Abstract:
The present invention provides a ballast circuit and method for fabricating the same for multi-electrode corona discharge arrays. The circuit comprises a conductive plastic material and at least one corona electrode protruding from the conductive plastic material. The distance between the plastic material and the corona electrode varies and controls the electrical resistance and determines the voltage breakdown of the circuit. Additionally, a particle collection surface may preferably be located within the conductive plastic material or preferably be separated from the material depending on the circuit design and configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/722,078 filed Sep. 29, 2005, the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to electrostatic particle collection systems, and more specifically to methods for fabricating ballast circuits for multi-electrode corona discharge arrays in electrostatic particulate collection systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     Highly efficient, low power particle collection devices have been demonstrated using multiple electrode corona discharge arrays. The advantages of multiple electrode corona discharge arrays for particle collection are described in “System and Method for Spatially Selective Particulate Deposition And Enhanced Particulate Deposition Efficiency”, filed Apr. 18, 2006, having an application Ser. No. 11/405,787, and in “Corona Charging Device and Methods”, filed Mar. 11, 2003 having an application Ser. No. 10/386,252, and in “Method And Apparatus for Concentrated Airborne Particle Collection”, filed Jun. 24, 2003, issued as U.S. Pat. No. 7,062,982 B2, all of which are herein incorporated by reference.  
         [0004]     A key circuit element needed for the proper operation of multiple electrode corona discharge arrays is a resistor electrically connected in series between the high voltage DC power supply and each corona electrode. This resistor is known as a ballast resistor. The main function of the ballast resistor is to limit the current through any individual corona electrode when the plasma is initiated and while operating at steady state.  
         [0005]     The voltage at which an electrical discharge is initiated is known to vary for each corona electrode in a multiple electrode system. Furthermore, the resistance of the air following the initial electrical discharge lowers dramatically such that the voltage needed to sustain the discharge is significantly lower than the initial breakdown voltage. Given these factors, it is therefore possible to deliver all electrical power to the corona discharge through a single or small number of electrodes. The resulting non-uniform plasma would defeat the primary benefits of a multiple electrode corona discharge system; that is, uniformity of electric field and charge density in the particle collection zone.  
         [0006]     Providing a ballast resistor for each corona electrode solves the plasma non-uniformity problem by limiting the power delivered to any single corona electrode. Power through a single electrode is limited by lowering the electrode voltage as more current passes through the ballast resistor to the electrode. The ballasting effect allows the power supply voltage to adjust to a voltage where other electrodes will initiate and sustain continuous plasma.  
         [0007]     This ballasting function places a number of electrical requirements onto the ballast resistor. The two key requirements are voltage breakdown between the resistor terminals and the resistance value. These requirements vary with electrode geometry and plasma power density. The value for the voltage breakdown of the ballast resistor used for the electrostatic radial geometry particle concentrator at is typically 9 kV. The resistance value for each of the ballast resistor used for this concentrator is 2 Gohm.  
         [0008]     Resistors having the above characteristics are produced commercially. However, the breakdown and resistance values are not usually in high demand for most electrical applications. As a result, these resistors are typically much more expensive than lower voltage, lower value resistors. As an example, a 50V, 100 kohm resistor in a surface mount package can usually be purchased for less than $0.01. The 10 KV, 1 Gohm resistors used in the radial collector are purchased in small quantities for about $1.00. For most commercial and industrial particle collection applications, the number of electrodes needed is typically greater than thirty and less than five hundred. The cost the plastic material needed to produce an equivalent of 108 1 Gohm, 10 kV resistors is about $0.50 yielding a 216× improvement in cost.  
         [0009]     Thus, there remains a need in the art for a highly-efficient, geometrically flexible and cost-effective material that provides for the resistive ballasting of multi-corona discharge arrays.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides a ballast circuit for an electrostatic particle collection system and the method for fabricating the same. The circuit comprises a conductive plastic material having a first end and a second end, such that the first end is connected to a power source. The circuit also comprises at least one corona electrode protruding from the second end of the conductive plastic material.  
