Abstract:
A centrifuge is employed to continuously remove particulates from a fluid. In one embodiment, the centrifuge removes small particles of soot from lubricating oil of large diesel engines. The fluid in introduced into the centrifuge through an inducer so that vortexes are not propagated in the fluid. Flow constrainers and flow straighteners maintain laminar flow of the fluid as it passes axially through the centrifuge. An exducer decelerates the fluid prior to its exit from the centrifuge. The exducer thus contributes to maintaining laminar flow conditions. Laminar flow may contribute to the soot-removal effectiveness of the centrifuge.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is in the field of centrifuges and, more particularly, centrifuges employed to remove particulates from lubricants. 
     Centrifuges have often been employed to remove various particulate contaminants from lubricating oil of internal combustion engines. The most common applications of centrifuges in this context have been in large diesel engines. Typically, lubricating oil of a large diesel engine may be continuously passed through a full flow filter and through a bypass centrifugal filter or centrifuge. While conventional centrifugal filters may be relatively costly, their cost is justified because engine life is improved when they are used. 
     Recent developments in environmental standards have introduced additional demands on filtering systems for diesel engine oil. Injector timing retardation is needed to meet more stringent air pollution standards. This results in increased production of carbon soot on the cylinder walls of an engine. Soot finds its way into the lubricating oil of the engine. Conventional full flow filters and conventional centrifugal filters do not adequately remove soot from the oil. Engine life is reduced in the presence of soot in the oil because the soot is abrasive and it reduces lubricating qualities of the oil. 
     Various efforts have been made to improve performance of centrifuges in attempts to introduce soot removal capabilities. Some examples of these efforts are illustrated in U.S. Pat. No. 6,019,717, issued Feb. 1, 2000 to P. K. Herman and U.S. Pat. No. 6,984,200 issued Jan. 10, 2006 to A. L. Samways. Each of these designs is directed to a problem of removing very small particles of soot, i.e., particles of about 1 to about 2 microns. Centrifuges separate particulates from fluids by exposing the particulates to centrifugal forces. Particulates with a density greater than the fluid are propelled through the fluid radially outward. But, in the case of soot particles suspended in oil, separation is difficult because soot particles have a density very similar to oil. Consequently, very high centrifugal forces may be required to move the soot particles through oil. Typically centrifugal forces of about 10,000 g&#39;s may be needed. These high forces may be produced by rotating a centrifuge at very high speeds. Alternatively, the requisite high g forces may be produced within a centrifuge having a very large diameter. However, as a practical matter, it is desirable to limit the diameter of a centrifuge to diameter of about 7 to 10 inches to meet space limitation on a vehicle and to limit rotational inertial effects. Also there is a practical limitation on the rotational speed that can be imparted to a centrifuge. Speeds of about 10,000 to about 12,000 rpm represent the limits of the current state of the art. 
     In attempts to capture small soot particles within these practical speed and size parameters, prior art centrifuges employ complex and labyrinth-like oil passage pathways. As oil traverses these complex pathways, it remains in a centrifuge for a relatively long time. In other words, it has an extended “residence time”. It has heretofore been assumed that improved soot removal is directly related to increased residence time. 
     But, in various efforts to increase residence time, prior art centrifuges have employed oil passage pathways that introduce multiple changes in direction of flow of oil. Many of these changes in flow direction may be abrupt. As oil flow makes these abrupt changes in direction, vortexes may be generated. These vortexes may propagate throughout the entire mass of oil that may be present in a prior art centrifuge. This may result in oil flow that is turbulent in nature. Turbulence in oil flow may produce additional difficulty in removing small particles from the oil. Whenever any one particle is propelled outwardly by centrifugal force in a turbulent flow, there is a high probability that the particle will encounter a reverse flow of oil in a vortex. Such a reverse flow may propel the particle inwardly and thus cancel the desired effects of centrifugal force imparted by the centrifuge. Thus, the particle has a high probability of remaining suspended in the oil. 
     It can be seen that soot removal effectiveness of centrifuges in the present state of the art is bounded by various limiting conditions. First there is a practical limit on a diameter of a centrifuge. Secondly there is a practical limit on the rotational speed at which a centrifuge may be operated. And thirdly, increased residence times may be attained at the cost of producing turbulent flow in a centrifuge. As described above, turbulent flow may offset or cancel any beneficial effects of increasing residence time. 
