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      Drying capacities calculation - Explanation          
                           
      1 Introduction:                
        - Drying is an important operation in cement plants.          
        - The raw material coming from the quarry often contains a certain percentage of moisture which should be eliminated
          in order to avoid handling problems and the generation of vapours which can clog auxiliary equipments.
        - All this without mentioning problems to grind such material in dry process.      
        - Moisture problems also exist for cement mills, but rarely, in the case of a component like limestone or pozzolana.
        - It is generally accepted that from 0,5% moisture, every 1% increase will cause a mill's efficiency drop of 10%.
      2 Kinds of drying methods:              
        - There are two types of drying methods: in an external device or with a grinding process, called drying/grinding,
          like raw mill, coal mill or cement mill.            
      2.1 External devices:              
        The most famous are:              
        - Rotary dryers are used for drying wet materials or powders (coal is a typical material for this application).
          The rotary dryer consists of a rotating drum with lifting blades which lift the material as the drum rotates. 
          Then, the material falls in the stream of hot air flowing through the drum.      
          The speed of the drum is normally between 2 and 6 rpm and its inclination 3 to 6 degrees.    
          Some rotary dryers have the entrance of the hot gases to the material discharge.    
         
     
         
               
               
               
               
               
               
               
               
               
               
               
               
               
        - Flash Dryers are pneumatic systems used to dry products requiring the removal of free moisture.  
          Flash Dryers are characterized by short residence time of few seconds.      
          The flash dryer consits of a duct in which the fresh material is conveyed by a hot gas stream.  
          Speed of gas is similar to a pneumatic system (20-25 m/sec).        
          Flowsheet of the flash dryer:            
         
     
             
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
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        Other dryers:                
        - Dispersion dryers: are similar to grinding chambers of ball mills.        
        - Rapid dryer: are provided with agitator paddles.           
        Dryer-crusher:                
        - The dryer-crusher is an impact hammer crusher combined with an airswept mill.    
          This concept is often called Tandem (KHD).          
          The drying process is realized in both crusher and mill and with a pneumatic system.    
          The mill and the crusher are connected in series.          
        Separator-dryer:              
        - A dynamic separator can also be used to dry material but it is limited because the short retention time.  
          Also, some wear parts like bearings avoid the use of excessive temperatures.      
      2.2 Drying with grinding:              
        - Internal dryers are in fact the drying chamber of ball mills.        
        - The drying chamber is equipped with flat lifters to mix the material in the hot gas flow.    
        - Drying chambers are possible with various types of ball mills:        
        Compound mill:              
        - This mill has one or two grinding compartments with end discharge configuration.    
        - It is really rare to encounter this type of mill for the raw material. Generally, they are cement mills with a certain 
          amount of wet additives.              
        - The grinding capacity of these mills is limited for various reasons: air speed over the ball charge limited to 2 m/sec
          and gas temperature limited to avoid problems to the bearings, balls and intermediate diaphragm.  
        - An other problem is the possible clogging of the slots of the intermediate diaphragm in case of insufficient drying.
        Central discharge mill:              
        - Also called birotator (doppel rotator), this mill is a good solution in case of hard material.    
        - A higher degree of moisture is also allowed.          
        - A drying chamber is followed by the coarse grinding chamber and the material leaves the mill by a central discharge
          through the holes of the shell. On the other side, there is the fine grinding chamber with the same central discharge.
        - This type of mill is known to be quite difficult to manage.        
        - A higher air speed over the ball charge is permitted up to 5-6 m/sec.      
        - The second chamber has generally fresh air for its ventilation but a configuration with hot gas is also possible.
        Airswept mill:                
        - This mill has one grinding compartment without outlet diaphragm.      
        - The transport of the material is effected by a pneumatic system.        
        - This kind of mill allows high air speed and higher moisture content.      
        - It is quite difficult to find a good compromise between the gas flow rate requirement and an adequate grinding
          efficiency.                
      3 Drying capacities calculation            
        - In thermal drying processes, the heat is transferred essentially by convection from the gas to the material
          and its moisture.              
        - The major possible heat sources are:            
          The kiln exhaust gases up to 350°C            
          The clinker cooler exhaust gases up to 350°C          
          The hot gas generator up to 800°C            
          The grinding heat (mill motor)            
        - The energy required is given by the following formula: E = Q x t x Cp,      
          where E is the required energy in Kcal/h,          
          Q is the gas quantity in Nm3/h,            
          t is the gas temperature in °C and            
          Cp is the gas specific heat in Kcal/Nm3.°C          
        - We propose two calculators to evaluate the gas and temperature requirements:     
          These calculators are based on the following methods:        
          1. A method developed by a german engineer for the dryers but relevant for ball mills    
          2. The method based on the mill heat balance          
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      3.1 Weber Method:              
        - This method is based on the following graph:          
     
     
                       
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
        - The example in red shows that choosing a temperature of 350°C which comes from the kiln exhaust fan  
          (specific heat of 0,35 Kcal/Nm3.°C), a dryer false air of 20% and a gas quantity of 1,2 Nm3 per kg of material,
          the dryer is able to dry up to 11,5% of moisture.          
        - See infographic which explains in detail the calculation:        
      3.2 Balance Method:              
        - The procedure is based on the mill heat balance.          
        - Mill heat balance procedure is explained on the next page:        
          http://www.thecementgrindingoffice.com/millbalexplan.html        
        - See infographic which explains in detail the calculation:        
      3.3 Comparison:                
        - First comparison for a monochamber + drying chamber mill with the following data:    
          Diameter: 3m              
          Grinding length: 6m              
          Speed: 18,8rpm              
          Volume load: 30% (53,3t of balls)            
          Absorbed power at terminals: 660kW            
          Specific power: 15kWh/t              
          Output: 44t/h              
          False air: 15%              
          Ambient temperature: 20°C            
          Residual moisture: 1%              
          Circulating factor: 3              
          Tails temperature: 80°C              
          Fresh feed temperature: 20°C            
          Outlet gas temperature: 90°C            
          Outlet material temperature: 90°C            
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          The diagram herebelow shows the gas requirement calculated with both methods for different moistures 
          (from 2% to 12%) and for gas temperatures of, respectively, 350°C and 500°C:      
         
     
                   
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
        - One sees that the balance method is more optimistic.        
        - The second comparison is for a big monochamber mill also with drying compartment:    
          Diameter: 5m              
          Length: 10m                
          Speed: 14rpm              
          Volume load: 30% (151t of balls)            
          Absorbed power at terminals: 2170kW            
          Specific power: 15kWh/t              
          Output: 145t/h              
          False air: 15%              
          Ambient temperature: 20°C            
          Residual moisture: 1%              
          Circulating factor: 3              
          Tails temperature: 80°C              
          Fresh feed temperature: 20°C            
          Outlet gas temperature: 90°C            
          Outlet material temperature: 90°C            
          The diagram herebelow shows the gas requirement calculated with both methods for different moistures 
          (from 2% to 12%) and for gas temperatures of, respectively, 350°C and 500°C:      
         
     
                   
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
                           
      4 Conclusion:                
        - Both methods are really interesting and show a different approach to a problem that is not always easy to solve.
        - The just quantity required is probably between them.        
        - Also, the calculation must take into account the grinding part of the installation and find a good compromise.
        - For dimensioning drying equipments, a deep study of the local conditions must be carry out with specialized 
          engineering offices.              
          Links to calculators:
     
             
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