Abstract:
A bag house air filtration system with minimal recirculation uses new inlet airflow patterns and cleaning processes. Inlet air enters the bag house. An input plenum changes the velocity profile. Dirty inlet air is split into two plenums, and then passed through guide vanes. A clean air plenum contains a series of individual compartments with a pre-set number of filter bags that are effectively cleaned during a single cleaning cycle. Each individual compartment has a door or louver mounted above the bag openings that is opened and closed during the cleaning cycle. An automatic control system continuously senses bag house pressure drop and activates the cycle when needed. The doors open and close in a pre-set pattern to drop the dust cakes from the bags and restore air flow to normal pressures. Closing a door mounted above the bags creates a reversal of pressure that removes the dust cake.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/606,133, filed Sep. 1, 2004, which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to the effective capture and removal of particulate material from dust laden air streams that must exit to atmosphere from many different process systems. Process systems include such varied applications as power plants, process kilns, cement plants, grain processing plants, foundries, steel mills, hot mix asphalt plants and many other industrial processes. 
   It is generally known and accepted throughout these industries that fabric filter collection systems (“bag houses”) provide the most economical and efficient “dry method” of removing small particulate material from process air streams. EPA air emissions laws governing the amount of particulate emissions allowed to exit into the atmosphere from any given process are now generally standardized throughout the United States and Canada. Additionally, these laws have remained reasonably stable for the past several years. The current accepted emissions level for particulate material permitted to atmosphere is “0.04 grains per dry standard cubic foot of exhaust air”. This law is commonly referenced by EPA agencies as 0.04 gdscf. In order to achieve an atmospheric loading of 0.04 gdscf or less, it is usually necessary to utilize fabric filters and achieve a cleaning efficiency of 0.99895% or greater. 
   Bag houses are manufactured in many sizes and configurations. Bag houses are typically available in “portable mode” or “stationary mode”. A portable bag house is limited in size by the availability of permits and the ability to transport the bag house on a highway. In contrast, stationary units are offered in almost any size with sections assembled on site to accommodate a specific process. 
   A standard bag house consists of a vessel, generally a rectangular container, having a dirty air inlet on one end leading into a lower dirty air section with a clean air outlet on the other end leading from an upper clean air section. Incoming dirty air must pass thorough the bag cloth in order to reach the clean air outlet. 
   A typical bag house contains a multitude of woven fabric bags that can be of varying types and sizes (typically 6″ diameter×12′0″ long). The filter bags are generally hung or mounted on a bag tube sheet by a snap band collar at the open end of the bag which fits tightly into holes in the tube sheet at the top of the bag section, thus providing air tight seals. Each fabric membrane bag is fitted over a separate wire cage or extruded cage. The cages provide support for the bag filter that allows the bag filter to remain open and to assume a specific shape during cleaning operations. 
   As the dirty air passes through the filter bags, particulate material from the dirty air stream is left on the outside of the cloth surface. As the dust accumulates and a dust cake forms on the outside of the bag, the dust cake becomes the actual filtration membrane. As the dust cake thickness increases, the airflow resistance through the filter media also increases. When a certain level of pressure drop (delta P resistance) is reached, the dust cake must be reduced in order to restore adequate airflow to the system. 
   There are a number of accepted cleaning methods used to “drop” the dust cake. These cleaning methods include compressed air pulse, vibration, shaking, reverse air blowing, and atmospheric venting. The most commonly used of these cleaning methods is the compressed air pulse method. In this method, a momentary burst of high-pressure compressed air is forced down into the throat of each bag, sending a shock wave down the length of the bag that dislodges the dust cake. The dust that is removed from the outer bag surface drops by gravity to a hopper collection area in the bottom of the bag house. The accumulated dust is then typically removed by a screw auger system. 
   Despite the widespread use of traditional bag houses, the science of particulate collection still suffers from shortcomings. During the operation of a typical bag house, cleaned air is inadvertently recycled through the system. Generally, air from the filter bags nearest the dirty air inlet flows in the correct direction. However, when the clean air from the bags closest to the dirty air inlet passes over the filter bags near the clean air outlet of the system, some of the clean air is drawn back down into the filter bags and passes through the filter bags in the wrong direction. This creates a re-circulation problem that reduces the efficiency of the bag house. 
