Headbox, sheet forming unit with a headbox and method for operating a sheet forming unit

Abstract

A headbox includes a feed device feeding the fibrous material suspension, a nozzle, and a turbulence-generating device. The fibrous material suspension is guided in the turbulence-generating device in sub-flows through turbulence-generating channels. Within the individual turbulence-generating channel of the turbulence-generating device, at least one region forming a fluidization region is provided, in which a pressure loss can be generated in the sub-flow of the fibrous material suspension guided through said region. The nozzle and the turbulence-generating device are designed and dimensioned such that they are suitable for setting a dwell time of ≦200 ms of the fibrous material suspension flowing through the same from a final fluidization region of an individual turbulence-generating channel before inlet into the nozzle as far as the outlet gap of the nozzle, and a pressure loss of ≧50 mbar in the final fluidization region upstream of the inlet into the nozzle.

1 . A headbox for a machine for producing a web of fibrous material from at least one fibrous stock suspension, the web being one of a paper web, a cardboard web, and a tissue web, said headbox comprising:
 at least one feed device feeding the at least one fibrous stock suspension;   a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, said nozzle having a first inlet; and   a turbulence generating device arranged directly upstream of said nozzle in a flow direction, said turbulence generating device including a plurality of turbulence generating channels which are configured for guiding therethrough, during an operation of the headbox, the at least one fibrous stock suspension in a plurality of partial flows, within an individual one of said plurality of turbulence generating channels of said turbulence generating device at least one region representing a fluidization region is provided in which a pressure loss can be produced in a respective one of said plurality of partial flows of the at least one fibrous stock suspension being guided through said fluidization region, said nozzle and said turbulence generating device arranged directly upstream of said nozzle being designed and dimensioned such that said nozzle and said turbulence generating device are suitable for setting a dwell time of ≦200 ms of the at least one fibrous stock suspension flowing through said nozzle and said turbulence generating device from a final said fluidization region of said individual one of said plurality of turbulence generating channels of said turbulence generating device before said first inlet into said nozzle as far as said outlet gap of said nozzle, and for setting said pressure loss of ≧50 mbar in said final fluidization region before said first inlet into said nozzle.   
     
     
         2 . The headbox according to  claim 1 , wherein said dwell time is one of ≦175 ms and ≦150 ms, and said pressure loss is one of ≧75 mbar, ≧100 mbar, and ≧150 mbar. 
     
     
         3 . The headbox according to  claim 1 , wherein said nozzle has a length (l D ) in a range of 100 mm≦l D ≦500 mm, and a distance between said final fluidization region inside said individual one of said plurality of turbulence generating channels of said turbulence generating device and said first inlet into said nozzle is ≦180 mm. 
     
     
         4 . The headbox according to  claim 3 , wherein said length (l D ) is one of 100 mm≦l D ≦400 mm and 200 mm≦l D ≦400 mm, and said distance is one of ≦150 mm, ≦120 mm, and 100 mm. 
     
     
         5 . The headbox according to  claim 1 , wherein said nozzle has a length which, under consideration of a stock consistency of the at least one fibrous stock suspension which is guided through said nozzle, meets the following requirements: l D ×SK one of ≦1000, ≦800, and ≦700, wherein l D =said length of said nozzle, measured in mm, and SK=said stock consistency in %. 
     
     
         6 . The headbox according to  claim 1 , wherein said nozzle includes a nozzle chamber, a first nozzle wall, a second nozzle wall, and a region of said outlet gap, said first nozzle wall and said second nozzle wall converging relative to one another in said flow direction and forming said outlet gap, said nozzle chamber being limited by said first nozzle wall and said second nozzle wall, an angle of convergence between said first nozzle wall and said second nozzle wall in said region of said outlet gap being one of between 5° and 45° and between preferably 10° and 20°. 
     
     
         7 . The headbox according to  claim 1 , wherein said turbulence generating device has a length (l TE ) viewed in a flow-through direction in a range of one of 100 mm≦l TE ≦500 mm, 100 mm≦l TE ≦400 mm, and 150 mm≦l TE ≦300 mm. 
     