     There has been no recognition in the prior art of a simple expedient to increase the soot removal effectiveness of centrifuges within the practical limits of centrifuge size and rotational speed. As can be seen, an improvement of soot removal effectiveness in a practical centrifuge would be desirable. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention a centrifuge for extracting particulates from a continuous flow of fluid comprises a rotor, a passage for constraining at least a portion of the flow of the fluid as laminar flow. The passage is adapted to direct the laminar flow orthogonally to centrifugal forces imparted to the fluid by rotation of the rotor. 
     In another aspect of the present invention a centrifuge adapted to capture soot from lubricating oil comprises a rotor with a laminar flow passage therein. The laminar flow passage is oriented parallel to an axis of rotation of the rotor. 
     In still another aspect of the present invention a method for removing particulates from a fluid comprises the steps of producing a laminar flow of the fluid and imparting centrifugal force on the fluid in a direction orthogonal to a direction of the laminar flow of the fluid to capture the particulates from the fluid. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is partial cross sectional view of a centrifuge constructed in accordance with the invention; 
         FIG. 2  is a cross sectional view of a portion of the centrifuge of  FIG. 1  taken along the line  2 - 2  showing various features in accordance with the invention; 
         FIG. 3  is a cross sectional view of a portion of the centrifuge of  FIG. 1  taken along the line  3 - 3  showing various features in accordance with the invention; 
         FIG. 4  is a cross sectional view of a portion of the centrifuge of  FIG. 1  taken along the line  4 - 4  showing various features in accordance with the invention; 
         FIG. 5  is a schematic representation of a portion of fluid flowing through the centrifuge of  FIG. 1  in accordance with the invention; and 
         FIG. 6  is a flow chart of a method of collecting particulates from a fluid in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Broadly, the present invention may be useful in improving effectiveness of particulate removal of a centrifuge. More particularly, the present invention may provide a simple expedient to improve soot removal effectiveness that can be applied to a centrifuge that is operated and constructed within the bounds of practical size and speed of conventional centrifuges. 
     In contrast to prior art centrifuges, among other things, the present invention may provide a centrifuge that operates with a fluid flow therethrough which is laminar, i.e. non-turbulent. A desirable improvement of soot-removal effectiveness may achieved by constructing a centrifuge in an inventive configuration illustrated in  FIG. 1 . 
     Referring now to  FIG. 1 , there is shown a sectional view of a centrifuge  10 . The centrifuge  10  may be comprised of a spindle  12 , a rotor  14 , a housing  16  and a driving device, such as a turbine  18 . A fluid such as lubricating oil may be introduced under pressure into a fluid inlet  16   a  to impinge on and rotate the turbine  18 . The turbine  18  and the rotor  14  may be attached directly to the spindle  12 . Thus the rotor  14  may be rotated by the turbine  18 . A portion, about 10% to about 15%, of the fluid introduced into the inlet  16   a  may bypass the turbine  18  and enter a hollow passageway  12   a  of the spindle  12 . The bypassed fluid may flow through a spindle passageway  12   a  and into the rotor  14 . The bypassed fluid is indicated by arrows  20 . 
     The fluid  20  may exit the spindle passageway  12   a  at spindle exit ports  12   b . The fluid  20  may then continue into the rotor  14  and proceeds to rotor exit ports  14   a . The fluid  20  may then proceed into the housing  16  through a return drain  16   b . As the bypassed fluid  20  flows through the rotor  14 , the fluid  20  may be subjected to centrifugal forces generated by rotation of the rotor  14  about a centrifuge axis  21 . The centrifugal forces are applied to the fluid  20  in a direction that is orthogonal to the axis  21 . 
     Operation of the inventive centrifuge  10  may be better understood by referring to cross-sectional  FIGS. 2-4 . 
     In  FIG. 2 , there is shown an inducer  22  that may be attached directly to the spindle  12 . The inducer  22  may be comprised of inducer vanes  22   a  and inducer exit ports  22   b . The inducer exit ports  22   b  may be contiguous with the spindle exit ports  12   b . The fluid  20  may pass through the ports  12   b  and  22   b  into acceleration regions, designated generally by the numerals  24 . Within the acceleration regions  24 , direction of the fluid  20  may be gradually changed from a radial flow direction to a tangential flow direction. 
     It can be seen that this change in flow direction may be made gradually and not abruptly. Fluid  20  emerging from the ports  22   b  may impinge on the inducer vanes  22   a  at an obtuse angle and there may be a gradual change in its direction of flow. The vanes  22   a  may be curved along an arc that generally merges from a radial direction toward a direction that is tangential. Rotational direction of the rotor  14  is shown by arrows designated by the numeral  26 . Fluid  20  may be propelled along the vanes  22   a  by internal pressure within the spindle passageway  12   a  and by centrifugal forces produced by rotation of the inducer  22 . As the fluid  20  progresses outwardly along the vanes  22   a , its flow orientation may become substantially aligned with a tangential flow of fluid  20  which may be produced by shear forces of the rotating rotor  14 . Fluid  20  thus may enter the rotor  14  without production of vortexes. Consequently the fluid  20  may be introduced into rotor  14  as laminar flow and not turbulent flow. 
