The most common split-range application is the heating and cooling of the barrel zones of plastics extruders (figure 1). On startup, the heat output of the controller takes the barrel zone up to working temperature. Heat delivery is usually modulated (that is, turned up or down) using the time-proportioning mode, by magnetic or solid-state contactors.

A calrod-type heater element would be embedded in the heat/cool unit (two half-cylinders of cast aluminum that clamp on to part of the barrel). The cool output could be water or heat transfer fluid, piped through coils in the same cast aluminum unit. Alternatively the cast aluminum unit would be finned, and heat is removed by blown air. Cooling would be modulated by pulsing a solenoid valve or fan motor contactor in the same manner as the heat is pulsed.

If screw-friction heat begins to overheat the zone from the inside, the controller will decrease its heat output -- to zero if necessary. If that is not enough, the cool output will take over and hold the polymer down to its working temperature.

Enlarge this picture Figure 2. Section A (top left) shows a balanced heat/cool zone. Section B (top right) shows cool gain two times too large, but maximum heat and cool capacities balanced. Section C (bottom left) shows cool capacity twice that of the heater and cool gain in need of adjustment. Section D (bottom right) shows the correct balance of proportional bands by making the cool gain equal to 0.5.

Figure 2 shows the heat growing to 100 percent as the temperature falls by one proportional band. Likewise, the cooling power grows to 100 percent as the temperature rises by one proportional band.

Figure 2A also shows the cool power matching the heat power for the same temperature deviation. When you have optimized the proportional band for heat delivery, the same proportional band will be optimum for positive temperature deviations; that is, if full cool output is equal to full heat output.

In the event of unbalance of heating and cooling capacity, an adjustment called cool gain is used to restore that balance for best control performance (figures 2B, 2C and 2D show unbalance).

This arrangement is versatile, well proven and cost effective. You can adapt it for other applications such as reactors and vessels that have contents generating exothermic heat.

I’ll have more on split-range applications next month.