When the heating system has a problem, what the site usually notices first is that the temperature will not rise, rather than that it is inaccurate. But there is one case that works the other way round: on the control panel the temperature reading sits steadily inside the set range, while the oil takes noticeably longer to heat up, with an unexplained interval in between. The temperature probe measures the oil temperature, and the oil temperature being reached does not mean the heating power has reached the oil, because there are two interfaces in between: the heating element and the heat transfer surface.
Accurate temperature and effective heating are two different things
The temperature probe sits in the oil and its reading reflects the actual oil temperature directly; this is normally not where the fault lies. The problem sits in the section where the heat travels from the heating element to the oil: the surface of the heating tube, the contact face between oil and tube wall, and the inner wall of the tank can all be points of resistance. Once the resistance grows, reaching the same set temperature takes the heat source a longer continuous output, and the temperature control system cannot tell the difference, because it is only responsible for the final oil temperature.
This is why a particular set of symptoms is seen on site: the setpoint has not changed, the set temperature has not changed, and yet the oil temperature curve has shifted back as a whole. Two hours to temperature yesterday, three hours today; or a slow first start and a slower second start. A single reading does not show the problem; comparing several days does.
Where fouling begins
Water and the impurities in the oil are the raw material of fouling. After the oil has been heated on the heating surface, the water dissolved in it vaporises at the local high temperature points, and the vaporisation process leaves traces on the metal surface. The suspended particles and oxidation products carried by the oil attach to those traces and build up layer by layer, and the heat transfer efficiency falls with it, so that the same heating power sends less and less heat into the oil. This is the underlying reason why the temperature looks normal while the oil heats slowly.
For a vacuum oil purifier this deposit causes trouble in another place as well, namely the filtration stage. The finer the filtration accuracy and the tighter the differential pressure alarm value of an element, for example the 150-500-5 fine element with a differential pressure alarm value of 0.35 MPa, the more it relies on the differential pressure to judge the resistance. If an oil circuit restriction and fouling on the heat transfer surface occur at the same time in the machine, slow heating and a falling flow rate appear together, and this is easily mistaken for a pump problem.
| Observed symptom | More likely cause | Next action |
|---|---|---|
| The setpoint is unchanged but the time to reach the set temperature gets longer each time | Increasing deposit on the heat transfer surface | Shut down, let it cool, then inspect and clean the heating section and the inner walls of the oil circuit |
| The oil output falls while the pressure gauge reading rises | Rising resistance in the filtration stage | Judge whether to clean or to change the element under the differential pressure management procedure |
| The oil temperature stays high for a long time and the deviation grows | Poor oil flow around the probe | Check the probe mounting position against the circulation path inside the tank |
The temperature control range is itself a line of defence
Keeping the temperature inside a sensible range is not only about making the oil usable; it also avoids creating the conditions for fouling. The temperature control range of the TYA series is 45–80 °C, and with a high water content the upper limit is pressed down to 70 °C; the ZYD-300 uses far-infrared radiation heating together with automatic constant temperature control, and the oil temperature is adjusted intelligently within a range of 20–80 °C. The upper limit is not an arbitrary figure: it constrains both the working intensity of the heating element and the time the oil spends at a high temperature.
- Pressing the upper limit down to 70 °C for oil with a high water content keeps the water from vaporising in quantity and keeps local overheating from occurring.
- Far-infrared radiation heating delivers the heat by radiation, which reduces the direct contact between the heating surface and the oil and slows the build-up of deposit.
- When the ambient temperature varies within a range of -20–45 °C, the time needed to reach the set temperature changes with it, and this is normal.
The order in which fouling is investigated matters too. First rule out changes in the external conditions: was more oil processed this time than last time, is the oil more viscous, is the ambient temperature lower, has the heating setpoint been changed. Only once these are ruled out does it become worth looking at whether the heating-up time is getting longer each time under the same conditions. The trend itself is the strongest evidence.
A normal temperature display only proves that the oil temperature has been reached; it does not prove that the heating system still has margin. The heating-up time is the true record of a change in heating capacity.
