Abstract:
A particulate filtration system for removing particulates suspended in a fluid include, in an exemplary embodiment, a filter element and a pre-collector body component operably connected to the filter element. The pre-collector body component is capable of receiving an electrical charge to attract and remove particles suspended in the fluid.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Provisional Patent Application No. 60/650,287 filed Feb. 4, 2005. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
       [0002]     None.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates generally to a filtration system and apparatus for removing particulate matter from a stream of gas or other fluid. More specifically, the present invention encompasses a filtration system that includes an electrostatic pre-collector apparatus and a fabric filter element for removing particulate matter from a stream of gas or other fluid.  
         [0004]     Fabric filtration is a common technique for separating out particulate matter in a gas stream. In an industrial setting, fabric filtration is often accomplished in a device known as a baghouse. Generally, a baghouse includes a housing that has an inlet for receiving dirty, particulate-laden gas and an outlet through which clean gas leaves the baghouse. The interior of the housing is divided by a tube sheet into a dirty gas or upstream plenum and a clean gas or downstream plenum, with the dirty gas plenum in fluid communication with the inlet and the clean gas plenum in fluid communication with the outlet. The tube sheet typically includes a number of apertures and supports a number of filter elements with each filter element covering one of the apertures.  
         [0005]     Generally, a filter element includes a support structure and a fabric filter media. The support structure, which is also called a core, typically has a cylindrical shape and is hollow. The walls of the support structure may be similar to a screen or a cage, or may simply include a number of perforations, so that a fluid may pass through the support structure. The support structure will have at least one end that is open and that is capable of being coupled to the tube sheet at an aperture. Customarily, the structure will extend from the tube sheet into the dirty gas plenum. There are several types of fabric filter media. A “bag” filter media is flexible and/or pliable and is shaped like a bag. A cartridge filter media is typically relatively rigid and pleated. Filter media are ordinarily mounted around the exterior or outer portion of the support structure.  
         [0006]     In operation, particulate laden or dirty gas is conducted into the baghouse, and more specifically into the dirty gas plenum, through the inlet. The gas then flows through the fabric filter media to the interior space within the filter cores. As the gas flows through the filter media, the particulate matter carried by the gas engages the exterior of the filter media and either accumulates on the filters or falls to the lower portion of the dirty gas plenum. Thereafter, the cleaned gas flows through the apertures in the tube sheet and into the clean gas plenum. The clean gas then flows out of the baghouse through the outlet.  
         [0007]     As particulate matter accumulates or cakes on the filters, the flow rate of the gas is reduced and the pressure drop across the filters increases. To restore the desired flow rate, a reverse pressure pulse may be applied to the filters. The reverse pressure pulse expands the filter media and separates the particulate matter, which falls to the lower portion of the dirty gas plenum. While filter material technology has advanced sufficiently to allow a given filter element to be cleaned in this manner tens of thousands of times before replacement is needed, further extension of a filter&#39;s useful life is economically desirable. Extended filter life not only saves the cost of filters, it also saves the cost of filter replacement, which is often difficult, costly and requires the baghouse to be taken out of service for a period of time.  
         [0008]     Another common technique for separating particulate matter from a gas stream is to use an electrostatic device, such as an electrostatic precipitator. In this device, particulate matter is electronically charged and then collected through the action of an electric field. A typical electrostatic device provides a discharge electrode that is maintained at a high voltage and a non-discharge electrode that is maintained at a relatively lower voltage or at ground. As the particulate-laden gas steam flows past the electrodes, the electric field present between the electrodes operates to charge a percentage of the passing particles and causes them to collect on the non-discharge electrode.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     In order to provide an efficient filtration system that has a longer useful life, in one embodiment the present invention is directed to an industrial filtration system that includes fabric filters and an apparatus that provides pre-collection of particulate matter. While an industrial baghouse is often thought of as useful for cleaning gas after a process, such as combustion, the filtration system of the present invention can be used in other applications, such as for gas turbine intake filtration.  
         [0010]     The baghouse filtration system of the present invention includes a housing that is divided into a first plenum and a second plenum by a tube sheet. The tube sheet is substantially planar and includes a number of spaced-apart apertures that extend through the sheet. The system further includes a number of filter elements, with each filter element coupled to the tube sheet at an aperture. The system also includes at least one pre-collector body component and at least one discharge electrode. Each body component is positioned in close proximity to the lower end of a filter element. Each discharge electrode is positioned some distance away from the body components. Preferably, each body component is coupled to the lower end of a filter element. Each body component and discharge electrode is coupled to a voltage source or sources so that there is an electrical potential present between them. In one embodiment, each body component is electrically coupled to ground and each discharge electrode is electrically coupled to a negative voltage. A body component may have any one of a number of shapes. In one embodiment, each body component is cylindrical and has a continuous sidewall. In a further embodiment, each body component sidewall is perforated.  
