Abstract:
The invention provides cyclonic separation apparatus containing a cyclone body having at least one fluid inlet and a fluid outlet, the fluid outlet being concentric with the longitudinal axis of the cyclone body. The cyclonic separation apparatus also contains a vortex finder projecting from an end surface of the cyclone body into the interior of the cyclonic separator, and a centerbody located partially within the vortex finder. The centerbody projects beyond the distal edge of the vortex finder so that the distance between the end surface of the cyclone body and the further end of the centerbody is at least twice the smallest diameter of the vortex finder, and the cross-sectional area of the centerbody is circular at any point along its length.

Description:
FIELD OF THE INVENTION 
     The invention relates to cyclonic separation apparatus, particularly but not exclusively to cyclonic separation apparatus for use in a vacuum cleaner. 
     BACKGROUND OF THE INVENTION 
     Cyclonic separation apparatus consists generally of a frusto-conical cyclone body having a tangential inlet at its larger, usually upper, end and a cone opening at its smaller, usually lower, end. A fluid carrying particles entrained within it enters via the tangential inlet and follows a helical path around the cyclone body. The particles are separated out from the fluid during this motion and are carried or dropped through the cone opening into a collector from which they can be disposed of as appropriate. The cleaned fluid, usually air, travels towards the central axis of the cyclone body to form a vortex and exits the cyclonic separator via a vortex finder which is positioned at the larger (upper) end of the cyclone body and is aligned with the central axis thereof. 
     The vortex finder usually takes the form of a simple tube extending downwardly into the cyclone body so that the vortex of exiting fluid is reliably directed out of the cyclone. However, the vortex finder has a number of inherent disadvantages. One of these disadvantages is the fact that there is a significant pressure drop within the vortex finder due to the high angular velocity of the exiting fluid. In an attempt to overcome this problem, centerbodies have been introduced into known vortex finders in combination with tangential offtakes in order to straighten the flow passing through and out of the cyclone. Some attempts have been made to reduce the swirl of the flow using fixed vanes. A variety of these attempts are illustrated in the paper entitled “The use of tangential offtakes for energy savings in process industries” (T O&#39;Doherty, M Biffin, N Syred: Journal of Process Mechanical Engineering 1992, Vol 206). Other arrangements incorporating centerbodies or vanes are illustrated in WO 97/46323, WO 91/06750 and U.S. Pat. No. 5,444,982. In all of these pieces of prior art, the centerbody is wholly contained within the vortex finder or, if it is not, it projects only to a very minor extent into the cyclone body. This is because the single aim of the centerbody or vane is to remove the swirl from the flow within the vortex finder, rather than to stabilize it. 
     Centerbodies have also been introduced to cyclonic separators for other reasons. One such reason, illustrated in U.S. Pat. No. 4,278,452, is to expand the outgoing fluid so that an outermost annulus of fluid containing any particles remaining entrained is recirculated through the separator. However, by necessity, the major part of the centerbody must remain outside the vortex finder and therefore is incapable of stabilizing the fluid flow inside the vortex finder. Another use of a centerbody is to support an electrode by means of which a Corona discharge is produced within the separation zone of the separator. This enhances the separation efficiency within the separation zone but, because the electrode must incorporate angular or pointed areas from which the Corona will discharge, there can be no stabilization of the exiting fluid. 
     In CH 388267, use is made of a centerbody projecting out of a vortex finder to prevent bubbles of gas escaping from the main outlet of apparatus for separating solid particles and gas bubbles from a liquid suspension. The centerbody has an essentially flat end. The gas bubbles, which migrate to the vortex core during operation, are caused to exit the apparatus via the cone opening, which forms an outlet for the cyclone. 
     Another problem associated with vortex finders is the fact that, during operation of the cyclonic separation apparatus, the vortex core precesses around the interior of the vortex finder causing a significant amount of noise. The provision of a centerbody wholly within the vortex finder has been recognized as contributing to the reduction of the noise associated with the exiting fluid to a certain extent but no attempt has been made to make use of a centerbody to reduce the noise still further. 
     SUMMARY OF THE INVENTION 
     In domestic appliances such as vacuum cleaners, noise is always undesirable and there is an ongoing desire to reduce the noise associated with the appliance as far as possible. It is therefore an object of the present invention to provide cyclonic separation apparatus, suitable for incorporation into a domestic appliance, in which the noise level is improved. It is a further object of the invention to provide cyclonic separation apparatus in which the pressure drop appearing across the vortex finder is as small as possible. It is a still further object of the invention to provide cyclonic separation apparatus suitable for use in a domestic vacuum cleaner. 
