WO2020165561A1

WO2020165561A1 - Screw compressor

Info

Publication number: WO2020165561A1
Application number: PCT/GB2020/050289
Authority: WO
Inventors: Terence William Thomas Young; John Michael Roll; Ashvin DHUNPUT; Maghmood VAN DER POLL
Original assignee: J & E Hall Limited
Priority date: 2019-02-11
Filing date: 2020-02-07
Publication date: 2020-08-20
Also published as: GB2581204B; GB2581204A; GB201901819D0

Abstract

A compressor includes a main rotor (4), formed substantially as a cylinder and rotatable about an axis of the cylinder, the main rotor having helical flutes, and a casing. An inlet port admits gas into a first part of the flutes at a first pressure. A star-shaped sealing gate rotor (12) meshes with the flutes and seals the first part of the flutes from a second part of the flutes. A first star-shaped compression gate rotor (6) meshes with the flutes in the first part for compressing the gas to a second pressure, higher than the first pressure. A duct feeds the gas at the second pressure to the second part of the flutes. A second star-shaped compression gate rotor (10) meshes with the flutes in the second part for compressing the gas to a third pressure, higher than the second pressure. The first and second parts of the flutes are formed by first and second angular sectors (2, 8), defined with respect to the axis of the main rotor (4). The second sector (8), between the sealing gate rotor (12) and the second compression rotor (10), has a smaller volume than the first sector (2).

Description

SCREW COMPRESSOR

Background to the Invention

[0001] This invention relates to a screw compressor.

[0002] A known single screw compressor has a main rotor and at least one meshing, star-shaped gate rotor. The main rotor has a number of helical screw threads, sometimes referred to as "flutes", which are cut with a globoid (or hour glass) shape to the roots of these threads. The flutes have a relatively large cross section at an input end and a significantly smaller cross section at a discharge end. Suction gas enters the flutes at the large openings at the input ends, in a generally axial direction with respect to the main rotor. The gas is then sealed into the flutes by the gate rotor(s) and the cylindrical casing as the rotor assembly rotates, the discharge ends of the flutes normally being closed by the end of the casing. Continued rotation causes the teeth of the gate rotor(s) to progress along the flutes causing a reduction in volume and thus an increase in pressure. The compressor is so designed that when the desired pressure increase has been reached the flute opens to a discharge port in the casing and continued rotation causes the refrigerant gas to be driven out through the discharge port. The design allows for this compression process to be mirrored on both sides of the main rotor by the use of two gate rotors.

[0003] A compound compressor, with two or more compression stages, can be based on the single screw concept described above. One or more extra main rotors are added to provide at least one additional compression stage.

[0004] Thus, in a single screw compressor there could be one low pressure stage main rotor and one high pressure stage main rotor. The low stage would normally have two matching gate rotors to provide two parallel compression stages. The high stage could have one similar diameter rotor with one gate rotor or one smaller size rotor with two small gate rotors. [0005] In such a compound compressor, two main rotors would be required with the appropriate number of gate rotors (minimum one) for each main rotor. One main rotor would form the low stage and one the high stage. Each main rotor casing/housing would be designed to provide the appropriate suction and discharge port, where the low stage discharge and high stage suction would both be at the intermediate pressure. Thus effectively two rotor housings are required to provide these separate pressure requirements, even if combined in one compound casing. These two rotor housings would be sealed from each other in order to provide the different pressure requirements for each stage. Thus the low stage rotor housing would be open to the low stage suction pressure. Similarly the high stage main rotor housing would be open to the high stage suction or intermediate pressure (as this is conventionally called).

Summary of the Invention

[0006] It is an aim of the invention to combine two separate compression stages at different pressures in the same single main rotor.

[0007] There are various problems involved in combining two or more stages of compression in a single rotor.

[0008] Each stage must be separated from the other. Only one stage should be open to the low stage suction. The low stage discharge should be conducted into the high stage suction. There should be a significant difference in the swept volume of the two stages - the high stage typically being one half to one third of the swept volume of the low stage.

[0009] The invention provides a means of achieving these requirements using a single main rotor.

