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       Mill Ventilation Measurement              
                         
    - The ventilation of a ball mill is extremely important and this for several reasons: removal of the very       
      fine particles, dedusting of the internal equipment, keeping an acceptable temperature, avoiding      
       coating on balls and/or linings, better drying of the material for the raw mills.          
    - Then, it is important to know the quantity of air or hot gas inside the mill.          
    - Four calculators are available to determine the required ventilation:          
      For cement mills:                
      1) Mill Ventilation and Mill Fan Sizing:              
      http://www.thecementgrindingoffice.com/millvent.html            
      2) Heat balance of the cement mill (Calculation of the water injection required)        
      http://www.thecementgrindingoffice.com/termbalwater.html          
      3) Heat balance of the cement mill (Calculation of the cement temperature at mill outlet)        
      http://www.thecementgrindingoffice.com/balancetemp.html          
      For raw mills:                  
      4) Calculation of drying capacities for raw mill              
      http://www.thecementgrindingoffice.com/drying.html            
    - Determine the measurement point(s) in the circuit:            
       The next step to ensure that the ventilation is the right one is the checking on site. The flow      
      measurement points depend of the configuration of the grinding circuit. Generally, the best points are      
      in a place where there are no elbows or other connections. Finally, where the flow of air is the least      
      disturbed.                  
      In fixing the measuring point, the following recommendations should be observed:        
      1) Constant shape and size of the pipeline              
      2) Length of the duct segment before the measuring point higher than 5D (D = duct diameter)      
      3) Length of the duct segment after the measuring point higher than 4D          
      See the figure below:                
     
     
                 
      Usually, the measurments are taken in the vertical duct at mill outlet and in the duct after the fan.      
      Other points of the circuit can also be measured in case of necessity like the separator ventilation.      
      Concerning the raw mill, we have to consider the problem of the high dust loaded pipelines and the      
      measurements points must be well-chosen.              
      The duct is often circular, but also may be square or rectangular.          
    - Location of measuring points in the plane:              
      Hydraulically smooth ducts don't show uniform velocity over the whole cross-section. For this reason,      
      the measuring section (plane) is divided into circular rings of equal area or according the         
      Log. Tchebycheff method.                
      See both figures below:                
     
     
             
     
     
         
      Or use the table before which gives the number of points in relation with the duct diameter:      
     
     
         
      The result will be the average of all measurements.            
    - Pitot L tube:                  
      The measuring device used is the Pitot L tube.              
     
     
             
      A Pitot tube is a small tube with its open end aligned into the flow so as to sense the full impact      
      pressure of the flowing fluid. The tube is also provided with static pressure holes.        
      The Pitot L tube gives both total and static pressures.            
      Nowadays, there are connected with a digital manometer.            
      The lecture can be the pressure and the velocity of the air. One can often choose the unity (English      
      or Metric).                  
      For further details, you must refer to the ISO 10780 standard (Stationary source emissions --      
      Measurement of velocity and volume flowrate of gas streams in ducts)          
      Here below, a representation of the Pitot tube operation:            
     
     
           
    - Equations used to know the air quantity (Bernoulli):            
     
     
                   
      where: ρdry air is the dry air density in kg/m3,   Find P:          
      P is the air pressure in Pa in function of the altitude, Altitude m      
      R is a specific gas constant for dry air = 287,05 J/(kg.K) and P (air pressure) Pa      
      T is the air temperature in °K.                
      Find ρdry air:                  
      Temperature °C              
      Air density kg/m3              
     
     
                   
      where: V is the air velocity in m/sec              
      p is the pressure given by the Pitot tube (dynamic pressure) in Pa and          
      ρdry air is the dry air density in kg/m3.              
      Find V:                  
      P (Pitot tube) Pa              
      Velocity m/sec              
     
     
                   
      where: Q is the air quantity (volumetric flow) m3/h,   10        
      S is the section (area) of the duct in m2 and     15        
      V is the air velocity in m3/sec.       20        
    25
      Find Q:         30        
     
      Duct diameter mm     35        
      Air flow (Q) m3/h     40        
                45        
    - Air quantity inside the ball mill:       50        
      False air
    % (at mill outlet)            
      Mill ventilation m3/h              
                         
    - In function of the results of the air flow measurements, the staff has to take the following actions:      
      1) Comparison with the theoretical need of the ball mill            
      2) Calibration of measuring devices in the control room            
      3) Calibration of the damper fan opening              
      4) Adjustment of the fan damper                
      5) Check up of the mill outlet if necessary (to decrease the false air)          
      6) Increase the fan capacity if necessary              
      End of the presentation                
                         
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