24 ANDRITZ


Pulp Paper & Logistics


Figure 1: Dewatering physics of the Screw Press (SCP) 3


around the complete screen circumference is highly uniform (see Figure 2). Similar to the horizontal


press, hydrostatic pressure increases from top to bottom of the vertical unit. Unlike the horizontal design, the increased hydrostatic pressure actually improves the dewatering effect, since higher pressure is required as the pulp suspension’s consistency increases down the length of the screw. The hydrostatic pressure is enhanced by the design of the screw shaft with lower volume at the end. This creates additional dewatering forces and results in extremely uniform dewatering around the complete circumference of the screen baskets over the complete length of the machine.


July/August 2018


Elimination of bending forces For screw presses, it is important to maintain a very small gap


between the screw flights and the screen baskets in order to achieve high dewatering efficiency. Mechanically speaking, the ‘dead weight’ of the screw shaft itself plus uneven dewatering forces on the shaft of a horizontal SCP create bending forces and possibly deflection of the shaft. Mechanical designers make


every effort to reduce the deflection to a minimum;


however, the deflection can be significant for longer screw presses (up to 10-metre shaft lengths). This deflection makes it difficult to maintain a small and uniform gap between the screw flights and screen baskets over the length of the shaft. In the vertical configuration, the dead weight of the screw shaft has no impact and the screw operates with symmetrical axial forces. The opportunity for


Figure 3: Increased throughput with hardwood kraft pulp (VST compared to SCP)


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