Siemens 331-2974 DAMPER ACTUATOR, PNEU NO 4, 2-3/8" STROKE, UNIT VENTILATOR HESITATION MODELSiemens 331 2974 Damper Actuator, Pneu No 4, 2 3 8" Stroke, Unit Ventilator Hesitation Model This 331 Number 4 Pneumatic Damper Actuator has a 2 3 8" stroke for unit ventilators and is a rugged, metal fabricated device that provides gradual or positive actuation of HVAC and fire smoke dampers. This hesitation model actuator has a nominal spring range of 2 to 3 and 8 to 13 psi, is fixed bracket mounted and provides minimum ventilation without separate