SCREW ROTOR DEVICE

Abstract

A screw rotor device includes a first rotor having a plurality of helical lobes about its periphery, a second rotor having a plurality of helical flutes about its periphery and a housing with a high pressure port and a low pressure port. The first and second rotors are rotatably mounted within the housing such that the lobes intermesh with the flutes for conveying, in use, a fluid between the high pressure port and the low pressure port. At least one of the rotors has a helical profile that changes along its length such that a gap is described between intermeshing lobes and flutes adjacent the high pressure port which is larger than a gap described between intermeshing lobes and flutes adjacent the low pressure port. The screw rotor device may be used for conveying a fluid.

Claims

1. A screw rotor device comprising a first rotor having a plurality of helical lobes about its periphery, a second rotor having a plurality of helical flutes about its periphery and a housing with a high pressure port and a low pressure port, the first and second rotors being rotatably mounted within the housing such that the lobes intermesh with the flutes for conveying, in use, a fluid between the high pressure port and the low pressure port, wherein at least one of the rotors has a helical profile that changes along its length such that a gap is described between intermeshing lobes and flutes adjacent the high pressure port which is larger than a gap described between intermeshing lobes and flutes adjacent the low pressure port.

2. Device according to claim 1, wherein the size of each flute adjacent the high pressure port is greater than its size adjacent the low pressure port.

3. Device according to claim 2, wherein the circumferential width of each flute adjacent the high pressure port is greater than its circumferential width adjacent the low pressure port.

4. Device according to claim 2, wherein the size of each lobe of the first rotor adjacent the low pressure port is greater than its size adjacent the high pressure port.

5. Device according to claim 4, wherein the circumferential width of each lobe adjacent the low pressure port is greater than its circumferential width adjacent the high pressure port.

6. Device according to claim 1, wherein the gap is described between driving surfaces of the intermeshing lobes and flutes.

7. Device according to claim 1, wherein the rotors are driven in a first respective direction and the gap is described between a trailing surface of each intermeshing lobe and a facing surface of a cooperating flute.

8. Device according to claim 7, wherein a further gap is described between a leading surface of each intermeshing lobe and a facing surface of a cooperating flute.

9. Device according to claim 1, wherein the rotors are driven in a first respective direction and the gap is described between a leading surface of each intermeshing lobe and a facing surface of a cooperating flute.

10. Device according to claim 1, wherein the at least one rotor has a helical profile that changes either gradually along at least part of its length or as a step transition part way along its length.

11. Device according to claim 1, wherein the at least one rotor has a helical profile that is constant along part of its length.

12. Device according to claim 1, wherein the at least one rotor comprises the first rotor.

13. Device according to claim 1, wherein the at least one rotor comprises the second rotor.

14. Device according to claim 1, wherein the device is free of lubricating oil.

15. Device according to claim 1, wherein the device is free of timing gears.

16. Device according to claim 1, comprising one or more further rotors each having helical lobes about its periphery and/or one or more further rotors each having a plurality of helical flutes, the further rotor or rotors being rotatably mounted within the housing such that the lobes or flutes intermesh with corresponding flutes or Lobes on another of the rotors for conveying, in use, a fluid between the high pressure port and the low pressure port.

17. Device according to claim 1, further comprising an expander, wherein the high pressure port comprises an inlet of the expander and the low pressure port comprises an outlet of the expander.

18. Device according to claim 1, further comprising a compressor, wherein the low pressure port comprises an inlet of the compressor and the high pressure port comprises an outlet of the compressor.

19. A steam generator comprising a screw rotor device comprising a first rotor having a plurality of helical lobes about its periphery, a second rotor having a plurality of helical flutes about its periphery and a housing with a high pressure port, the first and second rotors being rotatably mounted within the housing such that the lobes intermesh with the flutes for conveying, in use, a fluid between the high pressure port and the low pressure port, wherein at least one of the rotors has a helical profile that changes along its length such that a gap is described between intermeshing lobes and flutes adjacent the high pressure port which is larger than a gap described between intermeshing lobes and flutes adjacent the low pressure port.

