June 18, 2004
ISSUE #150

The function of a screw press is to separate liquids from solids by expelling the liquids through a screen that surrounds the compression screw. It takes pressure to make the fluid flow through the holes or slots in the screen. As screw presses have evolved, a number of mechanisms have been found useful in causing this separating action.

Compression can be achieved by gradually increasing the inner shaft diameter of the screw. This forces the material out against the screen so that liquid is expelled through the screen. For example, if a 16″ screw press has a 6″ shaft in the inlet, the flights will be 5″ tall. If this 6″ shaft diameter is increased to 12″ at the discharge, the flights will be only 2″ tall. The result is that the material will have been pressed from a 5″ opening down to a 2″ space.

Alternatively, compression can be achieved by tightening the pitch of the flighting. That is, if the flighting has a 16″ pitch at the inlet, material will move 16″ with each revolution of the screw. If the pitch is reduced to 8″ closer to the discharge, this same material moves only 8″ per revolution. Thus, with each turn off the screw there is more material being forced into the press than there is being removed. The consequence is compressive forces, which tend to push liquid through the screen.

A third way to achieve compression is to install a cone at the discharge. This cone is also referred to as a choke, stopper or door. In many screw press designs it is bolted into a fixed position, creating a fixed discharge orifice through with the press cake must pass. More commonly, the cone is pushed into the discharge opening by either an air or hydraulic cylinder. This creates a floating cone that exerts constant pressure against the solid cake that is being pushed from the press by the screw. The greater the air pressure, the greater the back pressure in the press, and the greater the dewatering that occurs.

Another mechanism for forcing liquid through the screen of a press is to force-feed the press with a Supercharger screw or a positive displacement pump. This creates a pressure differential that pushes liquid through the screen. The technique has limited applications because in many conditions the pressure simply plasters solids against the screen of the press, forming an impenetrable mat layer. Capacity can go down instead of up.

Similarly, differential pressure across the screen can be achieved by operating with a vacuum around the outside of the screen. This vacuum can draw high volumes of liquid through the screen. While successful in certain applications, the same problem of screen blinding due to a mat layer of fiber on the screen is possible.