A centrifugal separator, especially disc stack separator for separation of a suspension or mixture, has a feeding unit with an inlet and a stationary pipe. The pipe extends longitudinally from the top of the centrifugal separator through a stack of filter discs and has an opening directed to the accelerator disc a separator bowl of the centrifugal separator. The feed pipe is corrugated with spiral grooves running in the longitudinal direction of the pipe. The suspension flow is directed in the direction of rotation of the separator bowl and a soft infeed of the suspension to the accelerator disc and bowl without mechanically destroying particles or molecules of the suspension or mixture is achieved.