A sifter for separating coarse particles from a particle-carrying stream during the manufacture of wood fiber panels has a housing forming a chamber. The housing also has a material inlet for admitting the particle-carrying stream to the chamber, a front wall formed with an upper air inlet below the material inlet for admitting a respective upper air stream to the chamber and a lower air inlet below the upper inlet for admitting a respective lower air stream to the chamber, an exhaust-air outlet for conveying air and fine particles from the chamber, and a coarse-particle outlet for conveying coarse particles out of the chamber. The front wall of the housing has between the upper air inlet and the lower air inlet a portion with an inwardly concavely curved shape that forms a support vortex that supports the upper air stream entering through the upper feed air inlet in the chamber of the housing between the upper air inlet and the lower air inlet.