     
         8 . The headbox according to  claim 1 , wherein said final fluidization region before said first inlet into said nozzle is formed by a local graduated change of a cross sectional area of said individual one of said plurality of turbulence generating channels of said turbulence generating device, viewed in said flow direction. 
     
     
         9 . The headbox according to  claim 1 , wherein said final fluidization region before said first inlet into said nozzle is formed by a constant change of a cross sectional area of said individual one of said plurality of turbulence generating channels of said turbulence generating device, viewed in said flow direction. 
     
     
         10 . The headbox according to  claim 9 , wherein said constant change of said cross sectional area in said final fluidization region suits at least a medium fiber length of the at least one fibrous stock suspension which is used. 
     
     
         11 . The headbox according to  claim 10 , wherein a level of progression characterizing a cross sectional change in said final fluidization region suits at least said medium fiber length of the at least one fibrous stock suspension which is used. 
     
     
         12 . The headbox according to  claim 1 , wherein said individual one of said plurality of turbulence generating channels of said turbulence generating device is designed and dimensioned so that a maximum diameter (d hydr ) describing a cross sectional area is in a range of one of 5 mm≦d hydr ≦25 mm, 5 mm≦d hydr ≦20 mm, and 10 mm≦d hydr ≦20 mm. 
     
     
         13 . The headbox according to  claim 1 , wherein said individual one of said plurality of turbulence generating channels includes a second inlet, a hydraulic diameter (d hydr-8E ) describing a cross sectional area at said second inlet of said individual one of said plurality of turbulence generating channels of said turbulence generating device being in a range of one of 8 mm≦d hydr-8E ≦20 mm, 10 mm≦d hydr-8E ≦20 mm, and 10 mm≦d hydr-8E ≦15 mm. 
     
     
         14 . The headbox according to  claim 1 , wherein a number of rows of said plurality of turbulence generating channels of said turbulence generating device is such that a flow speed in a narrowest cross section of said individual one of said plurality of turbulence generating channels of said turbulence generating device is one of between 5 m/s and 20 m/s and between 7 m/s and 15 m/s. 
     
     
         15 . A sheet forming unit for a machine for producing a web of fibrous material from at least one fibrous stock suspension, the web being one of a paper web, a cardboard web, and a tissue web, said sheet forming unit comprising:
 a headbox including:
 at least one feed device feeding the at least one fibrous stock suspension; 
 a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, said nozzle having a first inlet; and 
 a turbulence generating device arranged directly upstream of said nozzle in a flow direction, said turbulence generating device including a plurality of turbulence generating channels which are configured for guiding therethrough, during an operation of the headbox, the at least one fibrous stock suspension in a plurality of partial flows, within an individual one of said plurality of turbulence generating channels of said turbulence generating device at least one region representing a fluidization region is provided in which a pressure loss can be produced in a respective one of said plurality of partial flows of the at least one fibrous stock suspension being guided through said fluidization region, said nozzle and said turbulence generating device arranged directly upstream of said nozzle being designed and dimensioned such that said nozzle and said turbulence generating device are suitable for setting a dwell time of ≦200 ms of the at least one fibrous stock suspension flowing through said nozzle and said turbulence generating device from a final said fluidization region of said individual one of said plurality of turbulence generating channels of said turbulence generating device before said first inlet into said nozzle as far as said outlet gap of said nozzle, and for setting said pressure loss of ≧50 mbar in said final fluidization region before said first inlet into said nozzle; and 
   a forming unit which is arranged downstream of said headbox and to which the at least one fibrous stock suspension is supplied from said outlet gap of said headbox in said free jet.   
     
     
         16 . The sheet forming unit according to  claim 15 , wherein said dwell time is one of ≦175 ms and ≦150 ms, and said pressure loss is one of ≧75 mbar, ≧100 mbar, and ≧150 mbar. 
     
