Damper control for preventing spread of fire and smoke through an induction mixing box

Claim 1

A damper control for preventing spread of fire into a controlled space through an induction mixing box wherein a flow of primary air through said box induces a flow of secondary air into and through the box, said flow of secondary air into the box being controlled by a normally closed secondary air damper variably opened by a pneumatic actuator in response to the variable effective pressure of branch air therein, a temperature sensor in said controlled space, and means for admitting the branch air under pressure into said actuator as a function of the sensed temperature in said controlled space, the damper control comprising a normally closed exhaust valve in communication with said actuator, a port in said valve sized to permit escape of the branch air therethrough in sufficient volume to remove all effective pressure of the branch air from said actuator, and a thermostatic element positioned to sense the temperature of said secondary air upstream from said damper and to open said valve upon sensing a predetermined excessive temperature, thereby removing the effective pressure of the branch air from the actuator, and thus permitting the secondary air damper to close and preventing flow of secondary air into said box.