SCREW ROTORS TYPE MACHINE

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: SCREW ROTORS TYPE MACHINE This invention relates to rotary positive displacement machines of the screw or helical rotors type, normally used as a fluid compressor such as an air compressor 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 a plurality of 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 a plurality of 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 αrooves on the female rotor and the walls of the casinσ 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 face of the rotors 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. The purpose for modifying the profile design is to reduce the leakage through the blow hole on both the discharge and induction sides of the compressor, and to reduce the leakage between the rotor tips and the bore which leakage path is especially important in the case of the slower rotating, usually female, rotor. A 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. 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. Normally when the machine is operated as a compressor there is no differential pressure between adjacent lobes 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. The requirement for sealinα on the induction side is less critical and the profile will not be designed to facilitate efficient sealing in this area. Simi] arly when the machine is operated as an expander there is no differential pressure between adjacent lobes on the discharge side which is at a low pressure. 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. It is the objective of this invention to produce a profile which reduces the size of 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. In the case of a machine which operates simultaneously as an expander and compressor both the induction side blow hole and discharge side blow hole should 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. According to the present invention, a rotary positive displacement machine comprises a pair of rotors provided with intermeshing screw or helical lobes housed in casing means; and drive means for separate driving of the rotors; a first of the rotors, the male rotor, having a lobe cf symmetric transverse profile, with varying centres of curvature; the profiles of the rotors being produced by a co-generation process while the opposite flanks of the female rotor lobe diverge at the periphery of the lobe. 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, 5T) 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 sav 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 line 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 may be 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 j yr and C R are the pitch circles of the male and female rotors respectively, while Cjyjo and Cpg re t e corresponding outside circles. Typically, 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 zone 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 at 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 choke 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 oor .

Claims: CLAIMS 1. A rotary positive displacement machine comprises 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) characterised in that a first of the rotors, the male rotor (2) , has a lobe (4) of symmetric transverse profile, with varying centres of curvature,- in that the profiles of the rotors (2, 3) are produced by a co-generation process. 2. A machine as claimed in claim 1, characterised in that the slower rotating rotor (usually the female) has a peripheral land (12) on the lobes (5) of relatively wide form providing a long leak path between the peripheral lands (12) and the casing means (6) . 3. 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, 4. A machine as claimed in any one of the preceding claims, characterised in that a root portion (RS) of the female rotor (3) between lobes corresponds to the tip of the male rotor 4, and the trailing and leading surfaces of each female lobe (5) have a first portion (5V) up to a transition point (V) of concave form, and a final portion (VW) to the female tip of substantially straight form, the final portions (VW) of each female lobe (5) diverging. 5. A machine as claimed in claim 4, 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 a corner point (W) of the female lobe tip. 6. A machine substantially as hereinbefore described with reference to and as illutrated in the accompanying drawings.

Inventors: MCGRUER JOHN, RIACH ALAN BRYSON, MCGRUER, JOHN, RIACH, ALAN, BRYSON
Applicants: FLEMING THERMODYNAMICS LTD, FLEMING THERMODYNAMICS LTD.
Application year: 1993
Main classification: F01C1
IPC codes: F01C1