Abstract: In a valve assembly, a main frame unit has an inner space defined by opposite side walls connected respectively to lateral walls of adjacent chambers, front and rear walls connected between the side walls, and top and bottom walls disposed on opposing top and bottom ends of the front and rear walls. Each side wall is formed with an access hole for passage of a workpiece into and out of the inner space. The rear wall is formed with a through hole and is mounted detachably with a cover plate for closing the through hole. A valve unit is disposed in the inner space and is movable so as to close the access hole in one of the side walls. A drive unit has a rotary axle disposed rotatably in the inner space and coupled to the valve unit to control closing movement of the valve unit.