A labyrinth seal for a bypass discharge-type motor-fan unit includes a motor unit from which extends a rotating shaft that passes through an aperture in an end plate to drive a fan unit. Disposed within the aperture is a labyrinth seal configured to prevent liquid entrained in a working airflow from entering the motor unit. The rotating shaft also includes a vacuum bore, such that the negative pressure generated by the operation of the fan unit draws any liquid out of an evacuation zone maintained between the motor unit and the end plate. As such, the components of the motor unit are protected from being exposed to the liquid, thereby extending the operating life motor-fan unit.