         [0011]     In operation, particulate-laden gas enters the first plenum through an inlet. Once in the first plenum, the gas passes adjacent to the pre-collector apparatus where the difference in the electrical potential between the discharge electrode and the body component causes at least a portion of the particulate matter in the gas to collect on the body component. Thereafter, the gas passes through a filter element causing particulate matter still carried by the gas to engage the exterior of the filter element and either accumulate on the filter element or separate from the gas and fall to the lower portion of the first plenum. The gas then passes from the interior space of the filter assembly through an aperture in the tube sheet and into the second plenum. The gas exits the system through an outlet. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     Further features of the present invention will become apparent to those skilled in the art to which the present invention relates from reading the following description with reference to the accompanying drawings, in which:  
         [0013]      FIG. 1  is a schematic view, partly in section, of a filtration system according to one embodiment of the present invention;  
         [0014]      FIG. 2  is an elevational view of one embodiment of a filter assembly and pre-collector apparatus according to the present invention;  
         [0015]      FIG. 3  is an elevational view of a second embodiment of a filter assembly and pre-collector apparatus according to the present invention;  
         [0016]      FIG. 4  is an elevational view of another embodiment of a filter assembly and pre-collection apparatus according to the present invention; and  
         [0017]      FIG. 5  is an exploded sectional view of the filter assembly and pre-collection apparatus illustrated in  FIG. 4 , taken approximately along the line  5 - 5  in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     A filtration system  10  according to one embodiment of the present invention is illustrated in  FIG. 1 . The filtration system  10  generally includes an enclosed housing  20  and a plurality of filter assemblies  100 . Each filter assembly  100  includes a filter element  120  and a pre-collector body component  130  extending below and attached to the filter element  120 . The system  10  also includes a pre-collector discharge electrode  160 . Dirty gas enters the housing  20  and clean gas leaves. More specifically, the dirty gas passes adjacent to the pre-collector body components  130  and discharge electrodes  160  which operate to remove at least a portion of the particulate matter in the dirty gas. Thereafter, the gas passes through the filter elements  120  where additional particulate matter is removed. Because of the operation of the pre-collector body components  130  and discharge electrodes  160 , however, the filter elements  120  have less particulate matter to remove and, therefore, the filter material of the filter elements will require fewer cleaning cycles for a longer useful service life.  
         [0019]     The housing  20  is divided into a first plenum  30  and a second plenum  40  by a tube sheet  50 . A suitable material for both the housing  20  and the tube sheet  50  is metal plate. The housing  20  also includes an inlet  60  that is in fluid communication with the first plenum  30  and an outlet  70  that is in fluid communication with the second plenum  40 . An accumulation chamber  80  is located at the lower end of the first plenum  30  and is defined by irregularly shaped and sloped walls. For example, the accumulation chamber  80  has a V-shaped cross-section, as illustrated in  FIG. 1 .  
         [0020]     At least a portion of the tube sheet  50  is substantially planar. The tube sheet  50  includes a plurality of apertures, such as aperture  90 , that extend through the planar portion of the sheet  50 .  FIG. 1  shows a number of filter assemblies  100  hanging from the tube sheet  50  and extending through the apertures  90  in the sheet  50 . Each filter assembly  100  is supported at its upper end by the tube sheet  50  and hangs downwardly in a substantially vertical direction. It should be understood that in operation, there is a filter assembly  100  associated with each aperture  90  in the sheet  50 . Also, the filter assemblies  100 , as illustrated, do not extend into the accumulation chamber  80 , but it should be apparent that filter assemblies could be made with a length that allowed them to extend into the accumulation chamber  80 .  
         [0021]     Each filter assembly  100  includes a filter element  120  and a pre-collector body component  130 . The pre-collector body component  130  is coupled to and supported by the lower end of the filter element  120 . A pre-collector discharge electrode  160  hangs vertically between the filter assemblies  100 .  
