GB2137537A - Centrifuge - Google Patents

Abstract

A centrifuge has a hollow rotor (12) which is open at its lower end and closed at its upper end by means of an upper wall (31) which, at least in a region which extends from the rotor side wall (30) towards the axis of rotation (11) to within a distance from the axis which is less than the distance from the axis to the peripheral edge of the lower end opening (18), is imperforate. The side wall (30) has a portion tapering towards the opening (18). In operation liquid containing solids in suspension is fed via a duct (19) and a cone (35). Separated solids accumulate on the internal surface of the wall (30) while clean liquid discharges via opening (18). To discharge the solids, the rotor is slowed down and the solids and any liquid still within the rotor fall down a receiver (21) and a tube (25). <IMAGE>

Description

SPECIFICATION Centrifuge and method of separating a suspension According to a first aspect of the invention, there is provided a centrifuge comprising a body and a hollow rotor mounted for rotation relative to the body about an axis which extends upwardly and downwardly when the centrifuge is in use, wherein the rotor has an opening at its lower end, has a side wall which converges downwardly towards the opening and has an upper wall which closes an upper end of the rotor, at least in a region which extends from the side wall towards the axis to within a distance from the axis which is less than the distance from the axis to the periphery of said opening.

When a centrifuge in accordance with the invention is in use and a fluid stream comprising more dense and less dense components is directed into the rotor to be carried round with the rotor at high speed, the more dense component will tend to move away from the axis of rotation and remain close to the side wall at a position away from the lower end of the side wall, whilst the less dense component will escape from the rotorthrough the opening at the lower end and then travel away from the axis of rotation. If the more dense component is dislodged from the side wall when the rotor is stationary or is rotating only slowly, that component will tend to fall downwardly through the opening at the bottom of the rotor and leave the rotor in a direction along the axis.Thus, the more dense component can be collected at a position near to the axis whilst the less dense component can be collected at a position further from the axis.

According to a second aspect of the invention, there is provided a method of separating a suspension wherein a hollow member having an open end is rotated about an upright axis with the open end lowermost, the suspension is fed into the rotating, hollow member, a less dense component escapes through the open end and travels therefrom initially in a direction away from the axis and a more dense component is retained in the rotating member and wherein subsequently the member is decelerated and the more dense component is permitted to fall downwardly through the opening.

The more dense component may simply fall downwards when the speed of rotation falls. Alternatively, there may be established a substantial difference between the rotational speed of the hollow member and the rotational speed of fluid present in the hollow member, so that the fluid scours from the rotating member more dense material which remains thereon. In this way, the more dense component can be removed from the hollow member without the use of a cleaning implement. If necessary, the direction of rotation of the hollow member can be reversed one or more times to promote the scouring action of fluid within the hollow member.

The more dense component which falls through the opening of the rotor may be in admixture with the less dense component.

An example of a centrifuge according to the first aspect of the invention and which can be used in a method according to the second aspect of the invention will now be described, with reference to the accompanying drawing which shows diagrammatically a cross-section of the centrifuge in a plane containing a vertical axis of rotation of the centrifuge.

The centrifuge comprises a hollow body 10 which is normally stationary during use of the centrifuge and within which there is mounted for rotation relative to the body about an axis 11 a hollow rotor 12. The body is formed in upper and lower parts 13 and 14 respectively connected by elements 15 which can accommodate slight relative movement of the upper and lower parts and which are capable of damping relative vibration of the body parts. There may be three or four of the elements 15 spaced apart around the axis 11. The elements 15 may be formed of an elastomer.

For driving the rotor 12 about the axis 11, there is provided an electric motor 16 which is mounted in the upper body part 13 to protrude from the upper end of the body. The motor is supported on a horizontal plate 17 comprised by the upper body part. This plate has a central hole through which a shaft of the motor extends along the axis 11. The rotor is suspended by its upper end from the shaft of the motor. By this, is meant that the rotor is supported at its upper end only and that a lower part of the rotor does not contact any other part of the centrifuge.

The rotor 12 is hollow and has at its lower end an opening 18. For feeding into the rotor a fluid stream comprising components of differing density which are to be separated, there is provided feed means including a duct 19 which extends from a side wall of the lower body part 14 towards the axis 11, upwardly through the opening 18 and upwardly within the rotor. The feed means also includes a valve 20, for example a solenoid operated valve, for closing the duct 19, preferably at its lower end.

The centrifuge further comprises a receiver 21 which is disposed within the lower body part 14 and lies directly below the rotor 12. The receiver 21 and feed means are fixed with respect to the body 10. At its upper end, the receiver defines an opening 22 which is of somewhat larger diameter than the opening 18 and is spaced downwardly therefrom by a gap 23. The centre of the opening 22 lies on the axis 11.

An upper part 24 of the receiver 21 has the form of a funnel which leads downwardly to a lower part 25 of the receiver having the form of a cylindrical tube which is co-axial with the rotor 12. At its lower end, the funnel 24 has a diameter somewhat less than that of the tube 25. The tube 25 extends through the bottom of the body 10 to a sludge tank or to a further tube (not shown).

