The directly heated air furnace applies the principle that the natural wind is preheated firstly around the furnace wall before entering the furnace chamber, works under the negative pressure of the fan, to get the effect of complete coal combustion and reducing energy consumption. Under the negative pressure of fans, the outside wind enters the furnace chamber from the top of the furnace and is preheated there before entering the hearth, and then the hot air enters the rotary drum dryer. The fuel can be coal, wood an so on. this air furnace is wlways matched with all kinds of rotary dryer line,such as brewer's grains rotary dryer line, coal slime rotary dryer line, bean dregs rotary dryer line, coal sludge rotary dryer and so on.