     
         17 . The sheet forming unit according to  claim 15 , wherein said forming unit includes a clothing, the at least one fibrous stock suspension being deposited in said free jet onto said clothing of said forming unit creating a line of impingement, said headbox and said forming unit being designed and arranged so that a dwell time of the at least one fibrous stock suspension from said final fluidization region to said line of impingement on said clothing is one of ≧30 ms to ≦300 ms, ≧50 ms to ≦200 ms, and ≧80 ms to ≦200 ms. 
     
     
         18 . A method to operate a sheet forming unit for a machine for producing a web of fibrous material from at least one fibrous stock suspension, the web being one of a paper web, a cardboard web, and a tissue web, said method comprising the steps of:
 providing that the sheet forming unit includes:
 a headbox including:
 at least one feed device feeding the at least one fibrous stock suspension; 
 a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, said nozzle having a first inlet; and 
 a turbulence generating device arranged directly upstream of said nozzle in a flow direction, said turbulence generating device including a plurality of turbulence generating channels which are configured for guiding therethrough, during an operation of the headbox, the at least one fibrous stock suspension in a plurality of partial flows, within an individual one of said plurality of turbulence generating channels of said turbulence generating device at least one region representing a fluidization region is provided in which a pressure loss can be produced in a respective one of said plurality of partial flows of the at least one fibrous stock suspension being guided through said fluidization region, said nozzle and said turbulence generating device arranged directly upstream of said nozzle being designed and dimensioned such that said nozzle and said turbulence generating device are suitable for setting a dwell time of ≦200 ms of the at least one fibrous stock suspension flowing through said nozzle and said turbulence generating device from a final said fluidization region of said individual one of said plurality of turbulence generating channels of said turbulence generating device before said first inlet into said nozzle as far as said outlet gap of said nozzle, and for setting said pressure loss of ≧50 mbar in said final fluidization region before said first inlet into said nozzle; and 
 
 a forming unit which is arranged downstream of said headbox and to which the at least one fibrous stock suspension is supplied from said outlet gap of said headbox in said free jet; 
   feeding the at least one fibrous stock suspension of said headbox across a machine width;   guiding the at least one fibrous stock suspension of said headbox by forming said plurality of partial flows in said plurality of turbulence generating channels of said turbulence generating device;   feeding the at least one fibrous stock suspension of said headbox to said nozzle from where the at least one fibrous stock suspension is delivered in said free jet into said forming unit;   adjusting, inside said individual one of said plurality of turbulence generating channels of said turbulence generating device, said pressure loss in the at least one fibrous stock suspension;   producing, in said final fluidization region of said individual one of said turbulence generating channels of said turbulence generating device upstream from said inlet into said nozzle, said pressure loss inside the at least one fibrous stock suspension of ≧50 mbar; and   guiding the at least one fibrous stock suspension from said final fluidization region to said outlet gap of said nozzle so that at least one of:
 (a) said dwell time of the at least one fibrous stock suspension in a region of the sheet forming unit extending from said final fluidization region as far as said outlet gap is ≦200 ms, and 
 (b) a dwell time in a region of the sheet forming unit extending from said final fluidization region to a line of impingement is ≧30 ms to ≦300 ms, wherein the at least one fibrous stock suspension of said headbox is fed to said nozzle from where the at least one fibrous stock suspension is delivered in said free jet into said forming unit onto a clothing of said forming unit under definition of said line of impingement. 
   
     
     
         19 . The method according to  claim 18 , wherein said dwell time of the at least one fibrous stock suspension in a region of the sheet forming unit extending from said final fluidization region as far as said outlet gap is one of ≦175 ms and ≦150 ms, and said pressure loss is one of ≧75 mbar, ≧100 mbar, and ≧150 mbar. 
     
     
         20 . The method according to  claim 18 , wherein said dwell time in a region of the sheet forming unit extending from said final fluidization region to a line of impingement is one of ≧50 ms to ≦200 ms and ≧80 ms to ≦200 ms.