Device and process for production of a fibrous web

Abstract

A machine and process for production of a fibrous web, with a forming section ( 2 ) in which the web is formed from a pulp suspension on a fabric ( 3 ) and with a final drying unit ( 4 ). In at least one further drying step, a rotatable dryer ( 5 ) with a hood ( 6 ) is located after the forming section ( 2 ) and before the final drying unit ( 4 ). The additional dryer ( 5 ) has a gas-impermeable exterior surface ( 7 ) which is directly heated by a combustion unit such a gas or oil fired burner ( 9 ).

1 . Machine for production of a fibrous web, comprising: a forming section in which the web is formed from a pulp suspension on a fabric; a final drying unit; and at least one intermediate dryer located after the forming section and before the final drying unit; wherein the intermediate dryer has a rotating, gas-impermeable external drying surface contacting the web, which is overlaid with a fabric such that the web and fabric co-rotate with the drying surface; and a combustion unit that directly heats the drying surface. 
   
   
       2 . Machine according to  claim 1 , wherein the drying surface is on the exterior of a rotating cylinder and the combustion unit is a burner located inside of the cylinder. 
   
   
       3 . Machine according to  claim 1 , wherein the final drying unit after the intermediate dryer is a Yankee. 
   
   
       4 . Machine according to  claim 1 , wherein the final drying unit after the intermediate dryer is a through-air dryer. 
   
   
       5 . Machine according to  claim 1 , wherein the final drying unit after the intermediate dryer is a through-air dryer and a Yankee. 
   
   
       6 . Machine according to  claim 1 , wherein the overlaid fabric produces sufficient pressure on the web to hold the web in continuous contact with the dryer surface and overcome vapor pressure of water escaping the web as the web is dried. 
   
   
       7 . Machine according to  claim 1 , wherein the overlaid fabric produces 700 to 14,000 Pa pressure on the web against the dryer surface. 
   
   
       8 . Machine according to  claim 1 , wherein the hood is a steam collecting hood. 
   
   
       9 . Machine according to  claim 1 , wherein the intermediate dryer is a cylinder defining the dryer surface with a diameter of 1 to 5 m. 
   
   
       10 . Machine according to  claim 1 , wherein the intermediate dryer has a cylinder shell defining the dryer surface and the shell has a wall thickness of 5 to 80 mm. 
   
   
       11 . Machine according to  claim 10 , wherein the cylinder shell is made of a material with thermal conductivity equal or greater than 25 W/(K*m). 
   
   
       12 . Machine according to  claim 10 , wherein the cylinder shell is made of steel, cast iron, copper, or aluminium metals, or alloys containing at least one of said metals. 
   
   
       13 . Machine according to one of  claim 1 , wherein the combustion unit is a gas-fuelled burner. 
   
   
       14 . Machine according to  claim 1 , wherein the combustion unit is an oil burner. 
   
   
       15 . Machine according to  claim 1 , wherein the dryer surface has a non-stick coating. 
   
   
       16 . Machine according to  claim 1 , wherein the dryer surface is cylindrical and the web overlays the dryer surface with a wrap angle of between about 270° and 330°. 
   
   
       17 . Process for production of tissue, in which a fibrous web is formed from a pulp suspension on a fabric in a forming section and this web is then dried on a final drying unit, with an additional, intermediate drying step for the web being implemented before the web reaches the final drying unit, wherein the improvement comprises that the additional drying step is performed with the web carried on a moving, imperforate drying surface that is directly heated by a combustion unit. 
   
   
       18 . Process according to  claim 17 , wherein steam generated during the additional drying is removed with a steam collecting hood over the drying surface. 
   
   
       19 . Process according to  claim 18 , wherein steam collected in the steam collecting hood is fed to further processes. 
   
   
       20 . Process according to  claim 19 , wherein steam generated in the steam collecting hood heats a Yankee hood. 
   
   
       21 . Process according to  claim 19 , wherein the steam generated in the steam collecting hood heat a Yankee. 
   
   
       22 . Process according to  claim 17 , wherein exhaust gas from the combustion unit pre-heats fresh air for the combustion unit. 
   
   
       23 . Process according to  claim 17 , wherein after forming, the web is dewatered and dried without mechanical pressing. 
   
   
       24 . Process according to  claim 17 , wherein the drying surface is at a temperature between about 150° C. and 400° C. 
   
   
       25 . Process according to  claim 17 , wherein the additional drying step is carried out with a plurality of additional dryers.