     The invention provides cyclonic separation apparatus containing a cyclone body having at least one fluid inlet and a fluid outlet having a vortex finder. The invention also provides a vacuum cleaner incorporating such cyclonic separation apparatus. Further and preferred features of the cyclonic separation apparatus include a centerbody having a circular cross-section and a hemispherical, conical or frusto-conical end which protrudes beyond the lowermost end of the vortex finder to a distance at which the furthermost end of the centerbody is at least twice the smallest diameter of the vortex finder from the end surface of the cyclone body reduces the noise associated with the exiting vortex to an appreciable degree. The reduction has been found to be significantly better than in the case when the vortex finder does not protrude out of the vortex finder to any significant extent. It is believed that precession of the vortex core when bounded by the walls of the vortex finder causes pressure perturbations within the airflow which are manifested as noise. Hence it is desirable to stabilize this rotation completely before the exiting air enters the vortex finder. The extension of the centerbody into the core&#39;s low pressure area before it reaches the vortex finder causes the core to stabilize before it reaches the vortex finder. The noise level is thereby reduced. Experimentation with specific apparatus has shown that, for specific dimensions of cyclone, vortex finder and centerbody, there are optimum distances from the upper surface of the cyclone to which the centerbody must extend. It will be clear from the description and examples which follow that it is not necessary for the centerbody to extend all the way up the vortex finder to the upper surface of the cyclone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described with reference to the accompanying drawings, wherein: 
     FIG. 1 shows, in cross section, cyclonic separation apparatus according to the present invention and suitable for use in a vacuum cleaner; 
     FIG. 2 a  shows, to a larger scale, the centerbody forming part of the apparatus shown in FIG. 1; 
     FIG. 2 b  shows a first alternative configuration of the centerbody of FIG. 2 a;    
     FIG. 2 c  shows a second alternative configuration of the centerbody of FIG. 2 a;    
     FIG. 2 d  shows a second alternative configuration of the centerbody of FIG. 2 a . 
     FIG. 3 is a cross-section through part of alternative cyclonic separation apparatus according to the present invention; 
     FIG. 4 is a schematic drawing of the test apparatus used to determine the results of the experiments described below; and 
     FIG. 5 is a graph showing a comparison in cyclone noise with and without an optimised vortex finder centerbody in place. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows cyclonic separation apparatus  10  suitable for use in a cyclonic vacuum cleaner. In fact, in this example, the cyclonic separation apparatus consists of two concentric cyclones  12 , 14  for sequential cleaning of an airflow. The remaining features of the vacuum cleaner (such as the cleaner head or hose, the motor, motor filters, handle, supporting wheels, etc.) are not shown in the drawing because they do not form part of the present invention and will not be described any further here. Indeed, it is only the innermost, high efficiency cyclone  14  which incorporates a vortex finder in this embodiment and therefore it is only the innermost cyclone  14  which is of interest in the context of this invention. It will, however, be understood that the invention is applicable to cyclonic separation apparatus other than that which is suitable for use in vacuum cleaners and also to cyclonic separation apparatus incorporating only a single cyclone. 
     The innermost cyclone  14  comprise a cyclone body  16  which is generally frusto-conical in shape and has a fluid inlet  18  at its upper end and a cone opening  20  at its lower end. The cone opening  20  is surrounded by a closed collection chamber  22  in which particles entering the cyclone  14  via the fluid inlet  18  and separated from the airflow within the cyclone body  16  are collected. The cyclone body  16  has an upper surface  24  in the centre of which is located a vortex finder  26 . The vortex finder is generally tubular in shape and has a lower cylindrical portion  26   a  which merges into an upper frusto-conical portion  26   b  which leads out of the cyclone body  16  to an exit conduit. The operation of cyclonic separation apparatus of the type described thus far is well known and documented elsewhere and will not be described in any further detail here. 
