GB2417757A - Vacuum pump with fewer rotors at exhaust stage - Google Patents

Abstract

A vacuum pump comprises a stator 2 which houses a multi-stage rotor assembly 3. Each stage 7-12 comprises intermeshing rotor components, each being mounted on a respective shaft 13, 14, 15. An exhaust stage 10-12 of the rotor assembly comprises fewer intermeshing rotor components than an inlet stage 7 of the rotor assembly. A reduction in the number of rotors permits the pump to have larger capacity, e.g. three rotor, inlet stages with smaller capacity, e.g. two rotor, outlet stages without excessive reduction in later stage axial length thus controlling power consumption. The inlet stages may have two flowpaths (Fig. 4) with separate inlets 4,5 which later merge.

Description

24 1 7757 Vacuum pump This invention relates to the field of vacuum pumps,

in particular to the field of multi-stage vacuum pumps with a plurality of parallel shafts.

It is known to provide a vacuum pump with a number of parallel shafts located within a pump housing, each shaft carrying a rotor assembly. Such rotor assemblies may comprise a number of rotor components which intermesh with corresponding rotor components on the other shafts during rotation in such a way that a pumping action is achieved. A pump with three shafts has a central rotor assembly intermeshing with two outer rotor assemblies. For a given volume of the pump housing, in comparison to a twin-shafted pump, a triple-shafted pump can increase pump capacity by providing a double flow path for pumped fluid passing through the pump from dual inlets to effectively provide two swept volumes, albeit in fluid communication with one another.

The present invention relates to problems associated with multi-stage vacuum pumps having three or more shafts. As discussed above, triple shafted pumps, with a dual inlet, have a high capacity for a compact pump, since capacity is governed by the fluid volume of the inlet stage which, in turn, is governed by the cross sectional area of the inlet. However, this increased cross sectional area is propagated throughout the length of the pump such that a higher capacity is also to be found at the exhaust stages. Such a high capacity at the exhaust stages leads to a device with a high power requirement, which is generally undesirable.

In view of this, it is desirable to keep the volume of the exhaust stages as small as possible. The capacity of each stage along the length of a pump, approaching the exhaust stages, is conventionally reduced by providing each subsequent stage with a shorter axial length than its upstream neighbouring stage. Since the cross sectional area of the swept volume of a triple shafted pump is increased, when compared to an equivalent twin-shafted pump, this reduction in axial length of each stage as the exhaust is approached needs to be more significant than in conventional twin-shafted pumps. This can lead to rotor and stator components that are particularly thin. Such components are not only weak but also prone to warping either during manufacture or during use. Materials with improved properties may be utilised in manufacture of these components, but generally these materials are expensive. These thin components have the additional disadvantage that fluid leakage experienced about the component begins to dominate the performance of the pumping stage as the quantity of leaked fluid approaches the quantity of fluid being actively pumped by the stage.

It is an aim of at least the preferred embodiment of the present invention to overcome, or at least minimise some of the aforementioned problems.

According to the present invention there is provided a vacuum pump comprising a stator housing a multi-stage rotor assembly, each stage comprising two or more intermeshing rotor components each mounted on a respective shaft, characterized in that an exhaust stage of the rotor assembly comprises fewer intermeshing rotor components than an inlet stage of the rotor assembly.

By reducing the number of shafts and therefore the number of intermeshing rotor components towards the exhaust stages of the pump and thus restricting the cross sectional area of the pumping volume at the exhaust stages, it is possible to achieve a pump having both the benefits of an increased inlet capacity and the benefits of a small exhaust capacity. Since it is the capacity of the exhaust stage that governs the power requirements of the vacuum pump, a pump may be provided with a substantial pumping capacity but without a substantial power consumption requirement.

The inlet stage of the pump may comprise three inlet stage rotor components with the corresponding exhaust stage comprising two rotor components. A single shaft may carry both an inlet rotor component and an exhaust stage rotor component. Alternatively, the pump may comprise a plurality of first shafts carrying the inlet stage rotor components and a plurality of second shafts carrying the exhaust stage rotor components. The first shafts may be driven by a first motor and the second shafts may be driven by a second l motor. The first shafts may carry gears which intermesh with gears carried by the second shafts to cause synchronized motion therebetween.