[0010] The invention provides a compressor including: a main rotor, formed substantially as a cylinder and rotatable about an axis of the cylinder, the main rotor having helical flutes; a casing; an inlet port for admitting gas into a first part of the flutes at a first pressure; a star-shaped sealing gate rotor, meshing with the flutes and sealing said first part of the flutes from a second part of the flutes; a first star-shaped compression gate rotor meshing with the flutes in the first part for compressing the gas to a second pressure, higher than said first pressure; a duct for feeding the gas at said second pressure to said second part of the flutes; and a second star-shaped compression gate rotor meshing with the flutes in the second part for compressing the gas to a third pressure, higher than said second pressure; the first and second parts of the flutes being formed by first and second angular sectors, defined with respect to the axis of the main rotor, with the second sector, between the sealing gate rotor and the second compression rotor, having a smaller volume than the first sector.

[0011] For improved sealing, the main rotor may extend beyond an inlet end of the casing.

[0012] In one embodiment, the casing is at the first pressure. The first and second compression gate rotors may have axes that are substantially mutually parallel. The sealing gate rotor may have an axis that is substantially perpendicular to the axis of the first and / or the second compression gate rotor.

[0013] In an alternative embodiment the casing is at the second pressure and the inlet port is at the first pressure.

[0014] The first compression gate rotor and the sealing gate rotor may have axes that are substantially mutually parallel. The second compression gate rotor may have an axis that is substantially perpendicular to the axis of the first compression gate rotor.

[0015] In this way, the invention allows two stage compression with low stage suction, intermediate and high stage discharge pressures all contained in one single rotor and rotor housing, and with a sealing mechanism to allow for this two stage compression with one main rotor and rotor housing. In addition the invention allows for the main compressor casing to be held at either suction (first) or intermediate (second) pressure, so that the design can be optimised for the operating conditions.

Brief Description of the Drawings

[0016] The invention will be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

[0017] Figure 1 schematically shows a compressor according to a first embodiment of the invention;

[0018] Figure 2 schematically shows a compressor according to a second embodiment of the invention;

[0019] Figure 3 is a schematic side view of the compressor of Figure 2; and

[0020] Figure 4 is a schematic perspective view of the compressor of Figure 2. Detailed Description of Particular Embodiments

[0021] Figure 1 shows a compressor in which the low stage side 2 of the main rotor 4 is open directly to the low stage suction in the normal manner. Thus the casing is at Low Stage Suction Pressure. The specially shaped casing around the rotor (not shown in Figure 1) seals the compression process, but leaves a triangular shaped section of the rotor open to the low stage gas, which can flow radially (and axially) into the flutes machined into the rotor 4. A first compression gate rotor 6 is in mesh with the main rotor 4 such that this gate rotor first seals the flute and hence the gas that has entered this flute. Rotation of the main rotor also rotates the gate rotor 6 which in turn reduces the volume trapped within the flute, thus compressing the gas within this flute. This compression continues until the flute volume has been reduced to that required for efficient compression and at this stage the flute is open, not to the normal discharge passage, but to an intermediate pressure gallery, not shown here, which takes the intermediate gas into the suction of the high stage compression. This high stage suction port is positioned radially at the suction end of the flute, but extends axially towards the discharge to provide the ideal high stage volume and pressure ratio.

[0022] As mentioned above there is an open triangular section of the rotor into which the gas can enter the rotor. This section also provides the means by which the gate rotor can be entered into the main rotor in a twisting action, during assembly. Such an opening in the high pressure side/stage of the compressor would leave the high stage open to low stage suction pressure and this opening cannot be closed due to the assembly requirement. Now the high stage volume 8 should, as stated previously, be substantially less than the low stage volume. However the same rotor flute will be used on both sides of the compressor and thus would provide equal volumes in both stages. This invention reduces the volume within the high side, leaves the assembly opening for the low stage gate rotor assembly and seals the high side compression chamber/ flutes.

