The temperature control range is 45–80 °C and the upper limit is pulled down to 70 °C when the water content is high. Most people remember both figures, but the decision that actually has to be taken every day is this: for the batch of oil in front of you, which end of the range does the set value go to. There is no uniform answer, because the four conditions that move it, namely how much water the oil holds, how much flash point margin is left, how far the viscosity has to come down and how much thermal stability margin the oil itself has, are not the same from one batch to the next.
The set point is the intersection of four conditions, not the point of best efficiency
Raising the temperature brings a chain of changes that happen at the same time: the solubility of water falls, the viscosity falls, and the partial pressure of the oil and water vapour in the gas phase rises. The first two help the degassing; the third puts a heavier gas load on the vacuum path and makes the working vacuum shallower. A high temperature also accelerates oxidation and shortens the time the oil can stay in service. The contribution of temperature to degassing efficiency follows a curve that is steep at the front and flat at the back. It is not a case of the higher the better, and it is not an efficiency factor that can be converted linearly.
The set point therefore has to satisfy four constraints at the same time: how fast the degassing task has to be carried out; whether the temperature needed for that speed lets the vapour load hold the vacuum back; how much flash point margin is left between the temperature and the flash point of the oil; and how large the actual effect of that temperature on the quality of the oil is. If one of them is missing, a set value copied from somewhere else fails on some batch or other.
Towards the lower end: when it is worth staying near 45 °C
There are several situations in which putting the set value at the lower end of the range is reasonable. First, the initial water content of the oil is already close to or below ≤3 ppm (GB/T 7600), so the degassing duty itself is light and the small efficiency gain from a higher temperature does not offset the cost of oxidation. Second, the oil has a small thermal stability margin and little flash point margin left, so raising the temperature narrows the safety boundary; in that case the temperature is pressed down and time is traded for efficiency. Third, the ambient temperature on site is low: the ambient temperature range of the TYA is -20–45 °C, and the machine may in any case sit for long periods only reaching the lower limit once it has been heated, so a set value close to the lower end is more realistic. Fourth, the value of this batch of oil is higher than the degassing time itself, and quality takes priority over efficiency.
It has to be said that the lower limit is not an energy-saving setting. A low temperature means a high viscosity, a heavier load on the pump and a lower flow rate, and holding the machine at the lower end of the range can instead make the processing time longer. Any gain in efficiency has to be counted together with the viscosity.
Towards the upper end: when the temperature can be pushed up
The opposite set of situations suits the upper end of the range. First, the oil holds a lot of water and the water has to be driven out quickly, with the degassing cycle itself the bottleneck; here the faster separation of water brought about by a higher temperature is a real gain. But when the water content is high the upper limit is already pulled down to 70 °C, and that is the constraint in force, not a recommended value. Second, the oil has a high viscosity and poor flow behaviour, and is especially difficult to start in the cold season; the oil only moves once the temperature is high enough. Third, the oil has ample flash point margin and a large thermal stability margin, so the oxidation cost of raising the temperature can be carried. Fourth, this batch of oil is not intended for long-term online service, so the trade of a short cycle against a high efficiency holds.
There is one further condition that is easily overlooked: temperature and vacuum level hold each other back. The target for gas content is ≤0.01% (GB/T 423). Once the temperature is pushed up, the vapour load gets heavier and the working vacuum becomes shallower, and if the dissolved gas is not removed cleanly this figure may fail. The set point is judged on the degassing result, not on the number on the temperature control panel.
Turning the conditions into figures: a decision that can be recorded
The conditions above can be organised into a chain of decisions. First look at the initial water content of the batch: if it is high, take 70 °C as the upper limit. Then look at the viscosity of the oil and the ambient temperature on site: if the viscosity is high and the start is difficult, move towards the upper end of the range. Then look at the flash point margin and the thermal stability margin: where the margin is small, press the temperature down. Finally use the flow rate to work back whether the machine is large enough. The flow rate of the TYA-50 is 2000 L/h and the TYA-100 is 4000 L/h, and the larger the tank the slower the warm-up, which leaves more leeway in the timing of adjustments.
| Site condition | Set point orientation | Main reason | Indicator to watch |
|---|---|---|---|
| The initial water content is already close to ≤3 ppm | Near 45 °C | The degassing duty is light and the gain from a higher temperature does not offset the oxidation | Colour value and acid value |
| The water content is on the high side and the degassing cycle is tight | Within 70 °C | Faster separation and vaporisation of the water, limited by the upper limit | Water content (GB/T 7600) |
| High viscosity, difficult start at low temperature | Upper end of the range | Lower flow resistance so that the pump can push the oil through | Differential pressure and actual flow |
| Small flash point margin, insufficient thermal stability margin | Lower end of the range | Narrows the safety and ageing margin | Flash point record |
| Winter ambient temperature close to -20 °C | Hold the 45 °C lower limit first | The heating capacity and the thermal mass of the tank determine the warm-up time | Warm-up and stabilisation time |
| Large difference in degassing result between batches | Check the set value first, then compare | The set value was not matched to the condition of the oil | The set value recorded for each run |
- Record the set temperature, the oil batch, the initial water content and the final water content for every run. Without all four the run cannot be reviewed afterwards.
- When the water content is high, first confirm that the upper limit is being applied at 70 °C, and only then judge how well the degassing worked.
- Do not carry a set value that suited one batch on one machine straight over to a different batch of oil.
- A set value that runs close to the upper limit for long periods means that the temperature capability of this machine does not match the condition of this oil.
Setting the point is not about finding the end of the range with the highest efficiency; it is about finding the compromise that suits this batch of oil in terms of water content, flash point margin and viscosity.