         [0011]     In one embodiment, a radial configured ballast circuit for an electrostatic particle collection system comprises a conductive plastic material having an inner surface and an outer surface, such that the outer surface is connected to a power source. The circuit also comprises at least one corona electrode protruding from the inner surface of the conductive plastic material, wherein distance between the inner surface of the conductive plastic material and the corona electrode varies electrical resistance and determines the voltage breakdown of the circuit.  
         [0012]     In another embodiment, a planer configured ballast circuit for an electrostatic particle collection system comprises a conductive plastic material having a top surface and a bottom surface such that the top surface is connected to a power source. The circuit also comprises at least one corona electrode protruding from the bottom surface of the conductive plastic material, wherein distance between the top surface of the conductive plastic material and the corona electrode varies electrical resistance and determines the voltage breakdown of the circuit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1A  is a schematic diagram illustrating electrostatic particle collection device according to an embodiment of the present invention.  
         [0014]      FIG. 1B  is a schematic diagram illustrating cross-section of the circuit of  FIG. 1A  according to one embodiment of the present invention.  
         [0015]      FIG. 2A  is a schematic diagram illustrating electrostatic particle collection device according to another embodiment of the present invention.  
         [0016]      FIG. 2B  is a schematic diagram illustrating cross-section of the circuit of  FIG. 2A  according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     As will be described in greater detail below, a conductive plastic material has been shown to meet the requirements for the resistive ballasting of multi-electrode corona discharge arrays. Typical ballast resistor electrical requirements are resistance greater than or equal to 10 9  ohm and voltage breakdown of greater than or equal to 10 kV across the terminals. Conductive plastics possess a unique combination of material properties that enable its use for this application. Use of this material will substantially reduce the cost to manufacture multi-electrode corona discharge arrays where a large number is (i.e. &gt;10 electrodes) of discharge elements is required.  
         [0018]     Furthermore, using a conductive plastic as the resistive element of a multi-electrode ballast circuit enables a large number of circuit designs and geometries that can be used to accommodate the variations of particle collection geometry. A brief description of the multi-electrode ballast circuit for cylindrical and planer configurations are provided herein below with respect to  FIGS. 1A, 1B  and  FIGS. 2A and 2B  respectively.  
         [0019]     Referring to  FIG. 1A  there is shown a schematic diagram illustrating electrostatic particle collection ballast device  100  according to an embodiment of the present invention. Note that this diagram is a schematic representation of a radial configuration of the device  100  and the device may preferably be constructed with other geometric configurations. Device  100  comprises a body  102  preferably of polycarbonate or similar mechanical grade plastic material, with a multi- electrode ballast circuit  104  disposed on the body  102 . The circuit  104  having a conductive plastic  106  as a resistive element partially surrounding the device body  102 . The circuit  104  further includes a corona array of corona electrodes  108  protruding from the conductive plastic  106  as shown in  FIG. 1A . Also, included is a collection surface  110 , preferably having a columnar shape, made of a conductive material, concentrically positioned with respect to the corona electrodes  108 . The collection surface  110  is situated opposed to the corona electrodes  108 . The collection surface  110  provides an area to initiate and sustain the electrical corona discharge from the corona electrodes  108 . The arrow  111  on the top of the device  100  indicates the direction of the flow of particle-laden air through the device. Additionally, shown is a hydrosol extraction unit  112  which pumps water to the center of the collection column  110  and then the water flows off from the collection column  110  to drain out the collected aerosol particulates as shown by the arrows  114  as shown in  FIG. 1A . Also, shown is a fan  116  which is used to draw in the ambient air through the device. A connection to a high voltage power supply (not shown) is made to the conductive plastic material,  106 , by a wire (not shown) connected to a conductive ring  118 , such as a strip of conductive tape or thin metal, that attaches to the surface of conductive plastic  106  as shown in  FIG. 1A .  