[0023] The high stage compression gate rotor 10 is located at 180 degrees or as appropriate from the existing low stage gate rotor. A third gate rotor 12 is then added assembled in the opposite direction to the two compression gate rotors. This sealing gate rotor is fitted at 90 degrees (or as required to match the actual compression process) from the high stage compressing gate rotor. This both reduces the volume 8 of the high stage compression process and leaves an opening on the non-sealing side of the sealing gate rotor adjacent to the low stage gate rotor 6 and thus permitting assembly of both the low stage gate rotor and the high stage sealing gate rotor into the casing via this same opening.

[0024] To simplify sealing of the high stage compression, the main rotor 4 can be extended with standard truncation removed. The flow of the intermediate gas via the intermediate gas passage as indicated previously will enter the sealed high pressure side 8 of the main rotor between the compressing and sealing high stage gate rotors 10, 12. This intermediate port will be open to the high stage flutes whilst these flutes are sealed from the low stage, but are increasing in volume. Compression in the high stage will start once the flute volume starts to be reduced by the combination of the rotation of the main rotor 4 and the compression and sealing gate rotors 10, 12.

[0025] The arrows in Figures 1 and 2 denote gas leakage paths.

[0026] Figures 2, 3 and 4 show a second embodiment of the invention, in which it is the low stage suction which is sealed from the casing 34 and the high stage suction (intermediate pressure) which is open to the casing 34. Thus the casing is at intermediate pressure. The sealing gate rotor 32 is positioned 180 degrees or as appropriate from the low stage compression gate rotor 26, to provide the required capacity. The low stage suction will flow directly into the low stage side of the rotor via a sealed gallery 36. The casing 34 is extended to seal this side of the rotor from the casing intermediate pressure with the sealing rotor 32 completing the sealing of this low stage compression process. The discharge from this low stage will be directly into the casing thus maintaining the casing at the low stage discharge or intermediate pressure. The high stage suction will enter the main rotor 24 rotor directly from the intermediate casing. Compression takes place as conventionally without the need for the sealing rotor, but the compression process effected by the high stage compression gate rotor 30 is reduced to 90 degrees, at 28, or that amount required for optimum efficiency. Discharge from this high stage is via a conventional discharge port 38.

[0027] In both arrangements, the sealing gate rotor 12, 32 will rotate in the opposite direction when viewed from the sealing face of the gate rotor compared with both the compression gate rotors 6, 10, 26, 30. The geometry of this sealing gate rotor is specially designed to match this requirement.

[0028] Actual number of flutes in the main rotor, teeth on the stars, teeth/ flute width and depth of engagement can be varied to provide the optimum design to match the required operating condition.

Claims

1. A compressor including a main rotor, formed substantially as a cylinder, rotatable about an axis of the cylinder and having helical flutes; a casing; an inlet port for admitting gas into a first part of the flutes at a first pressure; a star shaped sealing gate rotor, meshing with the flutes and sealing said first part of the flutes from a second part of the flutes; a first star-shaped compression gate rotor meshing with the flutes in the first part for compressing the gas to a second pressure, higher than said first pressure; a duct for feeding the gas at said second pressure to said second part of the flutes; and a second star-shaped compression gate rotor meshing with the flutes in the second part for compressing the gas to a third pressure, higher than said second pressure; the first and second parts of the flutes being formed by first and second angular sectors, defined with respect to the axis of the main rotor, with the second sector, between the sealing gate rotor and the second compression rotor, having a smaller volume than the first sector.

2. A compressor according to claim 1, wherein the main rotor extends beyond an inlet end of the casing.

3. A compressor according to claim 1 or 2, wherein the casing is at the first pressure.

4. A compressor according to claim 3, wherein the first and second compression gate rotors have axes that are substantially mutually parallel.

5. A compressor according to claim 3 or 4, wherein the sealing gate rotor has an axis that is substantially perpendicular to the axis of the first and / or the second compression gate rotor.

6. A compressor according to claim 1 or 2, wherein the casing is at the second pressure, the inlet port being at the first pressure and being sealed from the casing.

7. A compressor according to claim 6, wherein the first compression gate rotor and the sealing gate rotor have axes that are substantially mutually parallel.

8. A compressor according to claim 6 or 7, wherein the second compression gate rotor has an axis that is substantially perpendicular to the axis of the first compression gate rotor.