement 28. Alternatively the reduced backing pressure
could be
used to enhance the overall compression of arrangement 28.
In an alternative, the housing could be configured to include internal
structure
defining the flow path between the vacuum pumping arrangements, thereby
eliminating
the need for additional fittings and pipes. In other examples, the vacuum pump
can be
backed independently by a single or multiple backing pumps.
Figure 4 shows a section through the vacuum pump 10. The first vacuum
pumping arrangement is inserted into a bore 38 of the housing 12 through an
opening
40 in the underside of the housing and fastened in position with fastening
members (not
shown). An o-ring 42 seals the arrangement when in position. The first vacuum
pumping arrangement comprises in this example seven vacuum pumping sections
each
having a pumping mechanism and one or more pumping stages. The number of
sections, stages in each section and type of pumping mechanism may be selected
as
required depending on pumping requirements, such as capacity and compression.
The
arrangement 30 comprises a drive shaft 58 supported for rotation by upper and
lower
bearings 60, 62. Typically the upper bearing is a magnetic bearing (with a
back-up
bearing) and the lower bearing is a roller bearing. The upper bearing is
supported by a
spider 63 having a central hub from which three arms extend in a radial
direction. The
arms are fixed to the housing to provide support for the bearing and the
vacuum
pumping arrangement, whilst allowing space for gas to enter the most upstream
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pumping mechanism 44. The spider could also be machined into and form part of
the
housing (i.e. integral with the housing).
A motor 64 drives rotation of the drive shaft and is connected to a source of
electrical power. The first pumping arrangement is a high speed pump and is
typically
rotated at speeds of between about 10,000 and 100,000 rpm.
The sections 44, 46, 48, 50 of vacuum pumping arrangement 30 each comprise a
turbo molecular pumping mechanism. These sections comprise respectively four
stages,
three stages, three stages and two stages, but more or fewer stages may be
provided as
required. Although at least one section comprising a turbo molecular pumping
mechanism is required to generate the required vacuum pressure, the other
sections
may be replaced with other types of pumping mechanism. Sections 52 and 54 are
each
four stage drag pumping mechanisms, and section 56 is a two stage regenerative
pumping mechanism. The regenerative pumping mechanism is otherwise known as an
aerodynamic mechanism in which an array of blades on a rotor disc extend into
respective channels of a stator generating a vortex in the channels on
rotation which
compress the gas being pumped. Different types of pumping mechanisms may be
used
in the latter sections, as well as different numbers of stages, depending on
pumping
requirements.
The first vacuum pumping mechanism comprises four fluid inlets to the various
sections. Section 44 has a fluid inlet 66 connected for fluid communication
with the
housing inlet 24 by internal flow path 67. Section 46 has a fluid inlet 68
connected for
fluid communication with the housing inlet 22 by internal flow path 69.
Section 48 has a
fluid inlet 70 connected for fluid communication with the housing inlet 20 by
internal
flow path 71. Section 50 has a fluid inlet 72 connected for fluid
communication with the
housing inlet 18 by internal flow path 73. Flow paths 69, 71, 73 extend away
from the
housing inlets and through 90 degrees to their respective fluid inlets, but
may
alternatively be direct in line with the pumping mechanism (i.e. not through
90 degrees).
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Fluid entering through housing inlet 24 passes through all of the pumping
sections of the first vacuum pumping arrangement, namely sections 44 to 56.
Fluid
entering through housing inlet 22 passes through sections 46 to 56 only. Fluid
entering
through housing inlet 20 passes through sections 48 to 56 only and fluid
entering
through housing inlet 18 passes through sections 50 to 56 only. In this
example, housing
inlet 24 is evacuated to the lowest pressure and the evacuation pressure
gradually
increases as gas passes through fewer sections.
Pumping sections 52, 54, 56 provide a backing pressure for the turbo molecular
sections of the first vacuum pumping arrangement 30, since a turbo molecular
mechanism cannot, or at least cannot efficiently, exhaust at atmosphere.
Whilst in
some examples the most downstream section 56 may exhaust at atmosphere,
typically it
exhausts below atmosphere and is itself backed by a separate primary pump, or
alternatively another similar turbo molecular pump then a primary pump.
