Figure 1. An easy-to-control process such as this well-stirred water bath can be easily controlled with an on/off controller.

This month, I continue looking at normal and abnormal behavior of extruders, picking up with cooling issues and then beginning to look at control principles to minimize overshoot in extruders.

Cooling Issues

Remember that your extruder drive could be putting some 150 kW (200 hp), via the screw, into the polymer. The shear heat is enough to send some barrel zone temperatures too high. This is where the controller shuts down its heat and applies just enough steady cooling to hold the temperature down to setpoint.

Water Cooling Problem. The heat is off; the light indicates that cool is on; but the temperature will not come down to setpoint. Possible sources include:

  • No water pressure or the pump is stopped.
  • The pipe or solenoid valve is clogged. Cooling water is in a closed loop. Using demineralized water will minimize clogging.
  • The manual flow-adjusting valve is closed.

Alternatively, the solenoid valve could be failing to open. Check if its coil is energized by feeling if it is hot. If it is not energized, the controller cool-output relay or the power feed to it is at fault. If the coil is energized and the valve does not allow flow, change it. If you feel the valve, you should sense a click as the controller switches it on and off.

Another Water Cooling Problem. The heat is on but the temperature will not come up to setpoint. In this case, check whether the solenoid valve is stuck open or held open by a defective controller relay failing to switch it off.

Fan Cooling Problem. If the temperature is below setpoint and the fan will not turn off, check the fan contactor. Is it welded up or locked on? Is the controller relay locked on due to a faulty controller?

Another Fan Cooling Problem. If there is no airflow and the temperature is well over setpoint, check for a faulty controller, controller relay or fan contactor. Also check for motor failure or air path blockage.

Figure 2. More representative of common process heating applications, this water bath has thermal lags and is susceptible to temperature overshoot if on/off control is used.

Alarms and Warning Features

Controllers are available with built-in high, low and deviation alarms, but beware: a faulty controller usually means loss of both alarm action and warning signal.

In combination with SCRs and small donut current transformers, controllers can immediately detect heater failure or partial failure, heater on when not called for, and heater off when heater on called for. Where safety or costly damage or downtime are threatened, an independent cut-off controller or device is essential.

On processes where personal injury is not a threat, the buyer may opt to save the cost of a second device and bear the risk of material or equipment damage due to overheating.

Figure 3. If on/off control were used, an extruder barrel temperature zone also would overshoot and cycle.

Control Principles

On an extruder, you are trying to control the temperature of a few pounds of polymer inside one or two tons of steel. Is it any wonder that time lags make this difficult? You can see what kind of control you need according to the nature of the process.

Figure 1 shows an example of an easy-to-control process: a well-stirred water bath with a light-gauge electric heater, a fast responding bare thermocouple, and an on/off controller with a very small dead band. As soon as the temperature rises to setpoint, the control relay opens and the temperature stops climbing. When the temperature falls slightly -- say 0.1oC -- the controller switches the heater on again. The temperature graph is virtually a straight line.

Why so easy? There is not enough mass and heat capacity in the heater wire to make the temperature overshoot after switch off. Also, the thermocouple senses the water temperature immediately, and there is no delay in the control action. This is not a practical process.

A Real Water Bath: Not So Easy. A practical heater will be metal sheathed, heavy and could run 180 to 360oF (100 to 200oC) higher than the water temperature. Controller switch-off does not stop the heater from delivering heat into the water; overshoot is inevitable (figure 2). Worse yet, the thermocouple will be insulated and in a protection tube, so it is late telling the controller that the water has come up to setpoint and it’s time to switch off.

The result is that the temperature cycles continually above and below setpoint even though the controller is very precise in switching on and off exactly at setpoint. Any dead band in the controller would make the cycles even larger.

Another Hard-to-Control Process: An Extruder. An extruder barrel zone also would overshoot and cycle because there is a massive heater and barrel wall running hotter than the polymer when heat is being demanded (figure 3). Also the thermocouple is set deep because you have to sense and control the polymer temperature. This makes it late in responding to changes in heat input and reporting back to the controller.

This suggests that you need a controller that can throttle back the power well ahead of the temperature reaching setpoint. This provides a way to defeat temperature overshoot and cycling.

Next month, I’ll show the elements of a controller that would achieve this. PH