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      Filtres          
                   
      3   Bag filters (Suite 1)        
       3.6    Cleaning methods:        
        - Let's remind that there are three ways to clean the filter elements:      
          * The mechanical system (shaker)        
          * The reverse air system (reverse-air)        
          * The pulsed air system (pulse jet)        
        - The mechanical system:        
          It is performed by shaking or vibration of the bags.      
          There are three ways:        
          * Mechanical shock        
          * Oscillation        
          * Sound waves of low frequency        
          It is only used for the bags with internal absorption.      
          It is a system that requires much bags.        
          An illustration below:   Operating mode:  
         
     
           
        - The reverse air system:        
          A dust-free air passes in the opposite direction of normal operation.    
          This system requires the isolation of the bags to unclog.      
          An illustration here below:   Operating mode:  
         
     
           
        - The pulsed air system:        
          This system is only used with external absorption bags.      
          A jet of air at 6-7 bar is sent periodically within each bag and generates a pressure wave that causes the sudden
          enlargement of the filter cloth and detaches the pasted material.      
          It does not require the isolation of the filter bags to be processed.      
          The bags are poorly requested.        
          An illustration below:   Operating mode:  
         
     
     
         
        - The tables here below give an idea of the general parameters of cleaning for the 3 types:  
         
     
     
       3.7   Filter materials:        
        - From the point of view of the filter material, one can distinguish two types of materials:  
        - The woven media consisting of regularly perpendicular woven wires, with an aperture of square meshes.
          Example of filter cloth:        
         
     
           
        - Pressed media which is a set of small fibers arranged randomly (eg Teflon type).    
        - Felt example:        
         
     
           
        - The main difference between the two is in the process of filtration:    
        - The fabric retains relatively large particles forming a solid layer which also acts as a filter.  
        - Teflon, in addition to the surface action holds the particles inside the filter layer, which results in good efficiency
          on the fine particles.        
        - Cleaning Teflon is more difficult and has higher pressure drop, but in return it is more efficient.
        - After the cleaning, the fabric loses a part of the dust adhering to the frame and a short time later, the cleaning
          efficiency decreases.        
        - Different filter materials are distinguished by other key features such as:    
          * The size of the fibers        
        - * The mechanical strength of the fibers (at different temperatures)    
          * The chemical resistance of fibers        
          * The resistance to the passage of air        
          * The electrostatic charge        
          Filter materials must meet the following criteria to avoid problems:    
          * Good mechanical strength        
        - * Good dimensional stability at the operating temperature      
          * Good air permeability (low pressure drop)      
          * Good thermal stability at operating temperature      
          Application based on 3 types of cleaning:        
         
     
       
        - Main characteristics of different types of filter media:      
         
     
           
        - Some examples of filter bags:        
         
     
       
       3.8   Advantages and disadvantages of the three cleaning systems:      
       3.8.1   The mechanical system:        
        - Advantages:        
          * A low pressure drop for equivalent filtration efficiencies      
          * Has a high filtration efficiency for respirable dust      
          * Can use strong bags that can support the reinforced cleaning cycle    
          * Easy to use        
        - Disadvantages:        
          * Has a low filtration rate (0,45 to 0,6 m/min) (*)      
          (*) Filtration velocity = air-to-cloth ratio        
          * Can not be used at high temperatures        
          * Requires a large space        
          * Requires a large number of filter bags        
          * It consists of many moving parts and requires frequent maintenance    
          * Staff must enter the baghouse to replace bags, creating a significant risk    
          exposure to toxic dust        
          * May cause reduced efficiency of cleaning if there is a slight positive pressure inside the bags  
       3.8.2   The reverse current system:        
        - Advantages:        
          * Has a low pressure drop for equivalent filtration efficiencies      
          * Has a high filtration efficiency for respirable dust      
          * Is preferred for high temperatures due to the action of gently cleaning    
        - Disadvantages:        
          * Has a low filtration rate (0.3 to 0.6 m / min)      
          * Has no effective way to eliminate the accumulation of residual dust    
          * Requires frequent cleaning to compensate for the low cleaning      
          * Air cleaning must be filtered        
          * Staff must enter the baghouse to replace bags, creating a significant risk    
          exposure to toxic dust        
       3.8.3   The pulsed air system:        
        - Advantages:        
          * Can have a filtration velocity relatively high (1,8 to 3 m/min)      
          * Has a very good performance and minimal residual dust accumulation due to aggressive cleaning
          * Has a high filtration efficiency for respirable dust      
          * Can be cleaned continuously        
          * Can use resistant handles that can withstand a reinforced cleaning cycle    
          *Has a lower wear of the bags        
          * Has a a smaller size due to the higher speed of filtration      
          * Some models allow you to change bag without entering the filter      
        - Disadvantages:        
          * Can easily be used at high temperatures except with special fabrics    
          * Requires the use of dry compressed air        
          * Can not be used in case of high humidity in the gas      
       3.9   Comparison:        
        - A table gives a comparison between the three systems:      
         
