Formation of "Small Cooling Fan" rotating stall: fan impeller structure, dimensions are in accordance with the rated air flow (CFM) designed, when the normal air flow (CFM) work, the gas enters the impeller in the direction of air flow blade inlet Mounting Angle consistent, smooth gas may enter the impeller, when the "Small Cooling Fan" into the impeller of the gas flow is less than the rated flow, the radial velocity of the gas into the impeller is reduced, the gas flow into the direction of the relative angular velocity of the impeller is reduced, thus the blade inlet angle is not consistent with the installation. Leaves "Small Cooling Fan" fan in the processing and installation process, due to various reasons, the blades can not have exactly the same shape and mounting angles.