GB2178485A

GB2178485A - Method for compressing a gaseous flow media in a roots compressor

Info

Publication number: GB2178485A
Application number: GB08613999A
Authority: GB
Inventors: Hans Nissen
Original assignee: Aerzener Maschinenfabrik GmbH
Current assignee: Aerzener Maschinenfabrik GmbH
Priority date: 1985-07-30
Filing date: 1986-06-09
Publication date: 1987-02-11
Also published as: ES8706905A1; IT8620951A1; FR2585777B1; GB8613999D0; DE3527292C2; IT1191824B; IT8620951A0; FR2585777A1; ES556163A0; DE3527292A1; DD245929A5; GB2178485B

Abstract

In order to eliminate delivery-side pulses in a Roots compressor to the same degree for any rotational speed of the compressor, flow medium is introduced in a controlled manner from the delivery side D into the flow chambers formed by the pistons 1 and the cylinder wall 3, after closure with respect to the suction side, so that sub-pressure pulses are produced in the delivery-side flow chamber and the wave produced by these sub-pressure pulses cancels out the wave produced by over- pressure pulses caused on entry of the piston head of the one piston into the reduced-diameter region of the other piston. <IMAGE>

Description

SPECIFICATION Method for compressing gaseous flow media and a roots compressor for carrying out the method The invention relates to a method for the compression of gaseous flow media with a Roots compressor, in which flow medium is introduced, in a controlled manner, from the delivery side alternately into the flow chambers formed respectively by the Roots piston and the associated portion of the cylinder wall, after closure relative to the suction side.

The invention also relates to a Roots compressor for carrying out this method.

In Roots compressors, the general problem arises that a part of the flow medium which has already been compressed is returned to the flow chamber, as the flow chamber becomes opened to the delivery side. This reversal of flow leads to marked fluctuations both in or on the Roots compressor and also in the associated pipeline system of the delivery side.

Attempts have already been made to minimise these fluctuations, by introducing the flow medium from the delivery side into the flow chamber before it is opened to the delivery side (see for example DE-PS 1133500, DE-AS 1258543; Bulletin of JSME Vol. 24 No. 189 March 1981 pages 547-554). By the measures proposed, return of the flow medium from the delivery side into the flow chamber is not completely avoided, so that, with these known proposals, it is only possible, at best, to reduce the occurrence of such fluctuations.

Also, it is already known from US-PS 421 5977 that the pulses or fluctuations produced in the delivery-side flow duct, caused in the compression of gaseous flow media by Roots compressors, can be further reduced, if the portion of the flow medium required for compression is continuously and steadily removed from the flow medium supplied to the delivery side, before its introduction into the flow duct. In this, it is particularly important to maintain the operating conditions extremely exactly, but this is generally not possible in practice. A subsequent adaption to the operating conditions existing in practice is not possible with compressors of this kind.

A Roots compressor with which an adaptation to various operating conditions is possible, without great technical resources and expensive modifications, is known from DE-OS 3238015. In this type of Roots compressor, bar-like adjusting plates are moved in recesses on the front face of the supply chamber. With such a construction, a variation in the area opening function with time can be achieved in a simple way, while maintaining the opening characteristics determined over the angle of rotation in dependence upon the depth of insertion of the adjusting plates.

Thus, one and the same machine can be adapted easily to different operating conditions, without great technical resources and without expensive modifications. This is especially important if an adaptation of the compressor to special circumstances of the user is required, since the set operating data frequently do not agree with the required operating data in practice.

Measurements have now shown that, in using the solution set out in DE-OS 3238015 the effects achievable by the use of adjusting plates, in relation to the suppression of the delivery side pulses (considered over the range of rotational speeds of the compressor), are markedly different, that is the suppression of a pulse of an equal order of size is not achievable for every rotational speed.

The object of the present invention is therefore to provide a new method, as well as a Roots compressor for carrying out the method, by means of which suppression of delivery side pulses of the same degree for any rotational speed of the compressor can be achieved and, moreover, where the absolute value of the suppression lies above the optimum values attainable withthe construction according to DE-OS 3238015.

Thus, the invention provides a method for compressing a gaseous flow medium with a Roots compressor comprising Roots pistons mounted in a cylinder, in which flow medium is introduced in a controlled manner from the delivery side alternately into the flow chambers formed by the respective Roots pistons and the associated portions of the cylinder wall, after closure relative to the suction side, in which the introduction is controlled so that sub-pressure pulses are produced in the delivery side flow chamber and the wave produced by the sub-pressure pulses cancels the wave produced by the over-pressure pulses generated on entry of the head of one piston into the reduced-diameter region of the other piston.

