A housing for a centrifugal fan includes a surrounding wall extending upwardly from a periphery of a bottom wall with an air inlet, and including a surrounding wall body defining an opening, a protruding wall body extending outwardly from the surrounding wall body, corresponding to the opening, and having a dust-discharging hole, and a shielding plate disposed on the protruding wall body. The invention includes a process of fermenting starch-containing material into a fermentation product comprising a fermentation step wherein one or more enzymes are added before or during the fermentation step. A first apparatus is mounted within the primary duct imparts a swirling motion to the gas which enters through the inlet end of the duct. A first agent is deployed to the first target. The first fin structure has a first primary surface, a second primary surface opposite the first primary surface, and a plurality of edges between the first and the second primary surfaces including a first edge and a second edge opposite the first edge.