Abstract:
A filtering device has a filter element disposed between the inlet and the outlet of the device. The inlet surface of the filter element is prone to clogging by suspended particles, e.g. waste, carried with the incoming raw fluid. The invention provides a cleaning head for the filtering device, comprising a basis and a nozzle movably mounted thereon. The cleaning head basis is mounted on a driving mechanism so that the nozzle can scan the inlet surface parallel thereto, and can clean the inlet surface by means of a backwash flow passing through the filter element into the nozzle under a cleaning pressure differential created by connecting the nozzle to a low-pressure outlet. The nozzle maintains, during scanning, permanent contact with the inlet surface so that lateral flow directly into the nozzle is essentially prevented despite variations of distance between the cleaning head basis and the inlet surface during the scanning.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to self-cleaning and manually cleaned filters, in particular to filters using a suction head to clean clogged surface of a filter element.  
       BACKGROUND OF THE INVENTION  
       [0002]     GB 1485989 discloses a filter with an outward-flow cylindrical filter element which is cleaned by three axially and angularly spaced rotary nozzles. The nozzles are axially elongated and spring-loaded into contact with the inner surface of the filter element. They are connected by a central duct to a solids discharge valve, which opens to atmosphere so that the pressure outside the element causes reverse flow through the filter element opposite the nozzles. The duct is rotated by a motor so that the three nozzles sweep axially overlapping sections of the filter element. Nozzles are spring-loaded at each axial end or in their middle and are movable in radial direction. Each nozzle comprises a cloth-reinforced phenolic resin pad which engages the internal wall of the filter element and may be independently spring-loaded.  
         [0003]     GB2157964 describes a filter with a cylindrical filter body and a device for backwashing the filter body by reverse flow of the filtered fluid, positioned within the filter body. The device includes a rotary coaxial backwash duct for removal of the backwashing fluid and a few backwash heads (nozzles) pivoted by pins to the backwash duct. The backwash heads are elongated parallel to the filter axis and the pins are also parallel to the filter axis. The heads are biased into engagement with the filter body such that, upon rotary movement of the backwash duct, the heads sweep the filter body, the pivotal mounting absorbing irregularities in surface contour.  
         [0004]     U.S. Pat. No. 4,042,504 discloses a number of self-cleanable filters with cylinder filter body and a cleaning body (nozzle) movable along the cylindrical surface of the filter body for cleaning same by suction. In some of the embodiments, the cleaning body is mounted for both linear and rotary movements, resulting in scanning the filter body along a helical path.  
       SUMMARY OF THE INVENTION  
       [0005]     A filtering device has an inlet for raw fluid coming under high operative pressure, an outlet for filtered fluid, and a filter element disposed between the inlet and the outlet. The inlet is in fluid communication with an inlet chamber bounded by an inlet surface of the filter element and the inlet surface is prone to clogging by suspended particles, e.g. waste, carried with the incoming raw fluid.  
         [0006]     In accordance with the present invention, there is provided a cleaning head for the above filtering device. The cleaning head comprises a basis and a nozzle movably mounted thereon. The nozzle has an orifice defined in a rim, the orifice being connectable to a low-pressure outlet. The cleaning head basis is mountable on a driving mechanism so that the nozzle can scan the inlet surface parallel thereto, and can clean the inlet surface by means of a backwash flow passing through the filter element into the orifice under a cleaning pressure differential created by connecting the orifice to the low-pressure outlet.  
         [0007]     The orifice is compact, with largest dimension at least an order of magnitude less than any dimension of the filter element along the inlet surface, and the nozzle is movable to maintain, during scanning, permanent contact between the rim and the inlet surface so that lateral flow from the inlet chamber directly into the orifice is essentially prevented despite variations of distance between the cleaning head basis and the inlet surface during the scanning.  
         [0008]     The nozzle may be mounted to the cleaning head basis by a moveable joint allowing perpendicular movement of the nozzle with respect to the inlet surface in order to maintain the permanent contact. Preferably, the moveable joint comprises a biasing means urging the nozzle to the inlet surface.  
         [0009]     The moveable joint and the biasing means allow the perpendicular movement of the nozzle within a first range of distance relative to the cleaning head basis; the filter element has deviations of the inlet surface within a second range of distance relative to the cleaning head basis. Preferably, the second range, during the scanning, is substantially within the first range. More preferably, the middle of the second range is substantially in the middle of the first range.  
         [0010]     Preferably, the moveable joint of the cleaning head is isolated from the suspended particles in the raw fluid and in the backwash flow.  
