A failure to bring the oil temperature up is the most worrying fault in vacuum degassing, because it slows down the degassing, affects the vacuum level and raises the viscosity of the oil, and it ends in a site situation where several symptoms appear at once. The temperature control range is 45–80 °C, and the upper limit is brought down to 70 °C when the water content is high, so first establish what has actually been set, and only then decide whether the problem is in the heating or in the heat losses.
What follows is a check list to be used in order. The first four items do not require dismantling the machine; only the last two involve opening it up with the power off.
Item one: whether the heating element is actually energised
The heating uses far-infrared carbon fibre elements. To judge whether it is energised, do not rely on the indicator lamp alone; check three things: whether the control panel shows a heating output, whether the voltage of the heating circuit is normal, and whether there is a temperature rise at the heating end, which can be checked with an infrared thermometer or by hand. If the lamp is lit but the heating end is not hot, the fault is in the output circuit or at the terminals; if there is not even an indication, first check whether the set temperature is above the current oil temperature, because the system will not start the heating when the set value is below the measured value.
Item two: whether the temperature control probe is in proper contact
A poor probe contact is the item that is most easily overlooked. A probe that has overheated or whose reading drifts makes the system misjudge the temperature: if the probe has failed or has been inserted too deeply, the heating may stay off for a long time; if it is only loosely inserted, the reading swings up and down and the heating starts and stops with no pattern. The way to judge it is to measure one point in the oil with a reference thermometer and compare it with the panel display; a large difference means that the probe or the display is at fault. This item does not require dismantling the machine, only putting the thermometer into the oil and comparing.
Item three: whether the oil flow is high enough to carry the heat away
The heat of the heating element has to be carried away by the oil flow. A nearly closed inlet valve, an oil pump that is not running and a line blocked by the filter element all make the flow slow or nearly stationary, so the heat piles up locally and the average temperature in the tank cannot rise. How to tell them apart: check whether the pump is turning, whether the inlet flowmeter shows anything, and whether there is a clear difference between the outlet temperature and the temperature in the tank. If there is a temperature difference and there is flow, but the overall temperature does not rise, the focus goes back to the heating element; if the flow itself is close to zero, the circulation problem has to be solved first.
Item four: the oil level and the insulation
- The level is too high: with a large oil volume and a fixed heating power, more heat is needed to bring the whole batch of oil up to the target temperature, and the heating-up time is naturally longer. This is more pronounced when the water content is high, because evaporation absorbs heat.
- The insulation has been damaged: once the insulation on the casing has fallen off or become damp, heat is lost faster and the same input power can only hold a lower equilibrium temperature. Run the hand along the outside of the tank section by section; a distinctly cold patch is the place where the insulation has failed.
- The ambient temperature is too low: the ambient temperature range is -20–45 °C, and in a cold environment the heat is lost faster and a longer preheating time is needed. To decide whether this is the case, watch how far the oil temperature falls after the machine has been stopped and left standing for several hours; a large drop means that the heat is being lost quickly.
Items five and six: the two that can only be done with the machine stopped and the power off
Loose or oxidised terminals on the heating circuit, leakage caused by insulation that has become too thin after the wires have been stripped, and local failure of a heating tube caused by scale inside it: these two classes of fault cannot be seen from the outside and have to be checked by opening the machine up. Three actions have to be completed before the check: stop the machine, release the pressure, and disconnect the power and confirm it with a lock-out tag. The heating circuit is live and there may still be a residual capacitor discharge near the terminals; removing terminals without confirming that the power is off risks an electric shock. While opening the machine up, take care not to break the heating tube and not to splash oil onto the heating elements, because oil on the surface of a heating element under local high temperature is where the burnt smell comes from.
| Symptom | Most likely cause | How to confirm | Action |
|---|---|---|---|
| No output indication on the heating | The set temperature is below the current oil temperature, or the control circuit has not started | Compare the set value with the measured oil temperature | Correct the set value and check the control circuit |
| There is an output but the heating end is not hot | Loose wiring, a blown fuse or a failed heating tube | With the power off, measure the continuity and the resistance of the heating circuit | With the machine stopped and the power off, tighten the terminals or replace the heating element |
| The panel temperature differs a lot from the measured temperature | Poor contact at the temperature control probe | Compare with a reference thermometer in the oil | With the power off, refit and calibrate the probe |
| The flow is close to zero and the temperature does not rise | The inlet valve is nearly closed or the filter element is blocked | Watch the flowmeter and the difference between the outlet and the tank temperature | Open the inlet valve further and check the differential pressure of the filter element |
| A cold patch on the outside wall | The insulation has fallen off or become damp | Run the hand along the outside of the tank section by section and compare | Repair or replace the insulation |
| The oil temperature drops quickly after the machine is stopped | A low ambient temperature or excessive heat loss | Record the rate of temperature drop while standing | Extend the preheating time and improve the insulation of the surroundings |
When the oil temperature will not come up, first decide whether there is no heat or whether the heat is being lost. The first means checking the heating circuit, the second means checking the flow and the insulation.