     The invention takes the form of a vortex finder centerbody  30  which is located inside the vortex finder  26  and is shown in position in FIG.  1 . The centerbody  30  is also shown on an enlarged scale in FIG. 2 a . The centerbody  30  comprises a central elongate member  32  which is cylindrical along the majority of its length and has hemispherical ends  32   a ,  32   b . The hemispherical shaping of the ends  32   a ,  32   b  reduces the risk of turbulence being introduced to the airflow as a result of the presence of the centerbody  30 . The elongate member  32  carries two diametrically opposed tabs  34  which are generally rectangular in shape and extend radially outwardly from the elongate member  32  sufficiently far to abut against the interior walls of the vortex finder  26  within the cylindrical portion  26   a . The downstream edges of the tabs  34  have radiussed outer corners to reduce the risk of turbulence being introduced. Also, notches or grooves  36   a  are formed in the outer edges of the tabs  34  whilst corresponding tongues or projections  36   b  are formed in the interior walls of the cylindrical portion  26   a  of the vortex finder  26 . The tongues or projections  36 b are also diametrically opposed and are designed and positioned to cooperate with the notches or grooves  36   a  in the tabs  34  and so hold the centerbody  30  in position in the vortex finder  26 . It will be understood that the exact method of holding the centerbody in position is immaterial to the invention and the notches/grooves  36   a  and tongues/projections  36   b  can be replaced by any alternative suitable means for reliably holding the centerbody  30  within the vortex finder  26  so that the centerbody  30  will not be dislodged by the likely rate of flow of fluid through the cyclonic separation apparatus, nor subjected to unacceptable vibrations. A snap fitting method is regarded as particularly desirable because of its ease of manufacture and ease of use. 
     The length of the centerbody  30  and its positioning are sufficient to ensure that the end  32   a  of the centerbody  30  furthest from the upper surface  24  lies at a point whose distance below the upper surface  24  is equal to at least twice the smallest diameter of the vortex finder  26 . Thus the length of the protrusion of the centerbody  30  beyond the lower end of the vortex finder  26  added to the total length of the vortex finder  26  (below the upper surface  24 ) must be at least twice the diameter of the vortex finder  26 . If this criterion is satisfied, the noise reduction achievable is improved. In the embodiment shown in FIG. 1, the lowermost point of the centerbody  30  lies below the upper surface  24  at a distance which is equal to approximately 2.58 times the smallest diameter of the vortex finder  26 . Specifically, the lowermost point of the centerbody  30  lies 82.5 mm below the upper surface  24  and the smallest diameter of the vortex finder  26  is 32 mm. Furthermore, the length of the centerbody  30  is 60 mm and its diameter is 6 mm. The centerbody  30  projects below the lowermost edge of the vortex finder  26  to a distance of 16.5 mm. This arrangement succeeds in achieving a reduction in overall sound pressure level (noise) emitted from the whole vacuum cleaner product of 1.5 dBA. 
     In order for the centerbody  30  to function well, the cross-section of the centerbody  30  is made circular at any point along its length. The main body of the centerbody  30  is cylindrical, as mentioned above, but the upstream and downstream ends  32   a ,  32   b  can take various shapes. In the embodiment shown in FIG. 2 a , both of the ends  32   a ,  32   b  are hemispherical. However, one or other of the ends could be, for example, conical or frusto-conical, although a conical end will be preferable because this will reduce pressure drop and/or energy losses within the apparatus. An alternative centerbody  50  is shown in FIG. 2 b  in which the central portion of the elongate body  52  of the centerbody  50  is again cylindrical and the downstream end  52   b  is hemispherical, but the upstream end  52   a  is conical in shape. A further difference between the centerbody  50  shown in FIG. 2 a  and the alternative centerbody shown in FIG. 2 b  is the number of tabs  54  provided on the elongate body  52  for support purposes. In the embodiment shown in FIG. 2 b , four equiangularly spaced tabs  54  are provided. Corresonding tongues are then provided on the wall of the vortex finder  26  in order to support the centerbody  50  therein. 
     A further alternative embodiment is shown from two different angles in FIG. 2 c . In the Figure, the centerbody  70  is shown from two different perspective views so that the helical shape of the tabs  74  can clearly be seen. The helical shape is present so that the tabs  74  do not interfere with the rotational motion of the air exiting via the vortex finder. As in the embodiment shown in FIG. 2 a , the elongate body  72  is generally cylindrical in shape and the upstream end  72   a  is hemispherical. The downstream end  72   b  is planar. Each tab  74  is shaped at its distal end so as to include grooves  74   a  which cooperate with projections moulded into the vortex finder so that the centerbody  70  is held firmly in the correct position in the vortex finder. 
     An alternative configuration of separation apparatus is shown in part in FIG.  3 . The figure shows only the upper portion of the separation apparatus  80  which, as before, comprises an upstream, low-efficiency cyclone  82  and a downstream, high-efficiency cyclone  84 . The low-efficiency cyclone  84  has a cyclone body  86  which has an inlet  88  communicating with the upper end of the cyclone  84  and a cone opening (not shown) at the opposite end thereof surrounded by a collector (also not shown) in the same manner as shown in FIG.  1 . The cyclone  84  is closed at its upper end by an upper surface  90  from which depends a vortex finder  92  which extends into the interior of the cyclone  84  along a central axis thereof. The vortex finder  92  is cylindrical in shape for the majority of its length but flares outwardly at its upper end so as to merge smoothly with the upper surface  90 . 
