FIELD OF THE INVENTION

The present invention relates to the field of centrifugal separators for cleaning a gas containing liquid impurities. In particular, the present invention relates to cleaning crankcase gases of a combustion engine from oil particles.

BACKGROUND OF THE INVENTION

It is well known that a mixture of fluids having different densities may be separated from one another through use of a centrifugal separator. One specific use of such a separator is in the separation of oil from gas vented from a crankcase forming part of an internal combustion engine.

With regard to this specific use of separators, there can be a tendency for the high pressure gas found in the combustion chambers of an internal combustion engine to leak past the associated piston rings and into the crankcase of the engine. This continuous leaking of gas into the crankcase can lead to an undesirable increase of pressure within the crankcase and, as a consequence, to a need to vent gas from the casing. Such gas vented from the crankcase typically carries a quantity of engine oil (as droplets or a fine mist), which is picked up from the reservoir of oil held in the crankcase.

In order to allow vented gas to be introduced into the inlet system without also introducing unwanted oil (particularly into a turbocharging system wherein the efficiency of the compressor can be adversely affected by the presence of oil), it is necessary to clean the vented gas (i.e. to remove the oil carried by the gas) prior to the gas being introduced into the inlet system. This cleaning process may be undertaken by a centrifugal separator, which is mounted on or adjacent the crankcase and which directs cleaned gas to the inlet system and directs separated oil back to the crankcase. An example of such a separator is disclosed e.g. in U.S. Pat. No. 8,657,908.

Such separators usually comprise a number of separation discs, e.g. arranged in a stack or as axially extending surface plates, and the separation of oil from the gas takes place between such discs. However, during running of the separator, clogging between such discs may occur, which in turn decreases the separation performance of the separator.

There is thus a need in the art for arrangements that prevent clogging from occurring and for cleaning already clogged separation discs.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a centrifugal separator for cleaning gas that may prevent clogging and decrease the amount of clogging between separation discs.

As a first aspect of the invention, there is provided a centrifugal separator for cleaning gas containing contaminants comprisinga stationary casing, enclosing a separation space through which a gas flow is permitted,a gas inlet extending through the stationary casing and permitting supply of the gas to be cleaned,a rotating member comprising a plurality of separation members arranged in the separation space and being arranged to rotate around an axis (X) of rotation,a gas outlet configured to permit discharge of cleaned gas and comprising an outlet opening through a wall of the stationary casing,a drainage outlet configured to permit discharge of liquid impurities separated from the gas to be cleaned;a drive member, for rotating the rotating member; andwherein the centrifugal separator further comprisesa control unit which is configured to control the drive arrangement to rotate the rotating member at a first speed during a separation phase and at a second speed, which is higher than the first speed, during a cleaning phase to remove clogging on or between the separation members, wherein the cleaning phase is shorter in time than the separation phase.

The contaminants in the gas may comprise liquid impurities, such as oil, and soot.

Consequently, the centrifugal separator may be for separating liquid impurities, such as oil, from gas. The gas may be crankcase gas of a combustion engine. However, the centrifugal separator may also be suitable for cleaning gases from other sources, for instance the environment of machine tools which frequently contains large amounts of liquid impurities in the form of oil droplets or oil mist.

The stationary casing of the centrifugal separator may comprise a surrounding side wall, and first and second end walls, which enclose the separation space. The stationary casing may have a cylindrical shape with circular cross-section having a radius R from the axis (X) of rotation to the surrounding side wall. This radius R may be constant at least with respect to a major part of the circumference of the surrounding side wall. The stationary casing may also be slightly conical. The first and second end walls may thus form an upper end wall and a lower end wall of the cylindrical shaped casing.

The gas inlet of the centrifugal separator may be located through the first end wall or through the surrounding side wall close to the first end wall, thus at the top of the separator, such that gas entering through the gas inlet is directed to the separation space. The drainage outlet may be located in the second end wall, e.g. at the bottom of the separator. Thus, the drainage outlet may be arranged centrally in an end wall opposite the end wall through which, or at which, the inlet is arranged. The drainage outlet of the centrifugal separator may further be formed by a number of spot shaped through holes of the stationary casing or by a single drainage passage. The drainage outlet may be arranged at the axis of rotation or centered on the axis of rotation. The drainage outlet may also be in an annular collection groove at the inner end wall of the stationary casing. The gas outlet may be arranged in a surrounding side wall of the stationary casing or may for example be arranged in an end wall, such as the end wall opposite the end wall through which, or at which, the gas inlet is arranged.

