A temperature display that swings around the setpoint is a common sight on site. Its cause is often misread as insufficient heating capacity, which leads to checking the heating element, measuring the voltage and replacing the heater, while the real source is usually the temperature control logic itself: the controller stops heating when the setpoint is reached and starts again only when the temperature has fallen to a lower point. The distance between those two points is the control band, the hysteresis band.
The band exists to stop the controller from switching too often
If the heater restarted every time the temperature dropped a little, it would cycle at a high frequency. Frequent cycling brings two consequences: the contactor operates again and again and the inrush current recurs; and the temperature itself stays in a fluctuating state and never settles at any one value. The band exists so that the heater acts once at a sensible interval and keeps the temperature swing within a range that can be accepted.
A swinging temperature is therefore not a fault in itself. What matters is whether the amplitude and the period of the swing fall within a range the site can accept. The temperature control range of the TYA series is 45–80 °C, a continuously adjustable range, and the band width and the heating frequency needed for different setpoints inside that range are not the same.
A band that is too wide and one that is too narrow both carry a cost
When the band is too wide, the temperature swings across a wider range. As the oil temperature changes, the viscosity changes with it, the atomisation state in front of the nozzle changes in turn, and the degassing result varies from batch to batch. The heater starts and stops fewer times, but each heating run has to push the temperature across a longer distance, so the power draw does not fall; it rises, because every heating run has to make up a larger temperature difference. This is particularly visible when oil with a high water content has its temperature limit pulled down to 70 °C: the usable temperature margin is already small, so the swing takes a larger share of it.
When the band is too narrow the temperature looks stable, but the heater acts more often, the number of contactor operations and the inrush current both rise, and the life of the contactor is affected. The power draw is not necessarily lower either, because every heating run starts again from a lower temperature, and bringing the oil up from that temperature is less efficient than holding it.
| Band setting | Temperature behaviour | What comes with it |
|---|---|---|
| Too wide | Large swing amplitude | The atomisation state is unsteady and the degassing result differs between batches |
| Too narrow | Small swing amplitude | The heater cycles more often and the number of contactor operations rises |
| Matched to the duty | Steady within a small range around the setpoint | Both the degassing state and the power draw can be expected |
The ambient conditions play their part too. The ambient temperature range of the machine is -20–45 °C. Heat is lost faster in winter and more slowly in summer, so the same setpoint with the same band produces a different swing amplitude in a different season. Put the temperature curves of the same machine side by side and the seasonal factor becomes visible directly.
How the site separates the two cases
- Record the temperature curve over a period of time, noting the upper and lower limits, the swing amplitude and the period, rather than a single number.
- Record the number of heater starts and stops at the same time. Compared against the curve, this tells you whether the problem is the band setting or the heating capacity.
- Check the position of the temperature probe relative to the heated area. A change in the probe position rewrites the reading directly.
- Check whether the setpoint sits in the range that suits the oil in hand. For oil with a high water content the upper limit should be pulled down to 70 °C.
The starting point for investigating this kind of problem is to accept that it is not a fault. A machine whose temperature swings slightly around the setpoint and a machine whose temperature does not move at all although it keeps heating without effect are two different problems, and they are handled in different ways.
The swing itself is not the problem. The problem is a swing that does not match the duty.