     A centerbody  94  is immovably mounted within the vortex finder  92  and extends from a point above the level of the upper surface  90  right through the vortex finder  92  so that the centerbody  94  projects beyond the lower edge of the vortex finder  92 . The body of the centerbody  94  is generally cylindrical with a slight taper towards the upstream end  94   b . The upstream end  94   a  is hemispherical in shape but its downstream end  94   b  is merely planar. The centerbody  94  has three equiangularly spaced tabs or flanges  96  which extend outwardly from the upper end of the centerbody  94  to the inner wall of the vortex finder  92 . The outermost edges of the tabs or flanges  96  are shaped so as to follow the shape of the inner wall of the vortex finder  92  to assist with correct positioning of the centerbody  94 . 
     In this embodiment, the diameter of the centerbody  94  is 10 mm and the diameter D 1  of the vortex finder  92  is 30.3 mm. The length L 1  of the vortex finder is 50 mm and the distance L 2  between the lower end  94   a  of the centerbody  94  and the upper surface  90  is 64.4 mm. Hence the lowermost point of the centerbody  94  lies below the upper surface  90  at a distance of 2.13 times the (smallest) diameter of the vortex finder  92 . The centerbody  94  projects below the vortex finder  92  to a distance of 14.4 mm. 
     This invention will be better understood with reference to the following examples which are intended to illustrate specific embodiments within the overall scope of the invention as claimed. 
     Tests to determine the optimum position of the lowermost end of the centerbody in the apparatus shown in FIG. 1 have been carried out. The test method and apparatus will now be described with reference to FIG. 4 of the accompanying drawings. 
     A clear cyclone  100  with a variable-length vortex finder  120  and a variable-length centerbody  140  was mounted in an upright position using appropriate clamps and mounting devices (not shown). The cyclone  100  had a maximum diameter of 140 mm and a height of 360 mm. Suction was provided to the cyclone  100  by a quiet source connected via a first flexible hose  102  to ensure the minimum of interference from motor noise. A second flexible hose  104  connected to the cyclone inlet  106  took incoming air from a remote chamber (not shown) to avoid interference from the noise associated with air entering the hose opening. At the inlet  106  to the cyclone  100  a flow rate meter  108  was attached to allow the incoming flow rate to be measured accurately. 
     The variable-length vortex finder  120  consisted of a tube  122  of fixed length and fixed. diameter connected to the first flexible hose  102  and slidably mounted in the upper plate  110  of the cyclone  100  by means of a sealing and clamping ring  124 . In this case, the diameter of the tube was 32 mm. By clamping the tube  122  at different positions so that it projected into the cyclone  100  by different amounts, the length S of the vortex finder  120  could be varied. The variable-length centerbody  140  consisted of an elongate member  142  mounted in a knee  126  in the upper end of the vortex finder  120 . The elongate member  142  was slidably mounted in the knee  126  by means of a sealing and clamping block  144 . Further support was provided to the elongate member  142  by way of two tabs  146  extending from the elongate member  142  to the interior wall of the vortex finder  122 . The tabs  146  prevented the elongate member  142  from oscillating during the test procedure. By clamping the elongate member  142  so that it projected beyond the lower end  128  of the tube  122  by different amounts, the length L of the centerbody  140  could be varied. 
     In order to perform the experiment, the vortex finder length S was set to the required value and the end of the elongate member  142  was set flush with the lower end  128  of the tube  122  (ie, L=0). The suction source was activated and the flow rate measured and set to the required level by appropriate adjustment The centerbody  140  was then moved down in 5 mm stages and sound measurements taken at each stage. The optimum length of the centerbody being sought was the length at which the noise level was reduced to a minimum. When an approximate location of the optimum length of the centerbody  140  had been located 2 mm increments in centerbody length L were then used to pinpoint more accurately the optimum length. 
     Having determined the optimum length of the centerbody  140  for a given flowrate and a given vortex finder length S, the flowrate was then varied by adjusting the suction source and the incremental variation of the centerbody length L was repeated to determine the optimum centerbody length for that flowrate. Having determined the optimum centerbody length for each required flowrate and a given vortex finder length, the vortex finder length was then adjusted and a second series of experiments were carried out using the same set of flowrates to produce comparable results. The results obtained are set out below. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Flow Rate 
                 Vortex Finder Length 
                 Optimum Centerbody Length L 
               
