Batch Mode Screw Press Operation
November 9, 2006 ISSUE #180
It was in 2002 that Larry Hess, our sales representative, salvaged a Hewlett-Packard job by converting the screw press to batch operation. The application was to separate ink from shredded toner cartridge foam. The ink could be pressed from the flow, but the press cake would jam at the discharge cone. This was corrected by programming a PLC to open the cone momentarily every few minutes.
We attempted to use this same technique in dewatering carrot sludge from a brush peeler. Without hydrated lime, this material is even more difficult to dewater than potato peel. It was observed that when sludge was first admitted to the press, press liquor would flow readily through the screen. However, after a few minutes, the flow would stop. During this period no press cake would discharge past the cone.
The problem was that this waste could not be thickened, in the press, to the point where it was firm enough to push the cone open. Nevertheless, a well-dewatered cake was being formed in the press. The obvious solution was to put a cycle timer on the air pressure lines to the air cylinder. The timer was set to keep the cone shut for a few minutes, and then to open the cone for a few seconds. These time periods were arrived at by simple testing, manually opening and closing the cone. Unfortunately, the operation was too erratic to be commercially acceptable.
Nevertheless, the process has proved workable in other applications, particularly with pressing fish and shrimp waste. The controls have turned out to be very simple. A 110 volt timer and solenoid valve are all it takes. The timer actuates the solenoid. One output port of the solenoid valve goes to the closed side of the press' air cylinder, while the other port goes to the open side of the air cylinder.
Alternatively, a paper mill customer has installed a level switch in the inlet hopper of the screw press. When the hopper fills to a certain level, the cone is opened and shut in order to dump a load of press cake.
In yet another application, the screw presses tended to jam during periods of intermittent flow. This led to burst screens and folded flights. A solution was found in using a timer to open the cone, unloading the press, on a periodic basis.