         [0011]     In an embodiment of the cleaning head, the moveable joint is a telescope joint connecting the nozzle to the head basis. The biasing means may be one or more springs, preferably a cylindrical compression spring coaxial with the telescope joint. The spring preferably is preloaded.  
         [0012]     The telescope joint is preferably isolated by at least one sealing ring tightly and firmly mounted on the head basis and tightly sliding along the nozzle. The telescope joint further may comprise at least one annular wiper mounted on the head basis so as to wipe a portion of the nozzle surface that slides past the sealing ring. The annular wiper may be integral with the sealing ring.  
         [0013]     Alternatively, the telescope joint may be isolated by at least one elastic sleeve with one end tightly mounted on the head basis and with the other end tightly mounted on the moveable nozzle.  
         [0014]     Preferably, the orifice rim has contact surface less than 9 times orifice area, more preferably less than 4 times the orifice area. Preferably, the orifice has a substantially round form defined by an annular rim.  
         [0015]     In one embodiment of the cleaning head, the filter element has the shape of a cylinder with the inlet chamber inside, and the driving mechanism provides the scanning of said inlet surface along a helical path parallel to the inlet surface.  
         [0016]     The usage of a nozzle that both has a compact orifice and is biased towards the filter element surface allows optimization of the suction process. The compact orifice concentrates the available pressure differential on a small area for effective suction, while the biased nozzle maintains permanent contact with the filter element surface preventing parasite side flow. The biasing force is selected so that the contact friction and wear are limited. The orifice rim may be made with reduced width, further reducing the forces acting between the rim and the filter element. The inventive nozzle allows operation with cleaning pressure differential much higher than the pressure differential, achievable with prior art nozzles.  
         [0017]     Glossary:  
         [0018]     Cleaning pressure differential: the pressure difference between the exit (outlet) side of the filter element (higher pressure during flushing) and the inlet side of the filter element in front of the cleaning head orifice (lower pressure during flushing).  
         [0019]     Contact area: the maximal area of the orifice rim that may contact the inlet surface of the filter element  
         [0020]     Dimensions of the filter element: in the case of multiple nozzles used in one filter, these are dimensions of a section of the filter element scanned by a particular nozzle. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:  
         [0022]      FIG. 1  is a scheme of a filter with cylindrical filtering element and cleaning heads of the present invention;  
         [0023]      FIG. 2A  is a sectional elevation of the cleaning head of  FIG. 1 , in design operative position;  
         [0024]      FIGS. 2B and 2C  are sectional elevations of the cleaning head of  FIG. 1 , in two extreme operative positions;  
         [0025]      FIG. 3A  illustrates schematically the operation of a nozzle with wide rim and a gap, known from the prior art.  
         [0026]      FIG. 3B  illustrates schematically the operation of the nozzle of the present invention, in permanent contact with the filter element.  
         [0027]      FIG. 3C  illustrates schematically the operation of the nozzle of  FIG. 3B  but with a narrow rim. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     With reference to  FIG. 1 , there is shown schematically a filter  10 , e.g. for filtering irrigation water, comprising a housing  12  with an inlet port  14 , an inlet stop valve  16 , an outlet port  18 , and a filtering element, e.g. mesh  20 . The mesh  20  has the form of a straight circular cylinder and is supported from the outer side by a carrying skeletal structure  22  (shown in  FIGS. 2A-2C ). The mesh  20  may comprise layers of a fine soft mesh  24  supported from one or both sides by a rigid and coarser grid  26 .  
         [0029]     A rotary pipe  30  with radial cleaning heads  32 , each head having a carrying basis  40  and a movable nozzle  34 , is mounted coaxially with the filter mesh  20 . The pipe  30  is supported in the housing  12  also for axial translation so that a driving system (not shown) can both rotate and translate the pipe. By virtue of the rotation and translation movement, the cleaning heads  32  can scan the inlet (internal) surface of the mesh  20  along a helical line  33 , parallel to the inlet surface. The rotary pipe  30  has a discharge outlet  35  with flushing valve  36  which is normally closed.  
         [0030]     During normal filtering operation, raw water enters the filter  10  through the inlet port  14  under operative pressure Po and passes through the filtering mesh  20  from inside out. Filtered water leaves the filter under exit pressure Pe through the outlet port  18 . In the process, suspended particles are deposited on the inlet surface of the mesh and gradually clog the filter element. The pressure drop across the filter ΔPf=Po−Pe rises and, at some predetermined value ΔPb called “blockage pressure” and detected by an operator or an automatic controller, the filter performs a cleaning operation.  
