GB1571880A

GB1571880A - Rotary positive-displacement gas pump

Info

Publication number: GB1571880A
Application number: GB2596378A
Authority: GB
Original assignee: Balzers AG
Current assignee: OC Oerlikon Balzers AG
Priority date: 1977-06-06
Filing date: 1978-05-31
Publication date: 1980-07-23
Also published as: NL7708450A; DE2821903A1; CH621854A5; FR2393957A1

Abstract

The Roots pump has two parallel pistons (1, 2) which roll on one another without contact. At least two toothed bars (3) are arranged symmetrically at the perimeter of the main piston (1) and engage corresponding indentations in the counterpiston (2). The compressed gas is expelled, through discharge orifices arranged in the toothed-bar (3) flanks running forward in the delivery direction, via bores (11) in the main piston (1) and via the tubular shaft (12). Close behind the discharge orifices, outlet valves may be provided. The Roots pump can be designed both for dry running and as an oil-sealed pump. A higher pressure ratio and a lower clearance volume than hitherto can be achieved. If the pump is provided with a pre-stage equipped with Roots-type rolling pistons, it can be used economically as a vacuum pump for substantial amounts of gas and be used, for example, instead of the hitherto conventional steam jet ejectors. <IMAGE>

Description

(54) A ROTARY POSITIVE-DISPLACEMENT GAS PUMP (71) We, BALZERS AKTIENGESELL SCHAFT FUR HOCHVAKUUMTECHNIK UND DUNNE SCHICHTEN of FL 9496 Balzers, Principality of Liechtenstein, a company organised and existing under the laws of the Principality of Liechtenstein, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a rotary positivedisplacement gas-pump adapted for use as a vacuum pump or a compressor.

For the removal by suction of large amounts of gas, in particular for the generation of vacuum in chemical and metallurgical engineering, today preferably multi-step steam jet pumps are used. With suitable number of steps, the pressure range from atmospheric pressure to pressures under 10-3 mbars can be covered. They are robust and insensitive to soiling but require that steam of sufficiently high pressure and cooling water for condensation of the steam are available.

The setting up of such a pump system and the control of the various steps in dependence on the induction pressure require, particularly when a separate steam generator is needed, high procurement and installation costs. The operating costs for the energy to be applied and for the water supply and purification of the effluent are also considerable.

In this respect, mechanical pumping systems are more advantageous. However, they have hitherto been considered to only a slight extent. The essential reasons were misgivings about their reliability in the case of high incidence of dirt caused by their complicated structure and the fact that until today no sufficiently large units were available. The parallel arrangement of several pumps was therefore hitherto 'necessary, so that the investment and installation costs were not much lower than in the case of steam jet pumps.With the rising costs of energy and process water, however, the advantages of mechanical pumping systems, i.e. their higher efficiency, their lower requirement of cooling water and their simpler control, are making themselves felt more and more and the solution of the problem of the development of larger units with high reliability in service is therefore becoming increasingly urgent.

When a high specific suction capacity, i.e. a favourable price/performance ratio in the case of rotary pumps is to be achieved, high numbers of revolutions must be possible. These are only achievable if the internal friction losses are small, a requirement which hitherto is best fulfilled by the dry-running boosters of the Roots 'type with rotors which have a figure-of eight curve profile. Since the compression factory of this type of pump is too low against atmospheric pressure, they can however, be used only in combination with other types of pump which have higher compression factors in the case of the working pressures which are customary in chemical engineering.

The compression to atmospheric pressure of the gases pumped by the booster is hitherto effected by water ring pumps or oil-sealed preliminary pumps. Water ring pumps can hardly be any longer considered with today's high process water costs. They also have the disadvantage that, because of their relatively high final pressure, in most cases two-step booster units have to be used. Hence, there remain the oil-sealed rotary piston pumps, rotary vane pumps, multi-valve or trochoidal pumps; because of their high friction losses they must, in order to keep the heating within the admissible limits, but in comparison to rotary pumps with comparable suction capacity, be operated with lower number of revolutions and therefore have a substantially more unfavourable price/performance ratio.

