Method for operating a sheet-forming unit, and sheet forming unit

Abstract

A sheet forming unit including a headbox with at least one feed device, a turbulence generating device having a plurality of turbulence generating channels and a nozzle. The nozzle includes an outlet gap located downstream from the headbox forming a line of impingement in a flow direction. Immediately upstream from the nozzle is the turbulence generating device in which during operation of the headbox a fibrous stock suspension passes through the plurality of turbulence generating channels in partial flows. Inside a turbulence generating channel there is a final fluidization region in which a pressure loss is produced in the partial flow of fibrous stock suspension in the turbulence generating channel. The headbox and the forming unit are arranged so that a dwell time of the fibrous stock suspension from the final fluidization region to the line of impingement on the clothing is ≧30 ms to ≦300 ms.

What is claimed is: 
     
         1 . A sheet forming unit ( 3 ) for a machine to produce fibrous webs, in particular paper, cardboard or tissue webs, the sheet forming unit comprising:
 a headbox with at least one feed device feeding at least one fibrous stock suspension thereto;   a turbulence generating device having a plurality of turbulence generating channels; and   a nozzle including an outlet gap for dispensing said at least one fibrous stock suspension in the form of a free jet onto a clothing of a forming unit ( 2 ) located downstream from said headbox forming a line of impingement as viewed in a flow direction, located immediately upstream from said nozzle is said turbulence generating device in which during operation of said headbox the at least one fibrous stock suspension is led through said plurality of turbulence generating channels in partial flows, whereby inside at least one of said turbulence generating channels there is a final fluidization region in which a pressure loss (Δp) is produced in the partial flow of fibrous stock suspension in said at least one turbulence generating channel, said headbox and said forming unit ( 2 ) are arranged so that a dwell time (T V ) of said fibrous stock suspension from said final fluidization region to said line of impingement on said clothing is ≧30 ms to ≦300 ms.   
     
     
         2 . The sheet forming unit ( 3 ) of  claim 1 , wherein said dwell time (T V ) is in a range of at least one of ≧50 ms to ≦200 ms and ≧80 ms to ≦200 ms. 
     
     
         3 . The sheet forming unit ( 3 ) of  claim 1 , wherein said nozzle has a length l D  in the range of 100 mm≦l D ≦500 mm, and a distance (l 1 ) between said final fluidization region within an individual of said turbulence generating channels of said turbulence generating device and an inlet into said nozzle of ≦180 mm. 
     
     
         4 . The sheet forming unit ( 3 ) of  claim 3 , wherein said length l D  is in a range of at least one of 100 mm≦l D ≦400 mm and 200 mm≦l D ≦400 mm. 
     
     
         5 . The sheet forming unit ( 3 ) of  claim 4 , wherein said distance (l 1 ) is one of at least one of ≦150 and ≦120 mm. 
     
     
         6 . The sheet forming unit ( 3 ) of  claim 1 , wherein said nozzle has a length (l D ) which under consideration of a stock consistency of said fibrous stock suspension which is led through said nozzle during operation meets the following requirement:
     l   D   ×SK≦ 1000   whereby   l D =length of the nozzle, measured in mm; and   SK=stock consistency in %.   
     
     
         7 . The sheet forming unit ( 3 ) of  claim 6 , wherein said nozzle length (l D ) meets at least one of the following requirements:
     l   D   ×SK≦ 700 and  l   D   ×SK≦ 800.   
     
     
         8 . The sheet forming unit ( 3 ) of  claim 1 , wherein said nozzle further includes a nozzle chamber and an outlet gap, said nozzle chamber being defined by two individual nozzle walls converging in the flow direction and thereby forming an outlet gap, an angle of convergence (α) between said two individual nozzle walls in an area of the outlet gap is between 5° and 45°. 
     
     
         9 . The sheet forming unit ( 3 ) of  claim 8 , wherein said angle of convergence (α) is between 10° and 20°. 
     
     
         10 . The sheet forming unit ( 3 ) of  claim 1 , wherein said turbulence generating device has a length (l TE ) viewed in the flow direction in the range of 100 mm≦l TE ≦500 mm. 
     
     
         11 . The sheet forming unit ( 3 ) of  claim 10 , wherein said length (l TE ) is in a range of at least one of 100 mm≦l TE ≦400 mm and 150 mm≦l TE ≦300 mm. 
     
     
         12 . The sheet forming unit ( 3 ) of  claim 1 , wherein said final fluidization region before an inlet into said nozzle is formed by a local graduated change of the cross sectional area of at least one of said plurality of turbulence generating channel viewed in the flow direction. 
     
     
         13 . The sheet forming unit ( 3 ) of  claim 1 , wherein said final fluidization region before an inlet into said nozzle is formed by a local constant change of the cross sectional area of at least one of said plurality of turbulence generating channels viewed in the flow direction.