               
                 (liters/second) 
                 S (mm) 
                 (mm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 20 
                 66 
                 20 
               
               
                 22.5 
                 66 
                 22 
               
               
                 25 
                 66 
                 23 
               
               
                 20 
                 40 
                 45 
               
               
                 22.5 
                 40 
                 55 
               
               
                 25 
                 40 
                 49 
               
               
                 20 
                 80 
                 10 
               
               
                 22.5 
                 80 
                 6 
               
               
                 25 
                 80 
                 25 
               
               
                   
               
             
          
         
       
     
     The optimum length was further defined as being the length of the centerbody at which noise reduction reversed to a slight gain in noise level. The optimum length was therefore seen as a minimum overall sound pressure level, a point where no significant reduction is gained by continuing to extend the centerbody or a point where the tonal quality starts to deteriorate. In particular the fundamental frequency, identified using narrow band analysis, of the vortex precession was considered as being at its minimum at the optimum length. 
     Further tests revealed that, in a cyclone body having diameter of 140 mm, a height of 300 mm, a vortex finder diameter of 32 mm and a vortex finder length of 66 mm, the optimum protrusion of the centerbody  30  beyond the lowermost end of the vortex finder is 16.5 mm. This gives a distance between the lowermost end of the centerbody  30  and the upper surface  24  of 82.5 mm, which is 2.58 times the diameter of the vortex finder  26 . 
     Further tests were carried out using apparatus similar to that described above but with replaceable vortex finders having different diameters. In each case, the vortex finder length was 46 mm and a fixed flow rate of 27 litres/second was used. The centerbody used was similar to that described above but had a diameter of 10 mm. A method similar to that described above was used to find the optimum centerbody length for each vortex finder diameter. The results obtained are as follows: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Vortex Finder Diameter 
                 Optimum Centerbody Length 
               
               
                   
                 D1 (mm) 
                 L1 (mm) 
               
               
                   
                   
               
             
             
               
                   
                 38 
                 85 
               
               
                   
                 34 
                 88 
               
               
                   
                 30 
                 76 
               
               
                   
                 28 
                 64 
               
               
                   
                 26 
                 61 
               
               
                   
                   
               
             
          
         
       
     
     This clearly shows that the optimum centerbody length for a given flow rate and a given centerbody diameter decreases generally with the diameter of the vortex finder. 
     The centerbody  30  is preferably made from a plastics material and must be sufficiently rigid not to bend or oscillate when exposed to the flowrates likely to be passed through the separation apparatus. For a centerbody suitable for use in a vacuum cleaner, a suitable material is polypropylene and this allows the centerbody to be moulded simply and economically using any one of a variety of common techniques, for example, injection moulding. 
     Testing and research have shown that, depending upon the specific configuration of the cyclone, optimising the centerbody length can result in a reduction of between 2 and 6 dB of the overall sound pressure level of a cyclone. This is sufficient to achieve an audible difference in the overall noise levels of a domestic vacuum cleaner. FIG. 5 illustrates the difference in noise (sound pressure level) produced by the cyclone of a specific vacuum cleaner with and without an optimised centerbody in place. As can clearly be seen, the presence of the centerbody (noise level shown in bold lines) removes a significant tone which is present when the centerbody is absent (noise level shown in dotted lines). The advantages of reducing the noise level of a domestic vacuum cleaner are to improve consumer satisfaction and allow a user to hear other sounds and noises within the environment in which the cleaner is being used. This can improve the safety of the user when using the cleaner.