   It has been determined that current methods of bag house design, as well as current methods of bag cleaning, are grossly inefficient. Additionally, in some iristances the inefficiencies in bag house design and bag cleaning can lead to negative air effects that actually contribute to air emissions. Research by experts in the areas of computational fluid dynamics (CFD), as well as fluid dynamics, heat transfer, thermodynamics, and aerodynamics has confirmed these inefficiencies. 
   Computational fluid dynamics (CFD) is concerned with obtaining numerical solutions to fluid flow problems by using computers. Equations governing the fluid flow problem include conservation of mass, Navier-Stokes (conservation of momentum), and the energy equation. These equations form a system of coupled non-linear partial differential equations (PDEs). Because of the non-linear terms in these PDEs, analytic methods can yield very few solutions. In general, closed form analytic solutions are possible only if these PDEs can be made linear, either because non-linear terms naturally drop out (e.g., fully developed flows in ducts and flows that are inviscid and irrotational everywhere) or because nonlinear terms are small compared to other terms so that they can be neglected (e.g., creeping flows, small amplitude sloshing of liquid, etc.). If the non-linearities in the governing PDEs cannot be neglected, which is the situation for most engineering flows, and then numerical methods are needed to obtain solutions. With CFD the differential equations governing the fluid flow are replaced with a set of algebraic equations. This process is called discretization. Once the equations are discretized they can be solved with a digital computer to get an approximate solution. 
   The discretized equations are solved on a computational grid. The equations are parallelized using a Message Passing Interface (MPI) technique that allows the computational grid to be divided up onto different computers. The computers work together to solve the equations simultaneously, greatly decreasing the time required to achieve a solution. This technique was used to model the fluid flow inside of bag houses. The computer modeling was conducted over a period of 45 days using 64 computers. The computational grid consisted of 2 million grid points that corresponds to 12 million equations. Several overall designs and different types of inlet and outlet locations and orientations were tested. 
   Research discovered that an internal re-circulation phenomenon was present in all tested, existing bag house designs. The re-circulation phenomenon is related to the flow patterns in the bag house. The modeling showed that the direction of flow through the bags depend on their location in the bag house. For bags that are far from the clean air plenum outlet the air flows in an upward direction, the correct direction for filtering. However, in bags that are near the clean air plenum outlet the air moves in a reversed direction that prevents the bag from filtering. This also indicates that the air involved in the re-circulation phenomenon becomes re-contaminated and must again be cleaned in order to exit the unit to atmosphere. 
   The discovery of the re-circulation phenomenon tells us that current bag houses must be grossly oversized (4.0 to 1.0 air to cloth ratio) to compensate for the number of bags that are engaged in negative re-circulation. These bags do not contribute to the filtration process and can therefore only reduce bag house overall capacity while reverse flowing rather than online cleaning. There is also considerable evidence that the re-circulation phenomenon allows very fine dust that continuously migrates through micro pores of the filter cake to accumulate inside the bags during re-circulation. Then, this fine dust is lifted and emitted in periodic bursts to the atmosphere when disturbed by the shock pulse jet air cleaning cycle. These periodic bursts of fine dust during bag house stack tests can result in failure to pass the particulate loading and opacity air tests. This problem is relatively common with pulse jet type cleaning systems. The reverse flow of re-circulating cleaned air back into the interior of the bags is actually caused by a combination of the imbalance of vacuum exerted on the exit air plenum (vacuum influence by exhaust fan) and the internal bag section vortex vacuum influence generated inside of the dirty air entry plenum. The flow field in the bag house is complex and highly three-dimensional. 
   In the hopper region below the bags high air velocity can create strong vortices that influence the flow pattern through the bags. These vortices can create low-pressure regions below the bags that overcome the vacuum influence of the clean air plenum. This in turn can cause reverse flow through the bags. It is also reasonable to assume that the reverse flow phenomenon moves around the bag house and will change from bag to bag as the resistance changes due to dust cake loading and cleaning. The re-circulation phenomenon is further influenced by the relative position and configuration of inlet and outlet plenums and how they affect internal air flows, as well as the location, size and orientation of inlet blast plates. 
