Fluid Flow in degassing

Degassing can be carried out either by placing ladle containing molten steel under vacuum or by recirculation of molten steel in vacuum.

In ladle degassing, the effectiveness of degassing decreases from top to bottom of the molten steel bath. Bottom layers of steel are very much less affected by vacuum since these layers are under the influence of ferrostatic pressure due to column of liquid steel. Hence bath agitation would help exposing the entire content of molten steel to the vacuum.  Argon stirring is commonly employed during degassing. Argon bubbling during degassing of molten steel leads to massive volumetric expansion of bubbles due to temperature.( volume of gas becomes 6.3 times at 1873K). Moreover rising gas bubbles absorb dissolved gases. Radial expansion of gas bubble in vacuum processing impart to a radial motion to the surrounding fluid.

In recirculation degassing argon is also bubbled through porous plugs located at the bottom of the ladle. The speed of degassing increases with the increased rate of circulation (R) of liquid steel through the vacuum chamber. Typically R ranges in between 10t/min to 100t/min. Circulation velocity increases with an increasing argon gas flow rate.  

The circulation rate (R) can be determined by

(9)

                                                                 
Here R= circulation rate Kg/s
Q= argon injection rate .
 Pressure at the base of downleg
  Pressure in vacuum chamber
D= internal diameter of leg (m)
Material balance in degassing

Consider removal of hydrogen in recirculation degassing i.e. during circulation of molten steel in vacuum.