Abstract: Provided is an exhaust unit capable of preventing large pressure fluctuations within a process chamber due to atmospheric pressure changes. The exhaust unit includes a main exhaust duct and a supplemental exhaust duct that acts as a partial bypass. A flap is located at a downstream opening between the main exhaust duct and supplemental exhaust duct and controls the amount of bypassed gas flowing from the supplemental exhaust duct to the main exhaust duct. First and second plates of the flap are pivotally coupled to the main exhaust duct adjacent the downstream opening, the first plate colliding with gas flowing through the main exhaust duct and the second plate partially blocking bypassed gas flowing back into the main exhaust duct from the supplemental exhaust duct. When gas is exhausted through the main exhaust line and the supplemental exhaust duct, the flap passively controls the amount by which the supplemental exhaust duct is opened through fluctuations in atmospheric pressure.