Method and apparatus for drying of fibrous webs

Abstract

A machine and process for production of wet formed fibrous webs, in which the web is formed from a suspension on a fabric and subjected to a final drying process such as a Yankee dryer roll, while further subjecting the Web to an intermediate drying on a high-heat-transfer roll, with or without hood, before the final drying process. The intermediate dryer roll has a solid, gas-impermeable surface, and a localized heat source directly heats the dryer to a higher temperature than in the final drying process.

1 . Machine for production of fibrous webs with a forming section in which the web is formed on a fabric, and with at least one rotatable dryer which is located after the forming section, wherein the improvement comprises that the dryer has a solid surface and at least one heat source to heat the dryer, provided on the inside and or outside of the dryer. 
   
   
       2 . Machine according to  claim 1 , wherein the dryer is wrapped by a fabric, felt, foraminous member or other belt which produces pressure on the web to overcome vapor pressure of escaping water vapor and hold the web in continuous contact with the dryer. 
   
   
       3 . Machine according to  claim 1 , wherein the dryer is wrapped by a fabric, felt, foraminous member or other belt which produces 700 to 14,000 Pa pressure on the Web to overcome vapor pressure of escaping water vapor. 
   
   
       4 . Machine according to  claim 1 , wherein the dryer has a steam collecting hood that extracts steam generated by the dryer, while minimizing mixing with ambient air in order to make the steam available for other uses with or without further processing. 
   
   
       5 . Machine according to  claim 1 , wherein the dryer comprises a cylinder having a diameter of 1 to 5 m. 
   
   
       6 . Machine according to  claim 1  wherein the dryer comprises a solid cylinder shell with a shell wall thickness of 2 to 80 mm. 
   
   
       7 . Machine according to  claim 6 , wherein the cylinder shell is made of a material with thermal conductivity greater than 25 W/(K*m). 
   
   
       8 . Machine according to  claim 6 , wherein the cylinder shell is made of carbon fiber, or a steel, cast iron, copper, or aluminium metal, or alloys containing at least one of said metals. 
   
   
       9 . Machine according to  claim 1  wherein the heat source is a gas fuel burner. 
   
   
       10 . Machine according to  claim 1 , wherein the heat source is an oil burner. 
   
   
       11 . Machine according to  claim 1 , wherein the surface of the dryer has non-stick coating. 
   
   
       12 . Machine according to  claim 1 , wherein the web has a wrap angle of at least 180 degrees around the dryer. 
   
   
       13 . Machine according to  claim 1 , wherein the web has a wrap angle of at least 270 degrees around the dryer. 
   
   
       14 . Machine according to  claim 1 , wherein the web has a wrap angle in the range of 300 to 330 degrees around the dryer. 
   
   
       15 . Process for production of a wet formed web, in which the web is formed from a suspension on a fabric and subjected to a final drying process such as a Yankee dryer roll, wherein the improvement comprises subjecting the web to an intermediate drying on a high-heat-transfer roll, with or without hood, before the final drying process. 
   
   
       16 . Process according to  claim 15 , wherein steam generated during intermediate drying is removed by a steam collecting hood that reduces air contamination. 
   
   
       17 . Process according to  claim 16 , wherein the steam in the steam collecting hood is fed to further processes for energy savings or external energy recovery equipment. 
   
   
       18 . Process according to  claim 16 , wherein the steam generated in the steam hood is used to heat a hood in a downstream dryer. 
   
   
       19 . Process according to  claim 17 , wherein the steam is delivered to a sectionalized profile controlling vacuum box or other vacuum box to improve effectiveness of the box. 
   
   
       20 . Process according to  claim 16 , wherein steam generated in the steam hood is used to heat a dryer cylinder of the final drying process. 
   
   
       21 . Process according to  claim 15 , wherein the exhaust gas of the burner is fed a drying hood of a dryer cylinder of the final drying process. 
   
   
       22 . Process according to  claim 15 , wherein the exhaust gas of the burner is fed to a through air dryer of the final drying process. 
   
   
       23 . Process according to  claim 15 , wherein the exhaust gas from the burner pre-heats fresh air for the burner. 
   
   
       24 . Process according to  claim 15 , wherein after forming, the web is dewatered and dried without mechanical pressing. 
   
   
       25 . Process according to  claim 15 , wherein the intermediate dryer has a surface temperature between 101° C. and 450° C. 
   
   
       26 . Process according to  claim 15 , wherein the intermediate dryer has a surface temperature between about 200 and 220° C. 
   
   
       27 . Process according to  claim 15 , wherein the intermediate drying is carried out with several intermediate dryers, each intermediate dryer having the web held against the respective heated surface by either the forming fabric or another fabric.