There is formed in the tube 25 at one side of the axis 11, at a position somewhat below the funnel 24, an opening 26. There is mounted on the tube for pivoting relative thereto about an axis 27 which is perpendicular to and is offset from the axis 11 a diverter 28. A part of the diverter lies inside the tube 25 and a further part of the diverter lies outside the tube, the diverter extending through the opening 26.

When the diverter does not bear material which is not part of the diverter, it is biased by gravity into a first position illustrated by the full line in the drawing. In this position, an upwardlyfacing surface 29 of the diverter engages the funnel 24 so that upward movement of that part of the diverter which lies in the tube 25 is prevented. By pivoting about the axis 27, the diverter can move to a second position indicated by a broken line in the drawing. On movement of the diverter from its first position to its second position, that part of the diverter which lies in the tube 25 moves downwardly.

The rotor 12 comprises a side wall 30 and an upper end wall 31. The side wall defines the opening 18 at its lower end and diverges upwardly from the opening to a level approximately halfway between that opening and the upper wall. An upper part of the side wall is either cylindrical or diverges in the upward direction to a lesser degree than does the lower part of the side wall. An outwardly turned lip may be provided at the lower end of the side wall.

in the particular example illustrated, the upper wall 31 comprises two layers with the uppermost layer being flat and integral with the side wall 30.

The lower layer includes a central portion 32 of substantially conical form with its apex lowermost and lying on the axis 11. A peripheral portion 33 of the lower layer is flat and secured in face-to-face contact with the upper layer of the upper wall. The side wall 30 and the upper wall 31, at least in a region which extends from the side wall towards the axis 11 to within a distance from the axis which is less than the distance from the axis to the periphery of the opening 18 are imperforate. There is indicated in the drawing at34 a notional cylinder with the same radius as the opening 18 and an axis coincident with the axis 11.Whilst there may be one or more apertures in the upper wall nearer to the axis 11 than is the cylinder 34, such apertures would not normally affect operation of the centrifuge, provided fluid directed into the rotor is diverted away from them.

The upper end of the rotor is preferably completely closed.

The duct 19 of the feed means terminates just below the apex of the central portion 32 of the upper wall. The feed means further comprises a cone 35 which extends upwardly and outwardly from the upper end of the duct 19 so that there is defined between the cone 34 and the wall portion 32 an upwardly divergent, annular space 36 which constitutes a terminal portion of the feed passage and which has an open end 37 of annularform centred on the axis 11.

When the centrifuge is to be used to separate clean liquid from a suspension of more dense particles in a less dense liquid, the motor 16 is energised to rotate the rotor 12 in a first direction at a high speed, typically within the range 2500 to 3000 r.p.m. The liquid containing suspended solids is then admitted to the centrifuge by opening the valve 20 and is fed along the duct 19, either by gravity flow from a reservoir above the level of the centrifuge, or by means of a pump (not shown). When the liquid enters the space 36, friction between the liquid and the upper wall 31 causes the liquid to rotate about the axis 11 in the same direction as the rotor but, initially, at a lower speed than that of the rotor. As the liquid moves radially outwardly across the surface of the upper wall 31, its speed of rotation increases.

The liquid in which the solid particles are sus pended passes from the end 37 of the feed passage into the space inside the rotor and is accelerated further so that centrifugal action causes the more dense particles to move away from the axis 11 towards the internal surface of the side wall 30 and causes the less dense liquid to occupy a position somewhat nearer to the axis than do the separated particles.

There accumulates in the rotor 12 sufficient liquid to occupy the space between the notional cylinder 34 and the internal surface of the side wall 30 bearing separated particles. Liquid which cannot be accom modated in this space escapes from the rotor through the opening 18 and leaves the rotor, initially, in a substantially horizontal direction away from the axis 11. Thus, clean liquid enters the space defined by the lower body part 14, which forms a reservoir of liquid. A deflector 38 is provided in the upper body part near to the rotor 12 to deflect downwardly the stream of liquid leaving the rotor horizontally. An outlet 39 is provided for clean liquid to leave the lower body part.

Whilst the rotor 12 continues to rotate at a high speed, the major proportion of clean liquid leaving the rotor passes through the gap 23 into the reservoir. However, this liquid does not form a coherent stream and there is a certain amount of splashing and other erratic movement of the liquid which results in liquid entering the receiver 21 through the opening 22 at a relatively low rate. This liquid drains down the funnel 24 onto the surface 29 of the diverter 28 which obstructs flow of the liquid into the tube 25. Instead, the liquid is diverted through the opening 26 into the reservoir.

When solid particles which have accumulated in the rotor 12 are required to be removed, the motor 16 is energised to apply a turning force to the rotor 12 in a second direction. The side wall 30 decelerates rapidly, whilst the body of liquid within the rotor continues to rotate without substantial deceleration.