20. A method of manufacturing a screw rotor for a screw rotor device, the method comprising forming at least one helical lobe or flute having a constant depth and a circumferential width that varies along its length.

21. A method according to claim 20 comprising forming the helical lobe or flute with leading and trailing surfaces, wherein only one of the leading and trailing surfaces varies along the length of the rotor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

[0031] FIG. 1 is a schematic view of a rotor device according to an embodiment of the invention;

[0032] FIG. 2 is a perspective view of a first rotor incorporated in the device of FIG. 1;

[0033] FIG. 3 is a perspective view of a second rotor incorporated in the device of FIG. 1;

[0034] FIG. 4 is a perspective view of the rotors of FIGS. 2 and 3 in an intermeshing relation illustrating their use in an initial stage of an expander configuration;

[0035] FIG. 5 is a perspective view similar to that of FIG. 4 illustrating an intermediate stage of the expander configuration;

[0036] FIG. 6 is a perspective view similar to those of FIGS. 4 and 5 illustrating a final stage of the expander configuration;

[0037] FIG. 7 is a perspective view of the rotors of FIGS. 2 and 3 in an intermeshing relation illustrating their use in an initial stage of a compressor configuration;

[0038] FIG. 8 is a perspective view similar to that of FIG. 7 illustrating an intermediate stage of the compressor configuration;

[0039] FIG. 9 is a perspective view similar to those of FIGS. 7 and 8 illustrating a final stage of the compressor configuration;

[0040] FIG. 10 is an end view of the arrangement of FIGS. 4 to 9 from the low pressure end at the right of the Figures;

[0041] FIG. 11 is an end view of the arrangement of FIGS. 4 to 9 from the high pressure end at the left of the Figures;

[0042] FIG. 12 is a schematic representation of rotor helical profiles at each end of a rotor pair configured according to another embodiment overlaid to illustrate the differences therebetween; and

[0043] FIG. 13 is a perspective view of the first rotor being formed by a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0044] Referring now to the Figures, there is shown a screw rotor device 1 according to an embodiment of the invention. In this embodiment, the screw rotor device 1 includes a housing or casing 2, a first rotor 3 and second rotor 4 both rotatably mounted within the housing 2, a coupling shank 5 connected to the first rotor 3 and extending from one of its ends, a high pressure port 6 located at a first end of the housing 2 and a low pressure port 7 located at a second end of the housing 2. The screw rotor device 1 according to this embodiment may be configured to operate as either an expander, in which case the coupling shank 5 may be coupled to a generator (not shown), or as a compressor, in which case the coupling shank 5 may be coupled to a drive motor (not shown). The high pressure port 6 functions as an inlet 6 when the screw rotor device 1 is configured to operate as an expander or an outlet when configured to operate as a compressor. Similarly, the low pressure port 7 functions as an inlet when the screw rotor device 1 is configured to operate as a compressor or an outlet when configured to operate as an expander.

[0045] The first rotor 3 includes a shaft 30 with four helical lobes 31 arranged about the periphery of a central region thereof. Each lobe 31 includes a pair of concave surfaces 32a, 32b that converge to a peak 33. A helical trough 34 is defined between each lobe 31 of the first rotor 3. In this embodiment, each trough 34 includes a root surface 34a which is part-cylindrical. The shaft 30 includes a mounting portion 35 at each side of the central region for rotatably mounting the first rotor 3 within the housing 2. In this embodiment, the coupling shank 5 is formed integrally with the shaft 30 of the first rotor 3 and extends from one of its ends and out of the housing 2.

[0046] The second rotor 4 includes a shaft 40 with five helical flutes 41 arranged about the periphery of a central region thereof. Each flute 41 is described by a concave surface 42 between a pair of adjacent ridges 43. Each ridge includes a peak surface 44 which is part-cylindrical, having a similar shape to the root surface 34a of the first rotor 3 but a slightly smaller size such that the surfaces cooperate in use. The shaft 40 also includes a mounting portion 45 at each side of the central region for rotatably mounting the second rotor 4 within the housing 2.