Desirable would be a dry-running pumping system which effects compression to atmospheric pressure, achieves a high compression factor with few steps and can be operated with a high number of revolutions. It should, as far as possible, be of such a nature that all steps are present on one axis in a common housing and only one drive means is necessary. The present invention has set itself the target of solving this problem.

The limitation of the possibility of using boosters of the above described construction as vacuum pump results from the fact that no compression takes place in the interior of the pump. Instead, the volume of gas pumped from the intake side is filled up to the output pressure as soon as it communicates with the output side and must then be pumped out of the pump chamber against this pressure.

Substanitally more compression work is performed than in the case of the previously mentioned pumps with internal compression. Moreover, because there prevails practically continuously at the sealing gaps the full pressure difference between output pressure and intake pressure, the back-flow losses are very high. They lead to a further deterioration of efficiency and impair above all the compression factor at higher pressures. Against atmospheric pressures, therefore, even when the suction capacity goes against zero, a booster of Roots construction only has a compression factor of about 5 and generates such a great compression heat that it cannot be used in continuous operation without an additional cooling arrangement.

The invention is based on the task of finding a way whereby an internal compression to substantially higher compression factors, similarly to the case of the pumps mentioned, becomes possible so that the full pressure difference between output pressure and intake pressure becomes effective for only a short time at the sealing gaps. The gas pump should have no harmful dead space which would lead to the transference of a part of the compressed gases to the intake side (besides the gap losses).

The invention provides a rotary positivedisplacement gas pump adapted for use as a vacuum pump or a compressor and comprising two co-operating rotors, or socalled impellers, rotatable about parallel axes without contact with each other and synchronised by gears, at least two teeth constituted by bars distributed symmetrically on the periphery of one of the impellers and orientated parallel to the said axes, and recesses for receiving the bars provided in the periphery of the other impeller, the gas pump being so designed, that in operation each of the bars in turn delimits a compression space between itself and the said other impeller and compresses the gas which is being pumped and expels it through an opening in that side face of the bar in contact with the compressed gas through a valve arranged immediately downstream of said opening.

Preferably the tooth profile and the cooperating recess profile are so constructed that when the tooth profile reaches a partannular portion of the compression space the two profiles immediately come into engagement with one another and thus the back-flow of the pumped and compressed volume of gas into the following pumped and compressed volume is prevented.

A gas pump according to the invention may be constructed as a dry runner or it may be oil sealed.

In a gas pump according to the invention the depth space, even without oil covering, can be reduced to such an extent that it lies in the order of magnitude of 1000th of the delivery volume and is there fore almost negligible in comparison with the back-flowing gas volume.

The invention also provides a gas pump comprising two or more stages in which at least one of the stages is formed by a single stage gas pump according to the invention. The latter may be combined as outlet stage with a gas pump of the Roots type which has impellers with profiles similar to the figure-of-eight and serves as inlet stage.

If the gas pump is a two-stage pump both the stages may have a common housing, common mounting and gear. In the two-stage or multi-stage pump the impellers of the outlet stage may be mounted on the shaft of the inlet stage. The part of the housing in which the single-stage gas pump according to the invention is situated as one stage may also contain a by-pass 'line for the other stage.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a single-stage vacuum pump, or so-called one-step booster, in section along line A-A in Figure 2, Figure 2 is a section along line B-E in Figure 1, Figure 3 is a section through a tooth-like bar of Figure 1 on a larger scale, Figure 4 shows phases of the compression process, Figure 5 shows a two-stage vacuum pump, or so-called two-step booster system, in section along line A-A in Figure 6, each stage or "step" of which is formed by a one-step booster similar to that shown in Figures 1 and 2, Figure 6 is a section along line B-B in Figure 5, Figure 7 shows a two-step booster system in section along line A-A in Figure 8, the outlet step of which is formed by a onestep booster similar to that shown in Figures 1 and 2, and Figure 9 shows a detail of a toothlike bar and of the recess co-operatiny therewith.

