Abstract:
An dry horizontal flow electrostatic precipitator wherein a portion of the gas flow passing through the precipitator is extracted from the upper portion of the casing, such extraction flow having a lower particle concentration than gas flow at the bottom portion of the casing due to re-entrainment. The extraction gas flow is extracted by a manifold positioned at the top of the casing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application Ser. No. 60/275,538 filed on Mar. 9, 2001 by Arthur G. Hein, and the entire disclosure of this provisional application is expressly incorporated herein by reference. 
    
    
     NOTICE REGARDING COPYRIGHTED MATERIAL 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the public Patent Office file or records but otherwise reserves all copyright rights whatsoever. 
     FIELD OF THE INVENTION 
     This invention relates generally to electrostatic precipitators and in particular to precipitators in which the gas flows horizontally through the precipitator. 
     DESCRIPTION OF PRIOR ART 
     The use of electrostatic precipitators to remove suspended particles (i.e. dust) from gas flows is a well-known art and dry horizontal flow precipitators are commonly in service for such application. 
     The treatment zone of a common dry horizontal flow precipitator typically consists of a plurality of high voltage electrodes and collecting electrodes arranged to form gas passages that run parallel to the gas flow. The electrodes are enclosed in a casing through which the gas flows. The electrodes collect dust from the gas flow and rapping systems are used to dislodge the dust from the electrodes, which then falls due to gravity to the bottom of the casing. The bottom of the casing is equipped with a dust removal system to discharge the collected dust. 
     The gas flow is supplied to the precipitator through an inlet nozzle that connects an inlet duct to the casing inlet face and the gas flow is exhausted from the precipitator through an outlet duct that connects the casing outlet face to an exhaust duct. 
     In prior art precipitators, all of the gas flow treated by the precipitator enters the precipitator at the inlet face, flows through the precipitator treatment zone, and leaves the precipitator at the outlet face. 
     Optimum performance had been previously associated with uniform gas flow through the treatment zone. Absolutely uniform flow is not achievable and the Institute of Clean Air Companies, an association that includes major suppliers of electrostatic precipitators and which was formed to encourage improvement of engineering and technical standards, updated and reissued their standard ICAC-EP-7 in January 1997 to specify uniformity within acceptable limits. 
     The mathematical analysis leading to the conclusion that uniform gas flow provides best performance is based on the assumption that at any point within the precipitator the dust concentration does not vary from the top to the bottom of the treatment zone. Standards calling for uniform gas flow are also based on this assumption. However, this assumption is not correct, as re-entrainment of falling dust causes an ever-increasing difference in the dust concentration at the bottom of the treatment zone relative to the top as the gas flows through the treatment zone, with the result that at the precipitator outlet face, the dust concentration in the gas flow leaving the lower part of the precipitator treatment zone is much higher than from the upper part. 
     This was recognized in U.S. Pat. No. 3,733,785 to Gallaer and U.S. Pat. No. 4,695,297 to Hein that describe controlled non-uniform gas flows that improve precipitator collection efficiency over the uniform flow model. In the devices described in these patents, the gas flow remains entirely contained in the treatment zone from the inlet face to the outlet face. 
     SUMMARY OF THE INVENTION 
     The present invention involves a significant departure from the accepted theory and practice of containing all of the gas flow entering the precipitator in the casing through the entire treatment zone and exhausting it all at the outlet face of the precipitator. 
     As the gas flow passes through the treatment zone, the dust concentration in the gas flow becomes lower and reaches an average exit concentration at the outlet face. However, the dust concentration in the gas flow at the top of the precipitator is much lower than that of the gas flow at the bottom (because of re-entrainment) and the upper gas flow becomes cleaner than the average exit concentration before it reaches the outlet face. 
     A precipitator according to the invention extracts a portion of the gas flow (herein referred to as the “extraction flow”) from the main gas flow. The extraction flow is taken from the upper region of the treatment zone before the outlet face. The extraction flow can then be reintroduced to the remaining gas flow after the remaining gas flow passes through the outlet face. 
