Temperature gradients and heat flux measurements in hot pressing of paper

Impulse drying was simulated with a platen press equipped with a heated pressing head. The heat transferred to the wet paper sheet and the temperature gradient achieved in the thickness direction of the sheet during the press pulse were measured. The influence of the basis weight, the applied pressure, and the hot surface temperature on the temperature gradient and the total amount of heat transferred was investigated. It was found that at or below 0.5 MPa peak pressure, the wet paper sheet temperature never reaches temperatures significantly above 100 degrees C even if the applied hot surface temperature is as high as 300 degrees C. This is because almost all the load in this case is carried by the fiber network, leading to hydraulic pressures close to atmospheric. At higher press loads only the part of the wet paper sheet closest to the hot surface was heated to temperatures above 100 degrees C. The peak heat fluxes were in the range 2-8 MW/m(2), with the time-average heat flux being 0.8-3 MW/m(2) at surface temperatures ranging from 100 to 300 degrees C. The heat transfer increases with increased pressure at pressures below 3 MPa and surface temperatures above 150 degrees C, but at higher press loads or lower surface temperatures the applied pressure has a very minor influence on the heat transferred to the sheet. The basis weight was not seen to have any influence on the heat transferred.