March 15, 2023

There are two fundamental items which apply to screw presses:

#1 The press should be run at the lowest possible screw rpm which will not cause overfilling of the inlet hopper. Operating at low rpm maximizes the residence time in the press, giving the material being pressed the most time to drain its liquid.

#2 The objective is to keep the press always completely full of material. Liquid is expelled by the press because the screw squeezes material against itself. If the press is running half empty, the flow is not likely going to be squeezed sufficiently.

Here is a good way to achieve this performance:

If your press does not already have a level sensor over the inlet hopper, with wash-down water provision, it can be most useful. It can be used so that the press operates with a full inlet hopper, assuring that the press is always full.

The signal from the level sensor should be programmed to set the VFD which drives the press to operate in this fashion: On a signal from the sensor, the press should speed to a high speed, up to something like 80 Hertz. This will cause the level to drop down. Once the level has dropped, and the sensor is no longer detecting high level, the VFD should slow down the press to operate at a lower speed, like 20 Hertz. This will continue until the sensor indicates high level once again, at which time the high speed cycle will re-start.

The items which should be programmed into the VFD are (a) the high speed Hertz, and (b) the low speed Hertz. These two values need to be established by an engineer on site who works with the press to determine the practical values.

Some customers add an additional control loop, to stop the press if the level does not reach the sensor for an extended period. This happens when the flow going into the press is greatly diminished or even stopped. The trick is to program the system so that the press re-starts automatically when material is once again detected by the level sensor.

ISSUE #348