Figures 1 and 2 show a one-step booster according to the invention comprising twc' motors, or so-called impellers 1 and 2 rotatable about parallel axes without driving contact with each other. The ratio of the velocities of rotation of the impellers 1 and 2 is 2: 3. Three tooth? like bars 3 are distributed symmetrically on the periphery of the impeller 1. The tooth-like bars 3 are clamped slots of the impeller with securing bolts 4. The gas is sucked in at 5, compressed in the chamber 6 and forced via bores or longitudinal slots in the end face, which lies in the delivery direction, of the tooth-like bar 3 which bounds the compression chamber.

Figure 3 shows a section through a tooth-like bar 3 on a larger scale. The entrance opening 7 is here covered towards the inside by a semi-cylindrical, thin, resilient valve plate 8 which is secured inside the duct 9 by means of a clamping bar 10.

As soon as the compression in the chamber 6 (see Fig. 1) and subsequently during the entry of the tooth-like bar into the other impeller 2 (see Fig. 1) reaches a higher pressure than in the duct 9, the valve opens. The gas is forced through the bore 11 to the bore 12 in the shaft of the impeller and finally reaches the exhaust 13 (see Fig. 2).

Figure 2 shows the gearing with the gear wheels 15 and 16 accommodated in the space 14 on the left of the pump housing.

It causes the synchronous rotation of the two rotary impellers 1 and 2. The drive is effected via the axle 17.

On the right of the housing is the output chamber 18. The spaces 14 and 18 contain an oil sump. They communicate via the bore 19 (see Fig. 1). By means of centrifugal disks 20 and 21 the oil is transported to the bearings and gear wheels. Further centrifugal disks 22, 23, 24, 25 and baffle plates and oil-repellent rings 26 made of polytetrafluoroethylene such as Teflon (Registered Trade Mark) ensure that no oil can reach the booster chamber. For a still better oil and vacuum sealing, sliding ring seals may also be incorporated at the place where the shaft passes through out of the booster chamber. By means of bores 27 a pressure compensation between the spaces 14 and 18 is achieved.

In Figure 4 are shown various phases of the compression process. The positions a, b, c, d, e, f of the tooth axis correspond to the positions a', b', c', d', e', f' of the appropriate recess in the other impeller.

It is seen that the tooth in position a, as soon as it has reached the end 28 of the annular delivery chamber 6, immediately comes into engagement with the oppositely located recess (position a') so that the gas compressed in chamber 29 can no longer flow back into the chamber 6 which lies behind.

The gas volume 29 is further compressed.

In position e, e' the compression is complete. Up to position f, f', the suction intake chamber 30 is separated from the compression chamber 6 by the tooth flank.

From there on, sealing is again effected by the gap seal of the two rotary impellers at 31.

Until shortly before position e, e', the chamber on the reverse side of the tooth communicates with the chamber 6 in which up to this point in time only a slight compression has occurred so that after position f, f' has been passed no significant amount of gas can be conveyed out of this chamber across to the intake side.

From Fig. 4 is can also be seen that when the booster, upon reaching the final pressure, effects compression to atmospheric pressure, there is reached for only a very brief moment in the positions d, d' to e, e' a high pressure difference between the delivery side and the intake side. The back-ilowing amount of gas is also correspondingly low and the achievable final mmie, vc-ffie uigefob-.

pressure is correspondingly low.

Figures 5 and 6 show a 2-step version of the booster according to the invention.

The intake step 32 and the delivery step 33 are graded in their suction capacity in the ratio of 5 to 1. Figure 5 corresponds to the section AA' in Figure 6 and Figure 6 corresponds to the section BB' in Figure 5.

