The vacuum level in the dry suction boxes is higher than in the wet ones. In most cases the levels are between 10 and 30 kPa (1 −3 metres columns of water).The water can be removed only at a limited rate. If the suction increases too much, the wire is pressed harder against the blades of the suction box, and the friction becomes greater. The increased friction causes the wire and the blades of the suction box to get unnecessarily worn and the force moving the wire strongly increases.
The blades of the suction boxes are quite plane. Now the fibres are totally fixed and it is not necessary to create turbulence.
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