         [0031]     At the start of the cleaning operation, the flushing valve  36  is opened to the atmosphere or to a low-pressure enclosure. The driving system starts to rotate and translate the pipe  30  so that the nozzles  34  scan the inlet surface of the mesh  20 . As the pressure at the outlet side of the mesh is higher than the atmospheric pressure Pa, a pressure differential ΔP is established across the mesh in the area opposite the nozzle  34 , directed from the mesh to the discharge port  35  (see also  FIG. 3B ). ΔP is the cleaning pressure differential. The nozzles  34  start to suck the waste material deposited on the inlet surface of the mesh  20  and to dump it by a backwash flow  37  via the pipe  30  and the port  35  out of the filter. Each cleaning head  32  scans a section of the mesh  20  in close turns of the helical path  33  so that the entire surface of the mesh is cleaned up.  
         [0032]     The structure and the advantages of the cleaning head of the present invention will become clearer from the sectional elevations shown in  FIGS. 2A-2C . With reference to  FIG. 2A , the cleaning head  32  includes the tubular head basis  40  which is mounted radially, e.g. welded, to the rotary pipe  30 . The head base  40  accommodates the movable nozzle  34  and a cylindrical compression spring  42 . The nozzle  34  comprises a nozzle pipe  44  and nozzle cap  46  with an annular rim  47 . The tubular basis  40  is closed by a cover  48 . The nozzle pipe  44  is supported in the tubular basis  40  by an annular guide  50  and an opening in the cover  48  so as to form an axially movable telescope joint. The cylinder spring  42  urges the nozzle pipe  44  axially towards the inlet surface of the mesh  20 . (Note: the direction along the nozzle axis is “radial” direction for the cylinder mesh  20  and the rotary pipe  30 ).  
         [0033]     The shape of the cylinder mesh  20  always deviates from the perfect shape of a geometrical cylinder coaxial with the axis of rotation of the rotary pipe  30 , so that the distance between the basis  40  of the cleaning head and the mesh  20  varies during the scanning. The variation may be due to deviations of the carrying structure  22 , deviations of the mesh shape, inaccurate mounting of the driving mechanism with respect to the cylinder mesh, etc.  FIG. 2B  shows the extreme proximal position of the mesh to the cleaning head basis while  FIG. 2C  shows the extreme distal position of the mesh. The design operative position of the cleaning head  32  is shown in  FIG. 2A  where the spring biased nozzle  34  is in the middle between the two extreme positions, with ability to move towards either extreme position. The spring  42  maintains the nozzle  34  always in contact with the mesh  20 , following the deviations of the mesh shape, so that the gap between the rim  47  and the mesh  20  is virtually closed.  
         [0034]     It will be appreciated that the range of available motion of the nozzle perpendicular to the mesh surface must encompass the range of deviation of the mesh surface including mesh and filter structure tolerances, in order to maintain permanent contact and closed gap. The tolerances of the filter structure and the filter element are usually known beforehand and the nozzle motion range is designed to cover them. Preferably, the middle point of the motion range is in the middle of the tolerance range.  
         [0035]     The spring  42  is relatively weak in order to maintain low contact pressure on the mesh and to prevent excessive friction force, but it is strong enough to resist significant displacement of the nozzle together with the mesh under the action of the cleaning pressure differential ΔP. For this reason, it is important to protect the telescope joint and the spring from sticking with dirt. Thus, the telescope joint and the spring  42  are protected from the harmful particles by two tight sealing rings  52  and  54  mounted in the cover  48  and in the tubular basis  40  and sliding over the nozzle pipe  44 . The sealing rings have also annular lips (wipers) which prevent sticking of particles to the nozzle pipe surface before it enters the annular guides  50  and the sealed chamber.  
         [0036]     The advantage of the cleaning action in permanent but gentle contact with the mesh is seen with reference to  FIGS. 3A  to  3 C. In these figures, the cleaning pressure differential ΔP or ΔP 0  acting on the filter element during cleaning is plotted on the exit surface of the filter element.  
         [0037]      FIG. 3A  shows a nozzle  134  known in the prior art, without biasing means to urge it to the mesh surface. As the nozzle  134  scans the surface of the mesh  20 , during a substantial part of the scanning path the nozzle cap  146  is offset from the surface of the mesh  20  by a gap ΔY. The backwash flow  137  in this case comprises a side component  58  and a central component  59 . The central component  59  passes through the mesh  20  and is effective for cleaning it, while the side component  58  is wasted without contribution to cleaning. It will be appreciated also that if the gap ΔY between the nozzle rim  147  and the mesh  20  is large, the cleaning pressure differential ΔP 0  across the mesh is lower. For example, if the nozzle orifice is round and has radius r 0 , a gap of width r 0 /2 will reduce ΔP 0  to none.  
