GB2132702A

GB2132702A - Roots compressor

Info

Publication number: GB2132702A
Application number: GB08327282A
Authority: GB
Inventors: Hans Nissen
Original assignee: Aerzener Maschinenfabrik GmbH
Current assignee: Aerzener Maschinenfabrik GmbH
Priority date: 1982-10-13
Filing date: 1983-10-12
Publication date: 1984-07-11
Also published as: GB2132702B; IT8323275A1; FR2534638B1; DE3238015C2; IT8323275A0; IT1171754B; JPS59136594A; GB8327282D0; FR2534638A1; DE3238015A1

Abstract

In the compressor, gaseous material is fed from a suction side S to a pressure side D via working chambers F formed between the rotor pistons Z1, Z2. In order to avoid strong oscillations of the gaseous medium in the pressure side supply conduit with the resultant mechanical disadvantages, entry ports 2 into the working chambers, and/or the clearance between the pistons and the compressor casing, Figs 3 &4 (not shown), are so arranged and of such a size that the beginning of the supply of gaseous medium via a port 2 or piston clearance into the working chamber of one rotor occurs at the same time as the end of such supply of gaseous medium to the working chamber of another rotor. At the end of such supply of gaseous medium, the pressure in the respective working chamber and the pressure side correspond. The supply of gaseous medium, through the port 2 or via the clearance into the respective working chamber is taken from the pressure side of the compressor and is controlled in such a way that the gaseous medium in the pressure side supply conduit has a constant flow velocity. <IMAGE>

Description

SPECIFICATION Roots compressor The present invention relates to a process for compression of gaseous working medium with a Roots compressor (also known as a Roots blower), in which the working medium is fed in from the pressure side alternately into the pumping chambers formed at any one time by the respective Roots piston and the co-operating pumping chamber wall afterthe closing at the suction side. The invention further relates to a Roots compressorfor carrying outthe process.

In Roots compressorsthere exists in general the problem that a part ofthe already-compressed working medium blows back into the pumping or working chamber upon opening of the pumping chamber to the pressure side. This flow-back leads to strong oscillatory phenomena both in and adjacent the Roots compressor and also in the pressure side pipe system connected downstream.

Indeed, it has already been sought to reduce these oscillatory phenomena by introduction of working medium from the pressure side into the pumping or working chambers before their opening to the pressure side (see for example DE-PS 11 33 500, DE-AS 12 58 543; Bulletin of JSME, Vol 24, No. 189, March 1981, pages 547 to 554. Through the proposed measures however a blow-back ofthe medium from the pressure side into the working chamber cannot be avoided completely, so that with these known proposals at best only a reduction in the incidence of oscillations is possible.

It is an object of the present invention to provide a completely new kind ofprocessforcompression of gaseous working media with a Roots compressor.

It is a further objectto provide a Roots compressor for carrying outthe process, in which a strong oscillation ofworking medium in the pressure side supplyconduit and the consequent acoustical and mechanical disadvantages can be reduced or avoided.

According to the present invention there is provided a method of compressing a gaseous medium with a Roots compressor, the compressor having a suction side and a pressure side and comprising wall means defining a working volume in which Roots pistons co-operate with the wall means to form, between the wall means and respective ones ofthe pistons successive displacements chambers for conveyance ofthe gaseous medium from the suction side to the pressure side, the method comprising beginning supply ofthe gaseous medium to one displacement chamber substantially simultaneously with the end of supply of gaseous medium to another displacement chamber, controlling the pressure ofthe gaseous medium in the displacement chambers such that, at the end ofsupply of the gaseous medium to the said other displacement chamber, the pressure in the other displacement chamber corresponds to the pressure of gaseous medium at the pressure side ofthe com pressor, and maintaining substantially constantflow velocity of the gaseous medium at the pressure side of the compressor.

The invention also provides a Roots compressor having a suction side and a pressure side and comprising wall means defining a working volume in which roots pistons are movabletoform displacement chambers for conveyance ofthe gaseous medium from the suction side to the pressure side, and control means for controlling admission to the displacement chambers ofthe gaseous medium, the control means being adapted to ensurethatthe beginning of supply of the gaseous medium to one displacement chamber occurs substantially simultaneously with the end of supply of the gaseous medium to another displacement chamber, the pressure ofthe gaseous medium in the said other displacement chamber corresponds, at the end of supply of the medium thereto, to the pressure of gaseous medium at the pressure side ofthe compressor, and a substantiallyconstantflowvelocityofthegaseous medium at the pressure side is maintained.

