The invention provides an apparatus for adjusting the flow of a fluid within a conduit, comprising a slide plate with a flat body and a fixed plate with a flat body. The slide plate and fixed plate have openings on their bodies. At a fully closed position, the openings of the slide plate do not substantially overlap with the openings of the fixed plate. By rotation of an actuator shaft and drive pin disposed thereon, the slide plate may move relative to the fixed plate, whereby the openings of the slide plate begin overlapping with the opening of the fixed plate, thereby allowing flow of fluid through the openings. Sequentially or independently from the movement of the slide plate, rotation of the actuator shaft may cause the fixed plate to rotate relative to the walls of the conduit, whereby flow of fluid may pass through the opening between the conduit and fixed plate assembly.