EP0627041B1

EP0627041B1 - Screw rotors type machine

Info

Publication number: EP0627041B1
Application number: EP93904233A
Authority: EP
Inventors: Alan Bryson Riach; John Mcgruer
Original assignee: Sprintex Australasia Pty Ltd
Current assignee: Sprintex Australasia Pty Ltd
Priority date: 1992-02-19
Filing date: 1993-02-19
Publication date: 1999-09-29
Anticipated expiration: 2013-02-19
Also published as: EP0627041A1; DE69326606D1; DE69326606T2; WO1993017223A1; GB9203521D0

Abstract

A rotary positive displacement machine for operation both as a compressor and an expander comprises a casing (6) housing intermeshing male and female rotors (2, 3), the casing (6) including a fluid inlet and a fluid discharge for fluid to be compressed or expanded, and drive means (9, 10) enabling driving of the rotors (2, 3) separately from each other so that the machine can operate as a dry machine. The rotors (2, 3) intermesh via lobes (4, 5) whose profiles are produced by a co-generation process, the male lobe (4) having a symmetric transverse profile of varying curvature while the opposite flanks (5L, 5T) of the female lobe (5) are generally concave but with a substantially straight portion at the tip arranged such that each female lobe (5) diverges at the tip. The machine enables leakage blow holes to be reduced both for the compressing and expansion modes of operation.

