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Frequently, there is a need to adjust the percentage of solids in press cake from a screw press. This is done by changing the air pressure on the cake discharge cone of the press. The higher the air pressure, the drier the cake. For example, at maximum air pressure (60 to 100 psi) some specific material in a paper mill may produce cake with 40% to 50% solids. This controls well down to where, with a minimal air pressure of 5 to 10 psi, the press will make cake with 25% to 30% solids.
To get the solids lower than this, it is necessary to actuate the air cylinder so that it withdraws entirely from the cake discharge end of the press. Thus the screw press starts to work like an internally fed rotary drum screen. With close flow control, the press can be made to make “cake” with 10% or 20% solids. However, there is a tendency for the press to purge, with negligible dewatering. The same thing happens with a rotary drum screen if the flow into that screen is more than it can take.
With a given screw press, it may be difficult to control solids content of the press cake at either extreme. For this reason Vincent offers the Series CP and VP presses for high solids cake and the Series KP presses for lower solids content. The differences are achieved mostly by changing the screw shaft configuration, flight pitch changes, the number of stages of compression, and L/D ratio of the screen length to screw diameter.
Another common way to increase the solids content of the press cake is to reduce the speed (rpm) of the screw. This allows more residence time for the material in the press, allowing more liquid to drain out.
It should be kept in mind that there are adverse effects of increasing the solids content of press cake. At higher cone pressures, the amount of suspended solids that are driven through the screen into the press liquor will increase. At the same time, high cone air pressures tend to reduce the overall throughput capacity of the press. Similarly, the amps drawn by the press drive motor will increase as the air pressure is increased. Also, dry press cake is more abrasive than wet cake, which will increase the frequency at which the screw must be rebuilt.
Issue 183