Method, system and calendar for controlling the moisture profile and/or moisture gradient of a paper web, and a web
View Patent ↗The moisture profile and/or moisture gradient of a paper web of at least SC quality paper is controlled in a paper machine comprising a calender ( 1 ) which has at least two roll stacks ( 21, 22; 31, 32 ), one stack has at least three rolls and another has at least five rolls. The web is moisturized to a desired pre-moisture content M 1 by a pre-moisturizer ( 7 ) before the calender. At least one intermediate moisturizer ( 3 ) is arranged between two roll stacks moisturizes the web to a desired intermediate moisture content M 2 before the last roll stack ( 31, 32 ), in which the web is dried to a desired final moisture value M 3 . For continuously controlling and optimizing the moisture profile and/or moisture gradient of the web, the pre-moisturizing W 1 of the web is controlled by a pre-moisturizer ( 7 ) control parameter which corresponds to the final moisture value M 3 of the web.
1. A method for controlling the moisture gradient in production of an uncoated paper web of at least SC quality, having a cross direction, a running direction and a thickness direction, comprising the steps of:
pre-moisturizing, with a pre-moisturizer, the paper web in the cross direction substantially across the entire width of the web from an initial moisture content before pre-moisturizing to a selected pre-moisture content;
passing the pre-moisturized paper web through a multi-nip calender having at least a first roll stack and a last roll stack, each roll stack having at least three rolls, and wherein the multi-nip calender is situated before a slitter-winder of the web;
intermediate-moisturizing the web, with an intermediate-moisturizer, in the cross direction substantially across the entire width of the web before the last roll stack and after a first calendering nip of the first roll stack to a selected intermediate moisture content;
drying the web in the last roll stack to a selected final moisture value;
continuously controlling the moisture gradient of the web in the thickness-direction in the multi-nip calender by adjusting the pre-moisturizing of the web, based on a calculated or measured value of the selected final moisture value of the web.
2. The method of claim 1 , wherein the pre-moisturizer is controlled by the final moisture value of the web as measured after the last roll stack.
3. The method of claim 1 wherein the intermediate moisturizer is controlled by the final moisture value of the web as measured after the last roll stack.
4. The method of claim 1 wherein the pre-moisturizer or the intermediate moisturizer is controlled automatically.
5. The method of claim 1 wherein a final moisture value is calculated from the selected pre-moisture value of the web and from evaporation of moisture that has occurred in each roll stack; and the intermediate moisturizing of the web is carried out by the intermediate moisturizer of the web.
6. The method of claim 1 wherein evaporations from each roll and the additional or intermediate moisturizing of the web form a subtotal, and that said subtotal and the pre-moisture value of the web are passed as separate variables through a coupling means to serve as a control parameter of the pre-moisturizer.
7. The method of claim 1 wherein the final moisture value which has been either measured, or calculated in a coupling means, is passed by means of the coupling means to serve as a control parameter of the pre-moisturizer.