         [0022]     It should be understood that the filter assemblies  100  may be arrayed in a vertically extending matrix in a typical housing  20  as is known in the baghouse industry. The discharge electrodes  160  may be positioned in a number of different locations within the typical baghouse. For example, as indicated in  FIG. 1 , the discharge electrodes  160  may be positioned in their own rows and columns between and aligned with the filter assemblies  100 . Alternatively, the discharge electrodes  160  may be offset from those devices so that in effect, the discharge electrodes are positioned at the center of each square of four filter assemblies  100 . It is also within the scope of this invention for the discharge electrodes  160  to be positioned between every other or every third filter assembly  100  or square of four filter assemblies  100 . Of course, additional locations for the discharge electrodes  160  are also within the scope of this invention.  
         [0023]     In one embodiment, the filter assembly  100  ( FIG. 2 ) is coupled to the tube sheet  50  at an aperture  90  by a collar  180 . Although the filter assembly  100  is shown having a circular cross section, it will be apparent that any suitable configuration cross section could be used, such as an oval or a rectangle. The pre-collector body component  130  is coupled to the filter element  120  at a connection  200 , so that the pre-collector body component  130  extends co-axially with the filter element  120  upstream from the filter element  120 . The collar  180  and the connection  200  will be described below.  
         [0024]     The filter element  120  preferably includes a pleated filter media. The pleated filter media is formed in a substantially tubular shape with accordion folds at its inner and outer peripheries. The pleated filter media may be constructed of any suitable material for a desired filtering requirement.  
         [0025]     The pre-collector body component  130  preferably has a tubular shape. It should be understood that the pre-collector body component  130  is not limited to this shape and that other shapes are included within the scope of this invention, such as a rectangular or oval shape. The pre-collector body component  130  is made of any suitable electrically conductive material or, alternatively, is coated by any suitable electrically conductive material. Preferably, the pre-collector body component  130  is made substantially of metal such as conductive carbon steel. In this embodiment, the outer surface or sidewall  190  of the pre-collector body component  130  is substantially continuous, that is, it has no holes or perforations.  
         [0026]     The discharge electrode  160  extends vertically and is spaced a short distance horizontally from the pre-collector body component  130 . The discharge electrode  160  is made from an electrically conductive material such as a thin wire of stainless steel. In operation, as will be discussed later, the discharge electrode  160  is electrically coupled to a voltage source, so that it obtains and maintains an electric potential or charge relative to the pre-collector body component  130 . In one embodiment, the discharge electrode  160  is coupled to line voltage through a transformer and rectifier (not shown) so that the discharge electrode is maintained at a voltage potential between negative 20,000 and negative 50,000 volts DC. The discharge electrode  160  may be entirely or partially shielded (i.e. only that length that corresponds to the length of the pre-collector body component  130 ) to lessen the likelihood that electrical current will arc between the pre-collector body component  130  and the discharge electrode  160 . Shielding is suggested if the discharge electrode  160  is to be placed in close physical proximity to the pre-collector body component  130  or if the electric potential or charge provided to the discharge electrode  160  and/or the pre-collector body component  130  is notably large relative to the distance between the pre-collector body component  130  and the discharge electrode  160 .  
         [0027]     Another embodiment of a filter assembly  150  coupled to the tube sheet  50  at an aperture  90  by a collar  180  is illustrated in  FIG. 3 . In this embodiment, the filter assembly  150  includes a bag filter element  140  instead of a pleated filter element  120 . The bag filter element  140  is made from a flexible, pliable fabric. The fabric may be any suitable material for the desired filtering requirement. The pre-collector body component  130  is coupled to the filter bag element  140  at a connection  170 , so that the pre-collector body component  130  extends co-axially with the filter bag element  140  upstream from the bag filter element.  
         [0028]     A filter assembly  210  according to another embodiment includes a pre-collector body component  220  is illustrated in  FIG. 4 . In this embodiment, filter assembly  210  also includes a pleated filter element  240 . Also in this embodiment, the pre-collector body component  220  is a hollow tube that has a plurality of apertures or perforations  230  extending therethrough. Preferably, the pre-collector body component  220  has approximately 30% to 60% of its surface area occupied by the apertures  230 . The primary function of the apertures  230  is to reduce the weight of the pre-collector body component  220 . As with the previous embodiment, the pre-collector body component  220  may be made from or coated by any suitable electrically conductive material. One such suitable material that the pre-collector body component  220  could be made from is carbon steel.  