As discussed above, the exhaust 34 of the second vacuum pumping arrangement
28 is connected to an inlet 32 of the first vacuum pumping arrangement 30. The
section
of Figure 4 does not show these ports, however the exhaust 34 is located
downstream of
the most downstream pumping section of the second arrangement and the inlet 32
is
located upstream of at least one of sections 52, 54, 56 and downstream of at
least
section 50. In this way, the exhaust 34 is backed by one, two or three pumping
sections.
The backing section is preferably configured as a booster mechanism whereby
the
compression ratio of the section is between 10:1 and 1:1 (for example) so that
pumping
capacity is increased. A section configured as a booster is capable of pumping
a greater
amount of gas which is useful in the event that a large amount of gas is input
to the
mass spectrometer.
The second vacuum pumping arrangement 28 comprises a cartridge in this
example, although a cartridge envelope is not essential for arrangement 28 (or
both
pumps) and alternatively it may be integrated directly into the housing. The
cartridge 75
comprises a casing 74 for supporting the pumping mechanisms of the cartridge.
The
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casing is configured so that the cartridge can be inserted into and engage
with a bore 76
of the housing 12 to expose fluid inlets 78, 80 of the pumping mechanisms to
respective
housing inlets 16, 14. Figure 5 shows a perspective view of the vacuum pump 10
with
the cartridge prior to insertion in bore 76 through housing opening 82.
The inner surface of the bore 76 guides the cartridge 75 towards the fully
inserted position shown in Figure 4. The casing has an outwardly projecting
flange 83
which forms with housing end surface 84 abutment surfaces for locating the
cartridge in
the correct position and limiting the extent to which the cartridge 75 can be
inserted
into the housing 12. Fastening members 86 fix the cartridge in position when
it has
been inserted. The members engage in closed bores 88 of the housing, typically
by
threaded engagement.
As shown in Figure 4, the housing 12 is shaped so as to expose the bore at a
number of locations to allow fluid to enter the fluid inlets 78, 80 when the
cartridge 14 is
in the fully inserted position. Flow paths 79, 81 manufactured in the housing
guide flow
from the housing inlets 14, 16 to respective fluid inlets 78, 80 of the
cartridge. The
housing also defines a volume 89 for gas flow from housing inlet 16. Gas from
inlet 16
may therefore pass into the cartridge radially through fluid inlet 78 or
axially through a
fluid inlet 85. The provision of radial and axial fluid inlets increases
conductance into the
pumping arrangement. It is preferable in this case as shown that at least one
turbo
molecular pumping stage 87 (or array of rotor blades) is located at the fluid
inlet 85 to
draw gas into the cartridge. The provision of two fluid inlets at the housing
inlet 16
produces low conductance resistance to flow.
In more detail and as shown additionally in Figure 6, the cartridge casing
defines
a plurality of apertures 91 spaced about its circumference forming radial
fluid inlets into
the vacuum pumping arrangement 28. Four apertures 91 are provided in this
example
at 90 degrees to one another but other configurations are possible. The
apertures are
located in part of the housing that extends into volume 89 and therefore all
apertures
are exposed to the volume, including the apertures located furthest from the
housing
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inlet 16. The array of rotor blades 87 and the pumping section 90 are spaced
apart in an
axial direction by an amount which is approximately equal to the axial width
of the
apertures 91. This configuration allows gas to penetrate into the space 93
between the
pumping mechanisms towards the drive shaft to enable pumping by both radially
outer
and radially inner portions of the turbo molecular mechanism 90. Without such
a space
93 the gas would interact only with the radially outer portions of the
mechanism, or to a
much greater extent than with the radially inner portions, and less efficient
pumping
would be achieved.
The fluid inlet 85 of the cartridge casing is formed by a generally circular
aperture
which is open in the axial direction (to the right in Figures 4 and 6). The
casing
comprises a shoulder portion 95 which is located upstream of the apertures 91
and fully
within volume 89. The shoulder portion seats a bearing mount, or spider, 97
for
mounting bearing 98 in position, which is likewise received in volume 89. The
spider
comprises an outer rim fixed to the casing, an inner hub for supporting the
bearing and
three radial arms (or spokes) connecting the rim with the hub. This
configuration
provides three apertures extending through approximately 120 degrees forming
the
axial fluid inlet 85 into the vacuum pumping arrangement 28. The spider may
have
fewer or more radial arms as is not restricted to three.
