Abstract:
A duplex fluid filter assembly allows extended life between filter replacements, by automatically back-flushing one filter unit with clean fluid from another filter unit of the duplex assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of U.S. Provisional Application No. 61/824,467 filed May 17, 2013 for “Self-cleaning duplex filter”, the disclose of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Aspects of the present disclosure are in the technical field of fluid filtration, including both liquid filtration and filtration of air and other gases. 
         [0003]    In-line filtration to remove solid contaminants from circulating or flowing fluids, for example, from lubricants, coolants, and fuels is well known. However, it is generally necessary to interrupt the flow of fluid through a filter unit from time to time to replace the filter medium as it becomes clogged with solids. It has been proposed to provide a filter assembly including two filter units connected in parallel, so that one filter unit can remain in operation while the filter medium in the other is being replaced. However, replacing the filter medium is a messy and labor-intensive process. It is therefore desirable to increase the interval between filter replacements. 
       SUMMARY OF THE INVENTION 
       [0004]    The present disclosure teaches apparatus, methods, and programs for fluid filtration in which it is possible to filter a working fluid continuously for long periods without human intervention, by automatically back-flushing a filter unit without interrupting filtering of the working fluid. 
         [0005]    Embodiments of the present invention provide filter units, filter systems, methods of operating such filter units and filter systems, and computer programs (which may be stored on a non-transitory tangible storage medium) for causing a processor controlling such a unit or system to carry out such a method. 
         [0006]    One embodiment of the invention provides a filter unit comprising: a housing; a horizontal tube plate within the housing, separating the housing into upper and lower portions; at least one port in the tube plate, with a respective at least one connector for a respective at least one filter cartridge to be mounted in the housing upper portion; a fluid outlet below the tube plate and mounted within the housing lower portion, the at least one port communicating with the fluid outlet; a fluid inlet into the housing upper portion; at least one passage between the housing upper portion and the housing lower portion; and a drain communicating with the housing lower portion. 
         [0007]    The at least one port in the tube plate may comprise a plurality of ports, and the fluid outlet may then comprise a manifold communicating with the plurality of ports. 
         [0008]    The at least one passage may comprise one or more slots between an outer edge of the tube plate and a wall of the housing. 
         [0009]    The fluid inlet may be positioned to cause incoming fluid to flow across an upper surface of the tube plate. 
         [0010]    The filter unit may further comprise a respective at least one filter cartridge mounted on the at least one connector with an interior communicating through the at least one port with the fluid outlet. 
         [0011]    Valves may be operative to selectively open and close the fluid inlet, the fluid outlet, and the drain. 
         [0012]    An embodiment of the invention provides a method, and a controller and computer program operative to implement the method, to control the valves to place the filter unit in a selected one of a filtering mode in which at least one fluid inlet valve and at least one fluid outlet valve are open and a drain valve is closed, a back-flushing mode in which the fluid inlet valve is closed and at least one fluid outlet valve and the drain valve are open, and an offline mode in which at least one fluid inlet valve or fluid outlet valve is closed and the drain valve is closed. 
         [0013]    A sensor may detect clogging of the at least one filter cartridge and switch the filter unit from the filtering mode to the back-flushing mode when a level of clogging exceeds a predetermined threshold. The sensor may then provide an input to the controller in response to which the controller switches the valves. The sensor may detect a pressure drop across the filter unit. 
         [0014]    An embodiment of the invention provides a filter assembly comprising two filter units, each as mentioned above, having a common inlet and a common outlet, at least one inlet valve operative to selectively connect or disconnect the fluid inlet of each filter unit and the common inlet, at least one outlet valve to selectively connect or disconnect the fluid outlet of each filter unit and the common outlet, and drain valves to selectively open and close the drain of each filter unit. 
         [0015]    The method, computer program, and/or controller may then control the valves to place each filter unit in a selected one of a filtering mode in which the fluid inlet valve and the fluid outlet valve are open for that filter unit and the drain valve is closed for that filter unit, a back-flushing mode in which the fluid inlet valve is closed, the fluid outlet valve or another fluid valve connecting an outlet side of the filter medium with the fluid outlet is open, and the drain valve is open for that filter unit, and an offline mode in which at least one of the fluid inlet valve and the fluid outlet valve is closed and the drain valve is closed for that filter unit. 
         [0016]    The valves may be controlled: to place one filter unit in the filtering mode and the other filter unit in the offline mode; when clogging of the one filter unit is detected, to place the other filter unit in the filtering mode and the one filter unit in the back-flushing mode; and after back-flushing, to place the one filter unit in the offline mode and maintain the other filter unit in the filtering mode. 
         [0017]    An embodiment provides a fluid system comprising: a path or circuit in which fluid flows or circulates; and a filter unit as mentioned above connected in the circuit so that in operation fluid enters the filter unit at the fluid inlet and leaves the filter unit at the fluid outlet. 
         [0018]    An embodiment provides a filter assembly comprising two filter units, each comprising: a housing; a fluid inlet port into the housing; at least one connector for a respective at least one filter cartridge to be mounted in the housing; a fluid outlet port communicating with the at least one connector so as to be in communication with an interior of a filter cartridge mounted on the at least one connector; and a drain in a lower portion of the housing; the assembly further comprising: a common inlet; at least one inlet valve operative to selectively connect or disconnect the fluid inlet port of each filter unit and the common inlet; a common outlet; at least one outlet valve to selectively connect or disconnect the fluid outlet of each filter unit and the common outlet; drain valves to selectively open and close the drain of each filter unit; and a controller operative to control the valves to place each filter unit in a selected one of a filtering mode in which the fluid inlet valve and the fluid outlet valve are open and the drain valve is closed, a back-flushing mode in which the fluid inlet valve is closed, the fluid outlet valve or another fluid valve connecting an outlet side of the filter medium with the fluid outlet is open, and the drain valve is open, and an offline mode in which at least one of the fluid inlet valve and the fluid outlet valve is closed and the drain valve is closed. 
         [0019]    The valves may be controlled: to place one filter unit in the filtering mode and the other filter unit in the offline mode; when clogging of the one filter unit is detected, to place the other filter unit in the filtering mode and the one filter unit in the back-flushing mode; and after back-flushing, to place the one filter unit in the offline mode and maintain the other filter unit in the filtering mode. 
         [0020]    An embodiment of the invention provides a filter cartridge comprising a filter medium forming a tube and one or more shrunk-on, fitted, or clamped-on bands encircling the tube and restraining the tube against outward pressure. 
         [0021]    The filter medium may be pleated, with inner edges of the pleats being supported by a perforated tube, and the pleats being in compression between the one or more bands and the tube. The filter medium may be stainless steel. The bands may be heat-shrink fluoropolymer. 
         [0022]    The mentioned filter units and filter systems may be equipped with the mentioned filter cartridges. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above and other aspects, features, and advantages of the present invention may be more apparent from the following more particular description of embodiments thereof, presented in conjunction with the following drawings. In the drawings: 
           [0024]      FIG. 1  is a front elevation view, partly cut away, of one embodiment of a filter assembly. 
           [0025]      FIG. 2  is a schematic diagram of the filter assembly shown in  FIG. 1 . 
           [0026]      FIG. 3  is a perspective view of a filter cartridge suitable for use in the filter assembly of  FIG. 1 . 
           [0027]      FIG. 4  is a cross-section through the filter cartridge of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    A better understanding of various features and advantages of the present methods and devices may be obtained by reference to the following detailed description of illustrative embodiments of the invention and accompanying drawings. Although these drawings depict embodiments of the contemplated methods and devices, they should not be construed as foreclosing alternative or equivalent embodiments apparent to those of ordinary skill in the subject art. 
         [0029]    Referring to  FIG. 1  of the accompanying drawings, one form of filter assembly  10  comprises two filter units  12 ,  14 . Each filter unit  12 ,  14  comprises an upright cylindrical housing  16  with a removable lid  18 . A horizontal tube plate  20  is mounted across the housing  16 , dividing the interior of the housing  16  into a larger upper portion  22  and a smaller lower portion  24 . Housing lower portion  24  is larger than is conventional, in this embodiment approximately 25% of the volume of housing upper portion  22 . The upper and lower housing portions  22 ,  24  are in communication via slots  26  between the tube plate  20  and the wall of the housing  16 . The slots  26  extend round most of the circumference of the tube plate  20 , separated by relatively short bridges by which the tube plate  20  is joined to the housing  16 . Air vent valves  48  are provided in the top of the housings  16 . 
         [0030]    The tube plate  20  is provided with an array of ports  30  with connectors  32  on the upper side of the tube plate  20  for connecting to one or more filter cartridges  34 . The filter cartridges  34  may be of conventional design, and include a tubular filter surface, a closed upper end, and an open lower end (or an opening in the lower end) with a fitting that mates with the connectors  32  so as to provide a conduit from the inside of each filter cartridge to a port  30 . An O-ring seal  84  (see  FIG. 3 ) is preferred between cartridges  34  and port connectors  32 , because conventional flat gasket seals tend to leak under reverse pressure. However, the O-ring seal itself may be conventional. 
         [0031]    Referring to  FIG. 3 , in an embodiment, the filter medium of each filter cartridge  34  is a pleated stainless steel screen  80 , supported on the inside by a perforated metal tube  86 . On the outside, the pleats are encircled by bands  82 , preferably made from heat-shrunk tubing of fluorinated ethylene propylene (FEP) or other suitable fluoropolymer. The bands  82  are dimensioned so that, if unconstrained, they would shrink to approximately 60% of the external diameter of cartridge  34 . As a result, the bands  82  are in tension, and the pleats of screen  80  are placed under radial compression. As explained below, such a construction will restrain the pleated screen against outward pressure. The bands  82  are, therefore, considerably stronger than the cloth bands that are used on conventional pleated filter cartridges, which merely serve to stabilize the pleats against lateral movement. While the bands are preferably made from shrunk tubing, it is also contemplated that they could be clamped or crimped on or fitted onto the cartridges. The thickness of the stainless steel screen material is chosen to resist the compression force. 
         [0032]    On the underside of the tube plate  20 , the ports  30  open into a manifold  36  that isolates the ports  30  from the lower housing portion  24 . Thus, fluid flow into the housing upper portion  22  passes through the filter surfaces of the filter cartridges  34  into the interior of the cartridge and through the ports into the manifold  36 . 
         [0033]    Each filter unit  12 ,  14  has an inlet port  40 , positioned in a side of the housing  16 , with the bottom of the inlet port  40  preferably level with the upper face of the tube plate  20 . In operation, the position of the inlet port  40  causes a fluid current across the top of the tube plate  20  which tends to sweep any solids that may have settled on the tube plate  20  across to the slots  26 , allowing the solids to fall into the housing lower portion  24  or be directed through the filter media. The filter assembly  10  has a common inlet  42 , which is in communication with the inlet ports  40  of both filter units  12 ,  14  through a two position three-way valve  43  (see  FIG. 2 ) that acts as a transfer valve, directing incoming fluid to one or other of the filter units  12 ,  14 . The inlet ports  40  are connected by a cross-pipe  46  with a pair of solenoid-controlled valve  47 . Two oppositely facing solenoid-controlled valves  47  are provided to close the cross-pipe  46  against pressure differentials in both directions. Alternatively, a single valve of a type that seals against pressure differentials in both directions, such as a ball valve, may be used. 
         [0034]    Each filter unit  12 ,  14  has an outlet port  50 , positioned in a side of the housing  16  below the tube plate  20 , and communicating with the manifold  36 . The filter assembly  10  has a common outlet  52 , which is in communication with the outlet ports  50  of both filter units  12 ,  14  through a two-position three-way valve  53  (see  FIG. 2 ) that acts as a transfer valve, connecting one or other of the filter units  12 ,  14  to the outlet  52 . Three way valve  53  is ganged to three way valve  43  by a common shaft  45 , so that they operate together. The outlet ports  50  are connected by a cross-pipe  56  with a pair of solenoid-controlled valves  57 , similarly to valves  47 . In use, the common inlet  42  and the common outlet  52  are connected to an external fluid system  76  in which a fluid that requires filtering flows or circulates, so that the fluid enters the filter assembly  10  at the common inlet  42  and leaves the filter assembly  10  at the common outlet  52 . 
         [0035]    Each filter unit  12 ,  14  has a drain  60 , at the bottom of the housing lower portion  24 , which is controlled by a normally-closed solenoid-operated drain valve  62 . The drains  60  are connected to a recovery unit  64  that extracts fluid and solids from the housing lower portion  24  and allows the solids to be removed for disposal or further processing. 
         [0036]    Pressure sensors  66  are provided on the inlet side of each filter unit  12 ,  14 . At least one differential pressure sensor  67  is provided to measure the pressure difference between the common fluid inlet  42  and the common fluid outlet  52 . 
         [0037]    A controller unit  70  receives inputs from the pressure sensors  66 ,  67  and operates the valves  43 ,  44 ,  53 ,  54 ,  62  as described below. The controller unit  70  may be a programmable logic controller, a general purpose computer suitably programmed, or other suitable device having non-volatile storage for instructions, volatile or non-volatile storage for data indicating the configuration and current state of the filter assembly  10 , and a processor, logic circuitry, or other mechanism for executing the instructions in accordance with input data so as to operate the valves. 
         [0038]    In normal operation, one of the filter units, for example, filter unit  12 , is placed in a filtering mode and the other filter unit, in this example filter unit  14 , is placed in an offline mode by controller  70  setting the inlet transfer valve  43  and the outlet transfer valve  53  to direct fluid from the inlet  42  to the outlet  52  through the one filter unit  12  and not through the other filter unit  14 . The pressure equalization and backflush valves  47 ,  57  are all closed. The drain valves  62  are closed. The external circuit  76  operates, and fluid circulates through the active filtering unit  12 . The fluid passes in through the inlet port  40  into the housing upper portion  16  flowing around the outside of the filter cartridge  34  (or cartridges if there are more than one). The fluid flow passes radially inward through the filter media and into the center of the filter cartridge and downward to the lower opening in the cartridge. The fluid flows through the connectors  32  and the ports  30  in the tube plate  20  to the manifold  36  where it collects and is channeled through the outlet  50 . 
         [0039]    The filter media of the filter cartridges  34  separates out solid contaminants from the fluid in the usual way. During filtration the contaminated (dirty) fluid flows from the inlet  40  around the outside of the cartridge(s)  34  and passes through the filter media. The filter media captures or separates the solid contaminants in the fluid flow. With a stainless steel screen filter medium  80 , the solid contaminants are caught on the surface of the filter, rather than being embedded within its thickness. Some of the contaminants will fall from the filter medium and settle onto the tube plate  20  and, because of the flow of fluid from the inlet  40  across the tube plate  20 , those solids will be washed across to the far edge of the tube plate  20 , and fall through the slots  26 , so as to settle in the housing lower portion  24 . The large size of housing lower portion  24  provides both a region of almost stagnant fluid within which solids can settle out, and a space in which the solids can accumulate. Much of the solids, however, will become trapped in the filter media, gradually clogging the filter media thereby reducing its effectiveness and increasing the resistance to flow through the filter unit  12 . 
         [0040]    The pressure sensor  67  monitors the pressure drop across the filter cartridges  34 , which is indicative of the degree of clogging of the filter medium, by comparing the pressures at ports  42  and  52 . When the pressure drop exceeds a threshold, the controller unit  70  activates the back-flushing mode of the filter unit  12 . The threshold may be close to the threshold at which, in a comparable conventional filter unit, the cartridges would have been removed for washing. It is desirable to carry out the back-flushing procedure before housing lower portion  24  fills up with solid contaminants. However, if housing lower portion  24  does fill up, loose contaminants will be trapped in housing upper portion  22 , and will rapidly clog filter medium  80  and trigger a back-flush. 
         [0041]    In order to back-flush the first filter unit  12 , the controller  70  first checks the pressures in the housings  16  of the two filter units  14 , using the additional pressure sensors  66 , to ensure that they are approximately equal. If the pressures are not sufficiently nearly equal, they can be equalized by the controller  70  opening the inlet pressure equalization valves  47 . That may be necessary, for example, to reduce the torque required to operate three-way valve  43 . The second filter unit  14  is then placed in its filtering mode, by controller  70  changing over the transfer valves  43 ,  53 , thereby ensuring that operation of the external circuit  76  is not interrupted, and isolating the first filter unit  12 . The controller  70  then opens the two outlet back-flush valves  57 , and opens the drain valve  62  of the first filter unit  12 . If the external circuit  76  is pressurized, there is then a pressure difference from the common outlet  52  to the drain  60 , which causes cleansed fluid to flow in the reverse direction through outlet  50  up through the ports and into the center of the cartridges  34  of the first filter unit  12 . The flow then flows radially outwardly through the filter media of the filter cartridges  34 , thereby dislodging the contaminants on the filter media. As explained above, the bands  82  restrain the pleated filter medium  80  against the back-pressure. In normal operation, the gauge pressure in the outlet  52  is typically sufficient to back-flush the filter media without additional assistance. The reverse flow back-flushes the filter cartridges  34 , dislodging a large proportion of the trapped solids from the filter media. Since the inlet valve  42  of the first filter unit  12  is closed, after passing outward through the filter media, the fluid flows through the slots  26  and into the housing lower portion  24  with the solids. The dirty fluid flows out through the drain  60  carrying with it both the dislodged solids and the solids that have previously settled in the bottom of the housing lower portion  24 . The rate of flow of the back-flushing fluid is limited by the size of the cross-pipes  56  and the outlet  60 , and may be chosen to provide effective back-flushing without wasting too much fluid or starving the outlet side of the fluid system  76 . 
         [0042]    When back-flushing is completed, the controller  70  closes the drain valve  62  of the filter unit  12  that is being back-flushed, and closes the back-flush valves  57 . Back-flushing may be continued for a predetermined time, or until a predetermined amount of fluid has been used. The filter unit  12  is then in the offline mode, and the other filter unit  14  remains in the filtering mode. Eventually, the filter unit  14  in turn needs back-flushing, and the back-flushing process is carried out for the filter unit  14 , with the filter unit  12  becoming the unit in the filtering mode. 
         [0043]    The process may be continued for a considerable time, with the two filter units alternating in the filtering and offline modes, until the filter medium of the cartridges  34  either deteriorates or becomes clogged with a proportion of the solids that cannot be dislodged by back-flushing. At that time, either filter unit  12 ,  14 , or both in succession, may be renewed by isolating the relevant filter unit  12 ,  14  from the fluid circuit  76 , removing the lid  18 , and removing and replacing the filter cartridges  34 . The used filter cartridges, depending on their type and condition, may be processed in any appropriate way. For example, they may be cleaned by a more powerful procedure such as high-pressure washing or steam cleaning and reused, or they may be refurbished, recycled or discarded. 
         [0044]    In an example, 400 GPM (25 l/s) of hydrocarbon fuel at 65 psig (450 kPa), with a constant solids content, was passed through a test filter. In an embodiment, the filter housing  16  is approximately 20″ (500 mm) in diameter. There are six slots, each approximately 1″ (25 mm) wide and 9″ 225 mm) long, separated by lands 1½″ (40 to 50 mm) long. There are 15 filter cartridges, each 4¼″ (110 mm) in diameter. The cartridges are rated for a collapse strength of 150 psid in the forward (inward) direction, and a burst strength of 30 psid in the reverse (outward) direction. With a new filter, a clogging threshold of 15 to 20 psid (100 to 140 kPa) was reached in 90 to 100 minutes. It was found that back-flushing with 50 gallons (190 liters) of clean fuel (which takes about 5 minutes) every 15 to 20 minutes was sufficient to keep the pressure drop below the threshold, with no noticeable deterioration over at least 80 cycles. Thus, the time between filter changes can, be increased by at least a factor of 80. 
         [0045]    As may be seen from the above description, the described filter apparatus makes possible efficient and economical filtration, because the back-flushing process can be entirely automatic, so that human intervention is required only when the filter cartridges need to be replaced, and the possibility of repeated back-flushing allows for a much longer interval between replacements than in a conventional system. Also, because the back-flushing is driven by the existing system pressure, the back-flushing process can be carried out simply by manipulating valves that must already be present. Thus, the only additional hardware required is to upgrade valves from manual to electrical operation, and an electronic controller that can be very simple. 
         [0046]    While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention. 
         [0047]    For example, the filter assembly  10  shown in the drawings has two filter units  12 ,  14 . A filter assembly  10  having more than two filter units would also be possible. In that case, one or more filter units may be in each of the filtering, back-flushing, and offline modes, and/or two or more filter units may be in the same one of those modes, depending on the demands of the fluid system in which the filter assembly is installed. 
         [0048]    While the connectors are described in the above embodiment as a separate component, it is also contemplated that the connectors can be formed as an extension of the filter cartridge that mates with the port  30 , or an extension of the port  30  that mates with an opening in the cartridge. 
         [0049]    Accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.