Patent Publication Number: US-7219805-B2

Title: Sieve screen level sensor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   Reference is made to and priority claimed from U.S. Provisional Application Ser. No. 60/520,822, filed on Nov. 17, 2003, entitled: SIEVE SCREEN LEVEL SENSOR. 

   FIELD OF THE INVENTION 
   The present invention relates generally to the bulk handling of materials that are in the form of a fine powder or granulate. More particularly, the present invention is related to the process of sifting bulk toner, which is intended for use in electrographic copiers and printers, during or as part of the process of placing the toner into smaller containers. 
   BACKGROUND OF THE INVENTION 
   The toner used in electrographic copiers and printers is a blend of particles, including plastic resins, coloring pigments and other ingredients. Most toners are manufactured in bulk using a melt mixing or hot compounding process. Plastic resins, carbon black, magnetic iron oxides, waxes and charge control agents are blended together while in a molten state to thereby form a hot paste having a consistency similar to cake mix. This mixture is then cooled, typically by forming it into slabs on a cooling belt or by pelletizing the mixture and cooling the pellets. The raw toner is then ground or pulverized into a toner powder by jet mills or air-swept hammer mills. This process produces a powder having a wide range of particle sizes. The toner powder is sifted to remove over-size and under-size toner particles. The pulverized, sifted toner powder is then blended with additives to adjust flow and electrostatic properties. The finished toner powder has particle sizes that range from, for example, twelve microns (μ) to approximately eight microns and smaller. The bulk toner is typically placed into large-sized or bulk containers, such as, for example, large barrels. 
   The toner powder is typically repackaged from the large bulk containers into smaller intermediate or end-use containers that are suitable for sale to and/or use by end users. Repackaging the toner from the bulk containers into smaller containers generally involves gravity-assisted flow of the toner from the bulk container into a sieve, such as a vibratory sieve, and into the smaller containers. The sieve typically contains a mesh or screen filter through which the toner powder must flow. The filter is intended to prevent the passage of agglomerated toner particles and contaminants into the smaller containers. The mesh or screen filter has very fine openings, such as, for example, from approximately 200 to approximately 400 openings per inch, and is typically constructed of a metal, such as, for example, stainless steel. 
   The fine mesh filter occasionally becomes clogged or blinded due to an accumulation of agglomerated toner powder, oversized toner, and/or foreign particles thereon. The partial or complete clogging or blinding of the filter significantly reduces or stops the throughput of product through the screen, and a build-up of toner powder above the screen results. The weight of the built-up of toner powder bears directly upon the fine screen and may result in tearing of the screen. When a screen tears, the coarse material collected thereon is undesirably conveyed through the sieve thereby contaminating otherwise acceptable product. The contaminated product must be recycled, i.e., re-processed through the sieve. 
   In order to prevent the above-described overloading and tearing of sieve screens, the sieves must be shut down and preventive maintenance and cleaning of the screens performed. The performance of such preventative maintenance, and the resulting down time of the sieves, is costly and inefficient. Further, the preventive maintenance must be performed on a predicted minimum schedule, which may often be premature for a particular screen, thereby causing unnecessary down time of the sieves. 
   Therefore, what is needed in the art is a method and apparatus to detect a build-up of powder on the screen. 
   Further, what is needed in the art is a method and apparatus for detecting a blinded or clogged screen thereby indicating the need for cleaning and/or preventive maintenance. 
   Moreover, what is needed in the art is a method and apparatus that prevents overloading and/or tearing of filter screens, and which increases the useful life of a filter screen. 
   SUMMARY OF THE INVENTION 
   The present invention provides a filtering apparatus for filtering fine powder, and which reduces the occurrences of and/or detects torn and/or blinded sieve screens. 
   The invention provides, in one form thereof, a sieve having a material inlet and a material outlet. An inlet valve controls the flow of material into the sieve through the material inlet. A sieve screen is disposed within the sieve between the material inlet and outlet such that the material must pass through the sieve screen to enter the material outlet and thereby exit the sieve. A sieve screen level sensor assembly senses a level of material accumulated upon the sieve screen, and issues a level sense signal indicative of that level. A programmable logic controller receives the level sense signal and controls the inlet valve dependent at least in part thereon to thereby control the flow of material into the material inlet. 
   An advantage of the present invention is that a build-up of material on the sieve screen or a blinded sieve screen is detected to thereby reduce the occurrence of torn sieve screens. 
   A further advantage of the present invention is that a blinded or clogged screen is detected and the need for cleaning and/or preventive maintenance of the sieve screen is indicated, thereby avoiding premature preventative maintenance and/or cleaning. 
   A still further advantage of the present invention is the overloading and/or tearing of filter screens is reduced, thereby increasing the useful life of a sieve screen. 