     Referring now to  FIG. 3  there is a cross-sectional view taken along the lines  3 - 3  showing a flow constrainer  28  and flow straighteners  30 . The flow constrainer  28  and flow straighteners  30  may be interconnected with the spindle  12  and rotate with the spindle  12 . As fluid  20  flows through the rotor  14  it may be constrained to flow between an outer surface  28   a  of the flow constrainer  28  and an inner surface  14   b  of the rotor  14 . Additionally, fluid  20  may be constrained to flow in an axial direction by the flow straighteners  30  through a series of rotor passages  32 . It can be seen that each passage  32  may be bounded by the flow constrainer  28 , the rotor inner surface  14   b  and two adjacent flow straighteners  30 . 
     Cross-sectional areas of the passages  32  may be desirably selected to be consistent with a fluid flow therethrough that corresponds to a Reynolds Number (Re) less than about 1000. A Reynolds Number less than 1000 is typically definitive of laminar, i.e., non-turbulent flow. For any particular fluid flow Re is a function of various parameters in accordance with the following expression:
 
 Re=ρVDe/μ 
 
     where
         μ=Absolute Viscosity of a fluid   ρ=Density of a fluid   V=Velocity of flow   De=Equivalent Hydraulic Diameter.       

     Each of the passages  32  may be considered to have an Effective Hydraulic Diameter (De) and De may be chosen to provide a Reynolds Number less than about 1000 for the particular fluid flow passing through the centrifuge  10 . In other words spacing between adjacent ones of the flow straighteners  30  and spacing between the flow constrainer  28  and the inner surface  14   b  of the rotor  14  may be selected to assure that a Reynolds Number less than about 1000 is provided for a particular viscosity, density and flow rate of fluid. Thus, for example, the centrifuge  10  may be adapted to provide for soot removal of lubricating oils of various viscosities. 
     Referring now to  FIG. 4 , there is shown an exducer  34  that may be attached directly to the spindle  12 . The exducer  34  may comprise exducer vanes  34   a . The exducer  34  may be positioned over the rotor exit ports  14   a . The fluid  20  may pass through the rotor passages  32  of  FIG. 3  into deceleration regions, designated generally by the numerals  36 . Within the deceleration regions  36 , direction of the fluid  20  may be gradually changed from a tangential flow direction to a radial flow direction. 
     As in the case of the inducer  22  of  FIG. 2 , this change in flow direction may be made gradually and not abruptly. Fluid  20  emerging from the passages  32  may impinge on the exducer vanes  34   a  at an obtuse angle and there may be a gradual change in its direction of flow. The vanes  34   a  may be curved along an arc that generally merges away from a direction of rotation of the rotor  14 . Fluid  20  may flow along the vanes  34   a  and gradually lose its tangential velocity. As the fluid  20  progresses inwardly along the vanes  34   a , it passes into the rotor exit ports  14   a  and thus exits from the rotor  14 . Fluid  20  thus may exit the rotor  14  without production of vortexes. Consequently the fluid  20  may be removed from the rotor  14  as laminar flow and not turbulent flow. 
     It should be noted that the centrifuge  10  may be devoid of any elements for prolonging “residence time” of the fluid  20  in the rotor  14 . The soot-removal effectiveness of the centrifuge  10  may not be a function of residence time. 
     This may be better understood by referring to  FIG. 5 .  FIG. 5  is a schematic representation of various regions of fluid  20  that may exist within the passages  32  of the centrifuge  10 . A first region may be considered a flow region designated by the numeral  38 . The flow region  38  may completely fill the passages  32 . The flow region  38  may be considered to have a soot-capturing sub-region or capture region  38   a  during operation of the centrifuge  10 . The capture region  38   a  may be adjacent the inner surface  14   b  of the rotor  14 . In that regard the inner surface  14   b  may be considered a capture surface. 