One shaft may carry rotor components having a plurality of lobes, another shaft may carry rotor components having a plurality of sockets, the shafts being arranged such that the rotor components of one shaft cooperate with the rotor components of the other shaft. One shaft may carry one rotor component having a plurality of lobes and one rotor component having a plurality of sockets.

The invention is described below in greater detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a cross sectional representation of a pump incorporating a pumping mechanism with a triple-shafted inlet stage and a twin-shafted exhaust stage; Figure 2 shows a cross section of the pumping mechanism at A-A in Figure 1; Figure 3 shows a cross section of the pumping mechanism at B-B in Figure 1; Figure 4 shows the flow path through the pump of Figure 1; Figure 5 illustrates an alternative pumping mechanism with an increased number of lobes and sockets; and Figure 6 illustrates an alternative pumping mechanism with a non planar shaft configuration.

A schematic cross sectional view of a vacuum pump 1 is illustrated in Figure 1. The vacuum pump 1 comprises a housing unit 2 which serves as a stator.

The housing unit 2 defines a cavity within which a rotor assembly 3 is located, the rotor assembly 3 cooperating with the stator 2 to achieve a pumping action thus displacing fluid from an inlet 4, 5 of the pump to an exhaust 6 of the pump 1. In this example, the inlets are located on opposite sides of the pump 1.

The rotor assembly 3 comprises a plurality of pumping stages. An inlet stage 7 is provided proximate the inlets 4, 5 together with an exhaust stage 12 proximate the exhaust 6 and a number (here four) of intermediate stages 8, 9, 10,11 positioned therebetween. Each stage comprises a number of intermeshing rotor components, each mounted on a respective shaft 13, 14, 15. Additional details of example stages are given in Figures 2 and 3.

Figure 2 shows a cross sectional view of the inlet stage 7 of the pump 1.

Each of the three shafts 13,14,15 carries a respective rotor component 21, 22,23. The outer rotor components 21 and 23 each comprise a number of lobes 24a, 24b, 24c, 24d. Two lobes are illustrated on each rotor component in this embodiment but any number may be provided. These lobes 24a, 24b, 24c, 24d are configured in such a manner that, upon rotation of each rotor component, say 21, the lobes 24a, 24b cooperate with the housing unit 2 (acting as a stator) to form a cavity 26a into which a quantity of fluid is drawn through the inlet 4. A smaller secondary flow path is provided as fluid from inlet 4 is drawn into a socket 25a of rotor component 22. Through continued rotation of the components the fluid held in cavity 26a is displaced from the inlet 4 of the stage to an outlet conduit 16 (the primary flow path) and the fluid captured in socket 25a is displaced from the inlet 4 to an outlet conduit 17 (the secondary flow path). As the lobe 24b of the rotor component 21 meshes with the socket 25b of the cooperating component 22 fluid contained within cavity 26a is prevented from returning to the inlet 4 and the fluid is forced through the outlet conduit 16. Similarly, as the lobe 24d of rotor component 23 meshes with socket 25a the fluid previously contained within socket 25a is forced out towards outlet conduit 17 and is prevented from returning to inlet 5.

Simultaneously, fluid is drawn into the pumping mechanism through inlet 5 and is passed via cavity 26b and socket 25b to the outlet conduits 17 and 16 respectively. As fluid is passed through outlet conduits 16, 17 it is passed to the next pumping stage 8. This positive displacement of fluid through the pump causes the pump inlet pressure to be reduced.

As indicated in Figure 2, the three shafts 13, 14, 15 are arranged to be rotated such that each adjacent pair of rotor components (for example 21 and 22) contra-rotate with respect to one another. This allows the lobes 24 of one rotor component 21 to intermesh with the sockets 25 of the adjacent rotor component 22. Here, the two outer rotor components 21, 23 each travel in the same rotational direction and intermesh with the central rotor component 22 which is rotated in the opposite direction.