   Needs exist for bag houses that eliminate the re-circulation problem and allow for maximum air cleansing potential and effective removal of dust cakes. 
   SUMMARY OF THE INVENTION 
   The present invention is a design and method of construction and operation for a modular bag house structure with unique bolt on inlet and outlet face sections. Both dirty air inlet and clean air outlet plenum designs affect internal airflow characteristics, as well as the functionality of the up-draft pressure pulse bag cleaning method. Overall house filter capacity is determined by the number of modules used. The modules are identical in size and are fully interchangeable. Only inlet and outlet face plates may be altered if necessary to conform to certain field conditions. Filter house sizes may be altered in the field by simply adding or subtracting the number of modules per house. 
   A primary object of the present invention is to improve upon the efficiency of currently accepted designs of bag house vessels regarding internal airflows, and to improve upon the effectiveness and efficiency of bag cleaning science. The present invention creates an airflow pattern in the bag house with a more stable velocity pattern. Inlet air enters the bag house and is initially subject to gravity removal of large particulate matter. This large particulate matter is directly guided to the hopper section of the apparatus. After heavy particulate has been initially filtered, the dirty inlet air is split into two plenums on either side of the house. This divided air is then passed through guide vanes that create favorable velocity profiles for entering the bag chamber. The change in velocity profile creates a more stable airflow through the bag section of the house. 
   The overall bag house structure must be altered in order to provide for more effective straight line air movements within each chamber of the bag house. In order to first eliminate internal turbulence and stabilize pressures and flow pathways, it is necessary to design dirty air inlets to “straighten the air” and distribute the volume evenly throughout the hopper and bag section of the house, eliminating turbulence and vortices. By entering at the center of a large (frontal entry) distribution chamber, inlet air is directed to both right and left sides of the bag house splitting the flow volume and allowing it to be evenly directed down and under the bags. A large inlet plenum stabilizes inlet air pressure and also acts as a “primary knock out box collector” to capture and eliminate larger sized dust particulates from the air stream and deposit the material directly into the hopper. Removing large particulates from the air stream prior to entering the bag section helps to reduce abrasive wear and increases bag life considerably. After the dirty air stabilizes within the hopper and bag section, it is evenly vacuumed up through the bags in a continuous vertical plane into the clean air plenum. 
   With the new pressure pulse cleaning in up-draft bag houses, there is no recirculation occurring within any section of the bag house. One hundred percent of the dirty air entering the hopper and bag section of the house is drawn through the bag filters and exits through the clean air plenum with no downward recirculation of air. 
   The present invention also incorporates a method referred to as “up-draft bag house pressure pulse” cleaning. The use of the novel up-draft pressure pulse bag cleaning system simplifies the bag cleaning operation by eliminating the need for high pressure, high volume requirements of compressed air as used with pulse jet cleaning systems. The elimination of compressed air from the cleaning system also eliminates the ongoing maintenance requirement of the solenoid diaphragm valves needed to pulse each row of bags as well as the cost and maintenance of an air compressor. By eliminating the need for compressed air cleaning from the system, the danger of collected condensation within the air piping system that could freeze and damage piping is also eliminated. This is a common problem as many industrial applications operate in below freezing temperatures. 
   The up-draft pressure pulse cleaning system of the present invention is a simple yet efficient method of allowing the inertia of the exiting air flow in the clean air plenum to be momentarily stopped and then reversed on a selected compartment of bags creating a slight pressure pulse or reversal of flow in the bag. The momentary pulse or flow directional change is sufficient to allow the bag cloth to relax on the cage and alter its dynamic shape. The filter bags that are recommended for use with the present pressure pulse cleaning systems are slightly oversized to allow the bags to more easily change shape from tight online (under vacuum) shapes to relaxed offline shapes. The moment that the bag changes its dynamic shape (under vacuum shape) to a relaxed posture, the dust cake on the outside of the bag will drop off. The bag is then returned to normal operating vacuum and re-assumes its dynamic shape once again. 
   The clean air plenum contains a series of individual compartments that have a pre-set number of bags that can be effectively cleaned during each cycle. Each of these individual compartments has a door or louver located at the top of each plenum that is opened and closed during the cleaning cycle. An automatic control system continuously senses bag house pressure drop and activates the cycle when cleaning is required. The doors open and close in a pre-set pattern to drop the dust cakes from the bags and restore air flow to normal pressures. 