The vacuum-side main step 32 differs from the l-step version according to Figures 1 and 2 in that at the ejection openings in the tooth-like bars, which here are constructed as slits, no valves are arranged. Only the output ducts 11 are sealed off to the axle with valve 34 which come into action when the pressure between the two steps, e.g. during evacuation of a vacuum container, exceeds atmospheric pressure. As soon as the intake pressure falls below 0.2 bars, these valves remain closed and the gas transportation to the delivery step takes place solely via the bores 35 and 36 through the slits 37 and 38 into the chambers 39 and 40.

From the chamber 39 the gas passes via an interconnecting line 41 (see Fig. 5) in the housing of the main step into the intake chamber 42 (see Fig. 5) of the output-side step 33. The gas, after further compression, again leaves this step via output valves at 43 and finally reaches the output nozzle 45 after passing through the hollow shaft 44 and the chamber 47. Gear space 46 and chamber 47 communicate with one another analogously to the l-step version.

The ratio of the compression chamber volume to the volume of the housing in the case of boosters of this construction is somewhat more unfavourable than in the case of the known boosters with two figure of-eight curved impellers of equal size.

When, in a 2-step pumping system for intake pressures under 10 mbars, it is de sired to achieve an optimum ratio between suction capacity and volume or weight, it is therefore, at least when a very high suction capacity is aimed at, more favour Fable to combine the two types of booster with one another.

Figures 7 and 8 shows such a combination. Figure 7 corresponds to section AA' in Figure 8 and Figure 8 corresponds to section BB' in Figure 7. The larger vacuum-side step 48 has figure-of-eight shaped rotary impellers, whereas the rotary impellers of the output-side step 49 are constructed according to the invention and are mounted on the shaft ends of the rotary impellers of the large step.

The suction capacity of the output-side step is about 8 times smaller than that of the large step. Both steps are accommodated in a one-piece housing. The diversion line, customary in boosters, between the output side and the intake side, and which, as by-pass with a valve which can be adjusted in the contact pressure, serves to prevent an overloading of the drive motor, particularly during the starting-up process, is here included in the housing part of the smaller step, as a result of which inclusion a saving in weight is achieved.

In Figure 8 the gas enters at the intake nozzle 50. It leaves the output chamber 51 at 52 and passes into the housing of the second step 49. Figure 7 shows how the gas entering through 52 passes via the duct 53 to the intake side 54 of this step or, if the pressure difference between the intake side and the output side of the large pump step exceeds the specified value, passes via the valve 55 with adjustable contact pressure (see Fig. 8) back again to the intake side of this step.

The gas compressed to atmospheric pressure in the small step enters, via the valves incorporated in the tooth-like bars 56, into the radial bores 11 leading to the hollow and leaves the hollow shaft through slits 57, passes into the chamber 58 and finally to the output nozzle 59.

It is seen that only prolongation of the housing's middle part and the two rotary impellers of the small step which are mounted on the shafts are necessary in order to produce from a 1-step booster of the Roots type a booster system which effects compression to atmospheric pressure with high efficiency. Neither an additional mounting nor a further gear and drive unit is necessary. Nor does the drive have to be taken through the vacuum-tight manner, since the output chamber lies on this side. In comparison to the hitherto usual combinations of Roots boosters and oilsealed preliminary pumps there result, when it is additionally borne in mind that the interconnecting lines between the boosters are also dispensed with, considerable savings in material and in production costs. Also, the space required is substantially smaller.

A booster according to the invention may, of course, also be used as oil-sealed booster, though the number of revolutions has to be reduced because of the frictional losses. However, through the complete avoidance of a harmful dead space and smaller back-flow losses, there is achieved a still higher compression factor and, hence, a lower final pressure.