[0047] As illustrated in FIGS. 4 to 9, the first and second rotors 3, 4 are rotatably mounted within the housing 2 in a side-by-side relationship such that the helical lobes 31 are received within the helical flutes 41 in the normal way. In use, the first rotor 3 and second rotor 4 rotate in opposing directions causing the lobes 31 to intermesh with the flutes 41 and the direction of rotation of the rotors 3, 4 governs the conveying direction of the fluid along the screw rotor device 1.

[0048] Referring now to FIGS. 4 to 6, operation of the device 1 by rotating the rotors 3, 4 in a first direction will result in a conveying direction from the high pressure port 6 to the low pressure port 7. In this configuration, the device 1 operates as an expander. In this configuration, the first rotor 3 rotates in a clockwise direction when viewed along the longitudinal axis of the rotor pair 3, 4 from the end adjacent the high pressure port 6. The second rotor 4 rotates in a counter-clockwise direction when viewed from the same direction. The conveying direction is from the high pressure port 6 towards the low pressure port 7.

[0049] As illustrated by the darkened portions in FIG. 4, a pressurised fluid enters portions of the helical troughs 34 of the first rotor 3 and the helical flutes 41 of the second rotor 4 from the high pressure port 6. This high pressure fluid F forces rotation of the rotors 3, 4, as it is conveyed along the device 1, as illustrated in FIG. 5, and out to the low pressure port 7, as illustrated in FIG. 6.

[0050] Referring now to FIGS. 7 to 9, operation of the device 1 by rotating the rotors 3, 4 in a second direction, opposite the first direction will result in a conveying direction from the low pressure port 7 to the high pressure port 6. In this configuration, the device 1 operates as a compressor. In this configuration, the first rotor 3 rotates in a counter-clockwise direction when viewed along the longitudinal axis of the rotor pair 3, 4 from the end adjacent the high pressure port 6. The second rotor 4 rotates in a clockwise direction when viewed from the same direction.

[0051] As illustrated by the darkened portions in FIG. 7, a fluid F enters portions of the helical troughs 34 of the first rotor 3 and the helical flutes 41 of the second rotor 4 from the low pressure port 7. This fluid F is forced along the conveying direction by rotation of the rotors 3, 4, as illustrated in FIG. 8, and out to the high pressure port 6, as illustrated in FIG. 9. Rotation of the rotors 3, 4 is induced by a drive motor (not shown).

[0052] In accordance with the invention and as illustrated more clearly in FIGS. 10 and 11, at least one of the rotors 3, 4 has a helical profile that changes along its length to create a gap G described between intermeshing lobes 31 and flutes 41 at a first end 10 of each of the rotors 3, 4, corresponding to a region adjacent the high pressure port 6. The gap G at the first end 10 is larger than a gap g described between intermeshing lobes 31 and flutes 41 at a second end 11 of each of the rotors 3, 4, adjacent the low pressure port 7. In this embodiment, the smaller gap g is illustrated as a zero gap. More particularly, the first rotor 3 and second rotor 4 come into contact with one another in the region indicated by the arrow in FIG. 10, in the region adjacent the low pressure port 7. The contact region between the first rotor 3 and second rotor 4 extends along the rotor set for most, approximately 70% in this embodiment, of the threaded length from the end adjacent the low pressure port 7. Contact between the first rotor 3 and second rotor 4 is required along at least part of the threaded length to allow the rotation of one of the rotors 3, 4 to drive the other. It will be appreciated that the contact region may extend along any amount from 0% up to nearly 100% of the threaded length, depending on the specific requirements of a particular application.

