June 21, 2021
Revised August 25, 2022

One reason that screw presses are so easy to operate is that they have only two adjustments: the air pressure on the discharge cone and the speed (rpm) of the screw. Both of these are manual adjustments.

Frequently we get questions about automating operation of a screw press. Of the options, the most effective uses a control system which detects the level of material in the inlet hopper.

The primary advantage of using a level control is that the press will always operate with a level of material just over the screw in the inlet hopper. This means that the press operates in a full condition at all times, which results in maximum liquid separation and throughput.

As a rule, the signal from the level sensor is used either to adjust the speed (rpm) of the screw press, or the speed of the pump or conveyor being used to feed the press. Customers decide which of these options best suit their operation.

Five choices of level sensors are available: capacitance for a point level, and, for continuous level measurement, ultrasonic, photo optic, hydrostatic pressure sensor, and radar.

The capacitance point sensor has proven effective in several hundred installations. It has one or two electrodes in a fixed position. When the level reaches these probes, a short is caused which sends a signal indicating a level has been reached.

Thus the sensor is used to detect a specific level in the inlet hopper of the press. These are used in eggshell presses to speed up the press when a high level is detected. The press is programmed to slow back down once high level is no longer detected, after an adjustable period of operation. This is done to limit abrasive wear.

Radar is a newer type of level sensor. Using an electro-magnetic field, it is unique in that it can be programmed to ignore certain signals. Also, radar can accurately gauge level in applications which include steam vapor.

The ultrasonic sensor puts out a sound signal. It works by echoing sound off the material in the inlet hopper of the press. This echo indicates the depth of the material.

A photo-optic sensor works by reflecting light off the material in the inlet hopper of the press. Most of these use an infrared light beam. The reflection indicates the depth of the material.

A hydrostatic sensor works by sending an output signal which is proportional to the level of material in the hopper. The sensor is mounted low in the inlet of the press. The product weight presses on the sensor diaphragm, and this force is scaled to reflect a linear height of product. While we rarely recommend these, they have proven successful in vapor-tight applications with tall inlet chutes.

In many applications, being able to program a “sleep mode” can be valuable. In order to prevent undue abrasive wear, the press should turn off (“go into the sleep mode”) when the inlet hopper becomes empty. For example, the system can be programmed to shut off the press after two minutes of operating at minimum level, and turn back on upon reaching 80% of maximum level. An alarm (with a mute button) may sound on reaching 90% of maximum level.

An application using the sleep mode is the knots press at a paper mill. Usually these are continuously underfed. A knots press fills so gradually that it vibrates and overloads as it reaches the point where the cake finally breaks loose and comes out. At one mill slugs of cake came out only three or four times per hour. There is a danger of tripping out on overload with these presses (or ripping flights of the screw shaft or bursting the screen). A level control system which includes the sleep mode can be good for reducing abrasion, improving reliability, and conserving energy.

It is infrequent that the signal from a level sensor is used to either adjust the air pressure on the discharge cone or open the discharge cone. In practice such systems have rarely provided satisfactory results. We have had better luck with systems based on sensing amp draw of the drive motor.

The choice of a level detector depends on the material being fed into the screw press as well as the operating environment. Consider a press installed outdoors with 75 tons per hour of un-shredded corn husk falling into the inlet hopper, compared to an indoor press being pumped 10 tons an hour of a thick slurry of shredded produce waste, or 5 tons an hour of pre-thickened wash tank sludge at a plastics recycler. Operating conditions must be taken into consideration when selecting a level sensor.

Most screw presses operate in high humidity conditions. Systems using sensors vulnerable to condensation and splashing quickly get abandoned.

The signal from the level sensor must be wired to a control system. This will be either the customer’s PLC or a VFD. Sensors which operate with either a 110/220 volt power supply as well as 24 VDC are available.

ISSUE #335