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      Cement cooling in the separator            
                             
      1 Introduction:                
          - Cement leaving the ball mill typically has a temperature above 100°C.      
          - An for that reason, cement often needs to be cooled to avoid some problems ahead in the production line.
          - A problem that the cement manufacturer can meet is the formation of lumps in silos due to the release of water
            from the gypsum (dehydration).             
          - An other reason for cooling the cement is the bagging procedure and to handle bags at lower temperature.
          - The device generally used is the cement cooler which has a cylindrical body and tubes inside with a recirculation 
            of water.                
          - Cement passes through the labyrinth of tubes (from the bottom to the top) and is cooled.    
          - The cement cooler can decrease the cement temperature up to 60°C and has a specific consumption of  
            1-1,3 kWh/t of cement.              
          - Before investing in a cooler, it is essential to control if the separator is not able to perform this cooling function.
                             
      2 Separator as cooler:              
          - Separators can have good cooling capability or not depending on their characteristics:    
        2.1 Static separators and cyclones:            
          - Static separators used in series with the air are not able to cool.        
          - It is the same for cyclones.            
        2.2 First generation separators (turbos):            
          - All theses separators are designed with an internal air circulation.      
          - The main fan is inside.              
          - In these conditions, the cooling effect is negligible.        
          - In some cases, a secondary fresh air circuit has been added to the existing separator, but the cooling capacity
            is limited.                
        2.3 Second generation separators (cyclones):          
          - These separators have an external fan but the majority of the air is recirculated.     
          - A secondary fresh air circuit is always installed but has not been sized to efficiently cool the cement.  
          - This generation of separator due to their configuration with cyclones allow a better cooling than the 1st generation.
        2.4 Third generation separators (rotor):            
          - This type of separator is well suited for cooling because big air quantities are passing through it.  
          - In fact, separators circuits vary from one to another but there is always the possibility to adjust the air temperature
            at separator inlet.              
          - A certain quantity of air can be recirculated, the adjustment is often realized with the fresh air flap.  
          - At the start of the mill, it is better to recirculate the air to the separator in order to help the mill (with the rejects) 
            to reach its normal working temperature.          
          - After that, more fresh air is allowed in order to cool the cement.        
          - A heat balance is then necessary to specify the needs and the possibilities of cooling.    
                             
      3 Heat balance of the separator:            
        3.1 Introduction:                
          - As the mill heat balance, there must be an equilibrium between what goes in and what goes out of the system.
          - This principle is illustrated below:            
           
     
             
                     
                     
                     
                     
                     
                     
                     
          - What goes into the system:            
            1) cement (separator feed) at a certain temperature        
            2) air at a certain temperature            
            3) energy given by the separator motor          
          - What comes out of the system:            
            1) cement (fines) at a lower temperature          
            2) material (tails) at a lower temperature          
            3) air at a higher temperature            
            4) dissipation by the body of the separator          
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        3.2 Definitions of the parameters:            
            M = separation heat in kcal            
            F  = heat in total separator feed in kcal            
            A = heat in the air in kcal              
            S = heat loss by dissipation in kcal            
            P = heat loss by the fines at separator outlet in kcal          
            R = heat loss by the rejects at separator outlet in kcal        
            Ao = heat loss by the air at separator outlet in kcal          
            N = absorbed power of rotor at terminals in kW          
            f = fresh feed in kg/h              
            CF = circulating factor (A/F)            
            tF = temperature of the feed in °C            
            tR = temperature of the rejects in °C            
            tA = temperature of air at separator inlet in °C          
            tAO = temperature of air at separator outlet in °C          
            tP = temperature of cement at separator outlet in °C        
            V = volume of dry air in Nm3/h            
            Ss = separator area in m2              
        3.3 Equations of the heat balance:            
          - Solutions of all these equations are in: Kcal          
          - IN:                
           
     
               
                       
           
     
               
                       
           
     
               
                       
          - OUT:                
           
     
               
                       
           
     
               
                       
                       
           
     
             
                     
                     
           
     
             
                     
        3.4 Hyphothesis:                
          - Efficiency of energy = 85%          
          - Specific heat of cement (Kcal/kg °c) = 0,19          
          - Specific heat of air (Kcal/Nm3 °c) = 0,31          
          - Dissipation by the surface (Kcal/m2) = 500          
          - 1 KWh = 3600 KJ = 860 Kcal        
          - tR = tP + 5                
          - tAO = tP - 2                
      4 See the infographic here:
     
             
                             
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