The invention is based upon the discovery that the origin of the pulses and fluctuations on the delivery side, resulting in the production of over-pressure pulses, is due not only to return of the already-compressed flow medium into the flow chamber opened to the delivery side, but also to entry of the piston head of the one piston into the reduced-diameter region of the other piston. If order to eliminate the waves produced by these overpressure pulses, introduction of the flow medium takes place, according to the invention, so that a "counter-wave" is produced, which is so arranged with regard to wavelength, amplitude, direction and phase difference that the wave and the "counter-wave" cancel each other out. Because of this construction, reduction of the delivery-side pulses and fluctuations is achieved to a degree which has not been possible heretofore.In addition, for the first time, the pulses and fluctuations are not only optimally suppressed over a narrowly limited range of rotational speed, but instead over the whole range of rotational speeds.

Tests have shown that the method according to the invention can be carried out in a particularly advantageous way with Roots compressors having trilobal Roots pistons.

Thus, control of the "counter-wave" can, inter alia, be optimally carried out in relation to the phase relationship and amplitude practically without influence due to the cross-sections of the suction and pressure lines.

According to an advantageous preferred feature of the method of the invention, introduction of the flow medium into the region of the one Roots piston is staggered in time in relation to its introduction into the region of the other Roots piston. By this "asymmetry" of flow medium introduction, special control possibilities arise, which can be used for example for the correction of manufacturing tolerances, for the correction of dead-spots in the compressor housing or the like.

The method according to the invention may be carried out in a particularly advantageous way with a Roots compressor which is characterised in that, for control of the sub-pressure pulses, fixed and/or movable elements are provided, which cooperate with the Roots piston across a clearance gap ana are arranged in the cylinder wall in the region of the delivery side and by which the gap is variable in the axial and radial direction.

The elements can fundamentally be constructed in a wide variety of ways. It is particularly advantageous if the compressor includes elements for axial variation which are constructed as bar-like adjusting members insertable into the cylinder from the end surface and elements for radial variation which are constructed as slide plates. In this way, a continuous adjustment both in the radial and also in the axial direction is possible, without major mounting and setting operations of the control elements being required.

In a number of applications, it is not necessary to effect continuous adjustment in the axial or radial directions or both, since this would require a certain technical expenditure.

In these cases, it is advantageous to select a construction in which either the elements for axial variation or the elements for radial variation or both are constructed as replaceable inserts. In one preferred embodiment, the elements for axial variation are constructed as replaceable inserts and the elements for radial variation are constructed as slide plates. In another preferred embodiment, the elements for axial variation are constructed as bar-like adjusting members insertable into the flow chamber from the end surface and the elements for radial variation are constructed as replaceable inserts.

A particularly advantageous embodiment of the invention utilises slide plates which have a generally wedge-shaped cross-section and are movable around the cylinder wall, to vary the radial dimension of the gap.

If bar-like adjusting members are used, then it is preferable for their depth of insertion into the cylinder to be variable, so that the axial dimension of the gap can thus be changed.

The above-described possibilities relate to control in the region of the piston head and the cylinder wall. Control possibilities are also available according to the invention in the region of the end face of the piston and are also realisable in the piston housing. For this purpose, solutions according to Figures 1 and 2 of DE-OS 3238015 can be used with a correspondingly constructed and adjusted form of apparatus.

According to an advantageous embodiment, the elements for radial variation are constructed as rods extending parallel to the piston axis, which are supported in the cylinder wall and are insertable into and removable from the cylinder wall essentially in a radial direction. By such a construction, the surfaces required for control in dependence upon the angle of rotation are only realised approximately. The effect so obtained-as tests have shown-is still considerable, despite the merely approximate construction of the surface.

In the following, for further explanation and for better understanding of the invention, three embodiments of Roots compressors operating according to the method of the invention are described by way of example only and with reference to the accompanying drawings.

In order to produce the sub-pressure pulses necessary for the cancellation effect according to the invention and to be able to control them, in all three embodiments, elements are arranged in the region of the "over-flow position" in the flow chamber wall, with which the gap effective between the Roots piston and the flow chamber wall is adjustable in the axial and radial direction, that is an adjustment can be carried out which is achieved in both directions.