   When the bag house pressure drop is within the standard operating pressure range (approximately 2.0″ wc to approximately 4.0″ wc) the doors all remain fully open, allowing maximum filter flow. As the dust cake builds on the outside of the bags, the pressure drop will increase and when pressure exceeds preset limits the control system automatically activates the cleaning cycle. The cycle begins with a specific compartment and the door quickly closes on the bags in that specific compartment stopping and reversing flow instantly. The pressure pulse developed by this instantaneous blockage of airflow causes the bag to puff out and change its external shape, thereby dropping the dust cake. The door remains closed for 5 seconds and then is gradually opened and the cleaned bags are brought back on line. This cycle is repeated for each of the compartments until a full cleaning cycle is achieved. The automatic control then pauses to check the pressure drop and determine if the delta P has returned to normal range. If it has, the cleaning system stays at rest position with all doors open and all bags online. If the pressure is still elevated, the cleaning cycle will activate and the bags will again be cleaned. 
   It is the combination of the uniquely directed and controlled up-draft internal air flows within the vessel that eliminate re-circulation, along with the increased cleaning efficiency of the pressure pulse system that encompass the intent of this invention. 
   These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a typical bag house air filtration system. 
       FIG. 2  is a side view of a typical bag house filtration system demonstrating undesired re-circulation. 
       FIG. 3  is a top view of a filter bag and cage under vacuum. 
       FIG. 4  is a top view of a filter bag and cage in a relaxed atmospheric state. 
       FIG. 5  is a perspective cutaway view of a new bag house with controlled internal airflow deflectors. 
       FIGS. 6 and 7  are isometric views of an inlet side of a single module showing inlet and outlet face plates with an up-draft pressure pulse cleaning system and a clean air exit duct. 
       FIGS. 8 and 9  are elevation front and side views, respectively, of a typical module showing bags in a bag section with a clean air plenum and an up-draft pressure pulse system. 
       FIG. 10  is a detailed view showing the up-draft pressure pulse louver door and air cylinder in both closed and open positions. 
       FIG. 11  is an isometric view of the top of a bag house module showing bags with tube sheet clamps and up-draft plenum to clean air exit duct. 
       FIGS. 12 and 13  are isometric view of an up-draft pressure pulse bag house with a clean air duct. 
       FIGS. 14 and 15  are elevation front and side views, respectively, of an up-draft pressure pulse bag house with a clean air duct. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention relates to the effective capture and removal of particulate material from dust laden air streams that must exit to atmosphere from many different process systems. 
     FIG. 1  shows a typical bag house  11 . A standard bag house  11  consists of a vessel  13 , generally a rectangular container, having a dirty air inlet on one end  15  leading into a lower bag section  17  with a clean air outlet  19  on the other end  21  at the top leading from the upper clean air section  23 . Incoming dirty air  25  is deflected downward into the bag chamber  17  of the bag house  11  by a blast deflector plate  27 . The blast deflector plate  27  also protects bag filters  29  from being damaged by the force of the incoming dirty air  25 . After the dirty air  25  enters the bag chamber  17 , it must pass thorough a set of bag filters  29  in order to reach the clean air outlet  19 . 
   The bag house  11  contains a multitude of bag filters  29 . Each bag filter  29  consists of woven fabric bags  31  that can be of varying types and sizes (typically 6″ diameter×12′0″ long). The filter bags  31  are generally hung or mounted on a bag tube sheet  33  by a snap band collar  35  at the open end of the bag  31 , which fits tightly into holes in the tube sheet  33  at the top of the bag section  17 , thus providing air tight seals. Each fabric membrane bag  31  is fitted over a separate wire cage or extruded cage. The cages provide support for the bag filter  29  that allows the bag filter  29  to remain open and to assume a specific shape during cleaning operations. During cleaning, air flows inward through the bags, which are under a vacuum from the exhaust fan. 