The invention is based on thethought that the pulsations or oscillatory phenomena which occur in the pressure side supply conduit during the compression orconveyance of gaseous working material by Roots compressors or blowers are reduced in that, such an amount is drawn at any one time from the working medium supplied to the pressure side before the feeding in the supply conduit, thatthe amount remaining in the supply conduit continues flowing with a constantflow velocity. The amounts withdrawn are again introduced into the working ordisplacement chambers successively and without interruption, so that, at the end ofthis introduction, the pressure in the respective working chamber corresponds to the working pressure ofthe pressure side.The constant flow velocity arises from the time-standardised supply volume atthe suction side,the ratio ofthe clearance on the suction and pressure sides and the cross-sectional area of the supply conduit on the pressure side.

Through the measures in accordance with the present invention not only are those oscillations reduced, which arise in the known Roots compressors because of blow-back of the working medium into the working chambers, but also those pulsations which are attributed to the piston geometry.

Essentially,thecontroloftheintroduction of the working medium into the respective working or displacement chambers can resultfrom arrangements internal to or external of the compressor. A particularly simple and effective control is however achieved in that the cross-sectional area of the rotary Roots piston decisiveforthefeeding oftheworking material into the working chambers is itself controlled.

The feeding of working material into the working chambers can take place in various ways in Roots compressors for performing the process ofthe invention. Thus, ducts connected to the pressure side could be provided, which ducts lead into the respectiveworking chambers atthe cylindrical surface and/ortheend surface oftheworking volume in which the pistons rotate. There could however also be provided for introduction of the working medium openings in the cylinder surface which enlarge the normal clearance between the piston head or lobes and the cylinder surface.

The arrangement of ducts for feeding of working medium has the advantage that, if necessary, cooling arrangements could be arranged between the press ure side withdrawal andthefeeding into the respectiveworking chambers. In the event of cooling, the control must however be so designed that the working medium remaining in the pressure side supply conduit has the above-described constant flow velocity.

The arrangement of the inlet openings in the region ofthe end surfaces ofthe working space or volume has the advantage that relatively large cross-sections ofthe inlet openings can be freed with a small angular change of the Roots piston. This is particularlythe case ifthe supply openings in the region of the edges thereof effective for beginning of feeding are adapted as to theirform to the contoured piston edge which determines the control.

Insofar as there are provided in the cylinder surface ports for feeding of the working medium which enlarge the normal clearance between the piston head or lobes and the cylinder surface,these parts could extend over the entire length of the cylinder surface. If necessary it can however also be advantageous for the ports to extend over only a part of the length ofthe cylinder.

It is advantageous that the exit ports consist of two portions,which are arranged in the region oftheends ofthe cylinder surface. In such an arrangement, strip-like adjustable slides or dampers could be provided in the exit ports of the end surfaces ofthe working space, which makes possible an alteration of with time of the surface functions according to the insertion depth, with retention of the characteristics of the opening.

In a Roots compressor with input ducts in at least one ofthe end surfaces ofthe working volume or cylinder, it is advantageous that the supply openings arranged in the end surfaces extend over a predeter mined angle of rotation ofthe Roots piston and on their sides lying opposite the volumes, are so covered by a cap-like duct section as to determine the effective cross-sectional area for the piston contour decisive for the control and the inner surface of the wall of the duct section.It is achieved bythese means that after the respective supply openings are opened by the Roots piston, it is no longerthe cross section thereofthat is decisiveforthefeeding of supply medium into the working chamber, but the cross section extending substantially perpendicularly thereto, which is formed from the piston contour edge determining the control and the inner surface ofthe wall ofthe duct cross section. Such an arrangement has the advantage that the effective cross-sectional surface forthe supply can be controlied independently ofthe cross section of the inlet openings.

The process according to the present invention can be put into practice in a particularly advantageous manner with a Roots compressor with two three- lobed or-toothed Roots pistons. However, realisation ofthe process according to the present invention is possible also with Roots compressors with two two-lobed Roots pistons.

The realisation with Roots compressors having two three-lobed Roots pistons has however the advantage that, on one hand,thetried and tested casing arrangementsfortwo-lobed Roots pistons can be retained, and, on the other hand, however, the required area of angular rotation for the introduction ofthe medium into the working or displacement chambers remains available.

Afurtheradvantage ofthethree-lobed Roots piston istobeseen inthattheclearancelossesovera particularangularregionaremorefavourablethan in two-lobed Roots pistons. Moreoverthree-lobed Roots pistons havetheadvantagethattheyhave more favourable properties in respect of deflection.