     This results in at least two benefits. First, extracting a portion of the gas flow from the top of the precipitator encourages the rise of the remaining gas flow as the remaining gas flow proceeds toward the outlet of the precipitator and thereby improves the efficiency of the precipitator upstream of the point of extraction by reducing re-entrainment. Second, the velocity of the remaining gas flow after the point of extraction is reduced as the volume of the gas flow has been reduced by the amount of the extraction flow. Precipitator performance depends on treatment time which is the length of time the gas flow is in the treatment zone. Reducing the velocity of the gas flow increases the treatment time and therefore raises the precipitator collection efficiency after the point of extraction of the extraction flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following description of the invention when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross sectional side view of a dry horizontal flow electrostatic precipitator according to prior art; 
     FIG. 2 is a cross sectional side view of an extraction system according to a preferred embodiment of the invention; 
     FIG. 3 is a cross sectional view thereof taken along the lines A—A in FIG. 2; and 
     FIG. 4 is a cross sectional view of an alternative embodiment of a precipitator according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As best seen in FIG. 1, the casing  1  of a dry horizontal flow precipitator, according to the prior art, encloses the electrode systems. Collecting electrodes  7  are spaced to form gas passages parallel to the gas flow. High voltage electrodes  8  are spaced in the gas passages between the collecting electrodes and are supported from insulators. The zone formed by the gas passages in which the gas flow is exposed to the influence of the high voltage and collecting electrodes is referred to as the treatment zone  14 . Treatment zone  14  in FIG. 1 consists of two fields in the direction of gas flow. Each field has it&#39;s own electrical power supply. Large precipitators may have several electrical power supplies across the width of the precipitator as well as several in the direction of flow. The number of fields in a precipitator is measured by the number of power supplies in the direction of gas flow and varies from a single field to a plurality of fields. The bottom of the casing is equipped with a dust removal system, which may be a hopper system  2  as shown in FIG. 1 or may be another system such as a drag conveyor or wet sluicing. Collected dust is dislodged from the electrodes by high voltage system rappers  9  and collecting system rappers  10 , which may either be positioned externally to the casing as shown, or positioned inside the casing. 
     Gas is introduced to the precipitator through inlet duct  4 , to inlet nozzle  3 . The desired gas flow distribution across the precipitator inlet face is accomplished with vanes and diffuser screens  5 ,  6  that are positioned in the inlet nozzle  3  and at the precipitator inlet face. Gas is exhausted from the precipitator through an outlet diffuser screen  11  to an outlet nozzle  12  and an outlet duct  13 . 
     As best seen in FIG. 2, a precipitator according to the invention, extracts the extraction flow  15  from the upper region of the casing at one or more locations in casing  1 . The precipitator shown in FIG. 2 shows two such locations. At each location a manifold is utilized to gather extraction gas flow across the width of the precipitator from the space above a precipitator field or from the space between precipitator fields. The manifold  16  may be external to the precipitator casing  1  as shown in FIG. 3 or manifold  17  may be positioned inside casing  1  as shown in FIG.  4 . The extraction flow is preferably introduced back into the main gas flow after it leaves the precipitator outlet face or alternatively, is exhausted elsewhere. Fan  19  is used to assist in extracting the extraction flow, but there may be sufficient pressure available to remove the extraction flow without a fan. 
     As further shown in FIG. 2, exhaust ports  18  and  20  are located at positions in the casing. At least one exhaust port  20 , for example, creates a point of extraction P spaced a predetermined distance from the inlet, for first extracting the extraction gas flow when the gas flow in the upper portion of the casing contains a predetermined level of dust concentration. Note that such a location is the first point of extraction, i.e., the location where gases are first extracted from the casing such that gases are not extracted in front of or before reaching the point of extraction P. Thus, only gases which have been first sufficiently treated by the precipitator and have a lower dust concentration are extracted through the exhaust ports  18  and  20 . 
     In operation, the gas flow in the upper portion of the treatment zone is less susceptible to the re-entrainment of falling dust. This causes such upper gas flow to have a lower dust concentration than the lower gas flow as the gas flow progresses through the treatment zone. At the point that the upper gas flow reaches a desired level of dust concentration, the extraction flow is extracted from the precipitator allowing the remaining gas flow to pass through the remaining portion of the treatment zone with a slower velocity. This allows the remaining gas flow in the precipitator to spend more time in the treatment zone, causing lower levels of dust concentration when the remaining flow reaches the outlet nozzle. The remaining gas flow is also pressured upwards, making such gas flow less susceptible to re-entrainment. Therefore a precipitator according to the invention is more efficient than a standard horizontal flow precipitator. 
     Many prior art precipitators have aisles between the fields that are designed to give access to the electrodes and rapping systems. The space at the top of these aisles is often not important for access and would then be available for installing a manifold which could exhaust either through the top of the casing as shown in FIG. 4 or through the sidewall of the precipitator casing at the end of the manifold. Therefore, existing precipitators can be easily modified to perform according to the invention. 
     While the principles of the invention have now been made clear in the illustrated embodiments, it will be immediately obvious to those skilled in the art that many modifications may be made of structure, arrangements, and algorithms used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operational requirements, without departing from those principles. The claims are therefore intended to cover and embrace such modifications within the limits only of the true spirit and scope of the invention.