The fan housing has a generally circular outer peripheral contour and is provided with a plurality of substantially identical equally spaced radial projections and has at each axial end of the housing a boundary surface and including a tubular part extending in the axial direction of the fan housing intermediate the two aforementioned boundary surfaces. A drive motor is centrally mounted by radial spokes at one axial end of the fan. A fan wheel driven by the motor at least partly surrounds the motor and is provided with fan blades accommodated in an annular flow channel defined intermediate the tubular part of the fan housing and the drive motor. The fan blades extend substantially all the way to the other axial end of the tubular part of the fan housing. The tubular part at its middle has a circular interior cross-section closely surrounding the outer peripheral boundary surface of the fan wheel. The radial projections are located at at least one of the two axial ends of the fan, and the tubular part merges into them. The inner wall of the tubular part in the vicinity of the radial projections is set back radially outward from the outer peripheral boundary surface of the fan blades but is confined within the outer boundary surface of the fan housing. Stationary flow-guiding elements are provided on the set back parts of the tubular part of the fan housing.

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