SCR (Selective Catalytic Reduction) dilution fan capable of being quickly started

The SCR dilution fan comprises a fan shaft, an air inlet and an impeller arranged at one end of the fan shaft in a sleeving mode, an annular air inlet cavity is formed in the end, away from the impeller, of the fan shaft in a surrounding mode, the air inlet is formed in the upper side of the annular air inlet cavity, and an opening of the air inlet faces upwards; an annular flow collecting channel allowing air to pass through is arranged between the air inlet and the impeller, the tail end of the annular flow collecting channel and the impeller form an exhaust channel, a sealing air inlet isolation channel extending in the axial direction of the fan shaft is arranged between the annular flow collecting channel and the fan shaft and between the annular air inlet cavity and the fan shaft, and the sealing air inlet isolation channel is filled with sealing air. And the sealing gas flows from one end far away from the impeller to one end close to the impeller. And turning is not needed before starting, the fan sha