Press Controls

June 19, 2013

VFD (Variable Frequency Drive)

It is generally recommended that a frequency inverter VFD be used to start, protect, and operate the screw press.  With a VFD it is possible to establish the optimal combination of screw speed and discharge cone air pressure.  The VFD also can be used to reverse the press in case of a jam, or to slow it down or speed it up during upset conditions.

PLC (Programmable Logic Controller)

Nine presses out of ten will operate unattended, indefinitely, and just fine at line frequency of 50 or 60 Hertz.  However, in some cases the press will tend to jam, overload, and trip out on high amps.  In this situation it may be necessary to have the discharge cone automatically open on high amps, re-closing at a lower set point.  This arrangement requires a solenoid operated 4-way air valve, replacing the manual valve which is provided with the press.  This action may be combined with speed change of the screw and air pressure change on the discharge cone.  Usually a PLC is programmed to control such operation.

Cone Timer

In other cases of jamming, a simpler arrangement is to install a Cone Timer.  A timer is used to periodically open the cone.  The closed period is determined by the amount of time required for press cake to accumulate in the press.  The duration of the "cone open" period is long enough to dump most of the press cake that has formed (but not long enough to lose the plug).  This type of operation may be used if the press periodically experiences jamming or overload due to fluctuations in the amount of material being fed into the press.  Alternatively, it may be used with slippery or slimy material that cannot be dewatered to sufficient firmness to force the cone open.  Cone Timer panels are available from Vincent.

Level Sensor

The most recent trend in screw press controls is the use of level sensors.  The most common application is where there is the opportunity to maximize press performance by changing the screw speed to match inbound flow.  This requires a VFD to vary the speed.  Unfortunately there is a tendency for operators to set the speed on the high side so that they will not be bothered by an overflowing inlet hopper.  In these situations a level control can be used so that the VFD will automatically adjust the press to optimal performance.

Another good application for a level control is when the screw press is sized for handling upset conditions of large flow, while the normal flow is quite small.  Dewatering knots in a paper mill is a typical application where this occurs.  In these cases a level control is used, increasing the screw speed when the level goes up, and reducing it when the level goes down.  In extreme cases, a PLC can be programmed to turn off the press when a low level is reached in the inlet hopper, and re-start the press when a higher level is signaled. 

A port on the side of the inlet hopper, for mounting a level controller, is usually provided on larger Vincent presses.

Programmed Shut-Down

Some materials may set up and become hard, or freeze, within the press when the press is turned off.  This is especially true in the case of pressing coffee grounds or in outdoor installations.  For these applications it is advisable to open the discharge cone for a short period, a minute or so, before turning off the press.  This allows the press to partially empty itself, fluffing the material left in the press.  Vincent can supply a solenoid operated valve and control panel for opening the cone prior to shut-down.  Alternatively, the customer may program a PLC to perform a sequential shut-down.

Auto Reversing

Some applications require the use of an especially programmed variable frequency drive.  In this case the VFD is not used to change the speed of the press, but, rather to set it for auto-reversing operation.  By having the screw run backwards for three or four turns every few minutes, some difficult-to-dewater materials can be pressed much more effectively.  The mechanism is that the screen is brushed clear by the material being pushed backwards from the normal flow direction.  This operation can help a great deal with material which tends to blind (cover over) the openings in the screen.  Vincent has loaner VFD's if you want to give it a try.  The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

RTD (Resistance Temperature Detector)

All Vincent vapor-tight and explosion-proof presses come with RTD's and their corresponding transmitters.  These are mounted on bearings, bushings, and the gearbox.  (Motors are specified with internal thermistors to provide protection from overheating which might create an ignition source.)

Air Regulator

Last but not least, for over sixty years Vincent presses have been supplied with an FRL (Filter, Regulator, Lubricator) set for adjusting the air pressure on the discharge cone of the press.  These are manually adjusted and rarely need any attention after initial commissioning of the press.

Issue 256