The advantage of such an oil-sealed booster in comparison to the known boosters lies in that, merely through its geometry, it is balanced statically and dynamically, that no difficult processing operations are necessary and no surfaces which slide on one another under high pressure at present. It may therefore also be made of light metal without any fear of troublesome wear. It is therefore particularly advantageous, no matter whether with or without oil sealing, to exploit the difference of the expansion coefficients of aluminium alloys and steel to produce only the housing from light metal. The expansion of the rotary impellers produced from steel or grey cast iron is then, despite their temperature which is higher in comparison with the housing, not substantially greater than that of the housing. Consequently, it is possible to allow smaller gap widths and thereby reduce the back-flow losses or load the booster more highly without special measures for cooling.

A compromise between the two types of operation, with and without oil sealing, is obtained when such small amounts of oil are injected that the harmful dead space is only just completely eliminated and the tooth edge is sealed but the lateral sealing gap is not yet filled up. Even then, the compression factor is already considerably increased and relatively high numbers of revolutions are still admissible.

A booster according to the invention may be used not only for vacuum generation but also for the generation of excess pres .sure. The problem mentioned at the begining, which occurs in the case of the use of boosters of the Roots type at higher output pressures, namely the increased compression work through deficient internal compression, also occurs more or less in the known boosters. The proposed novel booster therefore brings important advantages in this field of application too.At high working pressures, however, it is advisible, as shown in Figure 9, to widen the tooth-like bar in the direction of the periphery so that a longer sealing gap between the tooth and the housing is formed, or at least to let a movable sealing strip into the tooth edge which is pressed against the inner surface of the housing by centrifugal force and should therefore be made of a plastics material with selflubricating properties. These embodiments are also particularly suitable for the pumping of large amounts of gas at moderate reduced pressure.

In Figlure 9 the tooth-like bar 60 is pinned to the rotor and secured with screws 61. The gas leaves the compression chamber via the exit slit 62, the bores 63 and the through-passing apertured plate 64 and the valve 65. From there the gas passes into the slit 66 and into the output bores 67 to the centre of the rotary impeller. With the large amounts of gas which are to be pumped, the waste gas lines must have large cross-sections. This is facilitated by the broadening of the tooth profile. For this reason, the valve 65 is expediently projected more towards the inside where a larger valve surface can be accommodated.

The valve may consist e.g. of a rubber slab with fabric insert which is pressed against the apertured plate by centrifugal force. It is secured with a strip in the output duct 66. The dead space is here admittedly larger than in the embodiment according to Figure 3. In the case of small compression factors, such as occur in the case of operation as compressor or in the case of moderate reduced pressure, it does not, however, play any so important a role.

In Figure 9, there is also shown, by broken lines at 69 and 70, the position of the tooth profile and of the oppositely located recess after the output cycle has been completed. It is seen that the compression chamber 6 and the intake chamber 30 are, in each phase, separated by high flow resistances.

WHAT WE CLAIM IS: - 1. A rotary positive-displacement gas pump adapted for use as a vacuum pump or a compressor and comprising two cooperating rotors, or so-called impellers, rotatable about parallel axes without contact with each other and synchronised by gears, at least two teeth constituted by bars distributed symmetrically on the periphery of one of the impellers and orientated parallel to the said axes, and recesses for receiving the bars provided in the periphery of the other impeller, the gas pump being so designed, that in operation each of the bars in turn delimits a compression space between itself and the said other impeller and compresses the gas which is being pumped and expels it through an opening in that side face of the bar in contact with the compressed gas through a valve through a valve arranged immediately downstream of said opening.

2. A gas pump according to Claim 1 wherein the tooth profile and the cooperating recess profile are so constructed that when the tooth profile reaches a partannular portion of the compression space the two profiles immediately come into engagement with one another and thus the back-flow of the pumped and compressed volume of gas into the following pumped and compressed volume is prevented.

3. A gas pump according to Claim 1 or 2 which is constructed as a dry runner.

4. A gas pump according to Claim 1, 2 Wor 3 which is oil-sealed.

5. A gas pump according to Claim 1 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, Figures 1 to 4, or Figures 1 to 4 as modified in accordance with Figure 9, of the accompanying drawings.

