GB1570266A

GB1570266A - Roots blowers

Info

Publication number: GB1570266A
Application number: GB1335078A
Authority: GB
Original assignee: Leybold Heraeus GmbH
Current assignee: Balzers und Leybold Deutschland Holding AG
Priority date: 1977-06-04
Filing date: 1978-04-05
Publication date: 1980-06-25
Also published as: DE2725299A1; CH628709A5; FR2393174A1

Abstract

The Roots pump has a suction chamber (2) with two pistons (3) and at least one oil-filled side chamber (7) which is divided from the suction chamber by a partition (5). Supported in the partition (5) there are the piston shafts (4). In order to prevent oil droplets and oil mists from crossing into the suction chamber (2) through the shaft bearings (10), a pressure equalisation duct (13) is provided, whose suction chamber-side mouth is situated at the point where the shaft (4) passes through. Part of the pressure equalisation duct may run outside the partition (5) and pass through a filter (17). <IMAGE>

Description

(54) ROOTS BLOWERS (71) We, LEYBoLD-HERAEUs GmbH & Co. Kommanditgesellschaft, a German Company of Bonner Strasse 504, 5, Koln- Bayental, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may Ibe granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to compressiblefluid rotary pumps of the Roots blower type, Roots blowers can be used both as vacuum pumps and as compressors. A roots blower comprises two co-operating impellers arranged with their rotation axes parallel in a pumping chamber and rotated in opposite directions.The impellers are mounted on shafts which pass through respective apertures in the side wails of the pumping chamber. The impellers are synchronised by gear wheels located in one of two side chambers contiguous with the outer surfaces of the side walls of the pumping chamber.

The side chambers are partially filled with oil, which serves to lubricate ;the pair of gear wheels and also ;the shaft bearings. The impellers do not contact each other or the peripheral wall of the pumping chamber.

In contrast to other rotary positivedispiace- ment vacuum pumps and compressors, no oil or other liquid is used as a sealing means in a Roots blower. Roots blowers therefore serve as oil-free or "dry running" pumps.

The seals between the side chambers and the pumping chamber of a Roots blower can be formed by conventional means, such as oil splash rings, combined with diaphragm glands and/or labyrinth seats, so that during steady suction and compression states there is practically no oil flow from the side chambers to the pumping chamber. When, however, Roots blowers having such cun- ventional seals do not operate under steady suction or compression states, i.e. if the suction pressure and/or the compression end pressure undergo fluctuations with time, then an oil flow from the side chambers to the pumping chamber can no longer be reliably prevented.A lowering of the suction pressure and also of the compression end pressure leads to a reduction in 'the pressure in the apertures through which the shafts pass from the pumping chamber to the side chambers. The pressure in the side chambers attempts to follow the pressure in the pumping chamber, and accordingly relatively high rates or speeds resulting in an entrainment of oil droplets and oil mist in the pumping chambers may occur in the con necting regions between the side chambers and the pumping chamber. This has a particularly unfavourabie effect since the gas flowing out from a side chamber to the pumping chamber has to pass through the bearing.The bearing is as a rule well lubricated, and accordingly the gas passing there- through necessarily absorbs oil droplets and oil mist.

The oil overflow in the case of nonsteady state operation can be considerably reduced by additionatly providing pressure equalising ducts between the pumping chamber and side chambers, as proposed in German Offrniegungsschrift 1 939 717. The two pressure equallsation ducts provided there extend from the interior of the respective side chambers to the region of the suction cross-section. The conductance of such pressure equalising 'ducts must be adjusted condsiderably higher than the conductance of the shaft seal.This oplerating arrangement in fact has the advantage that the oil over flow can be considerably redunced in the case of non-steady state operation. In the case of steady state operation this solution to the problem is, however, worse than the solution without any pressure eluiilsation Since different pressures prevail in the pumping chamber at the site lolf the aperture in the side wail' and at the opening of the pressure equalising 'duct, a permanent gas flow through the side chambers occurs m the case of steady state operation. The gas flowing through the side chambers becomes saturated with oil vapour, is enriched with oil mist, and returns to the pumping icham- ber.Especially when operating as a vacuum pump, undesirably high oil concentrations in the conveying medium are easily achieved in ;this in Roots blowers. There is also an undesirably high oil consumption under cettq n c3nditicns.

The object of the invention is thus to provide an improved cc,mpressible-uid rotary pump of the Roots blower type which avoids the disadvantages of the previously known solutions: which, for reasons of simple and cheap manufacture, does not involve the provision of other sealing elements; and which, for reasons of freedom from wear and tear and on account of the thigh service life, allows the preferred conventional use of a contactless seal via diaphragm glands and labyrinth seals to be continued.

