GB2156438A - Meshing-screw rotary gas-compressor - Google Patents

Abstract

The compressor has an adjustment slide 10 that projects through a guiding hole formed in the discharge end-wall of the compression chamber 4 and situated immediately underneath a discharge port 8 in the said end-wall. The slide has part-cylindrical surface portions for cooperation with the screw rotors not shown and is guided by an edge portion of the guiding hole. The aforesaid end-wall is constituted by a plate 24 that may comprise two parts, Fig. 3. <IMAGE>

Description

SPECIFICATION A screw compressor with at least one adjustment slide This invention relates to a screw compressor of the kind specified in the introductory part of claim 1.

It is well known that screw compressors may be provided with one or more regulation slides particularly a single capacity adjustment slide or both such a slide and a slide for separately controiling the so-called volume ratio, i.e. the degree of compression.

The slides generally operate in the manner that they constitute, each, an axially displaceable, part cylindrical strip portion of the wall of the compression chamber, whereby they are operable, by axial adjustment, to change the effective length of the screw rotors. What is important for the invention is not the type or the number of the slides, but the fact thay any slide having a surface portion, which should form part of the wall of the compression chamber, shall have to be guided extremely accurately in order to fulfill the narrow tolerances of the location of such wall portions immediately outside the outer portions of the rotors.

Thus, it is necessary to provide in the compressor housing a very accurate guiding for such a slide. The provision of such an accurate guiding is complicated by the fact that the compressor housing or the relevant portion thereof is normally constituted by a heavy piece of a casting, which is expensive to machine finely by operations other than a pure drilling or cutting. Normally, therefore, it is preferred to use a slide, which is shaped as or with a circular cylindrical body as provided with the necessary concave part-cylindrical surface portion or portions for cooperation with the rotor surface portions.

A screw compressor of this type is known from the Swedish Patent Specification No. 225.172.

From this publication it clearly appears that the slide is shaped as disclosed above and that an accurate guidance of the 2 slide is achieved partly by the slide being incorporated in a bore recess extending over somewhat more than 1800C, whereby the slide is prevented from displacing itself transversely against the rotors, and partly by the slide furthermore being prevented from rotational movement in the bore recess by means of a fixed bottom guide block as received in a longitudinal bottom groove in the slide; otherwise a pure rotation of the slide in the bore recess could result in the said part-cylindrical surface portions abutting one of the screw rotors at one side thereof.

The said Swedish Patent Specification also shows the end wall plate member in the compressor chamber which is referred to in the introductory clause of claim 1. This, however, is without any direct connection with the problems relating to the exact guidance of the slide, and it should be only briefly mentioned, therefore, that this end plate member serves the special purpose of forming a planar wall member at the discharge end of the rotors and hereby to possess the port opening constituting the discharge port for the gas as compressed in the compressor; ideally this discharge port has a rather complicated shape, and it is much easier to work out the port in a separate insert member rather than in the compressor housing casting itself.

The invention is based on the consideration that the said separate end wall plate is additionally and is highly advantageously usable as an active guiding element for the regulation slide, whereby the said special and in fact quite complicated arrangement for preventing rotational movement of the slide is entirely avoidable. The end wall plate being an insert member in the compressor housing, its outer edge shall have to be located very near the circymcylinder of the rotors.Also the said partcylindrical surfaces of the slide or slides should be located "very near" the relevant portions of this circumcylinder, and on this background it is possible to utilize the end wall plate as an active guide for the partcylindrical surfaces of the slide or slides this only requiring a high accuracy working of that or those edge portions of the end wall plate which extend transversely across the partcylindrical slide surfaces. When these surfaces slidingly engage the finely worked edge portions of the end wall plate the slide will be stabilized against both a general transverse displacement against the rotors and against a rotation partly with the same result, and it will be entirely unnecessary, therefore, to secure the slide against rotation by further and more complicated guiding means.

Accordingly the screw compressor of the present invention is characterized by the features stated in the characterizing clause of claim 1.

In the following the invention is described in more detail with reference to the drawing, in which Figure 1 is a length sectional view of a screw compressor according to the invention, Figure 2 is a cross sectional view thereof, and Figure 3 is a plan view of an end wall plate member of the compressor.

The compressor shown comprises a compressor housing 2 having a twin cylindrical space 4 for receiving a pair of non-illustrated, mutually engaging screw rotors of conventional design. In the left hand side of Fig. 1 an intake port is designated 6, and to the right is shown a discharge port 8 as also shown in dotted lines in Fig. 2. As shown in both Fig. 1 and 2 a pair of adjustment slides 10 and 12 is arranged underneath the twin cylindrical space 4. An outer slide 10 of this pair is received in a halfcylindrical guiding groove in the compressor housing 2 and has, itself, an inner guiding groove of undercut or over-halfcircular cross section for slidably receiving the other slide 12. The outer slide 10 is a capacity adjustment slide, which cooperates with the rotor surfaces along mutually separated surface areas 14.This slide is displaceable by means of a piston 16 in a control cylinder 18 such that the front end of the slide underneath the rotors, to the left in Fig. 1, may be displaced more or less away from the intake port 6 of the compressor housing and thereby restrict the effective operative length of the rotors. From its guiding area inside the compressor housing the slide 10 projects outwardly beyond the end wall area of the compression chamber 4 at the discharge end thereof.