The cartridge casing further comprises apertures 99 forming a fluid inlet to
the
pumping section 92. Unlike apertures 91 located in volume 89, only that part
of the
casing proximate the housing inlet 14 is exposed to receive gas and therefore
apertures
99 are provided only in an upper part of the casing.
The pumping arrangement as shown fits horizontally into the housing, but could
be arranged vertically ¨ similarly the first arrangement could be horizontal ¨
hence
providing four configurations. That is horizontal-horizontal, horizontal-
vertical, vertical-
horizontal, and vertical-vertical.
In this example, the second vacuum pumping arrangement 28 comprises three
pumping sections 90, 92, 94. Section 90 comprises three turbo molecular
stages.
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Section 92 comprises three Holweck stages. Section 94 comprises two Seigbahn
stages.
Different types of molecular drag or other pumping mechanisms may be used in
sections
92 and 94, or there may only two sections in total. The number of stages in
the sections
may be varied as required dependent on pumping requirements.
A drive shaft 96 supports the pumping mechanisms for rotation and is itself
supported by bearings 98, 100. In this example, bearing 98 is a dry magnetic
bearing
with roller bearing back-up and bearing 100 is a lubricated roller bearing.
The drive
shaft is driven by motor 102 connected to a source of electrical power at high
rotational
speeds for example between about 10,000 and 100,000 rpm.
Rotation of the pumping sections 90, 92, 94 causes gas to flow from housing
inlet
16 through fluid inlets 78, 85 passing through section 90 (and turbo molecular
stage 87),
section 92 and section 94. Gas is caused to flow from housing inlet 14 through
fluid inlet
80 passing through, of the sections, only the downstream sections 92, 94. The
pressure
chambers in fluid communication with the housing inlets 14, 16 are evacuated
at
different pressures; housing inlet is evacuated at a lower pressure because
gas passing
through it is conveyed through more pumping sections and section 90 is a
configured to
pump at lower pressures. More than two housing inlets may be evacuated by the
cartridge.
Gas is exhausted from the cartridge 75 through exhaust port 34 and passes
through a booster section (one or more of sections 52, 54, 56) of the first
vacuum
pumping arrangement prior to a separate backing or primary pump. If the first
pump
power is high the second pump could be used to back the first (switching the
backing
arrangement shown).
The cartridge type configuration of the second vacuum pumping arrangement 28
is advantageous in that it can readily be removed and inserted into the
housing, allowing
easy maintenance, repair or replacement. Since the casing of the cartridge
provides the
support required for operation (and rotation at high speeds) the vacuum pump
10 can
be more easily manufactured and assembled. The various components of the
cartridge
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are assembled outside the housing 12, without the need to manufacture
typically
intricate structures for supporting the mechanism inside the housing. In the
example of
vacuum pump 10, vacuum pumping arrangement 30 does not have a cartridge type
configuration and cannot be operated without support inside the housing,
particularly
without spider 63 and bearing assembly 60. However, vacuum pumping arrangement
30
may alternatively be formed as a cartridge type configuration similarly to
vacuum
pumping arrangement 28.
The vacuum pumping arrangements may have drive shafts having respective
axes of rotation which are angled one relative to another. In the example
shown the
drive shafts 58, 96 have first and second axes of rotation, and the first axis
is
perpendicular to the second axis. The axis 96 intersects axis 58, but in other
examples
the axes may be offset. In an earlier document of the present applicant
(EP2273128) an
axis of a cartridge type vacuum pumping arrangement is inclined by an angle 0
to the
direction of gas flow through the pressure chambers of a differentially pumped
system
and in further examples of the present invention a similar construction can be
adopted
whereby a pumping axis of one pumping arrangement is at an angle 0 as is the
case with
the earlier document.