         [0038]     The side component may be reduced by using a nozzle with thick walls (thick rim)  147  so that the side flow  58  through the gap would experience greater hydraulic resistance. However, then the force urging the filter element to the nozzle rim  147  will reach high values. This force is approximately the product of the pressure differential ΔP 0  and the nozzle area πR 0   2  including the orifice area and the rim contact area. If higher pressure differential ΔP 0  is applied, this force may be considerable and capable of bending the filter element towards the nozzle when it moves on to scan further the filter element. In addition to the friction wear, the filter element will be fatigued due to the cyclic bending deformation. This would prevent the usage of higher cleaning pressure differentials.  
         [0039]     As shown in  FIG. 3B , the nozzle  47  of the present invention is in permanent but gentle contact with the filter element  20  due to the adjustment action of the biasing spring  42 . Due to the permanent contact, the side flow  59  is virtually non-existent and the backwash flow  37  consists only of the central flow component  59  which cleans the filter element. The width of the nozzle cap  46  may be made smaller, preferably such that the rim contact area does not exceed 9 times the orifice area, and thus the force of the loaded spring  42  urging the filter element to the nozzle rim  47  may be reduced.  
         [0040]     The applied cleaning pressure differential ΔP should be a few times higher than the “blockage pressure” ΔPb in order to remove reliably all particles retained on the filter element. Such high cleaning pressure differential would create strong flow through the nozzle, with specific flow rate an order of magnitude higher than the specific flow through the filter element during normal filtration. Such flow is unattainable with known nozzles having large orifice area, e.g. nozzles extended parallel to the filter element axis. The inventive nozzle is very compact and allows to concentrate the cleaning flow through a limited area of the filter element and to work with cleaning pressure differential which is at least 3 times higher that cleaning pressure differentials used in known filters at a given operative pressure.  
         [0041]     A typical irrigation filter works under operative pressure Po between 2 and 10 bar at its inlet. The pressure differential across the filter element ΔPf depends on the flow rate, on the mesh geometry and on the degree of contamination. In the process of operation, ΔPf reaches the “blockage pressure” value ΔPb which is usually about 0.5 bar. Thus the exit pressure Pe=Po−ΔPb before the cleaning action is about 1.5-9.5 bar. When the flushing valve is opened to connect the nozzle to the atmosphere (reference pressure Pa=0), the total pressure differential ΔPt between the exit side of the filter element and the flushing valve outlet will be ΔPt=Pe−Pa≅Pe. The inventive “following” nozzle allows ΔPt to be used almost entirely for cleaning as the nozzle prevents side flow from the inlet chamber directly into the orifice. The inventors have established that the achieved cleaning pressure differential ΔP may be above 80% of the total differential ΔPt. It will be appreciated that the total differential cannot be entirely utilized for cleaning in principle, because of inevitable hydraulic losses in the flow path from the orifice to the atmosphere (in the piping, the flushing valve, etc.). When the operative pressure Po is low, the filtration may be stopped during flushing, e.g. by closing the outlet port  18 . Thus the exit pressure Pe may be raised to equal the operative pressure Po, and the cleaning pressure differential ΔP will be raised as well.  
         [0042]     With reference to  FIG. 3C , the rim width of a nozzle  34 ′ of the present invention may be further reduced by forming the nozzle cap face with a step, such that the height of step is at least half the orifice radius, and the rim area does not exceed 4 times the orifice area. Thus the force urging the filter element to the nozzle rim may be further reduced as well as the accompanying wear.  
         [0043]     The nozzle cap  46 ′ and the rim  47 ′ are made of material providing low friction and low friction wear, so that the filter element surface and the nozzle cap  46 ′ will properly function at least during a filter life time.  
         [0044]     The cleaning head of the present invention achieves efficient cleaning of the filtering element (mesh) and reliable prolonged operation under high cleaning pressure. The compact (e.g. round) nozzle orifice concentrates the total available pressure differential on a small area for effective suction. At that, the spring-biased nozzle maintains permanent contact with the filter element surface and limits the friction, mesh deformation and wear. The telescope joint and the spring element between the nozzle and the head basis are protected from contamination and sticking, which allows the usage of weak spring element and further reduction of contact wear. The rim with reduced width also helps to reduce the forces acting between the nozzle rim and the filtering element.  
         [0045]     Although a description of specific embodiments has been presented, it is contemplated that various changes could be made without deviating from the scope of the present invention. For example, the present invention could be modified and used for cleaning filter elements with different layouts, such as flat, conical, from inside or from outside of curved filter elements, etc. by using suitable drives for the scanning motion; in mesh filters and in disk filters; the joint of the nozzle may be other than telescope and may be protected by other means, i.e. a bellows sleeve.