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The fire damper of this invention is to protect sensitive areas and equipment from fires which originate in the process of a plant. The invention, for example, may have utility in the protection of bag house filters and the like. It will, however, by way of example only, be largely described herein as applied to an electrostatic air cleaner located on a process stack. In recent years such electronic air cleaners, often in the form of a two-stage electrostatic precipitator, have become quite popular. Such apparatus, however, is quite sensitive to high temperatures such as are occasioned by fires and may readily be destroyed when exposed to them. In this invention a temperature rise is sensed by means of electrical detectors and relayed to the control box of the fire damper. Here it is translated into a control signal which actuates mechanism which will withdraw a latch means which supports a vent door whereafter such door falls by means of gravity to block the duct leading to the air cleaner while at the same time venting the plant process air and fire to outside atmosphere. The mechanism which actuates the latch means such as by pulling a steel shaft or pin may be either electrical or pneumatic. A satisfactory electrical mechanism is an electric solenoid that moves the latch or pin release. A satisfactory pneumatic release is one which operates by means of a pressurized gas, preferably a dry and inert gas like nitrogen where there is no risk of freezing or flamability, to operate a pneumatic cylinder-piston arrangement to withdraw the pin which supports the vent door. The vent door is so located and pivoted that, when the latch pin is pulled, it will immediately swiftly move into fire damping position by means of gravity. Fire and heat are then simultaneously vented vertically to atmosphere through that opening in the fire damper thus uncovered by the door in moving to its fire damping position, by virtue of which any possible flashback into the process, due to the failure of a relief damper or the like to open when the duct fire damper door is closed, is prevented.