Abstract: A system for drying a wet web of fibrous material carried through a rotary dryer on an air pervious support, without damage to the rotary dryer or the air pervious support in the event of an upset to the system comprising in combination (1) heater means for producing hot air; (2) a dryer unit, (3) an air mixer or heat exchanger for providing hot air at the desired temperature to the input section of the dryer; (4) recycle means for carrying air exiting the air output section of the dryer unit to the air mixer or heat exchanger; (5) a first line for carrying the hot air from the heater means to the air mixer or heat exchanger; (6) a first normally open damper means on the first line; (7) a second line for venting hot air away from the system; and (8) a second normally closed damper means on the second line, wherein when an upset in the system occurs, the hot air produced by the heater means and normally carried to the air mixer or heat exchanger is vented away from the system by opening the second normally clo