Pulp Paper & Logistics


ANDRITZ 23 Taking it vertical


Often, the simplest ideas are the best. After evaluating several different design concepts to address these two limitations of the SCP in low feed consistency applications, the solution turned out to be a quite simple one: turn a horizontal SCP on its end and feed it from the top. In a vertical configuration, gravity works in a positive way: evenly distributing the pulp suspension across the full circumference of the dewatering screen and ensuring 100 per cent filling. In a VST, the pulp suspension is


fed into the top of the machine and gravity transports the pulp downwards. The risk of plugging the inlet screen at low feed consistency applications is almost eliminated. Similar to the SCP, the VST has a conical shaft and decreasing pitch so the pulp is compressed and dewatered as it moves downward. The pulp suspension is automatically and consistently refilled into the area between screw flights. The rotating shaft moves the pressed cake downwards to the outlet casing.


A pneumatically-controlled


VST in the Andritz pilot plant in Austria This pressure creates uneven


filtrate flows around the screen circumference. There is lower filtrate flow at the top of the press where the hydrostatic pressure is low and a higher flow at the bottom of the screen basket where the hydrostatic pressure is highest. This pressure profile is consistent


over the length of the dewatering screw. Particularly in applications with low feed consistencies, the hydrostatic pressure can already


be too high at the beginning of the dewatering process. The speed of the filtrate flow through the screen basket is at such a level that fibres can be dragged into the holes and build up a layer on the surface of the screen – reducing dewatering efficiency. Another limitation of a


horizontal design is the difficulty in obtaining uniform filling, and thus uniform dewatering, through the three dewatering zones (low-, medium- and high-pressure)


and over the full length of the screw. An uneven filling degree means that the available screen area is not fully utilised and that no dewatering force is applied to some of the pulp suspension and the dewatering efficiency is reduced. When observing an SCP during operation, areas along the screw shaft with good dewatering (where a lot of filtrate is squeezed out) and areas where less dewatering takes place are visible.


counter-pressure ring builds up the pulp plug quickly after startup and releases the pulp when the proper consistency is achieved. The ring pressure can be adjusted to fine-tune discharge dryness or can be quickly unloaded to avoid plugging. The result is that higher throughput can be achieved with the VST.


The importance of 100 percent filling The VST utilises gravity to achieve 100 per cent filling degree (100 per cent of the screen area is used for dewatering). From top to bottom the filtrate flow


4 July/August 2018


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