In accordance with the invention this object is achieved in a compressible-fluid rotary pump of the Roots blower type as defined hereinbefore and comprising a pressure equalising duct interconnecting the pumping chamber and one of the side chambers and openwig into the pumping chamber at, or in the immediate vicinity of, one of said apertures in the respective side wall.

These measures ensure ithat in the case of pressure equilisation processes between a side chamber and the pumping chamber, the gas and air flows do not pass through the bearing and the other regions of the apertures filled with oil miist. In addition, there can be no pressure difference between the aperture and the opening of the pressure equalising duct into the pumping chamber, resulting in a constant flow through the side chambers.

A particularly favourable arrangement is one where the pressure equalizing duct opens directly out of the aperture at the pumping chamber end in the side walli.

Further advantages and details of the invention will be described in more detail with reference to the accompanying drawings comprising Figures 1 to 3 each of which shows digrammaitically in section only one side of a compressible-fluid rotary pump of the Roots blower type as defined hereinabove embodying the invention.

Teach embodiment of Roots blower shown comprises a housing 1 which contains a pumping chamber 2 in which are located two impellers 3 (only one is visible) mounted on a shaft 4. The shaft 4 of the visible impeller 3 is supported in bearings 10 in a partition or side wall 5 which separates the pumping chamber 2 from a side chamber 7 partially filled with oil. Housing caps 8 a secured to the side wall 5, in a manner not sown in more detail, in order to form the side chamber?. In each case only the sodled "gear sliide" side chamber 7 iaillY filled with oil is shown.This can be oosed by the fact that the end of the impeller shaft 4 proieccing into the chamber 7 is provided with a toothed gear 9. This toothed gear drives the second impeller of the pump via a further toothed gear, not shown. The pair of toothed gears also serves to synchronise the two impellers.

Also provided in the side wall 5 around the impeller shaft 4 are an oil splash ring 11 and a combined diaphragm gland and labyrinth seal 12 which prevent oil penetrating into the pumping chamber 2.

According to the invention a pressure equalizing duct 13 interconnects the pumping chamber 2 and the side chamber 7 and opens into the pumping chamber 2 at, or in the immediate vicinity of, the pumping chamber end of the aperture, or so-called leadithrwgh, for the shaft 4 in the side wall 5. In the embodiments the pressure equalising duct 13 opens directly out of an annular duct 14 in the side wall 5 at the pumping chamber end of the aperture. The annular duct 14 is formed by an element 15 mounted on the shaft 4, and which is also a part of the gland and seal 12.

In the embodiment according to Figure 1 the pressure equalising duct 13 is formed by bores in the side wall 5. The opening into the side chamber 7 is provided with a protective cap 16 which prevents woil splashes reaching the Iduot 13.

In the embodiment according to Figure 2 the presure equalizing duct is formed partly by an external pipe and partly by a bore in the side wall 5. Such an embodiment has the advantage that la device for separat ing damaging constituents-as a rule oil vapours-from the air passing from side chamber 7 into the pumping chamber 2, can be employed in the external part d the pressure equalising duct 13.

Such an embodiment is shown in Figure 3. In this case a filter generally denoted by 17 is used in the external pant of the pressure equalising duct 13. Such a filter may, for example, act chemically, and be in the form of a zeolite trap or an activated charcoal filter.

As has previously been mentioned, only one side section of the entire pump is shown in each filter. The s pposi,te side, not illus trated, is formed essentially as a mirror image of the side shown with respect to the bearing of the shaft 4 and the pressure equalising duct 13. One pressure equalising duct 13 is conveniently allocated to each of the total of four shaft lead-throughs.

