Experimental Results with Vacuum Drying of Thick Soft Paper Insulation

The objective of the work reported in this paper was to study the influence of the thickness of the paper mass and of the inner temperature distribution on the vacuum drying process; particularly to study the ways in which these two factors affect the quantitative distribution of residual moisture perpendicular to the layers of the paper. The quantitative distributions of residual moisture perpendicular to the layers in soft paper masses 0.3 to 2.4 cm thick were determined. The results show that both the magnitude and distribution of temperature throughout the paper affect the degree of drying. Neither a low vacuum nor a prolonged drying time will compensate for an unfavorable temperature distribution. The magnitude and distribution of temperature predominate over the effect of paper thickness. With unfavorable temperature conditions, paper only 0.6 cm thick will retain an excessive amount of moisture. With favorable temperature conditions, paper 2.4 cm thick can be dried under vacuum to an acceptable moisture content. The upper limit of residual moisture content for transformer insulating paper that can reasonably be expected to be achieved is about 0.5 percent, not 0.1 percent or less.