The volume of each of the subsequent stages 8, 9, 10, 11, 12 becomes progressively smaller than that of the inlet stage 7. The reduction in volume is typically achieved by a shortening of the axial length of each subsequent stage. This conventional approach is followed for a number of stages (in this embodiment, stages 8 and 9) after which, a different approach is implemented. Following stage 9, the volume of the latter stages of the pump 1 (in this embodiment 10, 1 1, 12) are reduced by reducing the cross sectional area of the rotor assembly 3. In the illustrated embodiment, this is achieved by truncating one of the outer shafts 15 so that it does not extend along the entire length of the pump 1, as shown in Figure 1. As a result, in each of the latter stages of the pump 10, 1 1, 12 there are only two, rather than three, rotor components. A cross sectional representation of one of the latter stages 1 1 of the pump 1 (taken along line B-B) is illustrated in Figure 3.

The pumping stage illustrated in Figure 3 operates in a very similar manner to that described above in relation to the triple-shafted section of the pump.

There is only a single cavity 36 formed rather than the two cavities in the earlier stages. In order to give a continually decreasing pumping volume along the length of the pump 1, the first twin-shafted stage 10 typically has a similar axial length than the preceding final, tripleshafted stage 9 as illustrated in Figure 1. In this way a reduction in pumping volume of approximately half is achieved such that a fluid volume ratio of approximately 2:1 is achieved from stage 9 to stage 10.

Figure 4 illustrates the fluid flow path that may be used within pump 1. Each illustrated, two dimensional; slice represents a stage within the pump and indicates the primary flow path within that stage. Consequently, for the first three stages 7, 8, 9 two different flow paths are indicated, acting in opposite directions. For the inlet stage 7, as illustrated in Figure 2, the primary flow path defined by rotor components 21 and 22, mounted on shafts 13 and 14 respectively, is in the downward sense, as viewed in these Figures 2 and 4, and is indicated by the dashed line on the corresponding two dimensional slice in Figure 4. The solid line indicated on that same two dimensional slice (acting in the upward sense) represents the primary flow path defined by rotor components 22 and 23 of the inlet stage 7, mounted on shafts 14 and 15 respectively (shown in Figure 2).

In order to minimise the length of porting conduits between adjacent stages in the triple-shafted region of the pump, the fluid paths alternate laterally along the pump 1 such that in one stage the flow path acts in the upward sense and in the next it acts in the downward sense as illustrated in Figure 4. For clarity, the two primary flow paths through the triple-shafted section of the pump are described in more detail as follows.

A first primary flow path (represented by a dashed line in Figure 4) is defined as fluid enters the pump 1 through inlet 4 and is then passed down through stage 7 and into the outlet conduit 16 to stage 8 where fluid is passed up through the stage to conduit 31, which transfers the fluid to stage 9. After passing downwards through stage 9 conduit 32 then passes the fluid from the bottom of stage 9 to the top of stage 10, where it merges with the second primary flow path.

The second primary flow path (represented by the solid line in Figure 4) begins at inlet 5 in stage 7 and fluid is passed upwards through stage 7 to conduit 17 which connects the upper outlet of stage 7 with the upper inlet of stage 8. Fluid is then transferred downwards through stage 8 to conduit 30 which passes the fluid to the bottom of stage 9. Having passed upwardly through stage 9, conduit 33 passes fluid to the top of stage 10, where it merges with the first primary flow path.

A single flow path (represented by the dash/dot line in Figure 4) then continues through the latter stages 10, 11, 12 of the pump 1. In each stage 10, 1 1, 12 the fluid passes downwards through the stage as illustrated in Figure 3 (a schematic representation of stage 11). Porting conduits 34, 35 are then provided between the outlet at the bottom of each stage 10, 11 to transfer the fluid to the inlet located in the upper portion of the subsequent stage 11, 12. The fluid finally passes downwards through the exhaust stage 12 and out of the exhaust 6 of the pump 1.

In summary, since the third shaft 15 is truncated, the flow paths through the latter stages 10,11, 12 of the pump 1 are all in the same direction (here illustrated in the downward sense). Consequently, the porting 32, 34, 35 in this twin-shafted region of the pump 1 is more complex and tortuous in order to pass fluid from the outlet on the lower part of one stage to the next inlet which lies in the upper part of the adjacent stage.

