| 摘要 |
<p>Fibrous suspension jet (4) velocity (VStrahl) is calculated from the measured pressure (PSTA) in the nozzle chamber (3). The dynamic pressure (PStau) of a jet (4.1) is measured directly after penetration of the screen (8). The pressure differential (pd) (across the screen) is calculated as (PSTA) - (PStau). Screen velocity (Vsieb) is calculated from a summation of measured head box pressure (PSTA) and calculated differential pressure (pd). Finally, the required velocity difference (delta v) between jet and screen is calculated by subtraction: (VStrahl) - (Vsieb). Preferred features: Dynamic pressure (PStau) is measured with a probe, especially a Pitot tube. This is located at the screen support surface (10) at a section (11) following dewatering, when initial dewatering yield is high. The measurement location is within the sheet-forming region, at an edge strip which is or becomes, separated. Dynamic pressure of the jet is measured by a Pitot tube especially, the location being outside the sheet formation region. The jet (4.1) is of water, especially clean-, screen- or clarified water. The dynamic pressure probe is back-flushed to prevent contamination and/or blocking. Backwashing can take place during and in addition to the pressure measurement. With a multilayer headbox, flow velocities of individual fibrous suspensions are calculated. A mean jet speed is calculated from the results, taking respective individual throughputs into consideration, especially by weighting. The mean is employed as a basis in the further calculations.</p> |
