US2906448A - Roots type vacuum pumps - Google Patents

Description

Sept 29, 1959 A. LORENZ 2,906,448

ROOTS TYPE VACUUM PUMPS Filed Oct. 25, 1955 United States Patent O 2,906,448 Roors TYPE VACUUM PUMPS Albert Lorenz, Hanau, Germany, assignor to W. C. Heraus G.m.b.H., Hanau, Germany, a German body corporate Application October 25, 1955, Serial No. 542,566

Claims priority, application Germany October 28, 1954 4 Claims. (Cl. 230-210) The present invention relates to vacuum pumps of the Roots type which are suitable for operation at pressures of less than about 20 mm. Hg.

It is an object of the invention to produce a vacuum pump of Vthis type in which heating of the pump parts that come into contact with the compressed gas is` avoided.

It is another object of the invention to provide such a pump in which leakages are minimized or prevented.y

It is yet another object of the invention to provide such a pump in which the deleterious eects of vibration and gas oscillation is minimized or substantially prevented.

Other objects and advantages of the invention will become apparent in a perusal of the specification which follows.

In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which show schematically two embodiments thereof, and in which- Fig. 1 shows a cross-section through a Roots type vacuum pump and including a first embodiment of the invention,

Fig. 2 shows a portion of the discharge zone of the pump shown in Figure l andincluding a second embodiment of the invention, and Fig. 3 shows a portion of the inlet zone of the pump of Fig. 1 illustrating a further modification of the invention. 'l

Referring to the drawings, the characteristic feature of a Roots pump is a pair of hollow parallel pistons (sometimes also'called rotors or impellers) 14 and 15'which rotate continuously within a sealed casing in opposite 2,906,448 Patented Sept. 29, 1959 ice chamber of the piston 15 has been opened into the discharge space 2 on the pressure side. Actually the major part of the energy required by the pump is used for work` which the pistons continuously have to perform against the pre-vacuum pressure; ybut the heat is developed at the expense of the energy provided by the motor and consumes a large proportion compared, for example, with the gas supply. Most of this reaction takes place in the chamber 2 in front of the exhaust side. On the other hand, other actions, such as the washing action of sluiced through gases or expansion in the interval between pressure strokes due to a lower gas density and consequently lower heat capacity do not provide any significant cooling to balance the heating.

The operation of Roots pumps in vacuum is therefore inseparable from the considerable heating of the pump parts which come into contact with the compressed gas: these parts comprise the pistons 14 and 15 and the pump casing. By uneven heat expansion of the various pump parts troubles may occur in various ways such as, for example, leakages in the circulating passages or even obstruction of the pistons.

Finally, in this type of Roots pump, it is also important to observe that at the dischargeside 2 a comparatively large part of the gas volume oscillates upwards and downwards between the pistons and the pump casing in the operative chamber which opens four times during one rotation. At the time of the operation, as depicted in Figure 1, for example, one volume of gas enters the operative chamber 2a which is partly limited by the piston 15. Immediately afterwards the .operative chamber 13 is filled with gas which, to a large extent, issues from the chamber 2a when the chamber 13 is open towards the pressure side. vThis gas, however, is already warmed up `as a result of the previously produced compression and the effect of back pressure. Effective cooling of the piston surfaces is also prevented in this way.

The invention provides means to overcome these drawbacks in the operation of Roots pumps in a vacuum by which, on the one hand, complete vacuum tightness of the pumps is ensured under all circumstances, everi directions, as indioatedby the direction of the arrows,

the pistons being in contact whereby the surfaces thereof carry out a rolling movement on each other. The pistons cause if more wings were used in the construction, the 1 volume of the operative chambers 13 between the pistons and the inside walls of the casing would be decreased. Compared with operation at normal pressures for the supply of gases, the Roots vacuum pumps of this 'invention must produce and maintain comparatively high compression ratios. This requires rotation of the pistons-at speeds in excess of 1,000 r.p.m. and preferably up to 3,000 r.p.m. to obtain good performance.

Since Roots pumps operate without valves, in operation'the full dilerence in pressure between the chamber 2 on the pressure side and the suction 4side with its inlet port 1 continuously loads the pistons 14 and 15, and to overcome this the pistons have to do work. With each rotation the gas column on the pressure side 2 undergoes four pressure strokes at the time when the ga-s supplied to the operative vchamber is freed-towards the pressure side 2. Gas at the higher pressure rebounds from the pressure side into the opening oper-ative chamber and thereby undergoes heating. Figure 1 shows thel pump at" thev momenti-when the operative with considerable heating, and mechanical iniluences dueto the oscillation and compression of the gas, but, on the other hand, the heating up of the gas on the compression side and also the effects of the pump parts coming into contact with it are .counteracted These means comprise constructing the pump casing with two walls, the outer wall or casing 10 of which is highly vacuum-tight. These means, moreover, maintain a pressure within the chamber 8 which is considerably less than. that of the external atmosphere, the chamber 8 being dened by the outer wall or casing 10 and the inner wall or casing 12. Also, a cooling device for the supplied gas `is built into the discharge opening or outlet 2 of the inner casing 12.