The inner slide 12 is a volume ratio adjusting slide which is displaceable by means of a control cylinder 20. This slide projects outwardly through the end wall area of the compression chamber 4 at the intake end thereof and projects inwardly underneath the rotors to a front end area, which, by adjustment of the slide position, can be spaced more or less from the exhaust end of the rotors. The front end of the slide 12 is shaped with a radial discharge channel 22, which, by said adjustment, in well known manner conditions a decreasing volume ratio, i.e. a decreasing degree of compression, the higher the distance between the channel 22 and the discharge port 8 is. This slide, however, will not be described in more detail, as it is not of particular significance in the present connection.

As apparent from Fig. 1 the compressor housing 2 is of the type, in which the compression chamber portion is made in one piece with the high pressure end portion of the housing, while the intake or low pressure end portion is a separately produced, affixed unit. This design offers many advantages, but also the difficulty that it is extra complicated to produce a finely worked guide for the slide 10 other than by a pure drilling or lathing of the guide, this of course irrespective of the compressor having one or more adjustment slides. It is for this reason the slide will normally be of a circular-cylindrical cross section as far as its surface portion cooperating with the guide of the compressor housing is concerned.

The guiding groove of the compressor housing has to extend all the way through the end wall area of the compression chamber at the discharge end thereof, whereby at this place the required working should result in a whole-cylindrical hole.

In many known compressors this is a substantial difficulty, because it has to be accepted that the discharge port 8 cannot be worked out with the desired size and shape, inasfar as the wall material, in which the ideal port should be provided, has simply been entirely removed by the shaping of the guide for the slide.

It is known, however, according to the said Swedish Patent Specification, to let the high pressure end wall portion of the compression chamber be constituted by a special plate member insert, in which the discharge port is provided. Hereby an ideal shaping of the discharge port will be possible not only principally, but even practically in that a separate plate member insert is much easier to work than an area inside a heavy or half-heavy piece of casting.

The compressor according to the invention comprises such a plate member insert, which is designated 24 in Fig. 1 and is shown in a planar view in Fig. 3. The plate member may be unitary, or it may be devided e.g. in two parts as illustrated by a vertical middle line in Fig. 3.

This plate member is provided with the discharge port 8 in the ideal shape thereof.

Hereafter, again, should be considered the said slide guiding of the compressor housing as presenting problems which are quite independent of the ideal shaping of the discharge port. The slide has to be guided such that viewed from the rotors it constitutes a rigid, though axially displaceable wall portion of the compressor chamber, i.e. the surface portion or portions of the slide as facing the rotors should remain located extremely near the cooperating rotor outside or -sides, but without any possibility of getting radially displaced into contact therewith. This is a question of extremely fine tolerances.

A general transverse displacement of the slide against the rotors may be counteracted as disclosed in the said Swedish Patent Specification, viz. by mounting the slide in a guiding groove, the cross section of wHich stretches over more than 1800, but a remaining problem is that the slide will be turnable in the partcircularcylindrical guiding groove. By such turning the partcylindrical surface or surfaces of the slide as cooperating with one or both rotors will be displaced towards the rotor surface at one side or edge of the said partcylindrical surface or surfaces, and for this reason it has been - and is - of utmost importance to secure the slide against carrying out any turning or rotational movement in the partcylindrical guiding groove.

Traditionally, as also disclosed in a detailed manner in the Swedish Patent Specification, this is achieved by providing the slide with a longitudinal narrow guiding groove, into which a fixed guide member as projecting from the wall of the slide guiding groove projects. However, because of the required fine tolerances with respect to the transverse movability of the slide, even this traditional guiding arrangement should be worked out with a very high degree of precision, which is in practice very costly.

In connection with the invention this traditional and troublesome manner of securing the slide against rotational movement is entirely avoided, viz. on account of the simple circumstance that the slide can be totally stabilized in its transverse direction by arranging for the said partcylindrical surface or surfaces of the slide as cooperating with the rotor or rotors to guidingly engage the corresponding edge areas of the said insert plate member as already in advance extending across these partcylindrical surface portions. The sole difference will be that the relevant edge areas of the insert plate member should be finely worked for an active guiding engagement with the relevant slide surface portion or portions, while in the prior art they could be left unworked or coarsely worked so as to be substantially tightening against the surface portions of the slide. With the relevant fine tolerances, however, such a "substantial tightening" is in no way the same as a real mechanical guiding engagement, even though the difference by measure can be very small, and this is emphasized by the fact that the said Swedish Patent Specification prescribes a separate and traditional antirotation locking of the slide, while this kind of locking arrangement can be entirely avoided by the present invention.

According to the invention an equally good antirotation locking is achievable in a much simpler manner just by a fine working of the relevant edge portions of the insert plate member, such working being relatively easy to carry out just because the plate member is a separately workable insert member.

Claims (2)

1. A screw compressor of the type having at least one adjustment slide mounted in a guiding groove in the compressor housing and stabilized against movement crosswise to its sliding direction, and wherein the end wall of the compression chamber at the discharge end thereof is constituted by an optionally two-piece plate member insert, which is separately provided with notch portions constituting discharge port means, characterized in that the said end plate member additionally constitutes a means for stabilizing the slide, the edge area of the end plate member as stretching across the slide surface portion facing the screw rotors being finely worked for a direct guiding engagement with this surface portion.

2. A screw compressor substantially as herein described with reference to the accompanying drawings.