These valves made of aluminium/mild steel sheet spinning, are best applied to air exhaust specially for low air flow movements in circular ductwork. The adjustable central disc can be moved in and out to result in adjustment of air flow passage area thus the air flow rate can be varied according to the requirement. The valves provide high initial resistance with wide throttling capability which offers to offset the relatively high system resistance. Can be easily installed on sub frame fixed to the wall opening which also can be directly connected to the circular ductwork.