As jshown in Figure v1, the pump casing which is :of somewhat oval section has, for example, two walls consisting of an inner wall 12 and an outer wall 10. The casing is connected to the space to `be evacuated or to further intermediate high vacuum pumps by means of flanges 1a at `the induction opening or inlet 1 of the inner casing 1. On the pressure side, the Roots pump is connected by the flange 11 directly to the prevacuum pumps or an intermediate connecting line 9. It isproposed that the intermediate chamber 8, which is dened by the two walls 10 and 12 of the casing, be subjected to a pressure which does not greatly exceed the pressure at the discharge side 2 which is. about 20 mm. Hg. For this purpose it is either connected directly to the pressure side of the pump, as shown in Figure 1-through -the passage 7 or, 'if a still lower presvacuum pumps, the'pressure at thepres'sure-sideorl outi let ofthe pump of the invention'will be-inth'e'neighbor'-l hfood of 20 mm. Hg. In this manner, the dierenceV in pressure between the outside a'trx'io's'phereVv and the pump' operative chamber is sealed off oris completely transferred to the outer casing, leadingto'improvedvacuumv It is, however, m'uch easier to make" this vacuum-tight without too much mechanical difficulty. In-

tightness.`

this way, it is ensured that the pump itself, which is' surrounded by the inner casing, is"V working ina low pressure chamber. The o'w conditionsin a'- vacuum' at a pressure of less than 2O mm.Hg prevents,V under all circumstances, the influx o'fany appreciable amount of gas from the intermediate chamber' 8 into the inner pump operative chamber 13' or into the suction' side 1 at shafty passages and axle bearings for theA piston drive. It is much easier to make theoter wall 1'0 completely Vacuumtight because in this case all moving parts of the pump can be placed within the outer casing;l This refers preferably to the pumpdriving motors (noti shown), which are arranged in the front side portion ofthe intermediate chamber 8: The gears 17 and 16 which primarily maintain the parallel running of the pistons 14 and 15 are also located on the inner side of the' intermediate chamber S at the frontr side walls of the inner wall 12. Deletei-iousA vibrations are avoided if the rotor of the drivingv motor or motors1 with single drive of the piston are directly overhn'ngon the shafts of the piston axes. remains free from shaft openings, the' sealing of which otherwise would require special care which-,- in View of the compression and heating effects referred to during? the operation as a vacuum pump, would require very considerable precautions.

Finally, an effective improvement is the proposed cooling of the compressed gas stream on the pressure side'. An advantageous embodiment of the cooling device to be included at the discharge side 2 consists, for ex-- ample, of a set of parallel honey-comb or tubular' cooling ribs 4 located parallel to the gas stream together with al cooling tube system 3 with inlets and outlets 5 and 6; the latter take up the developed compression heat. They should be of such size that their whole surface is greater than that of the pistons and that of the casing walls to make the cooling effect as intensive as possible. The cooling device can be connected in series with the water cooling system of the motor so that it is not visible from the outside. It may extend into the outlet Z to a position such that the rotation of the pistons is not impeded. Compared with other pumps, the increase in weight is small. The measures to be taken for cooling the interior can further be completed, for example, by an actual cooling of the rotating pistons 14 and 15' themselves. The latter may, for example, be hollow and cooling Water beY conducted to and from the bores 14a and 15a by suitable means (not shown).

The construction of the Roots pump can be further improved, as shown in Figure l, for example, by adding to `the inner wall 12 on the ydischarge side 2 a special prismatic attachment or bafe 1S in such a position as just to avoid the pistons and to leave at least two slots 22 and 23, one for each piston for the passage of the g'as to be supplied: these slots are preferably symmetrically located with respect to the plane of syr'nmetry passing through the inlet 1 and the outlet 2. In this manner, both surfaces 12a and 12b of the inner wall are closed at the discharge side against the slots. The oscillation of a noncooled but strongly warming up part of the snpplied gas stream is therefore interrupted, and this part is supplied to the connecting member 9 under all circumstances through the cooling system.

In'thismanner the outer wall 10 .4 Only cooled gas is able to pass back into the opening operative chambers by way of the cooling system. The cooling eiect thereby becomes more intensive. The symmetrical additional member, which is advantageously made from good heat conducting material, will also cause a reduction of the sectional-outlet 2 compared with the width of the inlet 1 which is" adjusted to the pressure conditions.

The cooling insert with the cooling ribs 4 is advantageously directly fixed to the flanges of the connecting passage 9.