6. A gas pump comprising two or more stages in which at least one of the stages is formed by a gas pump according to any one of Claims 1 to 5.

7. A gas pump in which a gas pump according to any one of Claims 1 to 5 is combined as outlet stage with a gas pump of the Roots type which has impellers with profiles similar to the figure-of-eight and serves as inlet stage.

8. A gas pump according to Claim 6 or 7 which is a two-stage pump and in which both stages have a common housing, com

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. gap is not yet filled up. Even then, the compression factor is already considerably increased and relatively high numbers of revolutions are still admissible. A booster according to the invention may be used not only for vacuum generation but also for the generation of excess pres .sure. The problem mentioned at the begining, which occurs in the case of the use of boosters of the Roots type at higher output pressures, namely the increased compression work through deficient internal compression, also occurs more or less in the known boosters. The proposed novel booster therefore brings important advantages in this field of application too.At high working pressures, however, it is advisible, as shown in Figure 9, to widen the tooth-like bar in the direction of the periphery so that a longer sealing gap between the tooth and the housing is formed, or at least to let a movable sealing strip into the tooth edge which is pressed against the inner surface of the housing by centrifugal force and should therefore be made of a plastics material with selflubricating properties. These embodiments are also particularly suitable for the pumping of large amounts of gas at moderate reduced pressure. In Figlure 9 the tooth-like bar 60 is pinned to the rotor and secured with screws 61. The gas leaves the compression chamber via the exit slit 62, the bores 63 and the through-passing apertured plate 64 and the valve 65. From there the gas passes into the slit 66 and into the output bores 67 to the centre of the rotary impeller. With the large amounts of gas which are to be pumped, the waste gas lines must have large cross-sections. This is facilitated by the broadening of the tooth profile. For this reason, the valve 65 is expediently projected more towards the inside where a larger valve surface can be accommodated. The valve may consist e.g. of a rubber slab with fabric insert which is pressed against the apertured plate by centrifugal force. It is secured with a strip in the output duct 66. The dead space is here admittedly larger than in the embodiment according to Figure 3. In the case of small compression factors, such as occur in the case of operation as compressor or in the case of moderate reduced pressure, it does not, however, play any so important a role. In Figure 9, there is also shown, by broken lines at 69 and 70, the position of the tooth profile and of the oppositely located recess after the output cycle has been completed. It is seen that the compression chamber 6 and the intake chamber 30 are, in each phase, separated by high flow resistances. WHAT WE CLAIM IS: -

1. A rotary positive-displacement gas pump adapted for use as a vacuum pump or a compressor and comprising two cooperating rotors, or so-called impellers, rotatable about parallel axes without contact with each other and synchronised by gears, at least two teeth constituted by bars distributed symmetrically on the periphery of one of the impellers and orientated parallel to the said axes, and recesses for receiving the bars provided in the periphery of the other impeller, the gas pump being so designed, that in operation each of the bars in turn delimits a compression space between itself and the said other impeller and compresses the gas which is being pumped and expels it through an opening in that side face of the bar in contact with the compressed gas through a valve through a valve arranged immediately downstream of said opening.

3. A gas pump according to Claim 1 or 2 which is constructed as a dry runner.

4. A gas pump according to Claim 1, 2 Wor 3 which is oil-sealed.

mon mounting and gearing.

9. A gas pump according to Claim 6, 7 or 8 in which the impellers of the outlet stage are mounted on the shaft of the inlet stage.

10. A gas pump according to Claim 6, 7, 8 or 9 in which the part of the housing in which the gas pump according to any one of Claims 1 to 5 is situated as one stage also contains a by-pass line for the other stage.

11. A gas pump according to Claim 6 or 7 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, Figures 5 and 6 or Figures 7 and 8 of the accompanying drawings. - ~