Description

This invention relates to rotary positive displacement machines of the screw or helical rotors type, normally used as fluid compressors such as air compressors and to the rotors for use in such machines. The invention relates to the dry type screw compressor where no lubricating oil passes through the working zones of the machine and the rotors are timed by the use of timing gears positioned outside the working chambers of the rotors which allow the rotors to rotate without coming into contact with each other. This contrasts with wet type screw compressors where one of the rotors (usually the male rotor) engages and drives the other rotor and to facilitate this driving operation lubricating oil in this case is passed through the rotors of the machine - such a wet type screw compressor is described in U.S. Patent 4 673 344. The rotor profile of such a machine must therefore provide suitable driving surfaces.
The aforementioned dry type rotary machine includes a housing having at least one pair of intersecting bores therein. Inlet and outlet ports are provided at opposite ends of the casing bores. A rotor is mounted for rotation within each of the bores.
One of these rotors is of the male type which includes one or more helical lobes and intervening grooves which lie substantially outside the pitch circle thereof with the flanks of the lobes having a generally convex profile.
The other rotor is of the female type and formed so that it includes one or more helical lobes and intervening grooves which lie substantially inside the pitch circle thereof with the flanks of the grooves having a generally concave profile.
The lobes on the male rotor cooperate with the grooves on the female rotor and the walls of the casing to define chambers for the fluid. These chambers may be considered to be chevron shaped.
Fluid enters the casing bores through the inlet port filling the cavities which are generated as the rotors come out of mesh on the low pressure side of the rotors. Fluid normally continues to fill the generated chambers until the chambers reach maximum volume. The filling chamber is then cut off from communication with the inlet port and the gas is compressed within the chamber. The chamber boundaries consist of the leading and trailing flanks of the male and female lobes and the bores in which the rotors rotate.
In a dry type screw compressor the rotors must not be allowed to touch each other and therefore mesh together with a small clearance between the meshing flanks of the rotors and between the rotor tips and the case. These clearances cause leakage paths through which the gasses being compressed may escape from one chamber to another.
One leakage path is the sealing line between adjacent male and female rotor flanks as the rotors mesh. The product of the length of this line and the required clearance forms a leakage path as described in US patent 2 622 787.
Another important leakage path is between the rotor tips and the casing bore, especially in the case of the slower rotating rotor, in which case there is a relatively long time during which leakage flow can occur. The width of the rotor tips is an important factor as this defines the length of the path through which the gas must pass before leaking to an adjacent chamber. A long path provides an effective means of reducing the leakage.
A third major leakage path is the channel between the male and female rotors and the cusp of the intersecting rotor bores sometimes described as the blow hole or leakage triangle. This blowhole occurs at both cusps where the rotors mesh.
Among the known rotor profiles is one where the flanks of each rotor are substantially formed from a circular arc. Such a profile, illustrated in US patent 2 622 787 gives a short sealing line length, but results in a large blow hole. Another commonly used profile is the asymmetric profile, also described in US patent 2 622 787. In this profile, one flank of each female rotor flute is defined as a circular arc or a section of an ellipse. The remaining female flank is generated by points near the outside of the meshing male flank. Both male flute flanks are generated by points on the female rotor. The claimed advantages of this asymmetric profile are that it reduces the size of the blowhole at one cusp and that the chambers formed have a large volume for a given rotor diameter. This profile has a large blowhole at the other cusp but this is not important if the machine is used as a simple compressor or expander where compression or expansion occurs on one side only. When used in oil lubricated compressors there is a requirement for one rotor to drive the other by contact between the lobe faces. This normally requires the female rotor to have a significant addendum, that is the outer diameter of the female rotor is significantly larger than the pitch circle, and that the flanks of both rotors have smooth curves to transmit the driving force. The addendum of the female rotor is normally of convex curvature and is commonly formed by a radius linking the flank to the tip as shown in US patent 2 622 787 and GB patent 2 112 460. This radius increases the size of the blowhole and results in a rotor tip which is short in width and thus which has a short leakage path. The provision of driving flanks also normally requires the male rotor to be rounded at the tip, typically having a radius linking the flanks to the tip, further increasing the size of the blowhole. The compressor of GB-A-2 112 460 like that of US-A-2 622 787 has intermeshing male and female lobed rotors, and in this case the male lobe has an asymmetric transverse profile with flanks having varying centres of curvature while the flanks of the female lobe also define an asymmetric transverse profile with varying centres of curvature.
Normally when the machine is operated only as a compressor there is no differential pressure between adjacent chambers on the induction (low pressure) side of the rotors, indeed the bores may be scalloped back from the rotor tips allowing communication of fluid between adjacent lobes giving better lobe filling on the induction side. The blow hole size on the discharge (high pressure) side will be minimised to facilitate efficient compression of the fluid. If the machine is operated only as an expander the direction of flow and of rotation of the rotors is reversed. The blow hole size on the induction (high pressure) side will be minimised to facilitate efficient expansion of the fluid. The requirement for sealing on the discharge side is less critical and the profile will not be designed to facilitate efficient sealing in this area.
In the case of a machine which operates simultaneously as an expander and compressor, the expansion takes place on one side of the machine and the compression on the other side. Both the induction side blow hole and discharge side blow hole should therefore be kept to a minimum in order to reduce leakage between adjacent lobes. Such an application where the screw machine would be used as an expander and compressor simultaneously would be to regulate the mass flow of air delivered to an internal combustion engine replacing the throttle valve. The air is expanded inside the expander/compressor then recompressed and delivered to the engine.
A rotor profile proposed in GB patent 1 145 893 claims to reduce the blow hole on both sides of the rotor mesh. It has lobes which are symmetric about a line extending radially from the rotor centre line. The male rotor lobe is described as having a curved outer portion joining the flank to the tip, the remainder of the male rotor flank being generated by points on the female rotor. The said curved portion generates the majority of the female rotor flank except for a portion near the tip which is convex in nature and is normally constructed as a segment of an arc joining the flank to the tip. Because the majority of the rotor flanks are generated, the blowhole is reduced compared to other known profiles such as the circular profile described above. Also, because the lobes are symmetric, the blowhole will be equal on both sides. However, the provision of convex curved portions on the male and female rotors between the generated flanks and the rotor tip circle means that the rotors do not mesh perfectly at the cusps and a blowhole is still formed, as shown in figure 9 of the patent. GB-A-1 145 893 provides the preamble of appended claim 1.
It is the objective of this invention to produce a profile which substantially eliminates the leakage blow hole on the induction side and on the discharge simultaneously. A further objective is to reduce the leakage between the rotor tips and the bore by maximising the length of the leakage path across the tips.
This objective is met by the provision of a rotary positive displacement machine including the features set out in claim 1.
Preferably the slower rotating rotor (usually the female) has a peripheral land on the lobes of relatively wide form providing a long leak path between the peripheral lands and the casing means.
The basic rotor profile is a combination of point and line generated and is symmetric in nature.
Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings wherein:
Fig 1 - shows a plan view of a screw type positive displacement machine;
Fig 2 - is a transverse section through the rotors of the machine showing the rotor profiles as used in an engine supercharger/expander machine; and
Fig 3 - shows the profile of Fig 2 to a larger scale to highlight to rotor profile, while Fig 4 shows a detail of Fig 1.
Referring to Fig 1, a screw compressor or pump 1 comprises intermeshing male and female rotors 2, 3 having helical lobes 4, 5 respectively, housed in a casing 6. Each of the lobes 4, 5 has leading (4L, 5L) and trailing surfaces (4T, ST) respectively
The shafts of the rotors 2, 3 are carried by bearings 7, while a drive (not shown) is connectable to one of the shafts 8. Gears 9, 10 serve for individual driving of the rotors 2, 3, as distinct from an arrangement where one of the rotors is driven directly from the other via lobe engagement. This enables the machine 1 to operate in a dry mode i.e. without lubricant in the rotors as there is no requirement for torque (force) transmission between the rotors, and this is beneficial for the particular uses intended for compressor (pump) machines in accordance with the present invention.
Figs 2 and 3 show the rotor profile preferred for the supercharger of an internal combustion engine. The profile of the male lobe 4 is of sophisticated symmetric form, that is to say other than a simple semi-circular (symmetric) profile, symmetry being present about the mid-axis M-M and the profile curves having different centres of curvature.
The design of rotor profile can be categorised as one of the following:
1. A circular profile, which is in established use in air and gas compressors has the advantage that there are no trapped pockets which produce negative torque. As a result the lobe thickness of the female can be reduced, increasing volumetric displacement. The sealing line length is kept to a minimum reducing leakage and again improving volumetric efficiency. One of the major disadvantages of a circular profile however is that the displacement volume is smaller when compared with a generated profile. A further disadvantage is that blow hole sealing is not optimised in a simple circular profile.
2. Generated profiles can be point generated and, or, line generated. One profile section or point on either the male or female rotor generates the corresponding profile on the meshing rotor. The benefits of a point generated profile is that the blow hole size can be reduced and the volume of fluid which can be displaced is greater than for a comparable circular profile. However, the sealing line length will be greater.
Normally the generated profile is asymmetric in nature in order to minimise the blow hole on one side of the machine whilst simultaneously maximising displacement. This provides the best compromise for a normal compressor or expander application.
The lobe profiles of the present machine 1 are obtained by a co-generation process, that is the profile shape of one lobe is produced by the line traced out by a point or ine on the other intermeshing lobe as it moves.
In the example shown in Fig 3, the profile can be summarised as follows:
For the female rotor 3: the portion RS is generated by the male tip portion SV is generated by point N; and VW is a straight line.
For the male rotor 2: portion NP is generated by point V on rotor 3 portion PQ is generated by line VW on rotor 3; while QT is generated by point W.
C_MP and C_FR are the pitch circles of the male and female rotors respectively, while C_MO and C_FO are the corresponding outside circles.
The portions VW on the opposed lobe flanks of the female rotor 3 diverge at the periphery of the rotor to facilitate rotor manufacture.
In operation of the machine 1, gas is compressed by reaction against the closing lobe flanks as the rotors 2, 3 rotate, and the compressed gas will endeavour to escape via any leakage paths present. One such leakage route is constituted by blow holes or leakage triangles which occur at the tone marked B in Fig 1: Fig 4 shows the detail to a larger scale looking in the axial direction of the machine. The blow holes will be present at both the high pressure and low pressure sides of the machine, but with the machine operating as an ordinary gas compressor with a set operational direction leakage is critical only via one side (the high pressure side) and the size of the blow holes at the other side is not really critical. However, where the machine is to operate both as a compressor (more especially as a supercharger) and separately in an expanding mode for example as arranged for an internal combustion engine as described in US patent 4667646 for example, then it is desirable for the blow hole size to be reduced to the minimum at both sides. The above described machine in accordance with the present invention, especially by virtue of the rotor profile, enables the blow hole size to be reduced an both sides in comparison with prior art machines.
A further feature of the profile is that the peripheral land 12 on at least one of the rotors 2, 3 can be relatively wide so forming a choks gap between the rotor and the casing 6 thereby mitigating against leakage via this path: in the profile of Fig 2 the peripheral land on the slower rotating female rotor is relatively wide. Thus, the leakage between the rotor tips 12 and the bore of the casing 6 is reduced by the present invention.
For precise inlet airflow control, the above machine 1 can be provided advantageously with a suitable variable area inlet port.