Figure 2 shows a section of an advantageous combination of the cooling system= with an insert member having bafiles in the form of ribs, plates or honeycomb shaped cooling surfaces 19, 20 and 21, which have such a depth that they project into the inside of the pump up to the permitted limit, i.e. so that they project as far as possible into the interior of the pump but outside the path of' rotation of the pistons. In thisl way a large number of pasagewaysis left free but, nevertheless, oscillation of the gas in the' outlet 2 is prevented. The inner ends` 30 of the lateral coolingA surfaces are advantageously symmetrically bent slightly towards the outside inthe direction ofthe gas ow so thatthe resis-v tance to flow is small.

Owing to the cooling arrangements, vacuum pumps of the Roots type constructed according to the invention are able to work continuously against a higher pre-vacuum pressure compared with pumps; without such cooling' arrangements.

They can be used successfully as parts of pump aggregates in all ranges of pressurev below 2O mm. Hg and therefore canl replace all vacuum pumps hitherto known. As part of a two-stage aggregate with a prevacuum pump on the pressure side, Roots pumps according tothe invention can attain an end vacuum of 10-3 mm. Hg. They thus have, compared with the latter, the advantage that no rediffusion of fuelgases inthe spaces to be evacuated can occur because it is notl necessary with these Roots pumps to use lubricating and sealing means for the pistons and the other parts which come into contact with the gas. Roots pumps constructed in accordance with the invention may also be built in two or more stages. It is then possible mutually to match their suction capacities and sizes.

Y With the evacuated space between the double walls of the Roots pumps of this invention, cooling of the pump inner casing by heat loss to the atmosphere is reduced, and the pump casing and impellers tend to remain at substantially the same temperature, thereby reducing the problem of varying clearances during pump operation. The' heat exchanger at the pump outlet provides the necessary cooling to prevent the pump from overheating. In addition to the two features of double walls with an evacuated space in between, and cooling means at the pump outlet, the pumps may also have certain other improvements which are of some importance for reliable operation under vacuum conditions.

I claim:

1. In a vacuum pump of the Roots type, the combination which comprises an inner casing which defines a pumping space, the inner casing having an inlet and an outlet, lobed intermeshing rotors mounted in the inner casingY and in said pumping space, -an outer casing disposed aronnd the inner casing and separated from it by a chamber, a conduit connected to the chamber for evacuating it, cooling means mounted in the outlet of the inner casing, and means including at least one bathe adjacent the inner casing outlet and extending into the pumping space closely adjacent the path of the rotors to directa substantial portion of the gas volume that oscillates between the rotors and the inner casing through the cooling means.

2. In a vacuum pumprof the Roots type, the combination which comprises an inner casing which denes a pumping space, the inner casing having an inlet and an outlet, lobed intermeshing rotors mounted in the inner casing and in said pumping space, an outer casing disposed around the inner casing and separated from it by a chamber, a conduit `connected to the chamber for evacuating it, cooling means mounted in the outlet of the inner casing, a prismatic baille symmetrically located in the inner casing outlet and having two sides extending into the pumping space closely adjacent the path of the rotors, and a third side partially blocking the outlet while leaving two slots between the interior of the inner casing and the outlet to direct a substantial portion of the gas volume that oscillates between the rotors and the inner casing through the cooling means.

3. In a vacuum pump of the Roots type, the combination which comprises an inner casing which delines a pumping space, the inner casing having an inlet and an outlet, lobed intermeshing rotors mounted in the inner casing and in said pumping space, an outer casing disposed around the inner casing and separated from it by a chamber, Ia conduit connected to the chamber for evacuating it, cooling means mounted in the outlet of the inner casing, and means including a plurality of spaced bafes vadjacent the inner casing outlet and extending into the pumping space closely adjacent the path of the rotors, the bales being disposed to dene spaces between them extending substantially parallel to gas dow from the pistons yso as to direct a substantial portion of the gas Volume that oscillates between the rotors and the inner casing through the cooling means.

4. In a vacuum pump of the Roots type the combination which comprises a casing which denes a pumping space, the casing having an inlet and an outlet, lobed intermeshing rotors mounted in the casing and in said pumping space, cooling means mounted in the outlet of the casing and means including -at least one bathe, at least a portion of said baffle extending over the casing outlet and into the pumping `space closely adjacent the path of the rotors to direct a substantial portion of the gas volume that oscillates between the rotors and the casing through the cooling means.

References Cited in the iile of this patent UNITED STATES PATENTS 1,038,075 Berrenberg Sept. 10, 1912 1,148,809 White Aug. 3, 1915 1,905,521 Steiner Apr. 25, 1933 2,107,031 Evans Feb. 1, 1938 2,205,984 Kromas June 25, 1940 2,243,874 Lysholm June 3, 1941 2,489,887 f Houghton Nov. 24, 1949 2,578,305 Huet Dec. 11, 1951 2,667,046 Densham Jan. 26, 1954

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