Whilst the earlier figures illustrate rotors with only two lobes and sockets, other configurations are envisaged. Figure 5 illustrates outer rotors 41, 43 having four lobes 44 each and a central rotor 42 with six sockets 45.

The outer rotors may be of different sizes with different numbers of lobes, consequently running at different speeds. Such a configuration provides numerous benefits for example, whilst there is a certain amount of fluid communication along the secondary flow paths, the primary flow path will dominate the bulk transport capacity of any particular stage and therefore, by providing different pumping capacities for respective primary flow paths within a single stage, different pressures can be achieved at each of the inlets. In addition, since at least one of the shafts is rotated at an alternative speed, the pulses generated by the rotors of one shaft no longer coincide with those of the others and hence vibration of the pump is reduced. Furthermore, if the speed of at least one of the rotors is increased, the combined torque required by the pump will be reduced. Where the rotor assembly is driven by this faster shaft, i.e. the motor is directly attached thereto, a smaller motor is required, this in turn brings about the additional benefits of a smaller footprint required to accommodate the motor which allows a greater flexibility in choice of motor technology.

Of primary benefit is provision of a pump with increased capacity without introduction of a corresponding increase in power consumption or volume. An alternatively dimensioned pump may be achieved where the shafts 53, 54, 55 of the rotor assembly 3 are positioned in a non-planar configuration as illustrated in Figure 6. Such a configuration results in a different external envelope of the pump 1 which may be useful in circumstances where space is limited.

Claims (16)

1. Claims 1. A vacuum pump comprising a stator housing a multi stage rotor

   assembly, each stage comprising two or more intermeshing rotor components each mounted on a respective shaft, characterized in that an exhaust stage of the rotor assembly comprises fewer intermeshing rotor components, than an inlet stage of the rotor assembly.
2. 2. A pump according to Claim 1, wherein the inlet stage comprises three rotor components and the exhaust stage comprises two rotor components.
3. 3. A pump according to Claim 1 or Claim 2, comprising a first shaft carrying a first inlet stage rotor component and a first exhaust stage rotor component, and a second shaft carrying a second inlet stage rotor component and a second exhaust stage rotor component.
4. 4. A pump according to any preceding claim, wherein one shaft carries rotor components having a plurality of lobes.
5. 5. A pump according to Claim 4, wherein another shaft carries rotor components having a plurality of sockets arranged to cooperate with the lobes of the rotor components carried by the first shaft.
6. 6. A pump according to any preceding claim, wherein the inlet stage comprises a first rotor component having a plurality of sockets and two further rotor components having a plurality of lobes.
7. 7. A pump according to Claim 6, wherein the first rotor component of the inlet stage comprises two sockets and the two further rotor components of the inlet stage comprise two lobes each.
8. 8. A pump according to Claim 6 or 7, wherein the first rotor component of the inlet stage rotates in one direction and the two further rotor components of the inlet stage rotate in the opposite direction. r
9. 9. A pump according to any preceding claim, wherein the exhaust stage comprises a first rotor component having a plurality of sockets and a second rotor component having a plurality of lobes.
10. 10. A pump according to Claim 9, wherein the first rotor component of the exhaust stage comprises two sockets and the second rotor component of the exhaust stage comprises two lobes.
11. 11. A pump according to Claim 9 or 10, wherein the first rotor component of the exhaust stage rotates in one direction and the second rotor component of the exhaust stage rotates in the opposite direction.
12. 12. A pump according to any preceding claim, comprising additional stages located between the inlet stage and the exhaust stage.
13. 13. A pump according to Claim 12, wherein at least one additional stage comprises the same number of rotor components as the inlet stage.
14. 14. A pump according to Claim 12 or Claim 13, wherein at least one additional stage comprises the same number of rotor components as the exhaust stage.
15. 15. A pump according to any of Claims 12 to 14, wherein a plurality of flow paths defined by the inlet stage merge with a flow path defined by the exhaust stage.
16. 16. A pump according to Claim 15, wherein the merger of the flow paths occurs between two additional stages.