     The fluid  20  passes into and through the passages  32  as a result of incoming pressure at the inlet  16   a  of  FIG. 1 . As fluid  20  passes through the passages  32 , its rate of flow may be determinative of the thickness of the capture region  38   a . As the rotor  14  of the centrifuge  10  is rotated, centrifugal forces may be applied to soot particles suspended in the fluid  20  within the region  38 . Soot particles may be propelled outwardly at a velocity that is a function of the rotational speed and diameter of the rotor  14 . For any given rotational speed and diameter, there is a finite rate at which a soot particle may travel radially. Flow rate of the fluid  20  may be determinative of the time during which a soot particle may travel radially while being subjected to the centrifugal force of the rotor  14 . If flow rate of fluid  20  were to increase due to, for example, increased pressure at the inlet  16   a , time for radial soot travel would decrease. As time for radial soot travel decreases, there may be a corresponding diminishment of a distance that a soot particle may travel in a radial direction. The distance that a soot particle may travel radially during transit through the rotor may be considered a capture distance and is represented as the capture region  38   a  of  FIG. 2 . The capture region  38   a  may have a thickness of about 0.005 inches in a typical one of the inventive centrifuges  10 . 
     The soot-removal effectiveness of the centrifuge  10  may be not merely a function of the size of the capture region  38   a . As fluid flow rate increases, the capture region  38   a , of course, becomes thinner and less soot may be collected during axial travel of the fluid  20  through the rotor  14 . But, as flow rate increases, there may be an increase in the amount of axial travel of the fluid  20  for any given period of time. In other words there may be an increase in rate of introduction of mass of soot, i.e., flux of soot, into the centrifuge  10  when flow rate increases. This increase of flux of soot has been found to directly offset any diminishment of soot-removal effectiveness produced by a diminishment of thickness of the capture region  38   a.    
     In a particular example of operation of the centrifuge  10 , the centrifuge was applied to an engine lubrication system in which soot was generated at a rate of about 6 grams/hr. In this example, the centrifuge  10  was about 3 to about 4 inches in diameter and about 7 to about 10 inches long and operated at a speed of about 10,000 to about 12,000 rpm. It was found that an equilibrium concentration of about 1% by weight of small soot particles developed after about 380 hours of operation. In this case the particle size of interest was about 2 μm or less. The lubrication system size was about 40 liters. In other words, this exemplary engine operation proceeded through an initial operation cycle of 380 hours with a small particle (≦2 μm) soot concentration less than 1% and after 380 hours, the soot concentration never exceeded about 1%. 
     In this context, engine wear from soot may be substantially reduced, as compared with the prior art. Soot particles larger than about 2 μm may be removed from lubrication systems with more conventional filtration devices. But conventional filtration systems typically may not control small particle soot accumulation at an equilibrium concentration. In prior art engines, small particle-soot removal lags behind soot production. There is a gradual buildup of small-particle soot until it becomes necessary to replace the lubricating oil with new oil that is free of soot. Typically, replacement is needed when soot concentration exceeds 1-2%. 
     The inventive centrifuge  10  may extract small-particle soot at virtually the same rate that it is produced by the engine until an equilibrium concentration of about 1% or less is reached. After that point in time, the centrifuge  10  may control small-particle soot concentration at about 1% or less for an indefinite time. 
     The present invention may be considered a method for removing particulates from the fluid  20 . In that regard the method may be understood by referring to  FIG. 6 . In  FIG. 6 , a schematic diagram portrays various aspects of an inventive method  300 . In a step  302  the fluid  20  with suspended particles therein may be continuously introduced into the centrifuge  10  as a laminar flow. In a step  304 , the fluid  20  may be rotated to produce centrifugal forces on the suspended particles. In a step  306  the fluid  20  may be continuously propelled axially in the centrifuge during rotation thereof. Laminar flow of the fluid may be maintained during the axial propelling of the fluid  20 . In a step  308  a portion of the suspended particles may be captured during passage of the fluid  20  through the centrifuge  10 . In a step  310  the fluid  20  may be continuously removed from the centrifuge  10  in an amount that corresponds to an amount introduced in step  302 . 
     During performance of the method  300  it may be desirable to maintain a flow of the fluid  20  so that a Reynolds number associated with the flow is about 1000 or less. Additionally, it may be desirable to perform the rotating step  304  so that centrifugal forces equivalent to a centrifugal acceleration of about 10,000 g&#39;s are applied to the particles. 
     The method  300  may be particularly useful for capturing small particles of soot that are suspended in lubricating oil of an engine. In that context, the method  300  may be advantageously performed by conducting the rotating step  304  at about 10,000 to about 12,000 rpm. Additionally, the method may be advantageously conducted by performing the capture step  308  at a radius of about 3 to about 5 inches from an axis of rotation of the centrifuge. When employed in this context, the method  300  may provide for an equilibrium concentration of about 1% or less of soot particles less than about 2 μm in an engine lubricating system with a capacity of about 40 liters. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.