Most people's impression of the relation between temperature and degassing is that the higher the temperature, the thinner the oil, the more easily the bubbles come out and the higher the efficiency. That judgement holds over a certain range, but past a certain point the efficiency no longer rises and may even fall. Understanding the part viscosity plays on this curve is what explains why a higher temperature is not always better.
The first path through which viscosity acts: mass transfer resistance
Degassing is in essence the migration of the gas from the oil phase to the oil surface and from there into the vacuum chamber. The first step relies on molecular diffusion and on the convective renewal of the oil, and both of them are governed by the viscosity. At a high viscosity the oil molecules move slowly and the dissolved gas reaches the oil surface more slowly; at the same time the convective renewal of the oil weakens and the bubbles stay longer in the oil layer.
So the fall in degassing efficiency when the temperature comes down has its origin in the mass transfer resistance that the rising viscosity brings, not in any direct effect of the temperature itself on the gas.
The second path: the behaviour of the bubbles
When the viscosity falls the internal friction of the oil decreases, the drag on the bubbles in the oil becomes smaller and their rise speed increases, so the bubbles merge and grow more easily and float to the surface to be drawn off. A rise in viscosity has the opposite effect: the bubbles rise slowly, the interfacial film is stable, and long-lasting foam that does not dissipate easily forms.
The two paths run in opposite directions but are not synchronous: the mass transfer resistance worsens as the temperature falls, while the behaviour of the bubbles improves as the temperature falls. The net effect is that the degassing efficiency rises with the temperature, but the gain narrows gradually. The relation between temperature and vacuum efficiency is therefore not a straight line but a curve that is steep at the front and flat at the back.
A different risk at high temperature: cracking and the loss of light fractions
When the oil is heated, what escapes first is not only the water but also the light fractions that were dissolved in the oil. The higher the temperature and the longer the residence time, the larger the proportion of the light fractions that is drawn off. Those fractions are precisely the ones that maintain the flash point, the viscosity index and the lubricating performance of the oil, and once they have been taken away the figures of the oil all move down.
In the more extreme case, a locally high temperature makes the oil crack and produce new low-molecular-weight products. Once these products enter the gas phase they show up as a rising load in the vacuum chamber, a more complex composition in the condensate and faster contamination of the pump oil, and the maintenance frequency of the machine goes up accordingly.
Why the temperature has to follow the condition of the oil
Because the viscosity of the oil is itself changing. New oil, oil whose viscosity has not been affected, oil whose viscosity has risen after a long run and oil that has just been started up in the cold on site all begin in a different state and need a different starting temperature and a different warm-up strategy. The same set value applied to every batch is not reasonable.
In addition, the ambient temperature range of the TYA series is -20–45 °C. In winter the starting oil temperature is low and the viscosity is high, so the first stage of the degassing is slow under the same setting and the preheating time has to be allowed for more generously. Where the water content is high the upper limit is brought down to 70 °C, which is a compromise between the rate of dehydration and the rate of oxidation.
| Factor | What changes when the temperature rises | Conclusion |
|---|---|---|
| Viscosity | Falls | The mass transfer resistance decreases and the degassing speeds up |
| Rise speed of the bubbles | Increases | They merge more easily and are carried away more readily |
| Vapour pressure of the oil and the water | Rises | The working vacuum becomes shallower and the pumping speed is taken up |
| Loss of light fractions | Intensifies | The flash point and the viscosity figures fall |
| Rate of oxidation | Clearly increases | The acid value rises and resinous products increase |
| Risk of cracking | Appears at local overheating | Low-molecular-weight products are formed and the pump load increases |
- Raise the temperature in steps, so that the viscosity falls first and the main degassing stage is entered afterwards.
- In winter work, build the preheating time into the production plan instead of shortening the degassing time.
- Where the water content is high, bring the upper limit down to 70 °C and trade time for the quality of the oil.
- If an off smell appears, the colour value darkens quickly or the vacuum load rises: lower the temperature at once and check whether the oil is overheating.
- When the viscosity of the oil has changed noticeably, reassess the temperature setting instead of carrying over the value used for the previous batch.
Temperature cuts both ways: it improves the mass transfer and it accelerates the ageing at the same time, so both ends have to be looked at when it is set.