In this way, there is established a large difference between the rotational speed of liquid in the rotor and the rotational speed of particles accumulated on the side wall 30. The liquid dislodges these particles with a scouring action to form a slurry within the rotor. Friction between the rotor and this slurry reduces the speed of rotation of the slurry until the slurry falls downwardly through the opening 18 and into the receiver 21 through the opening 22.

The rate at which slurry enters the receiver 21 is such that a substantial weight of slurry impinges on the surface 29 of the diverter. This weight is sufficient to move the diverter to its second position, so that the slurry can slide down the diverter into the tube 25. When slurry ceases to drain from the rotor into the receiver 21, the diverteris returned by gravity to its first position.

If the particles are not easily dislodged from the side wall 30, the direction of rotation of the rotor 12 may be reversed a plurality of times. During dislodgement of particles from the side wall 30, the supply of fluid through the feed means may be continued to assist cleaning of the rotor. Each cleaning cycle may be initiated after a predetermined interval. Alternatively, a cleaning cycle may be initiated only when a predetermined mass of particles has accumulated in the rotor.

To determine whether such mass has accumulated, the supply of electric powerto the motor 16 may be terminated and the motor used as a generator to generate a train of pulses at a frequency which is dependent on the speed of the rotor 12. The rotor will be decelerated by friction and the rate of deceleration will be dependent on the inertia of the rotor and its contents. When the rate of change of frequency of the pulses generated by the motor 16 is indicative that the inertia of the rotor and its contents exceeds a predetermined value, a cleaning cycle is initiated by energising the motor 16 to apply torque in the second direction to the rotor 12.

Claims (18)

1. A centrifuge comprising a body and a hollow rotor mounted for rotation relative to the body about an axis which extends upwardly and downwardly when the centrifuge is in use and having an opening at its lower end, the rotor further comprising a side wall which converges downwardly towards the opening and an upper wall which closes an upper end of the rotor, at least in a region which extends from the side wall towards the axis to within a distance from the axis which is less than the distance from the axis to the periphery of said opening.

2. A centrifuge according to Claim 1 further comprising feed means for feeding a fluid stream into the rotor to contact the upper wall.

3. A centrifuge according to Claim 2 wherein the feed means includes a wall disposed within the rotor and spaced downwardly from the upper wall thereof to define, in conjunction with the upper wall, a terminal portion of a feed passage.

4. A centrifuge according to Claim 3 wherein the terminal portion of the feed passage diverges from said axis.

5. A centrifuge according to Claim 4wherein the terminal portion of the feed passage has an open end which forms an annulus around the axis.

6. A centrifuge according to any preceding claim further comprising a receiver which defines a receiver opening directly below the opening defined by the rotor and which is spaced downwardly from the side wall of the rotor by a gap through which a less dense component of said stream leaves the rotor when the centrifuge is in use and the rotor is rotated at high speed.

7. A centrifuge according to Claim 6 wherein the receiver defines a discharge passage leading downwardly from the receiver opening and the centrifuge further comprises a diverter mounted for movement relative to the receiver between a first position in which the diverter obstructs the discharge passage and diverts to the outside of the receiver material which falls through the receiver opening and a second position in which material which falls through the receiver opening can pass the diverter and continue down the discharge passage.

8. A centrifuge according to Claim 7 wherein the diverter is biased to its first position and is so arranged that pressure on a face of the diverter which lies in the discharge passage and faces towards the receiver opening tends to move the diverterto the second position.

9. A centrifuge according to Claim 8 wherein the diverter is mounted for pivoting relative to the receiver about a pivot axis perpendicular to said axis of the rotor and has an upwardly facing surface, approximately one half of which lies on each side of the pivot axis.

10. A centrifuge according to any preceding claim wherein a part of the side wall of the rotor adjacent to the rotor opening is more convergent than is a part of the side wall remote from the rotor opening.

11. A centrifuge according to any preceding claim wherein the rotor is suspended by its upper end.

12. A method of separating a suspension wherein a hollow member having an open end is rotated about an upright axis with the open end lowermost, the suspension is fed into the rotating, hollow member, a less dense component escapes through the open end and travels therefrom initially in a direction away from the axis and a more dense component is retained in the rotating member and wherein, subsequently, the member iszdecelerated and the more dense component is permitted to fall downwardly through the opening.

13. A method according to Claim 12 wherein the suspension is fed into an upper end portion of the rotating member.

14. A method according to Claim 13 wherein the suspension is fed into the rotating memberto impinge on an upper wall thereof, which wall is transverse to the axis of rotation.

15. A method according to any one of Claims 12 to 14 wherein, after deceleration, the direction of rotation of said member is reversed a plurality of times.

16. A centrifuge substantially as herein described with reference to and as illustrated in the accompanying drawing.

17. A method of separating a suspension substantially as herein described with reference to and as illustrated in the accompanying drawing.

18. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.