The new method of designing duct of balanced air suction by means of computer is suggested on the basis of analysing constant and decreasing velocities in the main collecting points.

The plurality of suction channels permits effective pick-up of dirt, which may be further enhanced by an air duct in the form of a slot (48), which draws air over some of the working edges (42, 44).

A cyclone flow generates in the blower duct (20) by the first air inflow hole (31) and the suction force of the blower box.