   The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become apparent and be better understood by reference to the following description of one embodiment of the invention in conjunction with the accompanying drawings, wherein: 
     The FIGURE is a schematic diagram of one embodiment of an apparatus for filtering fine powder having a sieve screen level sensor of the present invention. 
   

   The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
   DETAILED DESCRIPTION OF THE DRAWINGS 
   Referring now to the FIGURE, one embodiment of a bulk powder filtering or sifting apparatus having a sieve screen level sensor of the present invention is shown. Bulk powder filtering apparatus  10  includes sieve  12 , sieve screen  14 , sieve screen level sensor assembly  20 , and input valve  22 . 
   Sieve  12  is a conventional material or powder sieve, such as, for example, a vibratory sieve manufactured by Russell Finex Corporation, intended to sift or sieve a fine bulk powder material M, such as, for example, toner, carbon, silica, alumina, plastic resins, etc. Sieve  12  includes an inlet  24  and an outlet  26 . Input valve  22 , such as, for example, a conventional rotary air lock valve, controls the flow of powder material M through inlet  24  and into sieve  12 . Powder material M exits sieve  12  through outlet  26 . Sieve screen  14  is disposed between inlet  24  and outlet  26 . 
   A flow of pressurized gas G PURGE , such as, for example, air, nitrogen, or another inert gas, is supplied via a purge gas supply line or conduit  32  to the interior of sieve  12 . Typically, purge gas G PURGE  is an inert gas supplied through purge gas supply line  32  and into sieve  12  that is used to purge sieve  12  of air, and to blanket any cloud of powder particles existing therein to inhibit combustion and/or explosion. The flow of air and/or purge gas G PURGE  through purge gas supply line  32  and into sieve  12  is at a purge gas pressure P PURGE , such as, for example, from approximately 1.0 to 3.0 inches water column (In. WC). A purge vent  34  vents the inside of sieve  12  to other interconnected processing devices (not shown), such as, for example, a recycling or filtering apparatus to remove and recycle powder from the gas that is being vented from sieve  12 . 
   Generally, sieve screen level sensor assembly  20  senses a level of material M upon or above sieve screen  14 , and slows or discontinues the flow of powder material M into sieve  12  when that level exceeds a predetermined threshold level. When the sensed level of material M falls below a predetermined threshold level and/or preventive maintenance on sieve screen  14  is performed the flow of powder material M into sieve  12  is returned to full speed/volume and/or resumed. 
   Sieve screen level sensor assembly  20  is associated with and includes a conduit or sensing gas supply line  42  that supplies a sensing gas G SENSE  to sieve  12 . Further, sieve screen level sensor assembly  20  includes pressure regulator  52 , sense gas control valve  54 , flow meter  56 , pressure switch  58 , and a programmable logic controller (PLC)  60 . 
   Sensing gas supply line  42  provides flow of sensing gas G SENSE  to sieve  12 . More particularly, sensing gas G SENSE  flows from a source (not shown) through sensing gas supply line  42 , out orifice  62  thereof, and into sieve  12 . Orifice  62  has a predetermined dimension (radius or area) and is disposed at a predetermined level or height, such as, for example from approximately 0.25 to approximately 1.0 inches or more, above the inlet side (not referenced) of sieve screen  14  (i.e., the side of sieve screen  14  closest to or facing material inlet  24 ). Preferably, orifice  62  is oriented such that a centerline (not shown) of orifice  62  is parallel relative to sieve screen  14 . Of course, those of ordinary skill in the art will recognize that the level (and orientation) at which orifice  62  is disposed relative to sieve screen  14  is dependent upon many factors and will vary depending upon the parameters of any particular application of the sieve screen level sensor assembly  20  of the present invention. 
   One or more pressure or flow regulators  52  (only one shown) are operably associated with sensing gas supply line  42 , and regulate the pressure of sensing gas G SENSE  therein. Typically, flow regulator  52  steps down or regulates the pressure of sensing gas G SENSE  at a sensing pressure P SENSE . Sensing pressure P SENSE , such as, for example, from approximately 4 to approximately 7 In.WC, is a predetermined amount greater than P PURGE . 
   Sense gas control valve  54  is operably associated with sensing gas supply line  42 . Sense gas control valve  54  is electrically connected to PLC  60  and receives therefrom sense gas control signal  64 . Sense gas control valve  54  is responsive, i.e., opens and/or closes, to sense gas control signal  64  to thereby control the flow of sensing gas G SENSE  through sensing gas supply line  42 . Thus, sense gas control valve  54  controls the flow of sensing gas G SENSE  through sensing gas supply line  42  and into sieve  12 , and thereby the flow of sensing gas G SENSE  is shut off during shutdown and/or preventive maintenance of bulk powder filtering apparatus  10 . Sense gas control valve  54  is a conventional and commercially-available valve, such as, for example, a solenoid-operated valve suitable for use in low-pressure applications. 