The rotating member is arranged for rotation during operation by means of the drive member. The rotating member comprises a plurality of separation members arranged in the separation space. The separation members of the rotating member are examples of surface-enlarging inserts that promote separation of contaminants from the gas. The separation members may be a stack of separation discs. The separation discs of the stack may be frustoconical. A frustoconical disc may have a planar portion extending in a plane that is perpendicular to the axis of rotation, and a frusto-conical portion that may extend upwards or downwards. The planar portion may be closer to the rotational axis than the frustoconical portion. Further, the discs of the stack may be radial discs, in which substantially the whole disc extends in a plane that is perpendicular to the axis of rotation.

It is also to be understood that the separation members, such as separation discs, not necessarily have to be arranged in a stack. The separation space may for example comprise axial discs, or plates that extend around the axis of rotation. The axial discs or plates may be planar, i.e. extending in planes that are parallel to the axis of rotation. The axial discs or plates may also have a slightly or significantly curved shape, such as an arcuate or spiral shape, as seen in a radial plane.

During operation, gas to be cleaned may be directed centrally through the plurality of separation members, such as centrally through the stack of separation discs. In such a set-up, the rotating member may further define a central space formed by at least one through hole in each of the separation members. This central space is connected to the gas inlet and configured to convey the gas to be cleaned from the gas inlet to the gaps between the separation members, such as between the gaps between the discs of a stack of separation discs. A separation disc that may be used as separation member may comprise a central, essentially flat portion perpendicular to the axis of rotation. This portion may comprise the through holes that form the central space.

Thus, the centrifugal separator may be configured to lead crankcase gases from the gas inlet into a central portion of the rotating member. In this manner the crankcase gases may be “pumped” from the central portion of the rotating member into the interspaces between the separation discs in the stack of separation discs by the rotation of the rotating member. Thus, the centrifugal separator may work according to the concurrent flow principle, in which the gas flows in the disc stack from a radial inner part to a radial outer part, which is opposite to a separator operating according to the counter-current flow principle, in which the gas is conducted into the centrifugal rotor at the periphery of the rotor and is led towards a central part of the rotor.

The drive member may for example comprise a turbine wheel, rotated by means of an oil jet from the lubrication oil system of the combustion engine or a free jet wheel comprising a blow-back disk. However, the drive member may also be independent of the combustion engine and comprise an electrical motor, a hydraulic motor or a pneumatic motor.

The centrifugal separator further comprises a control unit configured to control the rotational speed of the rotating member via the drive arrangement, e.g. such that a limited number of speeds can be obtained or so that a continuous change of the rotational speed can be performed. The control unit may comprise a processor and an input/output interface for communicating with the drive arrangement and for receiving information from other parts of the separator, such as from sensors arranged on the separator, and/or from e.g. an engine to which the centrifugal separator is connected or mounted.

The control unit may further comprise computer program products configured for switching between separating phase and cleaning phase. The computer program may be for analysing received information from e.g. the engine or sensors and send operational requests based on such analysis to the drive arrangement.

The control unit is at least able to control the drive arrangement to switch from rotating at a first speed during the separation phase and at a second, higher speed, during the cleaning phase. The first speed during the separation phase may be the normal operational speed of the centrifugal separator during which the continuous separation of contaminants from the gas occurs.

The control unit is also configured to control the drive arrangement to increase its speed, i.e. to switch from the separation phase to the cleaning phase. The cleaning phase is shorter in time compared to the separation phase. During the cleaning phase, clogging that has stuck between the separation members may be removed. Switching to the cleaning phase may be performed as a stepwise increase in speed up to the speed of the cleaning phase, or it may be performed as a quick increase in speed up to the speed of the cleaning phase.

The control unit is also configured to control the drive arrangement to the decrease its speed when switching from the cleaning phase back to the separation phase. However, the separation phase may comprise running at different rotational speeds, all of which are lower than the rotational speed during the cleaning phase.

