Thermic calculations
Generalities
Basic rules for cartridge
Adjustment of the CCHC
Mould heating example
Heat loss by natural convection

 

 

Example of calculation
of power in order to heat a steel mould.


Technical data > Thermic calculations > Mould heating example > Calculation of the thermal balance

3. Necessary power to balance heat losses at the surface of the plates, of the mould and the frame of the press.
 

These losses are by :

  • conduction

  • convection

  • radiation  

    • Losses by conduction through the plates' insulated surfaces

    Dessin des pertes par conduction
    where :

    l       = coefficient of conduction of the rigid insulator
             = 0,65 W/m.K


    e        = insulator thickness
             = 0,015 m

    S       = total insulated surface ( upper and lower plates)
             = (0,25 x 0,4) x 2
             = 0,2 m²

    DT     = 180°C

    Losses by convection of the lateral surfaces of the mould and the plates:
    where :
    h.DT  = power dispersed per m² on an upright metallic surface multiplied by the DT (180°C).
             = 900W/m²

    S       = upright surface of the plates and the mould
             = (0,15 + 0,25) x 2 x 0,15 + (0,25 + 0,4) x 2 x 0,05 x2
             = 0,25 m²

     

    Losses by convection below the upper plate:

       

    Where :

    h.DT  = 850W/m²

    S       = (0,25 x 0,4) – (0,15 x 0,25)
             = 0,0625 m²

     

    Losses by convection above the lower plate :

      

    Where :

    h.DT  = 1650W/m²

    S       = 0,0625 m²

     

    Losses by radiation of every surface of the mould and the plates:

    Where:

    a        = coefficient of emission for hardening steel slightly oxidized
               = 0,7

    s        = constant of Stefan-Boltzmann
               = 5,675 . 10-8 W/m².K4


    K       = Kelvin = degree Celsius + 273

    T       = Temperature of the mould and the plates in K
             = 200 + 273
             = 473 K

    S       = total surface of the mould and of the radiant plates
             = 0,375 m²

     

    Total loss per hour:
    Conduction Fa = 1560 W
    Convection lateral side Fb = 225 W
    Convection lower side  Fc = 53 W
    Convection upper side Fd = 103 W
    Radiation Fe = 745 W
    Total loss F3 = 2686 W