   As the dirty air  25  passes through the filter bags  29 , particulate material from the dirty air stream  25  is left on the outside of the cloth surface. As the dust accumulates and a dust cake forms on the outside of the bag, and the dust cake becomes the actual filtration membrane. As the dust cake thickness increases, the airflow resistance through the filter media also increases. When a certain level of pressure drop (delta P resistance) is reached, the dust cake must be reduced in order to restore adequate airflow to the system. 
   After a cleaning operation, the dust that is removed from the outer bag  31  surface drops by gravity to a hopper collection area  37  in the bottom of the bag house  11 . The accumulated dust is then typically removed by a screw auger system  39  and exits the system  41 . 
   Despite the widespread use of bag houses  11 , traditional designs suffer from shortcomings. During the operation of a typical bag house  11 , clean air is inadvertently recycled through the system.  FIG. 2  shows a typical bag house  11  with improperly circulating air. Generally, cleaned air from the filter bags  29  does flow in the correct direction  43 . However, studies have now proven that when the clean air from the bags passes through the clean air plenum  19  of the system, some of the clean air  45  is drawn back down into the filter bags  29  and passes through the filter bags  29  in the wrong direction. This creates a re-circulation problem that reduces the efficiency of the bag house  11 . 
   The re-circulation phenomenon is further influenced by the relative position and configuration of inlet and outlet plenums and how they affect internal air flows, as well as the location, size and orientation of inlet blast plates. 
     FIGS. 3 and 4  show typical bag filters  47  used in existing bag houses  11  and in a new bag house  49  of the present invention.  FIG. 3  shows a bag filter  47  while on line (under vacuum). The filter bag  47  consists of a wire frame, mesh, cage or other similar structure  51  with a bag  53  surrounding the wire frame  51 . As the dirty air passes through the bag  53 , dust is deposited on the outer surface of the bag  53  and forms a dust cake  55 . This dust cake must be removed when the pressure drop between across the filter bag  47  becomes too great.  FIG. 4  shows a bag filter  47  in a relaxed state at atmospheric pressure or during cleaning. The bag  53  expands from its vacuum state, thus shaking off the dust cake  55 . 
   The present invention is a design and method of operation for a bag house structure with unique inlet and outlet plenum designs that affect internal airflow characteristics, as well as a unique design and functionality of a pressure pulse bag cleaning method. 
     FIG. 5  shows a new bag house  49  in partial cutaway to reveal the inner structure of the bag house  49 . In order to eliminate internal turbulence and stabilize pressures and flow pathways, the dirty air inlet plenums  79  must “straighten the air” and distribute the volume evenly throughout the hopper section  59  and bag section  57  of the house  49 , eliminating turbulence and vortices. By entering at the center of a large (frontal entry) distribution chamber  77 , inlet air duct  63  is directed to both right and left sides of the bag house  49  splitting the flow volume and allowing it to be evenly directed down and under the filter bags  73 . A large inlet plenum  77  stabilizes inlet air pressure and also acts as a “primary knock out box collector” to capture and eliminate larger sized dust particulate from the air stream and deposit the material directly into the hopper  59 . Dirty air enters into the distribution header  77  and is divided into two plenums  79  that enter the bag section  57  separately on each side of the bag house  49 . Guide vanes  83  assist in distributing the flow uniformly through the plenums  79 . Other modifications are possible to assist in improving flow distribution and reducing the velocity magnitude. After the dirty air stabilizes within the hopper  59  and bag section  57 , it is evenly vacuumed up through the bags  73  into the clean air plenum  81 . Dust removed from bags falls to the hopper  59 , collecting in the screw conveyor  61  for removal. 
   The present invention is also a method of cleaning the new bag house  49 . The pressure pulse cleaning system of the present invention is a simple yet efficient method of using the inertia of the exiting air flow in the clean air plenum to act as the pulse force by momentarily stopping and then reversing flow on a selected compartment of bags creating a pressure pulse or reversal of flow in the bag  73 . The momentary pulse or flow directional change is sufficient to allow the bag cloth to relax on the cage and alter its shape. The filter bags that are recommended for use with the present pressure pulse cleaning systems are slightly oversized to allow the bags to more easily change shape from tight online (under vacuum) shapes to relaxed offline shapes. The moment that the bag changes its dynamic shape (under vacuum shape) the dust cake on the outside of the bag will drop off. The bag is then returned to normal operating vacuum and re-assumes its dynamic shape once again. 
