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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. It is mainly used in heat source industry which has no strict requirement for the product appearance. 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, then the hot air enters the rotary drum.