Figure 1 shows diagrammatically a first embodiment, in which movable elements both for axial and for radial adjustment are provided; Figure 2 further shows diagrammatically a second embodiment, in which movable elements are inserted for radial adjustment and fixed replaceable elements are inserted for axial adjustment; Figure 3 shows diagrammatically a third embodiment, in which special elements merely giving an approximate result are provided for radial adjustment and fixed elements are provided for axial adjustment.

In all three embodiments, the section plane is so situated that the cross-section on the right-hand side runs approximately through the middle and the cross-section on the left-hand side runs approximately through the peripheral region of the elements provided for control of the sub-pressure pulses.

In all three embodiments, the Roots compressor includes two Roots pistons 1 interengaging in known manner, which are mounted in a cylinder 2. The cylinder 2 includes a cylinder wall 3 associated with the Roots pistons 1, on which end plates 4 are mounted at the ends. The cylinder 2 is provided with a suction connection 5 and a delivery connection 6, which define respectively the suction side S and the delivery side D.

In all three embodiments, the cylinder wall 3 in the region of its half facing the delivery side D is provided with openings or windows F, which are closed to the outside by means of a cover 7. In the region of these windows F, fixed and/or movable elements are arranged, for control of the sub-pressure pulses already mentioned, which are described individually in more detail in connection with the respective embodiments.

As in known Roots compressors, in the present embodiments, the flow medium is alternately introduced in a controlled manner from the delivery side D into the flow chambers, after their closure with respect to the suction side 6, the flow chambers being formed by the respective Roots piston 1 and the associated portion of the cylinder wall 3 (including the end plates 4). As already described at the beginning, in accordance with the invention, the introduction is so controlled that a subpressure pulse is produced by the introduction process in the delivery side supply chamber, the wave from which cancels out the wave produced by reason of the over-pressure pulse, on entry of the piston head of the one piston into the reduced-diameter region of the other piston.

In the embodiment according to Fig. 1, the elements for axial and for radial adjustment are both movably arranged. The elements provided for axial adjustment consist of bar-like adjusting members 8 which are inserted from the front face into the cylinder in the region of the windows F, through slots 8a in the end plate 4. Thus the gap Sp can be given a predetermined width, considered in the axial direction of the compressor, depending upon the depth of insertion of the member 8.

For radial adjustment of the gap Sp, in the embodiment illustrated in Fig. 1, a so-called coaxially rotatable member 9 is provided. This rotatable member 9 consists of a slide plate 10, which can be moved concentrically to the cylinder wall 3, with its outer side 11 on an insert 12. The insert 12 rests in the window F and is covered on its outer side by the cover 7.

The insert 12 includes a web 13 with a longitudinal slot 14. On both sides of the web 13, guide plates 15 are arranged, which are secured to the outer side 11 of the slide plate 10. The guide plates 15 have bores extending through them as well as through the longitudinal slot 14, into which a guide rod 16 is inserted.

In the present embodiment, the surface of the bar-like plate 8 facing the Roots piston 1 is thus so constructed that it continues the internal contour of the cylinder wall 3 up to its connection with the pressure side D.

The outer surface of the plate 8 is further constructed so that it corresponds to the contour of the surface of the slide plate 10 facing the respective Roots piston in its extended position. In the region of the side of the window F facing the suction side S, the cylinder 2 is provided with a recess 17, which is so dimensioned that the side of the slide plate 10 facing the suction side S can be shifted in the direction of the arrow 18 into the recess 17. Since the thickness of the slide plate 10 decreases in the direction of the delivery side D, that is the slide plate has been formed with generally wedge-shaped cross-section, movement of the slide plate 10 in the direction of the arrow 18, from the position shown in Figure 1, allows the gap Sp to be varied in the radial direction.Thus, in the embodiment according to Fig. 1, a continuous adjustment both in the axial and also in the radial direction is possible, without major mounting and adjustment operations of the control elements being required.

In the embodiment illustrated in Figure 2, movable elements are provided for radial adjustment and fixed ones for axial adjustment.

The movable elements, as in the case of the embodiment according to Fig. 1, are constructed as movable members 9. Instead of the bar-like plates 8 insertable into the front face, fixed inserts 19 are provided which, as shown at the left-hand side of Figure 2, are arranged in the region of the ends of the slide plates 10 of the rotatable members 9. If the width of the gap Sp-considered in the axial direction of the rotors-is to be altered, the inserts must be taken out, together with the members 9, after removal of the cover 7 and replaced by other inserts dimensioned to correspond with the desired width.The inserts 19 thus have for this purpose, in the case of the embodiment according to Fig. 2, recesses whose cross-section corresponds to the cross-section of the slide plate 10, which can have a variable depth depending upon the width, as seen in the axial direction of the rotary piston. The surface of the inserts 19 facing the Roots piston 1 is thus so shaped that the inner contour of the cylinder wall 3 is continued up to the connection to the delivery side D.