Consequently, the centrifugal separator according to the invention is operating in a cycle comprising the separation phase and the cleaning phase.

Clogging refers to contaminants stuck between the separation members that obstruct or impede the flow of gas between the separation members. The clogging may originate from contaminants in the gas. The clogging may be formed by sticky matter and may comprise oil and particles.

The first aspect of the invention is based on the insight that clogging that may arise between separation members of a gas separator can be removed using a temporary increase in rotational speed. By increasing the speed also the centrifugal forces acting on the clogging increase, which may then come loose and be forced out from between the separation members. Thus, such a temporary boost in centrifugal force may clean the separation members from e.g. oil mist and particles that are clogged between the separation members. Further, the increase in speed may also prevent the formation of clogging.

In embodiments of the first aspect of the invention, the control unit is configured to switch from the separation phase to the cleaning phase after a predetermined period of time.

Thus, the control unit may be configured to control the drive arrangement to rotate the rotating member at the first speed for a predetermined time. After the predetermined time in the separation phase, the control unit may automatically initiate a cleaning phase, whereby clogging is removed from between the separation members. A predetermined time could be manually set by an operator. However, it could also be calculated from operating parameters of the centrifugal separator measured by various sensors, such as sensors registering a pressure at the gas inlet and/or gas outlet of the separator.

In embodiments of the first aspect of the invention the control unit is configured to receive a signal that is related to a stop of the engine to which the centrifugal separator is connected, and wherein the control unit is further configured to switch to the cleaning phase after a predetermined number of stops of the engine.

Thus, the control unit may be configured to count the number of stops of an engine to which the centrifugal separator is connected and then switch to the cleaning phase when the counted number of stops reaches a predetermined value. The predetermined value may for example be a count of 20 stops, a count of 50 stops etc.

In embodiments of the first aspect of the invention, the control unit is configured to receive a signal that is related to the time during which an engine to which the centrifugal separator may be connected has been running, and wherein the control unit is further configured to switch to the cleaning phase when the time exceeds a threshold value.

Thus, the control unit may be configured to register the operating time of an engine to which the centrifugal separator is connected and then switch to the cleaning phase when the operating time exceeds a threshold value, such as 100 hours, 150 hours etc.

In embodiments of the first aspect of the invention, the control unit is configured to receive a signal related to the gas pressure at the gas inlet and switch to the cleaning phase when the gas pressure at the gas inlet is above a predefined threshold value.

Consequently, the centrifugal separator may comprise at least one pressure sensor at the gas inlet. An increased gas pressure at the inlet may be a good indication that clogging has occurred between the separation members.

The gas pressure at the gas inlet may be measured at a specific rotational speed of the rotational member, and when this pressure is over a specific value, switching to the cleaning phase may occur.

The control unit may further be configured to receive several parameters, such as two or more parameters selected from the group consisting of a parameter related to the number of stops of the engine, a parameter related to the operating time of the engine, a parameter related to the operating time of the centrifugal separator and a parameter related to the gas pressure at the inlet. The control unit may then be configured to switch to the cleaning phase when one of those parameters exceeds a threshold value specific for that parameter.

According to another embodiment of the invention, the control unit is configured to control the drive arrangement to rotate the rotating member at the second speed for a predetermined time. A predetermined time could be manually set by an operator. However, it could also be calculated from operating parameters of the centrifugal separator measured by various sensors, such as sensors registering a pressure at the gas inlet and/or gas outlet of the separator. After the predetermined time, the control unit may be configured to switch back to the separation phase.

As an alternative, the control unit may be configured to switch from the cleaning phase back to the separation phase after receiving a signal related to the gas pressure at the gas inlet. As an example, the control unit may be configured to switch back to the separation phase when receiving a signal indicating that the gas pressure at the gas inlet is below a predetermined value.

In embodiments of the first aspect of the invention, the cleaning phase comprises rotating the rotating member during a time period that is between 30-180 s.

As an example, the cleaning phase may comprise rotating the rotating member during a time period that is between 60-120 s.

In embodiments of the first aspect of the invention, the cleaning phase comprises rotating the rotating member at a speed that is higher than 12,000 rpm.

