Separator

1. A separator for separating contaminants from a fluid stream which includes entrained particulate contaminants, comprising:

a first chamber having a cylindrical wall circumscribing a central axis, a first inlet through which a fluid stream enters the first chamber, to flow axially along the first chamber, the cylindrical wall having a plurality of apertures defined by edges, formed in the cylindrical wall through which the fluid stream can pass out of the first chamber,

an impaction surface located downstream of the first chamber, separated from an interior of the first chamber by the cylindrical wall, which is arranged to deflect the fluid stream after the fluid stream leaves the first chamber through the apertures, such that contaminants are separated from the fluid stream, and

an actuator which is arranged to move along an actuator axis which is parallel to the axis of the first chamber to adjust the open cross-sectional area of the apertures in the cylindrical wall according to a pressure differential between fluid pressure in the first chamber and a pressure reference by moving along the cylindrical wall so as progressively to occlude the apertures, the actuator comprising a diaphragm which separates the first chamber from a pressure reference, and comprises a tubular membrane which is fastened towards one end of the cylindrical wall and is arranged to fold and unfold progressively in response to a change in the differential pressure across the diaphragm so as respectively to uncover and cover the apertures, with the apertures having straight edges which extend perpendicular to the actuator axis in a circumferential direction around the cylindrical wall,

in which a first and second ones of the apertures (a) are spaced apart so that there is no axial overlap between them along the actuator axis, and (b) are the closest-spaced to one another along the actuator axis of any pair of the apertures which are spaced apart along the axis, and in which a ratio of the spacing between a closest of the edges of the first and second apertures along the actuator axis to an axial height of a smaller of the first and second apertures is not more than about 1.5, wherein the closest edges of the first and second apertures are straight along their lengths and extend perpendicular to the actuator axis.

2. A separator as claimed in claim 1 , in which each of the first and second apertures is approximately rectangular.

3. A separator as claimed in claim 1 , in which a ratio of a width of each of the first and second apertures measured transverse to the actuator axis to the axial height of the respective aperture is at least about 1.

4. A separator as claimed in claim 1 , in which each aperture has a center, and the center of the second aperture is displaced along the cylindrical wall transversely relative to the center of the first aperture.

5. A separator as claimed in claim 4 , in which the first and second apertures are spaced so that there is no transverse overlap between them along a direction perpendicular to the actuator axis.

6. A separator as claimed in claim 1 , in which the first and second apertures are located close to one another on one side of the cylindrical wall.

7. A separator as claimed in claim 6 , which includes at least a third aperture which is spaced apart from the first and second apertures around the a periphery of the cylindrical wall.

8. A separator as claimed in claim 7 , which includes a fourth aperture which is located close to the third aperture, spaced apart around the periphery of the cylindrical wall from the first and second apertures.

9. A separator as claimed in claim 1 , in which an internal diameter of a cylinder which is defined by the cylindrical wall is at least about 10 mm.

10. A separator as claimed in claim 1 , in which an area of a smaller of the first and second apertures is at least about 0.2 mm 2 .

11. A separator as claimed in claim 1 , wherein the apertures are formed in the cylindrical wall in a direction that is perpendicular to a tangent to a surface of the cylindrical wall.

12. A separator as claimed in claim 11 , wherein the angle between circumferentially spaced apart edge walls of the first aperture or the second aperture and the tangent to the surface of the cylindrical wall at each of the edge walls is not more than about 20°.

13. A separator as claimed in claim 1 , in which the ratio of the spacing between a closest of the edges of the first and second apertures along the actuator axis to an axial height of a smaller of the first and second apertures is at least about 0.05 and not more than about 0.075.