PURPOSE:To eliminate incompatibility between an operation condition of an exhuast fan and an operation number of a ventilator unit, by discriminating a number of ventilator units being operated during a fixed period of time and controlling a speed of the exhaust fan by an operation unit discriminator. CONSTITUTION:A smoke discharging device is constituted with a large number of ventilators to discharge gas into a common flue and an exhaust fan provided at the end of the common flue. Only the ventilator being operated during a fixed period of time is detected by a detector 6 and only a comparator circuit 15 relating to the detector is electrified. An output signal from each of the comparator circuits 15 is counted by a counter 16 which generates output corresponding to total pulse numbers from the comparator circuits 15. An operating number of units discrimination circuit 18 generates voltage in proportion to size of an input signal, through which speed setting of a speed controller 19 is varied and a speed of the exhaust fan is varied automatically. With this, the exhaust fan is operated properly according to a number of operation unit of the ventilator.
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