In the embodiment illustrated in Figure 3, two rods 20 and 20a are provided in each window F, for radial adjustment, which extend parallel to the axis of the Roots piston and are mounted for movement into and out of slots 21 in an insert 22, in an essentially radial direction thereto.

On their sides away from the Roots piston, the rods 20 and 20a are provided with threaded bolts 23, on which setting nuts 24 are threaded which are supported on a web 25 in the insert 22. On rotation of the setting nuts, the rods 20 and 20a can thus be moved outwardly from the slots 21 by a predetermined amount according to requirements or, in case of need, they can be returned into the slots.

For axial adjustment, as shown at the lefthand side of Fig. 3, fixed inserts 26 are provided which are equipped with gaps or slots 27 for the rods 20 and 20a. For adjustment of the width of the gap, seen in the axial direction of the Roots piston, the fixed inserts 26 merely have to be adjusted to the desired width, as is required in the case of the embodiment according to Fig. 2.

The number of rods 20 and 20a as well as their position in the window F is selected according to existing requirements. Consideration is taken of the fact that by such a construction the necessary surface for control in dependence upon the angle of rotation is only realised approximately.

Claims

1. A method for compressing a gaseous flow medium with a Roots compressor comprising Roots pistons mounted in a cylinder, in which flow medium is introduced in a controlled manner from the delivery side alternately into the flow chambers formed by the respective Roots pistons and the associated portions of the cylinder wall, after closure relative to the suction side, in which the introduction is controlled so that sub-pressure pulses are produced in the delivery side flow chamber and the wave produced by the sub-pressure pulses cancels the wave produced by the over-pressure pulses generated on entry of the head of one piston into the reduced-diameter region of the other piston.

2. A method for compressing a gaseous flow medium with a Roots compressor having trilobal Roots pistons mounted in a cylinder in which flow medium is introduced in a controlled manner from the delivery side alternately into the flow chambers formed by the respective Roots pistons and the associated portions of the cylinder wall, after closure relative to the suction side, in which the introduction is controlled so that sub-pressure pulses are produced in the delivery-side flow chamber and the wave produced by the sub-pressure pulses cancels the wave produced by the over-pressure pulses generated on entry of the head of one piston into the reduced-diameter region of the other piston.

3. A method according to claim 1 or 2, in which the introduction into the region of one Roots piston is displaced in time with respect to the introduction into the region of the other Roots piston.

4. A method for compressing a gaseous flow medium with a Roots compressor, substantially as hereinbefore described with reference to the accompanying drawings.

5. A Roots compressor for carrying out a method according to any preceding claim, in which fixed and/or movable elements are arranged in the cylinder wall, and cooperate with the Roots piston across a clearance gap, for controlling the sub-pressure pulses, by means of which the dimensions of the gap are variable in the axial and radial directions.

6. A Roots compressor according to claim 5, including elements for axial variation which are constructed as replaceable inserts and elements for radial variation which are constructed as slide plates.

7. A Roots compressor according to claim 5, including elements for axial variation which are constructed as bar-like adjusting members insertable into the cylinder from the end surface and elements for radial variation which are constructed as slide plates.

8. A Roots compressor according to claim 6 or 7, in which the slide plates have a generally wedge-shaped cross-section as viewed in the axial direction and are movable around the cylinder wall, whereby the radial dimension of the gap can be varied.

9. A Roots compressor as claimed in claim 8, in which the slide plates are movable in recesses provided in the cylinder wall.

10. A Roots compressor according to claim 5, including elements for axial variation which are constructed as bar-like adjusting members insertable into the cylinder from the end surface and elements for radial variation which are constructed as replaceable inserts.

11. A Roots compressor according to claim 7 or 10, in which the depth of insertion of the adjusting members into the cylinder is variable whereby the axial dimension of the gap may be varied.

12. A Roots compressor according to claim 5, including elements for axial variation and elements for radial variation which are both constructed as replaceable inserts.

13. A Roots compressor according to claim 5, including elements for radial variation which are constructed as rods running parallel to the piston axis and mounted so as to be movable into and out of the cylinder wall in an essentially radial direction.

14. A Roots compressor substantially as hereinbefore described with reference to any of the accompanying drawings.