The surface temperature of the tube beside the heating tank and the oil temperature shown on the control panel are not the same quantity. The principle of far-infrared carbon fibre heating is to deliver the heat to the oil by radiation, so the surface temperature of the heating element is necessarily higher than the temperature of the oil it heats. Reading a normal instrument as proof that the heater is not hot is the result of merging those two quantities into one.
Each of the two temperatures has its own purpose
The oil temperature is a process parameter. It decides the viscosity of the oil, the atomisation state and the degassing performance. The temperature control range of the TYA series is 45–80 °C, with the upper limit pulled back to 70 °C where the water content is high, and those set points refer to the temperature of the oil body inside the tank; the controller works around them. The surface temperature belongs to the state of a component, and it decides the environment in which the heating element, the seals and the filter material close by sit for a long time.
An acceptable oil temperature does not mean the surface temperature sits in a suitable range either. The oil takes away part of the heat as it passes the heating element, which spreads the oil temperature out, while the local temperature on the element surface can still be a good deal higher. The gap is larger when the oil level is low: the part below the surface is wrapped in oil and sheds heat well, while the part above the surface or close to the tank wall sheds it poorly and stays at a higher temperature.
What the gap leads to
An uneven temperature distribution affects the oil first. The oil close to the element is hotter than the oil further away, its viscosity is lower, and the droplets formed on atomisation differ in size. The oil leaving a tank has actually been through a temperature distribution rather than a single point temperature.
The components come next. Seals that sit for a long time at a raised local temperature age faster than they would at ambient temperature. The position of the filter material inside the tank also has to be taken into account if it is close to the heating area. Far-infrared heating delivers the heat in a concentrated way in the first place, and the energy distribution is not the same as that of a conventional coil heater.
The third layer is the basis of the judgement. A temperature control probe reads the oil temperature at its own position, and a different position gives a different reading, so two temperatures in the same tank from two measuring points is a common sight. When the probe sits away from the heating area the reading falls below the actual temperature near the element, and the action of the controller then sits apart from the actual state of the heating.
How the site judges local overheating
| What is seen | Likely cause | How to confirm |
|---|---|---|
| The set temperature is reached but the oil temperature stays below it | The measuring point is away from the heating area | Set a second measuring point near the heating area and compare |
| The oil heats up noticeably faster at a low oil level | The heat shedding conditions have changed | Record the heating time at different oil levels |
| Clear temperature difference between the upper and lower layers of the same batch | The oil is not circulating properly | Stop the machine, let it stand and take layer samples for comparison |
| A stronger oil smell near the heating element | A higher local temperature | Stop and cool the machine, then check the state of the oil in that area |
The last row of the table is worth a word more. The deterioration of oil at a high temperature happens locally, and if the oil at one position smells clearly stronger than the rest of the tank, that oil has spent time at a higher temperature. Such a difference is often invisible once the tanks are mixed, so the check has to be made with the machine stopped and before anything is stirred.
What to do about it
In most cases no special treatment is needed. The right approach is to let the temperature control reading represent the area that actually has to be controlled: mount the probe on the path the oil circulates along rather than hard against the tank wall; keep the oil level in the normal working range so that the heating element is not left out of the liquid for long; and do not set the temperature above the upper limit the process allows. The ambient temperature range is -20–45 °C, and the heat shedding conditions differ between winter and summer, so the behaviour at the same set point is bound to differ a little. That part of the difference belongs to the environment, not to a fault in the machine.
The oil temperature is the temperature of the process and the element temperature is the temperature of a component; each reading answers only for its own question.
