Gas extraction system for fume cupboards - has movable damper in duct whose position is changed in accordance with door position sensor signal

摘要

Each fume cupboard (2) is closed by a door (4) and connected by a respective duct (5) and a common manifold (3) to a common air extraction fan (24). Within each duct (5) there is a rotatable damper (6) connected to an electric motor which incorporates a potentiometer. Incorporated in a sensor member adjacent the door are several magnetically actuated switches each associated with a respective resistance of a resistance ladder (10) and with a magnet which normally holds the switch closed. As the door is opened a plate secured to it progressively comes between the switches and their associated magnets thus permitting the switches to open and increasing the resistance of the resistance ladder (10). This resistance, which is indicative of the position of the associated door (4), is connected to an amplifier which moves the electric motor and thus the damper (6). The outputs of the potentiometers are summed by a summing amplifier (26) whose output varies the speed of the fan (24).