         [0020]     Referring to  FIG. 1B  is a schematic representation of a cross-section of the ballast circuit  104  of the device taken  100  through the corona electrodes  108  in  FIG. 1A . Note, the ballast circuit  104  is configured to be of radial shape. Thus, this ballast circuit  104  can preferably be used for radial particle collector configurations. As shown in  FIG. 1B  is the conductive plastic  106  is shown as doughnut shape having an inner surface  106   a  and an outer surface  106   b.  The conductive plastic material  106  may preferably be acetyl, polycarbonate, or polystyrene. Also, four corona electrodes  108  are shown embedded or firmly enclosed in the conductive plastic  106  protruding from the inner surface of the conductive plastic. Although only four electrodes are shown as an example in the figure, more or less than four electrodes can preferably be enclosed in the conductive plastic. The electrodes  108  in this radial configuration are equally spaced from the conductive plastic material  106 . As shown in  FIG. 1B , the particle collection post  110  is firmly situated within the conductive plastic  106  as shown. The collection post  110  is a conductive material that is concentrically positioned with respect to the corona electrode  108 . It is electrically connected to a voltage near electrical ground and is used form the electric field between its surface and the tips of the corona electrode. The electric field is needed to initiate and sustain the electrical corona discharge. The post electrode also provides a surface upon which the captured particles will land. The connection to the high voltage DC power supply (not shown) is preferably provided from the outer surface  106   a  of the conductive plastic  106  via a high voltage conductive ring  118  as shown in  FIG. 1B . Note that the connection is preferably an insulating connection for providing a safe electrical operation.  
         [0021]     As discussed above, the schematic shows only four corona electrodes, however, the number of corona electrodes is normally much greater than four. Typical design rules allow a minimum pitch between corona electrodes of approximately 0.1 inch. Additionally, the schematic also shows a single level of corona electrodes, however, multiple levels of corona electrodes may preferably be used for some applications of particle collection.  
         [0022]     The key design parameter for the configuration of  FIG. 1B  is the distance from the outer surface  106   a  of the conductive plastic  106  to the corona electrode  108  surface that will be embedded into the plastic  106 . This distance provides a penetration depth of corona electrode  108  into the conductive plastic material  106 . The greater penetration depths produce lower values of ballast/electrical resistance. The distance comprises in the range between about 0.01 inches and about 0.5 inches. The distance will preferably be typically greater than 0.1 inch and less than 0.5 inches. This distance is controlled preferably during manufacture of the ballast resistor assembly  104 . This distance will vary the electrical resistance between the outer surface  106   a  of the conductive plastic  106  and each corona electrode  108  and will also determine the voltage breakdown of the device  100 .  
         [0023]     Other design parameters preferably include bulk resistivity of the conductive plastic, shape and orientation of power supply connection to plastic and as discussed above, option to insulate power supply connection. Bulk resistivity will preferably range typically between 10 8  ohm-cm-10 10  ohm-cm By varying the bulk resistivity of the conductive plastic, the bulk resistance and the voltage breakdown can be controlled. Higher bulk resistivities will produce higher ballast resistivities given identical geometries. Higher bulk resistivities will also produce higher breakdown voltages across the material. This is due to the fact that most materials have a breakdown voltage that is a nonlinear function of voltage. That is, if the voltage across the material is raised beyond the material&#39;s breakdown voltage, the current passing through the device will increase significantly for small changes in voltage, like a diode. Conductive plastics in the bulk resistivity range applicable to this application are primarily the pure plastic with a small amount of conductive doping material. Pure plastics such as acetyl, polycarbonate, and polystyrene have high breakdown voltages. This property is significantly lowered when conductive dopants are added to the pure material. Therefore, higher bulk resistivity materials tend to have higher breakdown voltage properties. Also, by varying penetration depth of power supply contact/connection into the conductive plastic, the bulk resistance can be varied/controlled. The penetration depth of the power supply connection is the distance from the power supply connection to the conductive plastic which is preferably typically greater than 0.1 inch and less than 0.5 inches. As mentioned above, the greater penetration depths produce lower values of ballast resistance. Furthermore, patterning the power supply connection in various shapes and orientations, the bulk resistance of the ballast circuit can preferably be controlled. For example, connecting at multiple points along the perimeter of the plastic material or varying the penetration connection distance and width and/or length of the connection surface can increase or decrease the bulk resistivity.  