   When the bag house pressure drop is within the standard operating pressure range (approximately 2.0″ wc to approximately 4.0″ wc) the doors  71  all remain fully open, allowing maximum flow. As the dust cake  55  builds on the outside of the bags  73 , the pressure drop increases and when pressure exceeds preset limits the control system automatically activates the cleaning cycle. The cycle begins with a specific compartment and the door  71  quickly closes on the bags  73  and bag openings  87  in that specific compartment  85  stopping and reversing flow instantly. By mounting the pulse doors above the bag openings, a pressure pulse is developed by the instantaneous blockage of airflow, causing the bag  73  to relax and puff out, changing its external shape, and thereby dropping the dust cake  55 . The door  71  remains closed for 5 seconds and then is gradually opened and the cleaned bags  73  are brought back on line. This cycle is repeated for each of the compartments until a full cleaning cycle is achieved. The automatic control then pauses to check the pressure drop and determine if the delta P has returned to normal range. If it has, the cleaning system stays at rest position all doors  71  open and all bags  73  online. If the pressure is still elevated, the cleaning cycle will activate and the bags  73  will again be cleaned. 
     FIGS. 6 and 7  are isometric views of an inlet side of a single module  87  showing an inlet plate  89  and an outlet face plate  91  in an up-draft bag house with a pressure pulse cleaning system and a clean air exit duct  93 . A dirty air inlet  95  allows dirty air into the module  87 . The dirty air inlet  95  feeds into an inlet plenum  97 , which then feeds into a bag section  99 . Clean air exits from the clean air duct  93 . Other parts of the system include a dust auger  101  and supports  103 . Plates  105  on the top of the bag section  99  allow access to bags  107 , as shown in  FIG. 8 . A pulse louver  109  moves to change airflow in the system. 
     FIGS. 8 and 9  are elevational front inside and side views, respectively, of a typical module  87  showing bags  107  in a bag section  99  with a clean air plenum  111  in an up-draft bag house pressure pulse cleaning system. A hopper section  113  is located above the conveyor  101 . 
     FIG. 10  is a detailed view showing the up-draft bag house pressure pulse cleaning louver system  109 . The louver system  109  is located on a neck  121  between the body of the bag section  99  and the clean air exit duct  93 . The louver system  109  includes an air cylinder  117  attached to a pivot  118  on an outer end  120  of an extension  119 . The pivoted air cylinder  117  moves a piston  122  and a door  115  from a closed position  124  while running to an open position  126  while cleaning. When in a cleaning position  124 , a louver  123  is closed by a crank arm  130  connected to pivot  132  on door  115 . The crank arm  130  is pivoted around bearing  134 , which is mounted on the inside  136  of the vertical wall of the neck  121 . In normal operation, the air cylinder  117  extends a piston  122  to close the door  115  and to open the louver  123 . Periodically, to clean bags  107 , the air cylinder temporarily pulls piston  122  to close louver  123  and to open the door  115  to supply an atmospheric pressure pulse to the inside of the bag. 
     FIG. 11  is an isometric view of the top of a bag house module  87  with lids  105  removed for showing bags  107  with tube sheet clamps  125  and up-draft plenum  111  to supply clean air exit duct  93  through the neck  121 . 
     FIGS. 12 and 13  are, respectively, outlet and inlet end isometric views of a pressure pulse cleaned up-draft bag house  127  with a series of bag house modules  87  joined together with a clean air duct  93  form of a series of joined modules. One extension arm  119  is shown on the first module. The similar extension arm doors  115  and door and louver opening and closing mechanisms have been removed to sow the atmospheric vent openings  128  in modular necks  121 , which are separately, periodically opened to pulse atmospheric pressure to the interiors of bags in single modules. 
     FIGS. 14 and 15  are right (extension arm) side and inlet end views, respectively, of a pressure pulse cleaned bag house  127  constructed with a series of joined bag house modules  87  and clean air duct  93  modules. 
   The combination of the uniquely directed and controlled internal air flows within the vessel eliminates re-circulation, along with the increased cleaning efficiency of the pressure pulse system that encompass the intent of this invention. 
   While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention.