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      Filters          
                   
      2   Electrofilters        
          This document provides basic elements.        
          For details, consult specialized books!        
       2.1   Definition:        
        - In two words the electrostatic precipitator - also known as ESP - is a dust extraction device of laden gases by electrical
          ionisation.        
       2.2   Constitution:        
          The electrostatic precipitator is mainly composed of:      
        - A large metal casing.        
        - Of wires, also called discharge electrodes, installed vertically.      
        - These wires receive an electrical current with a high negative charge and a high voltage of 30-100 kV.
        - The discharge electrodes are located and suspended between parallel plates.    
        - There are different types of discharge electrodes available:      
         
     
     
         
        - The plates are mounted in parallel and vertical and are most often made of stainless steel.  
        - These plates are connected to the ground at potential 0 and are also called collecting electrodes (with a positive charge).
        - The distance between wires and plates can vary from 150 to 250mm depending on the nature of gases.
        - The plates usually have a maximum height of 9m.      
        - There are different types of collecting plates available:      
         
     
           
        - The gas velocity is around 1 m/sec.        
        - Diagram:        
         
     
         
       2.3   Different types:        
        - There are three types of electrostatic precipitators:      
          * One stage filters (high voltage)        
          * The 2-stage filters (10-12kV)        
          * The wet electrostatic filter        
        - The discussion here is limited to the study of the one stage electrostatic precipitator.  
        - Example of ESP:        
         
     
     
       
       2.4   Principle of operation:        
        - The principle of operation of an electrostatic precipitator is based on the force applied by an electric field to a
          limestone particle or other.        
        - The particle of limestone is naturally charged but its charge is too low to give rise to a sufficient attractive force
          to be exploitable.        
        - It is therefore necessary that the particle be exposed, by an external process, to have a sufficient ionized charge.
        - This charge, useful and negative, is created by ionization of the carrier gas, obtained by the discharge electrodes
          raised to a high potential and giving what it is called the corona effect.     
        - The corona can be positive but it seems that the performance is less satisfactory in that case.   
        - So, the particles across this electric field are negatively charged.       
        - They are then deviated by the field and collected on the collecting electrodes.    
        - With time, the particles begin to deposit on the collecting surface, the thickness of the layer of material thus increases.
        - A collecting plate takes care of 12-25 cm of particles before being removed by shaking (hammers). 
        - An endless screw is often used to remove the material collected in the bottom of the filter.  
        - Diagram of the principle:        
         
     
     
        - The capture of the dispersed particles in the ionized gas is thus carried out in three successive steps:
          * The dust electrical charge        
          * The migration of the charged particles in the electric field toward the collecting electrodes where they precipitate
          by electrical discharge        
          * The evacuation of the purified gas and the removal of collected dust.    
        - It should be noted that the total length of these three steps must be shorter than the residence time of the gas
          through the filter.        
        - Below is another diagram of the operation:        
         
     
       
        - Summary of separation steps:        
          * Generation of an electrical charge (corona effect)      
          * Ionization of gases        
          * Charging of the particles        
          * Migration of the particles        
          * Captation of the particles        
          * Cleaning of collector plates (and electrodes)      
        - Without going into detail, it should be noted that the cleaning of charging electrodes and collecting plates
           is often realized via a system of impact hammers electromagnetic pulse device as shown on the photo below:
         
     
           
       2.5   Efficiency of the ESP:        
        - The efficiency of an electrostatic precipitator depends on many factors such as:    
          * dust concentration        
          * the amount of gas        
          * the collecting surface        
          * the temperature        
          * the moisture        
          * the particle size        
          * the resistivity of the particles        
        - Here below is a table giving an idea of ​​the influence of these parameters:    
         
     
         
        - The main characteristics of the particles affecting the efficiency are:    
          * the particle size        
          * the resistivity        
        - Regarding the size, here is a typical chart of efficiency as a function of particle diameter:  
         
     
         
        - Some resistivity according to the temperature (source Wikipedia)      
         
     
         
        - Regarding the resistivity of cement dust, here's a chart showing its relationship with humidity and temperature:
         
     
         
        - It should be noted that the resistivity of the raw meal is very high (up 10¹⁴Ω.cm) and the dust has sometimes difficulty
          to precipitate.        
        - Suitable devices can provide, for many industrial emissions, an overall efficiency of dust removal > 99%.
        - The commonly used formula to calculate the efficiency of an electrostatic precipitator looks like this:
         
     
           
                 
                 
          where:        
          η is the efficiency        
          ω is the migration velocity of the particles in m/s      
          S is the collecting surface in m2        
          Q is the quantity of gas in m3/s        
        - This equation is called formula of Deutsch.        
        - After, it was found that the efficiency was much more influenced by the variability of the rate of migration
          of the particles, especially the finest, and the Deutsch formula has been modified as follows (Allander, Matts
          and Ohnfeldt):        
         
     
           
                 
                 
        - With a coefficient called m, between 0,4 and 0,7 (0,5 being the standard value generally accepted).
                   
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