Occasionally we are asked what the pressure is on a material as it is being pressed inside a screw press. A variation on the question has to do with the compression ratio designed into a press.

The answer to the question is roundabout. Rather than design our screw presses for a compression ratio, we design them for torque. This is determined by horsepower of the drive motor and the output speed of the reducer gearbox. This torque that is designed into the press establishes the maximum burst forces that will be seen by the screen and the maximum shear forces that will be experienced by the screw shaft and its flights.

Usually these questions about compression come from people that are experienced with plastic extruders. Both a plastic molding machine and a Vincent dewatering screw press have superficial similarities. Both have motors and gearboxes driving horizontal screws which, by their design, compress material that is fed into the machine.

A key difference is that a dewatering press has the screw surrounded by a screen through which liquid is expelled. In contrast, the screw of an extrusion press is surrounded by a solid sleeve, probably heated, which confines material within the press. More fundamentally, an injection press is fed solid plastic pellets and it discharges molten plastic, while a dewatering press is fed wet material that is separated into a flow of press liquor and press cake.

A comparison of two types of material illustrates the fallacy of thinking in terms of a compression ratio. In one case, slippery and slimy materials fed into a dewatering press blind the screen and simply co-rotate with the screw. The material can be observed to circulate within the inlet hopper of the press without much, if any, feeding through the press. No cake comes from the discharge, no liquid comes through the screen, and the pressure exerted by the press is near zero.

In contrast, this very same screw press exhibits radically different characteristics when fed material such as either paper fiber and water, or properly limed and reacted orange peel. The pressure on the material in the press is obviously great: liquid is forcefully expelled through the screen and horsepower drawn by the motor increases significantly.

In summary, the compression of a dewatering press is not so much a mechanical design ratio as it is a function of the physical characteristics of the material being pressed.

Another observation is that the area on the outside of the screen of a dewatering press is open to the atmosphere. The only way there can be pressure inside the press is if the holes in the screen are blinded off. Since liquid is coming through the screen holes, they are not blinded, so the pressure must be close to atmospheric, at least at the holes in the screen. Clearly a pressure gradient, rather than a specific pressure, exists within the press.