Method of drying a paper board or paper web

Abstract

A method for drying a fibre web such as a paper or paper board web, wherein air is first removed from the web and a dryer felt, whereafter they are led together between two parallel metal bands moving at the same speed as the web and the felt. For drying the web, the band on the side of the web is heated by means of a heating device before the band reaches the drying zone and, correspondingly, the band on the side of the dryer felt is cooled before the band reaches the drying zone and comes into contact with the dryer felt. In the method, for adjusting the mechanical Z compression exerted peripendicularly on the surface of the web, the temperature of the cooling band is adjusted so that the difference between the condensation pressure of the steam condensed on the surface of the band on the side of the dryer felt and the pressure acting on the outer surface of the bands is substantially equal to the desired mechanical Z compression. Further, the drying rate of the web is adjusted by adjusting the temperature of the heated band by means of the heating device in such a way that a desired amount of water can be evaporated from the web within the drying zone. Since the pressure loss created by the steam flowing through the dryer felt is substantially insignificant with respect to the condensing pressure of the steam condensing on the surface of the band, and the temperature difference between the heated band and the cooled band affects substantially this pressure loss only, these adjustments do not substantially affect each other and they can be carried out in desired order and adjusted suitably in view of the properties of the web to be dried.