As shown in the Figures the axis 58 is vertical and the axis 96 is horizontal,
with
respect to gravitational force. It is preferable in the present construction
that the axis
58 is vertical in order to cancel out the effect of gravity acting on the
moving parts (rotor
parts) of the pumping arrangement. Particularly, upper bearing 60 is a non-
contact
bearing and therefore allows a small amount of radial movement of the rotor
(limited by
the back-up bearing). Therefore the drive shaft can to an extent be considered
cantilever, but as gravitational force is largely cancelled and does not cause
significant
radial movement. It will also be appreciated that there are many pumping
sections,
seven in total and four turbo molecular sections (requiring high speed
rotation), thereby
contributing to the overall length of the drive shaft. Hence in structures
with a
multiplicity of pumping sections (more than four) or a multiplicity of turbo
molecular
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sections (more than two) it is desirable, but not essential, that the drive
shaft is vertical
for improved rotor dynamics.
The drive shaft 96 of the second vacuum pumping arrangement is horizontal,
because the problems explained above in relation to the first vacuum pumping
arrangement are less apparent, and a horizontal orientation is advantageous
for
conserving foot-print or pump size. In a one-piece housing construction there
is also the
issue of mass as the vacuum pump is typically mounted by suspending it from
the
underside of a vacuum system. A horizontal drive shaft reduces the size of the
housing
and also its mass.
As described above in detail with reference to the Figures, the housing 12
houses
a plurality of compound pumping arrangements 28, 30 supported for independent
rotation one from another on respective drive shafts 58, 96 by separate motors
64, 102.
Independent rotation and drive allow improved versatility for application to
multiple
different pumping requirements. For example, operators of mass spectrometer
systems
are increasing the sample size of gas introduced to upstream pressure chambers
of the
system and accordingly there may be a cyclical or transient requirement during
operation for the second vacuum pumping arrangement to pump large quantities
of gas.
In these circumstances, the power drawn from the motor 102 is increased to
overcome
increased resistance to rotation in sections 90, 92, 94. However, the first
vacuum
pumping arrangement 30 is independent and continues operation without
interruption
or increased power requirement.
The power supply for the vacuum pumping arrangements 28, 30 may be shared
so that electrical power is supplied to the motors 64, 102 by a single supply
unit (not
shown). In this case, the unit may provide the electrical power and control of
the units
may be performed at the pumping arrangement or in the unit. That is two
frequency
converters may be provided in the unit or one in each of the pumping
arrangements.
The advantage of a single supply unit is that the requirement for electrical
power is
spread across or shared by both pumping arrangement so if one pumping
arrangement
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is subjected to high load conditions and the other to low load conditions the
overall
power consumption does not increase. Such conditions may occur for example if
both
vacuum pumping arrangements are operating at ultimate pressure and a sample
gas is
introduced to the vacuum system ¨ the pressure and amount of gas in the
upstream
chambers increases, with a commensurate increase in the load on the second
vacuum
pumping arrangement 28, but there is a delay prior to increased pressure in
the
downstream chambers and increased load on the first vacuum pumping arrangement
30. Alternatively, each vacuum pumping arrangement can be provided with a
dedicated
source of electrical power.
In some known vacuum pumping systems, a plurality of pumps are provided in
respective housings for differentially evacuating a vacuum system. However, in
these
known systems the pumps are arranged so that gas exhausted from a lower
pressure
pump is conveyed to an inlet of the higher pressure pump. Therefore the pumps
are
arranged in series. In vacuum pump 10, however, the two vacuum pumping
arrangements are not in series and instead evacuate the pressure chambers in
parallel.
A series relationship is partly provided but only between the exhaust of the
second
vacuum pumping arrangement and the booster section 52, 54, 56.
The housing 12 is manufactured to include the required internal structure both
for seating the vacuum pumping arrangements in the correct positions and for
guiding
fluid flow from the housing inlets to the fluid inlets of the vacuum pumping
arrangements. The internal structure is configured to have a relatively simple
shape
which permits formation by casting or moulding without the requirement for
extensive
subsequent machining. The internal structure is configured to include a
plurality of
shaped flow conduits defined by internal partitioning walls for directing flow
to the fluid
inlets of both vacuum pumping arrangements. The provision of a single one-
piece
housing for directing gas flow removes the requirement for forelines and other
pipework thereby avoiding the use of additional components, the time required
for
assembly and a potential cause of leakage.