WHAT WE CLAIM 1S:- 1. A compressible-fluid rotary pump of ithie Roots blower type as defined hereinbefore and comprising a pressure equalising duct interconnecting the pumping chamber and one of the side chambers oDeninV into the pumping chamber at, or in the immediate vicinity of, one of the said apertures in the respective side wall.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. in ;this in Roots blowers. There is also an undesirably high oil consumption under cettq n c3nditicns. The object of the invention is thus to provide an improved cc,mpressible-uid rotary pump of the Roots blower type which avoids the disadvantages of the previously known solutions: which, for reasons of simple and cheap manufacture, does not involve the provision of other sealing elements; and which, for reasons of freedom from wear and tear and on account of the thigh service life, allows the preferred conventional use of a contactless seal via diaphragm glands and labyrinth seals to be continued. In accordance with the invention this object is achieved in a compressible-fluid rotary pump of the Roots blower type as defined hereinbefore and comprising a pressure equalising duct interconnecting the pumping chamber and one of the side chambers and openwig into the pumping chamber at, or in the immediate vicinity of, one of said apertures in the respective side wall. These measures ensure ithat in the case of pressure equilisation processes between a side chamber and the pumping chamber, the gas and air flows do not pass through the bearing and the other regions of the apertures filled with oil miist. In addition, there can be no pressure difference between the aperture and the opening of the pressure equalising duct into the pumping chamber, resulting in a constant flow through the side chambers. A particularly favourable arrangement is one where the pressure equalizing duct opens directly out of the aperture at the pumping chamber end in the side walli. Further advantages and details of the invention will be described in more detail with reference to the accompanying drawings comprising Figures 1 to 3 each of which shows digrammaitically in section only one side of a compressible-fluid rotary pump of the Roots blower type as defined hereinabove embodying the invention. Teach embodiment of Roots blower shown comprises a housing 1 which contains a pumping chamber 2 in which are located two impellers 3 (only one is visible) mounted on a shaft 4. The shaft 4 of the visible impeller 3 is supported in bearings 10 in a partition or side wall 5 which separates the pumping chamber 2 from a side chamber 7 partially filled with oil. Housing caps 8 a secured to the side wall 5, in a manner not sown in more detail, in order to form the side chamber?. In each case only the sodled "gear sliide" side chamber 7 iaillY filled with oil is shown.This can be oosed by the fact that the end of the impeller shaft 4 proieccing into the chamber 7 is provided with a toothed gear 9. This toothed gear drives the second impeller of the pump via a further toothed gear, not shown. The pair of toothed gears also serves to synchronise the two impellers. Also provided in the side wall 5 around the impeller shaft 4 are an oil splash ring 11 and a combined diaphragm gland and labyrinth seal 12 which prevent oil penetrating into the pumping chamber 2. According to the invention a pressure equalizing duct 13 interconnects the pumping chamber 2 and the side chamber 7 and opens into the pumping chamber 2 at, or in the immediate vicinity of, the pumping chamber end of the aperture, or so-called leadithrwgh, for the shaft 4 in the side wall 5. In the embodiments the pressure equalising duct 13 opens directly out of an annular duct 14 in the side wall 5 at the pumping chamber end of the aperture. The annular duct 14 is formed by an element 15 mounted on the shaft 4, and which is also a part of the gland and seal 12. In the embodiment according to Figure 1 the pressure equalising duct 13 is formed by bores in the side wall 5. The opening into the side chamber 7 is provided with a protective cap 16 which prevents woil splashes reaching the Iduot 13. In the embodiment according to Figure 2 the presure equalizing duct is formed partly by an external pipe and partly by a bore in the side wall 5. Such an embodiment has the advantage that la device for separat ing damaging constituents-as a rule oil vapours-from the air passing from side chamber 7 into the pumping chamber 2, can be employed in the external part d the pressure equalising duct 13. Such an embodiment is shown in Figure 3. In this case a filter generally denoted by 17 is used in the external pant of the pressure equalising duct 13. Such a filter may, for example, act chemically, and be in the form of a zeolite trap or an activated charcoal filter. As has previously been mentioned, only one side section of the entire pump is shown in each filter. The s pposi,te side, not illus trated, is formed essentially as a mirror image of the side shown with respect to the bearing of the shaft 4 and the pressure equalising duct 13. One pressure equalising duct 13 is conveniently allocated to each of the total of four shaft lead-throughs. WHAT WE CLAIM 1S:-

1. A compressible-fluid rotary pump of ithie Roots blower type as defined hereinbefore and comprising a pressure equalising duct interconnecting the pumping chamber and one of the side chambers oDeninV into the pumping chamber at, or in the immediate vicinity of, one of the said apertures in the respective side wall.

2. A Roots pump according to Claim 1,

wherein the opening of the pressure equalising duct opens directly out of the aperture at the pumping chamber end in the said side wall.

3. A Roots pump according to Claim 1 or 2, wherein the pressure equalising duct ds formed by bores in the side wall.

4. A Roots pump according to Claim 1 or 2, wherein a part of the pressure equalising duct is formed by an external pipe.

5. A Roots pump according to Claim 4, wherein means for separating gaseous, vapour or liquid components from the pas- sing air is employed in the external part of the pressure equalizing duct.

6. A compressible4Iuid rotary pump of the Roots blower type substantially as hereinbefore described with reference to any one of the Figures of the accompanying draw ings.