Claims

A rotary positive displacement machine comprising a pair of rotors (2,3) provided with intermeshing screw or helical lobes (4,5) housed in casing means (6); and drive means (8,9,10) for separate driving of the rotors (2,3) wherein a first of the rotors, the male rotor (2), has a lobe (4) of symmetric transverse profile, with varying centres of curvature; and the profiles of the rotors (2,3) are produced by a co-generation process, a root portion (RS) of the female rotor (3) between lobes being generated by the tip of the male rotor lobe (4) and the trailing and leading surfaces of each female lobe (5) having a first portion (SV) up to a transition point (V) of concave form, characterised in that a final portion (VW) to the female lobe tip is of straight form, the final portions (VW) of the trailing and leading surfaces of each female lobe (5) diverging from each other in the radially outward direction.
A machine as claimed in claim 1, characterised in that a peripheral land (12) on the lobes (5) of the female rotor (3) is of relatively wide form providing a long leak path between the peripheral lands (12) and the casing means (6).
A machine as claimed in claim 1 or 2, characterised in that the basic rotor profile is a combination of point and line generated and is symmetric in nature.
A machine as claimed in claim 1, 2 or 3 characterised in that said first portion (SV) of the female lobe flanks (5L, 5T) is generated by a tip corner (N) of the male lobe (4), while the flanks (4L,4T) of the male lobe (4) have a first portion (NP) from the male tip generated by said transition point (V), of the female lobe, a second portion (PQ) generated by the straight line (VW) of the female lobe, and a final portion (QT) generated by the corner point (W) of the female lobe tip.