Abstract: A heater treater apparatus includes a vessel, a burner for heating a material contained in the vessel, an air intake passage for directing air to the burner, a flame arrester operatively associated with the air intake passage for preventing flames from the burner from extending past the flame arrester, and an air damper for controlling a flow rate of intake air drawn through the flame arrester. The air damper includes a fixed circular first plate having a first plurality of circumferentially spaced plate openings therethrough, and a similarly constructed rotatable circular second plate. The second plate may be rotated relative to the first plate from an open first position to a second position wherein the plate openings of the first plate are at least partially blocked by the second plate.