Atthesame rate of revolution, the typical basic frequencies of Roots compressors with two threelobed roots pistons lie manytimes higherthan in Roots compressors with two two-lobed Roots pistons.

This leads to the fact that, resonators can be installed for a further reduction of residual noise, which against resonators for two-lobed Roots pistons have, as regards their structural size less than 50% of their volume.

Forcarrying outthe process according to the invention with a Roots compressor with two twolobed Roots pistons, it is advantageous to arrange the inlet and discharge openings ofthe compressor in respective end surfaces ofthe working volume or pumping chamber. With a two-lobed Roots piston with very broad piston heads a part of the suction opening can also be formed as an inletslit in the cylinder surface ofthe respective working chambers.

In a three-iobed Roots piston, on the other hand, the conventional arrangement of the suction and pressure connections in the region ofthe cylindrical surfaces can be retained.

Afurtheradvantage of Roots compressors with three-lobed Roots pistons consists in that it is possible, in a similarsize ofstructure, to achieve a throughput per revolution of the magnitude of Roots compressors with two-lobed Roots pistons.

The process according to the present invention is not restricted to Roots compressors with two or three-lobed Roots pistons. Therefore the process of the present invention can if necessary at anytime be put into effect with roots compressors which have two, four- or more-lobed Roots pistons.

For a further explanation and fora better understanding ofthe invention, there are described below with reference to the accompanying drawings illustrative details of an example of a Roots compressorwith two three-lobed Roots pistons a few ofthe control possibilitiesforthe introduction of pressure medium into the respective working chambers.

Figures 1 and 2 show embodiments in which the introduction of the working medium takes place by way of supply openings arranged in the end surfaces oftheworking volume, and Figures 3 and 4 show embodiments in which the introduction ofthe medium takes place away overthe head ofthe piston through inlet ports arranged inthe cylindrical surface.

For the sake of simplicity, only a part of a three lobed orthree-toothed Roots-piston, together with sections ofthe housing ofthe compressor necessary for control, are shown.The direction of rotation ofthe respective Roots piston 1 is in each case shown by an arrow.

In all four embodiments, the rotary Roots piston 1 is in a position in which the working or displacement chamber Fformed between the lobes or teeth Z1 and Z2 opposite the suction side S (Figure 1) has closed. In all four embodiments the Roots piston 1 is also shown in a position in which gaseous medium is being supplied from the pressure side D into the working chamber F.

lnthecaseoftheembodimentaccordingto Figure 1, the gaseous medium is drawn from the pressure side through a duct (not shown) which communicates through a inletopening2intheendsurfaceofthe working volume of the compressor into the working or displacement chamber F. In the embodiment according to Figure 1, this inlet opening 2 has an approximately T-shape, and the open condition of a half of the cross-section of the opening 2, which is free from the lobe ortooth Z2 ofthe Roots-piston 1, is shown hatched.

As the drawing allows to be seen clearly, the edge 3 ofthe inlet opening 2 that is effective in beginning supply of gaseous medium is exactly adapted into a form which is definitive for control of the piston contouredge4. An arrangementofthis kind has the advantage that, with a very small angular change or rotation ofthe Roots piston 1, a relatively large cross-sectional surface of the supply inlet 2 can be opened.

The design ofthe supply opening 2, as mentioned in the introduction, is so chosen that, at the end ofthe charging, the pressure in the respective displacement chamber F corresonds to the working pressure of the pressure side, and for the charging at anytime, such an amount ofthe medium is drawn in that the amount remaining in the supply conduit continues to flow with a constant flow velocity.

In all fourembodimentsthearrangementaccording to the invention is moreoversuch thatthe end ofthe charging of the working chamber F coincides with the beginning of charging in the oppositely-lying (not shown) working chamber of the Roots compressor so that pressure medium is charged to the working chambers alternately and without interruption.

In the embodiments shown in Figure 2 the charging of pressure medium takes place likewise overthe end surface of the working volume. In contrast to the embodiment shown in Figure 1 howeverthe inlet openings 5 arranged in the end face of the working volume do not define the effective flow cross section.

Rather, in this embodiment, a cap-like duct section 6 is arranged over the supply opening 5, which is so shaped that the effective cross-sectional area for the supply is determined by the contoured piston edge 7 of the lobe or tooth Z2 which determines the control and the inner surface ofthe wall of the duct section 6.

This effective cross-sectional area 8 extends approximately perpendicularlytothe end face ofthe Roots piston and is shown hatched for a clearer understanding. An arrangement of thins type has the advantage thatthe cross-sectional area 8 which decides the supply ofthe working medium can be adapted to respective characteristics by a corresponding shaping ofthe inner surface of the walls ofthe duct cross section 6. Therefor it is for example possible to hold the effective cross-sectional area 8 constant over the region of a predetermined angle of rotation of the Roots piston despite the enlarged cross section of the supply opening 5 orto allow the area to increase according to the predetermined criteria.

In the embodiment shown in Figure 3, the introduction ofthe medium takes place both through a duct9 connected with the pressure side and also through inlet openings 10in the cylindrical surface of the working volume which openings enlarge the normal clearance between the piston head, or free ends ofthe piston lobes, and the cylindrical surface.

Basically, the inlets 10 can extend over the entire cylindrical surface. According to the necessary crosssectional area, it can however be arranged that the outlets consist oftwo sections which are located in the region of the ends ofthe cylindrical surface. In such a case, as shown in Figure4, adjustable strip-like dampers or slides can be positioned in the outlets 10.

In the embodiments shown in Figure 4these strip-like adjustable dampers 11 are displaceable in this way from the end surface ofthe working chamber, whereby in a relatively simple way, an alteration with time ofthe surface functions according to the insertion depth with retention ofthe characteristics of the opening is possible.

Claims

1. A method of compressing a gaseous medium with a Roots compressor, the compressor having a suction side and a pressure side and comprising wall means defining a working volume in which Roots pistons co-operate with the wall means to form, between the wall means and respective ones of the pistons successive displacements chambers for conveyance of the gaseous medium from the suction side to the pressure side, the method comprising beginning supply ofthe gaseous medium to one displacement chamber substantially simultaneously with the end of supply of gaseous medium to another displacement chamber, controlling the pressure of the gaseous medium in the displacementchamberssuch that, at the end of supply ofthe gaseous medium to the said other displacement chamber, the pressure in the other displacement chamber corresponds to the pressure of gaseous medium atthe pressure side of the compressor, and maintaining substantially con stantthe flow velocity ofthe gaseous medium atthe pressure side of the compressor.

2. A Roots compressor having a suction side and a pressure side and comprising wall means defining a working volume in which Roots pistons are movable to form displacement chambers for conveyance ofthe gaseous medium from the suction sidetothe pressure side, and control means for controlling admission to the displacement chambers of the gaseous medium, the control means being adapted to ensure that the beginning ofsupply of the gaseous medium to one displacement chamber occurs substantially simul taneously with the end of supply of the gaseous medium to another displacement chamber, the pressure of the gaseous medium in the said other displacement chamber corresponds, at the end of supply of the medium thereto, the pressure of gaseous medium at the pressure side ofthe compressor, and a substantially constantflow velocity ofthegaseous medium at the pressure side is maintained.

3. A compressor according to claim 2, wherein the control means comprise port means for admission of the gaseous medium into the displacement chambers and the pistons and the port means are shaped to co-operate to control the admission.

4. Acompressoraccording to claim 3, wherein the port means comprises openings arranged in end walls oftheworking volume, the compressor further comprising ducts connecting the openings to the pressure side and the pistons having respective edges shaped to co-operate with the openings to control the admission.

5. Acompressor according to claim 3 or claim 4, wherein the port means comprises a clearance between at least one Roots piston and the wall means, the compressor further comprising duct means can necting the clearance to the pressure side of the compressor

6. A compressor according to claim 5, wherein the clearance extends over the entire length ofthe wall means.

7. A compressoraccording to claim 5, wherein the clearance extends over part of the length ofthe wall means.

8. A compressor according to claim 7, wherein the clearance has two portions each extending over portions of the working volume adjacent opposite ends thereof.

9. A compressor according to any one of claims 5 to 8, comprising means for adjustment ofthe effective cross-sectional area of the clearance or clearance portions.

10. A compressor according to claim 9, in which the adjustment means comprises at leastonestrip-like adjustment damper adjustably mounted in the clear ance for adjustment of the effective cross-sectional area ofthe clearance or clearance portions.

11. A compressor according to claim 4, wherein the openings each extend over a predetermined angle of rotation of at least one Roots piston, the duct means comprising a cap-like portion extending over each opening and having an innerwall surface which, together with an adjacent edge ofthe piston controls the admission of gaseous medium to the displacement chambers.

12. A method of compressing a gaseous medium with a Roots compressor, the method being substan tally as herein before described with reference to any Figure of the drawings.

13. A Roots compressor substantially as hereinbefore described with reference to any figure of the drawings.