[0053] In the region adjacent the high pressure port 6, the first rotor 3 and second rotor 4 do not contact in the corresponding area, indicated by the arrow in FIG. 11. When the first rotor 3 rotates counter-clockwise and second rotor 4 rotates clockwise the gap G is formed between one of the concave surfaces 32a, 32b of the helical lobe 31 and the facing surface of a cooperating flute 41. It will be appreciated by those skilled in the art that the gap G is defined between the driving surfaces 32a, 42 of the rotors 3, 4. More particularly, in the expander configuration the second rotor 4 drives the first rotor 3 and the concave surface 32a describing part of the gap G is on a trailing, driven side of the lobe 31. In the compressor configuration, the concave surface 32a describing part of the gap G is on a leading, driving side of the lobe 31.

[0054] In this embodiment, absence of contact between the first rotor 3 and second rotor 4 is achieved by the second rotor 4 having a helical profile that changes gradually along its length, beginning at a location approximately 70% therealong. It twill be appreciated, however, that the change in helical profile may be sudden or abrupt and may occur at any point along its length. The change in helical profile in this embodiment is achieved by increasing the size, more particularly the circumferential width, of the flute 41 of the second rotor 4 in the region adjacent the high pressure port 6 relative to the size of the flute 41 in the region adjacent the low pressure port 7.

[0055] Referring now to FIG. 12, there is shown a cross-sectional view of the first rotor 103 and second rotor 104 according to another embodiment, which is more particularly suited to functioning as an expander. The dashed line represents the cross-sectional profile in the region adjacent the low pressure port 7, while the solid line represents the cross-sectional profile in the region adjacent the high pressure port 6. The first rotor 103 according to this embodiment differs from the first rotor 3 according to the embodiment described above in that its profile also varies along its length. More particularly, the gap G in this embodiment is created in part by a difference d in the concave surface 132a adjacent the high pressure port 6 relative to the concave surface 132a adjacent the low pressure port 7. Moreover, the second rotor 104 differs from the second rotor 4 according to the embodiment described above in that the profile of the second rotor 104 is inverted as compared to that of the second rotor 4 described previously. The profile of the second rotor 104 according to this embodiment is better suited to the expander configuration due to the profile of the driving surfaces 142.

[0056] FIG. 13 illustrates a milling operation for manufacturing the first screw rotor 103 using a cutting tool 8 coupled to a driving means (not shown) configured to rotate the cutting tool 8 about its centre as indicated by the arrow. In use, a solid metal bar (not shown) is mounted such that it is rotated about its longitudinal axis. The cutting tool 8, while rotating about its centre, makes a pass along the longitudinal axis of the metal bar to create a helical profiled screw rotor 103 through the relative rotation between the cutting tool 8 and metal bar. To make a change to the profile of the screw rotor 103 during the milling process, a small change can be made in the orientation of the rotational axis of the cutting tool 8 relative to the longitudinal axis of the metal bar 9 and/or the cutting path may be changed. Other methods are also envisaged.

[0057] It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention. For example, although the helical profile change is described as being applied to the second rotor 4 only this need not be the case and the helical profile change may be applied to the first rotor 3 only or both first rotor 3 and second rotor 4.

[0058] Additionally, the helical profile change of the second rotor 4 may be applied through an increase in the depth of the flutes 41, an increase in the circumferential width of the flutes 41 or a combination of the two. Additionally or alternatively, the helical profile of the first rotor 3 may be changed by reducing the size of the helical lobes 31, for example in the region adjacent the high pressure port 6, such as by reducing their circumferential width or height or a combination of the two.

[0059] The first and second rotors 3, 4 may have any number of helical lobes 31 and flutes 41 respectively and they may be of any profile shape. Additionally, the contact portion need not extend along 70% (or even most) of the length of the rotors, for example the contact portion may be any other single continuous length of contact or there may be multiple sections of intentional contact and non-contact along the rotor pair 3, 4. The contact portion need not be continuous and may extend between 0 and 100% of the length of the rotors.

[0060] It will also be appreciated by those skilled in the art that any number of combinations of the aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.