The temperature control panel shows a single number, and that number represents only the point where the probe sits. Oil is a medium with volume and a temperature gradient, so what the probe reads depends on which layer of oil it is immersed in. A situation that occurs often on site is that the panel shows the set temperature has been reached and the heating has stopped, yet the oil sample drawn off is still cold, or the other way round, the panel temperature has not been reached while the oil in the tank is too hot to touch. The probe position is the most common cause of both.
Far-infrared heating stratifies the oil vertically
Far-infrared carbon fibre heating radiates heat from the tank wall inwards. The heat acts first on the surface layer of oil close to the wall, then conducts from that layer towards the centre and spreads from top to bottom. In this process the oil near the wall heats up fastest and the centre of the tank, especially the bottom, heats up slowest. Waiting for the temperature of the whole tank to even out depends on the circulation of the liquid itself to distribute the heat. Without circulation, or with weak circulation, this temperature difference stays in place.
With the probe fitted close to the heat source the reading is high, the control loop acts on that high value, the heating time is shortened and the actual average temperature of the whole tank is low; with the probe fitted in a corner away from the heat source the reading is low, the heating runs longer and the local temperature may go above the set value. Neither arrangement means the probe is faulty, it means the sampling point was chosen in the wrong place.
A continuous range and an upper limit are two different things
The temperature control is made a continuous range of 45–80 °C rather than a few fixed settings, so that the operator can adjust within a range according to the oil and the condition of the incoming oil, instead of guessing which setting the manufacturer has preset. When the water content is on the high side the upper limit is brought down to 70 °C, because with more water present a temperature rise to 80 °C makes the moisture vaporise faster, the vapourised water drawn off together with the oil and gas adds to the load on the exhaust side, and the water vapour separated during the degassing is less likely to be recovered completely in the condensing section. Bringing the upper limit down is not caution for its own sake, it protects the efficiency of the degassing step.
Viewed the other way round, the ambient temperature range of -20–45 °C points at the same thing: at the same set value, the actual temperature response of the whole tank differs with the ambient temperature. In a cold environment the machine loses more heat and the warm-up takes longer; in a hot environment heat is lost slowly and the heating circuit may never reach full load.
The differences in judgement that the probe position creates
| Probe position | Reading characteristic | What it can tell you | What it cannot tell you |
|---|---|---|---|
| Close to the tank wall or the heating surface | Rises quickly, reads high | Whether the heating circuit is working | Whether the whole tank has reached the set temperature |
| Main flow area in the middle of the tank | Rises slowly, close to the average | The general temperature level of the whole tank | The instantaneous difference between the surface layer and the bottom |
| Bottom or dead corner | Low and noticeably lagging | Whether cold oil is still left at the bottom, not yet displaced | The temperature of the oil at the top |
How this is confirmed on site
- Record where the probe is fitted on the tank and hand this over when the machine or the tank is changed
- Take an independent measurement with a thermometer at the sampling point and compare it with the panel reading
- When a temperature deviation is consistently present, suspect the position and the circulation before suspecting the probe itself
- When the water content is on the high side, run at the 70 °C upper limit and leave the margin within the limit to the degassing efficiency
The temperature control reading is the reading at the position of the probe, not a report card for the whole tank of oil; only when the probe position and the reading are looked at together does it become clear how the number should be used.