to the classifying chamber by the air flow from suction fan. The high-speed rotation of classifier wheel would create a strong centrifugal force to separate coarse powder and fine powder. Qualified fine powder would go through the wheel vane then into the cyclone separator or dust collector, while mixture of coarse powder and fine powder would lose its speed and fall down along the house wall to the secondary air inlet. The secondary air flow would disperse it further and separate coarse and fine powder. Fine powder would go up to the classifying chamber for further classifying, while coarse powder would fall down and go out from the bottom outlet.