30 most of the dust gas 35 in the drum 20 flows toward the feeding chamber 29 when the drum 20 is under a positive pressure the discharge chamber 7 is also filled up with the dust gas 35 this dust gas 35 would leak out into the rear negative pressure dust collecting chamber 5 from the gaps between the sealing device 8 and the drum 20 and flow into the dust collecting apparatus 1 through the dust collecting port 4 and the dust collecting pipeline 2 under the negative pressure of the dust collecting apparatus 1 so as to be purified the drum 20 passes from the front mechanical room 23 to the rear mechanical room 18 the thermal energy from the surface of the drum 20 will heat up the front and rear mechanical rooms 23 and 18 so as to increase the temperature thereof in order to avoid the temperatures within the front and rear mechanical rooms 23 and 18 too high and affecting the normal operation of the supporting roller 14 and the drive motor 13 or the like a ventilation pipe 9 is installed within the front and rear mechanical rooms 23 and 18 an air inlet 16 of the ventilation pipe 19.