The utility model provides a control device used for controlling the pressure difference between a steam inlet of a screen mesh dryer and an outlet pipe line, and the utility model comprises an outlet valve which selectively controls the sweeping steam, the condensing fluid and the non condensing gas flown out from the dryer. A speed sensor of the dryer generates a first control signal, and a condensation rate sensor senses the accumulation rate of a condensing fluid layer to generate a second control signal. The controller can operationally be connected to an outlet valve driving mechanism and selectively stimulate the driving mechanism. The controller compares the signals and then determines the relatively best setting of the outlet valve, thus preventing the dryer overflowing due to the condensing fluid, while the pressure difference between the inlet and outlet pipe lines are maintained as low as possible. |