         [0029]     The filter assembly  210  is illustrated in  FIG. 5  as partially installed. The filter assembly  210  extends through an aperture  260  in the tube sheet  50  and through a resilient mounting band  250 . The band  250  ensures that the filter assembly  210  may be used with apertures that have not been precisely cut. The band  250  includes resilient metal, such as a stainless steel, and is covered with fabric. The band  250  is constructed with an outer diameter substantially equal to the inner diameter of the aperture  260  and may be easily deformed and inserted into the aperture  260  so that the exterior surface of the band  250  will snugly engage the surface defining the aperture  260 . The band  250  provides a seal between the filter assembly  210  and the aperture  260  in the tube sheet  50 . The band  250  is described in detail in U.S. Pat. No. 5,746,792, which is assigned to the assignee of the present invention and fully incorporated herein by reference.  
         [0030]     Filter assembly  210  also includes a mounting sleeve  270  located at its upper end to attach the filter assembly  210  to the tube sheet  50 . The mounting sleeve  270  is made from a suitable material, such as stamped, drawn or otherwise formed metal. The mounting sleeve  270  defines an open end of the filter assembly  210  for fluid communication with the clean gas plenum  40 . The mounting sleeve  270  is shaped so as to include a channel  280  for receiving a part of the band  250  when the filter is moved into an operational position. The mounting sleeve  270  further includes a tubular portion  290  that is adapted to be located within and extend through the aperture  260  in the tube sheet  50  and through the band  250 .  
         [0031]     A core  310  is fixed to and extends from the mounting sleeve  270 . The core  310  is made from a suitable material, such as perforated sheet metal, expanded metal, or mesh screen. A radially inner channel  300  in the mounting sleeve  270  receives an upper end of the core  310 . The upper end of the core  310  and the mounting sleeve  270  sleeve are connected together in a suitable manner, such as by welds, rivets, fasteners, or metal deformation. Thus, a relatively strong connection and structure exists that is capable of supporting the weight of the filter assembly  210  as it hangs from the tube sheet  50  even when the filter assembly has a relatively heavy accumulation of particles. Additionally, the connection establishes electrical communication between the mounting sleeve  270  and the core  310  so that these structures will have the same electrical potential.  
         [0032]     A pleated filter element  240  is located concentrically around the core  310 . The pleated filter element  240  is formed in a substantially tubular shape about the perimeter of the core  310  with accordion folds at its inner and outer peripheries. The pleated element  240  may be constructed of any suitable material for a desired filtering requirement. The upper end of the pleated element  240  is also located in the channel  300  of the mounting sleeve  270  and placed in a potting material  320 , which acts to seal the pleated element and the mounting sleeve. The pleated element  240  could be located radially inward of the core  310 .  
         [0033]     The filter element  240  and the pre-collector body component  220  are connected by a threaded connection  200 . The threaded connection  200  includes a collar  350  located at the lower end (as viewed in  FIG. 4 ) of the filter assembly  210 . The collar  350  has a receiving internally threaded portion  360 . The collar  350  is fixed to the core  310  and/or filter element  240  in a suitable manner such as by welds, rivets, fasteners or metal deformation and, as in this embodiment, it may also be sealed to pleated filter element  240  by an potting material  330 . The connection between the collar  350  and the core  310  establishes electrical communication between these two structures so that they have the same electrical potential.  
         [0034]     The threaded connection  200  also includes a collar  370  located at the upper end of the pre-collector body component  220 . The collar  370  has an externally threaded tubular portion  380  for threaded engagement with the receiving portion  360 . The collar  370  is fixed to the pre-collector body component  220  in a suitable manner such as by welds, rivets, fasteners or metal deformation. Thus, electrical communication is established between the collar  350 , the collar  370  and the pre-collector body component  220  so that these structures have the same electronic potential.  
         [0035]     A compressible gasket  390  is located between a lower end surface of the collar  350  of the filter assembly  210  and an upper end surface of the collar  370  of the pre-collector body component  220 . The gasket  390  compresses as the filter element  240  and the pre-collector body component  220  are connected together when filter element  240  and the pre-collector body component  220  are relatively rotated about a longitudinal central axis A to engage the threaded portions  360 ,  380  and thread the collar  370  into the collar  350 . The connection device  200  has a size capable of fitting through the effective size of the apertures  90  or  260  in the tube sheet  50  and a strength sufficient to support the operational weight of the pre-collector body component  220 . Of course, it should be understood that other connection devices, such as a clamp or the like, may be employed in alternate embodiments to connect the filter assembly and pre-collector apparatus body component.  