As an example, the cleaning phase comprises rotating the rotating member at a speed that is higher than 14,000 rpm, such as higher than 15,000 rpm.

In embodiments of the first aspect of the invention, the separation phase comprises rotating the rotating member at a speed of between 7,500 and 12,000 rpm.

Thus, in embodiments of the first aspect of the invention, the second speed is more than 2000 rpm higher than the first speed, such more than 3000 rpm higher than the first speed, such as more than 4000 rpm higher than the first speed, such as more than 5000 rpm higher than the first speed.

Consequently, in embodiments of the first aspect of the invention, the cleaning phase comprises rotating the rotating member at a speed that is more than 2000 rpm higher than the speed of the separation phase.

In embodiments of the first aspect of the invention, the drive member comprises an electrical motor.

Thus, the control equipment may be configured for driving such an electrical motor at different speeds. The electrical motor may principally be of any suitable kind, for instance a direct-current motor or an alternate-current motor (synchronous motor or asynchronous motor). As an example, the electrical motor may be a synchronous motor, such as a brushless electric motor, having a rotor that includes permanent magnets having a rotor that includes permanent magnets.

The electrical motor may be arranged within the stationary casing or outside the stationary casing.

Furthermore, an electrical motor which does not have separate bearings for the journalling of the rotor of the motor may be used. The already present and necessary bearings for the centrifugal rotor may instead be used for this journalling.

As an example, the electrical motor may be arranged within the stationary casing and have a stator that is supported by the stationary casing, and a rotor, that is constituted by part of the rotating member of the centrifugal separator and which is journalled relative to the stator only through the bearings.

Consequently, in embodiments of the first aspect of the invention, the rotating member is journalled in the stationary casing through bearings situated at only two bearing places axially spaced from each other, and the drive member is an electrical motor arranged within the stationary casing and having a stator, that is supported by the stationary casing, and a rotor, that is constituted by part of the rotating member and is journalled relative to the stator only through those bearings.

Such a configuration is for example explained in more detail in the international patent application published as WO 2004/001201.

As a further example, the control unit may be integrated with the electrical motor.

Thus, the control equipment may be configured for driving the electrical motor at different speeds. The control unit may thus be in the same unit as the electrical motor, which may be arranged within the stationary casing or outside the stationary casing. However, the control unit may also be a separate unit than the electrical motor. Thus, the control unit may be a separate unit arranged outside the stationary casing whereas the electrical motor is arranged within the stationary casing, or as an alternative, both the electrical motor and the control unit are arranged outside the stationary casing, but as different units.

As a second aspect of the invention, there is provided a method for removing clogging in a centrifugal separator, comprising the steps ofproviding a centrifugal separator according to the first aspect of the invention,running the centrifugal separator in the separation phase at a first speed,switching to running the centrifugal separator in the cleaning phase at the second speed to remove clogging on or between the separation members.

Terms and definitions used in relation to the second aspect of the invention are as discussed in relation to the first aspect of the invention above.

The step of running the centrifugal separator in the separation phase and in the cleaning phase comprises rotating the rotating member of the centrifugal separator.

The method may further comprise the steps ofintroducing gas containing contaminants into gas inlet of the centrifugal separator; anddischarging cleaned gas through the gas outlet of the centrifugal separator and discharging contaminants separated from the gas through the drainage outlet of the centrifugal separator.

In embodiments of the second aspect of the invention, the step of switching to running the centrifugal separator in the cleaning phase is initiated after a predetermined period of time.

In embodiments of the second aspect of the invention, the method is further comprisingreceiving a signal related to a stop of the engine to which the centrifugal separator is connected and wherein switching to running the centrifugal separator in the cleaning phase is initiated after a predetermined number of received stops.

In embodiments of the second aspect of the invention, the method is further comprisingmeasuring a pressure at the inlet of the separator and wherein switching to running the centrifugal separator in the cleaning phase is initiated when the gas pressure at the gas inlet is above a predefined threshold value.

As a further aspect of the invention, there is provided a computer program product, comprising program code instructions for executing the method according of the second aspect of the invention, when said program is executed by a computer. As an example, the separator control unit may comprise such a computer program product.