     
           
        - So we see that the pulsed air system is the most interesting because it is operating practically continuously
          and that for a lower space required.        
       3.10   Elements of sizing:        
       3.10.1 Filtration velocity:        
        - This is probably the best known and no doubt the most important parameter.    
        - It is also called: Air-to-Cloth ratio         
        - The formula defining this parameter is the following:      
         
     
           
                 
                 
          With:        
          V is equal to the velocity of filtration in (m3/min)/m2, or in m/min    
          Q is equal to the quantity of gas to be treated in m3/min      
          A is the filter surface (the filter bags so) in m2      
        - For this calculation, we consider the net filter area, we will see later what that means.  
        - The tables below provide filtration velocities typically used for various materials:  
         
     
         
         
     
         
               
        - And to summarize in the case that interests us:      
         
     
           
        - Knowing the amount of air to be treated, it is easy to calculate the required filter area:  
        - For example        
          * Air quantity: 65 000 m3/h = 1 083.3 m3/min      
          * Material: cement        
          * Filter tpe: reverse air with felt        
          We must solve the equation:        
         
     
           
                 
                 
        - For pulse jet filters, an equation has been developed by the manufacturers:    
         
     
           
                   
                   
          With:        
          V is filtration speed in m/min        
          A is a coefficient of material (see table below)      
          B is a coefficient of application (see table below)      
          T is the temperature in °C        
          C is dust concentration in g/m3        
          D is the mean diameter by mass of the particles in μm      
          Constraints to respect:        
          T must be between 10 and 135 °C        
          For T outside this area, use 10 or 1325        
          C should be between 0,1 and 229 g/m3        
          For C outside this zone, use either 0,1 or 229      
          D must be between 3 and 100 microns        
          For D outside this range, use either 0,8 or 1,2 if smaller or if greater than 100    
          Coefficients A and B for applications in cement field:      
         
     
         
         
     
         
        - For example        
          * Filter of a cement mill        
          * Temperature of cement: 102 °C        
          * Dust concentration: 40 g/m3        
          * Average diameter: 50 microns        
          Solution:        
          A = 10        
          B = 0,9        
         
     
           
                   
                   
         
     
           
                 
       3.10.2 Net area of filtration/gross area of filtration:      
        - One must calculate first the net filtration area, so that we need really.    
        - As filters having a cleaning system by shaking or reverse air have always a part offline  
          for cleaning, one must multiply the net area by a coefficient to determine the total area available for filtration.
        - The coefficients to be applied are:        
         
     
         
        - For pulse jet filters whose elements generally remain online during the cleaning phase, the net and gross filtration
          areas are equal.        
        - Finally, to close this chapter, here is a table showing the number of sections required depending on the filter surface:
         
     
         
       3.10.3 Pressure drop:        
        - The pressure drop of a dust collector is of course a factor to consider.    
        - The pressure drop is often given by the following formula:      
         
     
           
                 
          With:        
          ΔP is the total pressure drop        
          ΔPf est la perte de charge du média filtrant        
          ΔPp is the pressure drop of the filter cake        
          ΔPs is the pressure drop of the filter structure      
        - ΔPs is often neglected and then we have:        
         
     
           
                 
        - As the gas flow takes place at a low Reynolds number (Re = 1), we can apply Darcy's law (*) and we have:
         
     
           
                 
                 
          With:        
          ΔP is the total pressure drop        
          Df is the depth or thickness of the filter media      
          Dp is the depth or thickness of the material cake coating the filter medium    
          η is the viscosity of the gas        
          V is the speed of filtration        
          Kf is the permeability of the filter medium        
          Kp is the permeability of the cake coating the filter media      
          (*) Darcy's law is concerned with the flow in porous media      
        - As the pressure drop of the filter media is more or less constant, the total pressure drop of the filter depends
          directly on the thickness of material around, which increases with time.    
        - Small illustration of the phenomenon:        
         
     
         
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