Determination of Exhausting Speed in Vacuum Drying Tank of Electric Power Cable

Oil impregnated kraft paper is exclusively used for the insulation of high tension electric cable.In this case, for the sake of electrical properties high dryness is required so that vacuum drying process is adopted for this purpose to minimize the processing period and deterioration of insulating material.We briefly described here a theory to determine the exhausting speed of vacuum pump system.The exhausting speedG (l /mm) is given as follows;G=π/S0 (1 + d/Δ) L T1/T2 where S0 represents specific resistance of moisture diffusion in paper layer (min·cm/l), d : core diameter (cm), Δ : insulation thickness (cm), L : cable length (cm).T1 means the theoretical drying time, when the exhausting speed is supposed infinite, and it depends only upon insulation thickness when the initial and final moisture contents are given. Time T2 depends on the exhausting speed and total amount of paper to be dried. Vacuum drying time T is represented as ;T= T1+ T2In calculating the value of G, it is recommended to select T2/T1 epual to about 0.1.The vacuum pump system of the cable drying equipment should have sufficient exhausting speed above the pressures of 2-10 ×10-2 mmHg, and have to be designed so as to be able to treat a large quantity of water without trouble.