         [0036]     It should be understood that the sidewall of pre-collector body component  220  is electrically coupled to the tube sheet  50 . This electrical connection is accomplished through a series of physical connections. First, the sidewall of body component  220  is in contact with the collar  370 , which, in turn, is in contact with the collar  350  when the pre-collector body component  220  is installed on the filter assembly  210 . The collar  350  is in physical contact with the core  310 , and the core  310  is in physical contact with the mounting sleeve  270 . Mounting sleeve  270 , in turn, is in contact with the mounting band  250 , which is in contact with the tube sheet  50 . Finally, the tube sheet  50  is in contact with the housing  20 . Thus, the pre-collector body component  220  will have the same electric potential as the housing  20 . As stated earlier, preferably, the pre-collector body component  220  is also grounded since the housing  20  is grounded.  
         [0037]     For operation, the discharge electrode  160  and pre-collector body component  220  ( FIG. 3  or  4 ),  130  ( FIG. 2 ) have a difference in voltage potential. Thus, as stated earlier, in one embodiment the discharge electrode  160  is coupled to line voltage through a transformer and rectifier (not shown) so that the discharge electrode  160  is maintained at a voltage potential between negative 20,000 and negative 50,000 volts DC and the body component  220  ( FIG. 3  or  4 ),  130  ( FIG. 2 ) is grounded. It should be understood that the discharge electrode  160  could be provided with a positive electrical potential or that the voltages may be reversed. Of course, precautions such as insulation and shielding prevent electrical contact between the discharge electrodes  160  and the pre-collector body component  220  ( FIG. 3  or  4 ),  130  ( FIG. 2 ), the tube sheet  50 , and/or the housing  20 .  
         [0038]     In operation, particulate-laden gas enters the first plenum  30  ( FIG. 1 ) through the inlet  60 . A fan (not shown) may be utilized to cause the gas to move through system  10 . Once in the first plenum  30 , the gas passes adjacent to the pre-collector body components  130  and the discharge electrodes  160 . The pre-collector body components  130  and discharge electrodes  160  are separately coupled to a power source or to ground so that an electrical potential difference exists between these components. This electrical potential difference causes at least a portion of the particulate matter in the gas to collect on the pre-collector body components  130 . No electrical field or potential is intentionally created across the filter element  120  of the filter assembly  100 .  
         [0039]     Thereafter, the gas passes through the filter elements  120  ( FIG. 2 ) and into the interior of the filter assemblies  100 , which will cause particulate matter carried by the gas to separate by the filter elements and either accumulate on or in the filter elements or separate from the gas and fall to the lower portion  80  of the first plenum  30 . Next, the cleaned gas passes from the interior of the filter assemblies  100  through an aperture  90  in the tube sheet  50  and into the second plenum  40 . Ultimately, the cleaned gas will exit the system  10  through the outlet  70 .  
         [0040]     As stated, the pre-collector body components  130  and discharge electrodes  160  are separately coupled to a power source or to ground so that an electrical potential difference exists between these elements. Typically, particles have a negative charge and, therefore, will be repelled by a negatively charged item. Thus, preferably, the pre-collector discharge electrodes  160  are electrically coupled to a large negative voltage and the pre-collector body components  130  are electrically coupled to ground, which should tend to cause particles to collect on the pre-collector body components.  
         [0041]     Conventional baghouse filtration systems require a diverse mix of particles sizes to exhibit acceptable pressure drop values across the filter elements. As the size distribution of incoming particles decreases in a conventional fabric filter, the system pressure drop will increase and pulse cleaning intervals will decrease. In other words, as the particle size becomes more uniform, the system pressure drop increases requiring more frequent cleaning. Fine dust tends to create a very compact dust layer on the surface of the filter elements, which drives up system pressure drop.  
         [0042]     The electrically stimulated fabric filtration system of the present invention overcomes this problem by charging the incoming particles. The relatively larger particles are more readily charged than the relatively smaller particles, and, therefore, these larger particles are more likely to attach to the pre-collector bodies  130 , leaving smaller “like” charged particles to gather on the surface of the filter element. These “like” charged smaller particles tend to repel one another on the surface of the filter element, which creates a more porous dust layer. As a result, combining pre-cleaning of the particle burden with an electrically stimulated fabric filter reduces or eliminates the pressure drop problems experience by conventional pulse jet filter systems and thereby increases the useful life to the filter elements.  
         [0043]     Particles may accumulate on the pre-collector body components  130 ,  220  over time. It is contemplated that such accumulation of particles may require periodic cleaning. One suitable way to clean the pre-collector body components  130 ,  220  is to dislodge the accumulation of particles with acoustic energy by known acoustic horns.  
         [0044]     From the above description of preferred embodiments of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.