         [0024]     Referring to  FIG. 2A  there is shown a schematic diagram illustrating an electrostatic particle collection ballast device  100  according to an embodiment of the present invention. Note that this diagram is a schematic representation of a planer configuration of the device  100  and the device may preferably be constructed with other geometric configurations. Device  100  comprises a body  102  preferably of polycarbonate or similar mechanical grade plastic material, with a multi- electrode ballast circuit  104  disposed preferably inside the device body  102 . The circuit  104  having a conductive plastic  106  as a resistive element with an corona array of corona electrodes  108  protruding from the conductive plastic  106  as shown in  FIG. 2A . Also, included is a collection surface  110 , preferably a plate having a planar surface, preferably made of a conductive material, separated from the conductive plastic  106  as shown. The collection plate  110  is situated across from the conductive plastic  106 , preferably opposed to the corona electrodes  108  as shown in  FIG. 2A . In this embodiment, there is a separate structure (not shown) that positions or supports the plate  110  with respect to the conductive plastic  105  and the corona electrodes  108 . The collection surface  110  provides an area to initiate and sustain the electrical corona discharge from the corona electrodes  108 . Also, shown is the planar conductor, such as conductive tape or a thin metal strip  118 , covering the conductive plastic  106  as shown, to provide a connection to the power supply (not shown) via a high voltage wire (not shown).  
         [0025]     Referring to  FIG. 2B , there is shown a schematic representation of a cross-section of the ballast circuit  104  in the device  100  taken through the corona electrodes  108  in  FIG. 2A . Note, the ballast circuit  104  is configured to be of planer shape. Thus, this ballast circuit  104  can preferably be used for planer particle collector configurations. As shown in  FIG. 2B , is the conductive plastic  106  also preferably of planer shape having a top surface  106   c  and a bottom surface  106   d.  Additionally, twenty-one corona electrodes  108  are shown protruding from the bottom surface  106   d  of the conductive plastic  106 . Although, twenty one electrodes are shown as an example in the figure, more or less than twenty-one electrodes can preferably be enclosed in the conductive plastic. The electrodes  108  in this planar configuration are equally spaced from each other. The configuration shown in  FIG. 2B , illustrates the particle collection plate  110  preferably of planer shape is separated from the conductive plastic  106 . The connection to the high voltage DC power supply is preferably made through the top surface  106   c  of the conductive plastic  106  via the high voltage conductive tape/strip  118  as shown in  FIG. 1B . Note that the connection is preferably an insulating connection for providing a safe electrical operation.  
         [0026]     As discussed above, the schematic shows only twenty-one corona electrodes, however, the number of corona electrodes is normally much greater. Typical design rules allow a minimum pitch between corona electrodes of approximately 0.1 inch. Moreover, the schematic also shows a single level of corona electrodes, however, multiple levels of corona electrodes will be used for some applications of particle collection.  
         [0027]     The key design parameters for this configuration is the distance from the top surface  106   c  of the conductive plastic  106  to the corona electrode  108  surfaces that will be embedded into the plastic. Similar to the radial configuration described with respect to  FIG. 2A , this distance of the planer configuration in  FIG. 2B  provides a penetration depth of corona electrode  108  into the conductive plastic material  106 . The greater penetration depths produce lower values of ballast/electrical resistance. The distance comprises in the range between about 0.01 inches and about 0.5 inches. The distance will preferably be typically greater than 0.1 inch and less than 0.5 inches. This distance will be controlled during the construction of the ballast circuit assembly  104 . This distance will vary the electrical resistance between the outer surface  106   c  of the conductive plastic  106  and each corona electrode  108  and will thus determine the voltage breakdown of the device  100 .  
         [0028]     Other design parameters include bulk resistivity of plastic, shape and orientation of power supply connection to plastic and as described above option to insulate power supply connection. As described above with respect to the radial configuration in  FIG. 1A , the bulk resistivity for the planer configuration in  FIG. 1B  will preferably range typically between 10 8  ohm-cm-10 10  ohm-cm. By varying the bulk resistivity of the conductive plastic, the bulk resistance and the voltage breakdown can be controlled.  
         [0029]     Although the present invention describes only radial and planer configurations of the ballast circuits, note that other geometrical configurations may also be provided to accommodate the variations of particle collection geometry provided the configuration maintains the constraints required by the electrostatic particle collection device . Even though various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings without departing from the spirit and the scope of the invention.