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      Filters          
                   
      3   Bag filters        
          This document provides basic elements.        
          For details, consult specialized books!        
       3.1   Definition:        
        - Bag filters are dedusting devices used in industrial applications.      
        - They are composed of a metal casing in which are arranged filter bags that when they are traversed by the gas will
          retain dust.        
       3.2   Constitution:        
          The bag filter is mainly composed of:        
        - A large metal casing which can be made of several sections.      
        - Filter media, the most used are the cylindrical bags.      
        - A dust collection hopper        
        - A dust removal system which is the most frequently an endless screw such as electrostatic precipitators.
        - A filter media cleaning system.        
         
     
     
     
       3.3   Different types:        
        - There are three types of bag filters according to their cleaning method:    
          * filters with mechanical cleaning (shaker)        
          * filters with cleaning by reverse air flow (reverse-air)      
          * filters with cleaning by pulsed air (pulse jet)      
        - The trend today is to use rather long vertical bags (> 5 m), with a pulsed air filter cleaning system, a capture of the
          dust by the external side and with an air flow from the bottom to the top.    
        - Today, the reverse air and pulsed air systems are the most used in cement plants.  
        - Operating diagram (credit: Iowa Department of Natural Resources):    
                   
       3.4   Principle of operation::        
        - The dust laden gas enters the filter from the top or bottom of the box.    
        - The gases pass through the filter bag from outside to inside (outside absorption) with a speed of about 1,5 m/s and
          is sucked by the fan located downstream of the dust collector.      
        - In an alternative system, the gas can also pass through the filter bags from the inside to the outside (inner absorption).
        - The number of installed bags of course depends on the gas flow to be treated.    
        - With time therefore, it forms a kind of cake around the dust filter bags.    
        - And the cake of material helps, unlike what one might think, to improve the filtration because it is porous and only
          extra-fine particles can pass through.        
        - The pressure drop across the filtration surface increases with the deposition of solids retained and when it
          reaches a limit, the gas flow is interrupted and the bag is cleaned and sends the dusts in the bottom of the recovery
          hopper.        
        - To avoid stopping the flow of gas to be treated during the cleanup phase, the filter is usually performed in several
          sectors, each of which being subject to the cleaning step in rotation while the other are in phase of operation.
        - Dusts are finally collected in an endless screw and are sent either to the finished product, or to the bucket elevator
          to be processed again in a separator.        
        - On the other hand, clean air (dusted) exits through a chimney using an induced draft fan (often centrifugal).
        - Below, we can see the two cases where the dust is absorbed outside or inside of the filter bag:
          
     
       
         
     
           
       3.5   Filtration mechanisms:        
        - The filtering is performed by the combined effect of five mechanisms:    
        - Gravity:        
          Only applicable to the coarser particles.        
        - Impact or inertia:        
          Solid particles entrained by the gas, due to their mass, do not follow the path of the gas when it follows a curve
          to avoid an obstacle but go on the obstacle itself (predominant for particles > 1 micron).  
        - Interception:        
          The solid particles of the gas along the path around the obstacle and located at a distance less than their radius
          with respect to the obstacle are intercepted (mainly particles between 0.2 and 1 micron).  
        - Brownian effect:        
          Very fine particles, due to their Brownian motion, may collide with other gas molecules, deviate from their path
          and may touch the obstacle (mainly for particles <0.2 m).      
        - Electrostatic forces:        
          The presence of an electrostatic charge on the particles and the filter can increase the capture of the dust.
        - The obstacle is, of course, the filter bag coated of its cake of material already intercepted.  
        - The diagram below illustrates these filtering mechanisms:      
         
     
         
        - This drawing is inspired by Klingspor and Vernon, 1988.      
        - Typical diagram of filter efficiency as a function of the 5 forces:      
         
     
         
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