   Flow meter  56  is also operably associated with sensing gas supply line  42 . Flow meter  56  measures, and thereby provides a visual indication of, the flow of sensing gas G SENSE  through sensing gas supply line  42 . Flow meter  56  is a conventional and commercially-available flow meter capable of measuring a range of flow from approximately 4 to approximately 50 standard cubic feet per hour (scfh), such as, for example, model RMB-52-BV manufactured by Dwyer Instruments, Inc. of Michigan City, Ind. 
   Pressure switch  58  is also operably associated with sensing gas supply line  42 . Pressure switch  58  detects an increase or rise in sensing pressure P SENSE  above a certain level or predetermined threshold as will be more particularly described hereinafter. Pressure switch  58  issues level sense signal  66  to PLC  60  when P SENSE  equals and/or exceeds that predetermined threshold. Pressure switch  58  is also a conventional and commercially-available large-diaphragm or low-pressure pressure switch having a range of approximately 1.0 to 4.0 In.WC, such as, for example, Model No. 1640-2 also manufactured by Dwyer Instruments, Inc. of Michigan City, Ind. 
   PLC  60  is a conventional programmable logic control. PLC  60  is electrically connected with and issues sense gas control signal  64  to sense gas control valve  54 . As discussed above, sense gas control valve  54  is responsive to sense gas control signal  64 , i.e., the valve opens and/or closes in response to sense gas control signal  64 , to thereby control the flow of sensing gas G SENSE  through sensing gas supply line  42 . PLC  60  is also electrically connected with and receives level sense signal  66  from pressure switch  58 . As also discussed above, pressure switch  58  issues level sense signal  66  to PLC  60  when P SENSE  equals and/or exceeds a predetermined threshold. PLC  60  is further electrically connected with and issues an input valve control signal  68  to input valve  22 . Responsive to input valve control signal  68 , input valve  22  controls the flow of material M through inlet  24  and into sieve  12 . Although not shown, PLC  60  may monitor and control various other functions within bulk powder filtering apparatus  10 . 
   In steady-state use, input valve  22  provides a generally constant and continuous rate of flow of material M through inlet  24  and into sieve  12 . Material M then drops onto sieve screen  14 . As those skilled in the art will appreciate, a given input flow rate of material M having known properties, such as, for example, particle size, will flow through sieve screen  14 , also having known properties, such as, for example, mesh size, at a predictable and/or known rate that is generally if not substantially constant. Thus, the desired flow rate of material M through inlet  24  of sieve  12  is predetermined and established through the control of input valve  22  via PLC  60  and input valve control signal  68  issued thereby. The desired flow rate ensures that material M does not accumulate or back-up on sieve screen  14  in quantities or weights sufficient to blind or tear sieve screen  14 . Also in steady-state use, and as described above, sensing pressure P SENSE  is established and regulated at a level that is a predetermined amount greater than purge pressure P PURGE . 
   Also in steady-state use, and as described above, sieve  12  is pressurized with purge gas G PURGE  at a purge gas pressure P PURGE , such as, for example, from approximately 1.0 to 3.0 In.WC. Similarly, sensing gas G SENSE  flows through sensing gas supply line  42 , out orifice  62  thereof, and into sieve  12  at a sensing pressure P SENSE , such as, for example, from approximately 4 to approximately 7 In.WC. Thus, sensing pressure P SENSE  is a predetermined amount greater than purge gas pressure P PURGE . Sensing pressure P SENSE  is maintained at a level that is a predetermined amount greater than purge pressure P PURGE  to reduce the likelihood that a rise or spike in purge gas pressure P PURGE  equals or exceeds sensing pressure P SENSE , thereby reducing the likelihood of a false indication of a reduced or blocked flow of sensing gas G SENSE . 
   As agglomerated particles of material M, other coarse and/or foreign particles accumulate upon sieve screen  14 , the rate of flow of material M through sieve screen  14  is adversely affected. Typically, the adverse affect that occurs is a relatively gradual decrease in the rate of flow of material M through sieve screen  14 . However, relatively drastic decreases in the rate of flow of material M through sieve screen  14  also occur. In any event, whether the adverse affect takes the form of a gradual or a relatively drastic decrease in the rate of flow of material M through sieve screen  14 , the decrease in the rate of flow of material M through sieve screen  14  is generally difficult to predict or forecast, is highly variable, and is dependent upon many factors. Thus, in conventional sieve screening systems the sieve screens may be torn or preventive maintenance is performed at a less-than-optimal interval. The sieve screen level sensor assembly  20  of the present invention, however, detects such a build-up of material M on sieve screen  14  and reduces or stops the flow of material M into sieve  12  to thereby reduce the incidence of torn screens and/or signal the need for timely preventive maintenance. 
   More particularly, as agglomerated particles of material M and other coarse or foreign particles accumulate upon sieve screen  14 , thereby reducing the rate of flow of material M therethrough, material M begins to accumulate upon sieve screen  14 . When the level of accumulated material M rises to the level of orifice  62 , the flow of sensing gas G SENSE  therethrough is restricted or substantially blocked. Thus, the pressure of sensing gas G SENSE  within sensing gas supply line  42  increases above sensing pressure P SENSE . When the pressure of sensing gas G SENSE  within sensing gas supply line  42  exceeds P SENSE  by a predetermined threshold, pressure switch  58  issues or activates level sense signal  66  which is received by PLC  60 . Responsive thereto, PLC  60  issues or activates input valve control signal  68  which is received by input valve  22 . Responsive to input valve control signal  68 , input valve  22  slows or stops the flow of material M into sieve  12  and, thus, onto sieve screen  14 . 
   The continued operation of sieve  12  with the flow of material M reduced or stopped will clear the accumulated material M from screen  14  in a relatively brief time period when sieve screen  14  is only partially blocked or merely backed-up, and will thereby cause material M to drop below the level of orifice  62 . Thus, sensing gas G SENSE  will again flow normally and in a relatively unrestricted manner through sensing gas supply line  42 . The pressure of sensing gas G SENSE  therefore returns to sensing pressure P SENSE , and pressure switch  58  resets. Upon the reset of pressure switch  58 , level sense signal  66  also resets or returns to its default or inactive state. Responsive to the resetting of level sense signal  66 , PLC  60  deactivates or resets input valve control signal  68  to thereby return input valve  22  to normal operation, or to an intermediate or restarting mode of operation for resuming the flow of material M into sieve  12 . 
   The continued operation of sieve  12  with the flow of material M reduced or stopped will only gradually, if at all, clear the accumulated material M from screen  14  when sieve screen  14  is substantially blocked or blinded. Thus, the level of material M upon sieve screen  14  decreases, if at all, at a very gradual rate. PLC  60  is configured to issue or activate sense gas control signal  64  and a preventative maintenance signal PM signal  70  upon the expiration of a predetermined period of time following the activation of level sense signal  66  (indicating a backed-up condition). Responsive to sense gas control signal  64  being activated, sense gas control valve  54  stops the flow of sense gas G SENSE  through sense gas supply line  42  in preparation for the shut-down and preventative maintenance of bulk powder filtering apparatus  10 . PM signal activates an indicator or alarm, such as for example, a red light or audible buzzer, to alert maintenance personnel to the need for maintenance to be performed on bulk powder filtering apparatus  10 . PLC  60  is further programmed to shut down bulk powder filtering apparatus  10  in such a situation to enable maintenance personnel to commence preventative maintenance and/or cleaning of apparatus  10  and sieve screen  14 . 
   It should be particularly noted that during normal operation of sieve  12  and sieve screen level sensor assembly  20 , material M can migrate into and upstream within sensing gas supply line  42 . Such migration may result in accumulation of material M within sensing gas supply line  42 , thereby reducing the inside diameter thereof and causing an increase in sensing the pressure of sensing gas G SENSE  flowing therein. Such a condition may, if the pressure of sensing gas G SENSE  within sensing gas supply line  42  exceeds pressure P SENSE , result in a false indication of a blocked sieve screen  14 . Flow meter  56  provides a visual indication of the flow of sensing gas G SENSE  through sensing gas supply line  42 , and is monitored (manually or automatically monitored, such as, for example, by PLC  60 ) to indicate when an accumulation of material M is present in an amount sufficient to require cleaning and/or preventative maintenance of sensing gas supply line  42 . 
   While this invention has been described as having a preferred configuration, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the present invention using the general principles disclosed herein. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims. 
   PARTS LIST 
   
       
         10 . Filtering Apparatus 
         12 . Sieve 
         14 . Sieve Screen 
         20 . Sieve Screen Level Sensor Assy. 
         22 . Input Valve 
         24 . Material Inlet 
         26 . Material Outlet 
         32 . Purge Gas Supply Line 
         34 . Purge Vent 
         42 . Sensing Gas Supply Line 
         52 . Pressure Regulator 
         54 . Sense Gas Control Valve 
         56 . Flow Meter 
         58 . Pressure Switch 
         60 . Programmable Logic Ctrl. 
         62 . Orifice 
         64 . Sense Gas Control Sig. 
         66 . Level Sense Signal 
         68 . Input Valve Control Sig. 
         70 . PM Signal 
       M—Powder Material 
       G PURGE —Purge Gas G SENSE —Sense Gas 
       P PURGE —Purge Gas Pressure P SENSE —Sense Gas Pressure