GB2082680A

GB2082680A - Meshing screw compressor

Info

Publication number: GB2082680A
Application number: GB8120089A
Authority: GB
Original assignee: Aerzener Maschinenfabrik GmbH
Current assignee: Aerzener Maschinenfabrik GmbH
Priority date: 1980-08-22
Filing date: 1981-06-30
Publication date: 1982-03-10
Also published as: SE8104116L; GB2082680B; DE3031801B1; DE3031801C2

Abstract

In a double-entry screw compressor i.e. having two pairs of oppositely pitched rotors disposed end-to-end, both the male or main rotors H1 and H2 and the female or secondary rotors N1 and N2 are mutually angularly offset. The offset is up to one half of the screw pitch; and the rotor profiles are asymmetric. This arrangement ensures that fluid being compressed is not trapped in closed pockets T on one side of the abutment plane of the rotors, since the pockets T are vented via an opening(s) D1 and/or D2 to the grooves in the rotors on the other side of the said plane. <IMAGE>

Description

SPECIFICATION Screw-type compressor The present invention relates to a screw-type compressor of the kind in which oppositely pitched first and second main rotors mounted in end to end abutment on one common shaft mesh with corresponding oppositely pitched secondary rotors in end-to-end abutment on another common shaft.

Screw-type compressors of this kind (generally known as double gentry screw-type compressors or compressors with herring bone gearing) have long been successfully used in a wide range of applications. Moreover, such main and secondary rotors are joined in end-to-end abutment on a shaft in an identical manner and have a symmetrical profile.

Admittedly, the symmetrical profile in compressor construction compared with present day generally asymmetrical profile shapes has a number of disadvantages with respect to the degree of efficiency. However, owing to the formation of so-called trapped pockets, asymmetrical profiles have nevertheless so far only been used in single-entry designs, since this type of compressor offers the possibiiity of implementing the essential purging of trappedpockets in the area of the pressure-side face of the rotor more particularly with reference to fluid injection compressors (see for instance DE-O!S 28 it 570).

The object of the present invention is to further develop a screw-type compressor of the aforegoing type in such a manner that an asymmetrical profile can be used for the main and secondary rotors.

This purpose is achieved according to the invention, in that the main rotors and the secondary rotors are respectively mutually offset in the region of the plane of abutment on the shaft by an angle, and have an asymmetrical profile shape.

As a result of the features of the invention, purging of the pockets formed between meshing teeth on the pressure side, can be ensured in a technically simple manner and without complex and expensive purging ducts. As a result of the offset arrangement of main rotors and secondary rotors respectively in the area of the frontal plane of abutment, it is ensured that any trapped pockets on one side of the plane are able tc void into the still open pitch on the other side.

Dangerous high pressures or fluid knock (in the case of fluid-injection operation compressors) are thus excluded. Any machinery failure ascribabie to that type of effect is thus excluded. Finally, the invention rests upon the knowledge that as a result of the non-identical arrangement of the main and secondary rotors in the region of the frontal plane of abutment, the trapped pockets of a rotor pair may be purged into the open pitch of the subsequent opposing rotor pair, without the need for complex shaped arrangements and costly purging ducts.

Furhermore, as a result of the measures according to the invention, further smoothing of the reiatively steady flow in this type of compressor can be achieved. This also has a beneficial effect upon freedom from noise.

The angle oF offse' the main and secondary rotors may reach vaices of up to a half pitch division. The choice of angle is determined by the type of asymmetrical profile empioyed. An angle of the order of niagnitude of a half pitch division is nevertheless the optimum with reference to the discharge effect.

From rne technical viewpoint, the arrangement of inlet and outlet contour matching the angular offset presents no particular difficulties.

The length and the pitch angle of each rotor pair may be exactly the same. In this way a fully symmetrical design about the plane of abutment of the rotor pa;rs is achieved. In order to set the position of the rotors definitely, axial stressing is required, for instance in the form of a spring arrangement, so as to prevent any oscillating axial forces during operation.

Definite and satisfactory axial setting of the rotors depending upon design and technical requirements is nevertheless achieved according to an advantageous development of the invention, if the length of the rotor pair on the one connecting plane is greater or smaller than the length of the opposing rotor pair.

This effect is also achieved with identical rotor pair lengths by if the rotor pairs have different pitch angles.

Where necessary and for tne reasons outlined above, it may be useful to have rotor pairs cf different lengths and with different pitch angles.

An embodiment of the present invention will now be desK;bed, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic frontal section of mutually staggered abutting ends of main and secondary rotors of a screw-type compressor according to the present invention and, Fig. 2 s a diagrammatic plane view partly in section of the screw-type compressor of Fig. 1.

As may be seen from the drawings, a first main rotor H1 of a herringbone toothed double screw is anguiarly offset in relation to a second main rotor H2 by an anyie V. The rotors H 1 and H2 are located on a common shaft W. Like the main rotors H1 andH2, secondary rotors N1 and N2 which mesh with and are co-extensive with the main rotors H1 and H2 respectively, are also mutually offset by a given angle and located on another con' mown shaft W extending parallel to the first-mentioned shaft W. The angle V is such that the rotors are offset by up to half the screw pitch.

In the event of a trapped pocket T (see Fig. 1) forming in plane E-E (Fig. 2) of the interfaces between the main rotors H1 and H2 and the secondary rotors N1 and N2, this pocket can vent into the still open pitch of the opposite side of the compressor by way of areas D1 and D2, as can be seen from Fig. 1.

On further rotation of the rotors out of the position indicated in Fig. 1 and in the direction of the arrow, the head K of the main rotor H1 projects further into the associated groove of the secondary rotor N1, so that the volume of the trapped pocket T is further reduced (theoretically down to zero). Whereas at the beginning of the formation of the trapped pocket T, purging is achieved by way of both areas D1 and D2, the area D2 becomes increasingly smaller until it disappears, as the rotation continues. From that point onwards, only the continuously changing area D1 is effective during the course of rotation, and also disappears and becomes ineffective as the rotation continues, as soon as the volume of the trapped pocket reaches zero. A pressure increase in the trapped pocket T cannot occur because of this particular design.

Claims

1. A screw-type compressor having oppositely pitched first and second main rotors mounted in end-to-end abutment on a common shaft and meshing with respective oppositely pitched secondary rotors mounted in end-to-end abutment on another shaft, wherein the main rotors and the secondary rotors are respectively mutually offset angularly in the vicinity of the abutment plane and have an asymmetrical profile form.

2. A screw-type compressor as claimed in claim 1, wherein the offset is such that the rotors are angularly offset by up to one half of the pitch of the rotors.

3. A screw-type compressor as claimed in claim 1, or 2 wherein the length of the rotor pair on the one side of the abutment plane is greater or smaller than the length of the rotor pair on the other side of the abutment plane.

4. A screw-type compressor as claimed in claim 1 or 2 wherein the rotor pair on one side of the abutment plane has the same length but a different pitch angle to the rotor pair on the other side of the abutment plane.

5. A screw-type compressor as claimed in claim 3 wherein the pitch angles of the rotor pairs are different.

6. A screw-type compressor substantially as hereinbefore described with reference to the accompanying drawings.