The three numbers 45, 70 and 80 on the temperature control panel are often taken for three selectable temperature settings. They actually mark three boundaries: the minimum requirement for flow and dehydration, a compromise upper limit for oil with a high water content, and the highest value that the viscosity and the thermal stability allow. Only when each number is understood can the temperature be adjusted on site with confidence.
Lower limit 45 °C: to get the oil moving and the water out
At a low temperature the viscosity of the oil is high. A high viscosity has two direct consequences: first, the load on the pump rises and the flow rate falls, so the passage of the oil through the elements gets worse; second, the diffusion of the gas from the oil phase into the gas phase slows down and the degassing rate drops noticeably. For an oil that is already viscous, running below 45 °C can leave the equipment for a long time in a state where it cannot pull the oil and cannot remove the gas.
45 °C is at the same time the practical starting point for the dehydration. The solubility of water in oil falls as the temperature rises, so the higher the temperature, the more readily the water comes out of the dissolved state and vaporises. If the temperature is not high enough, both the separation and the vaporisation are slow, the amount of condensate is small and the water content curve flattens out.
Upper limit 80 °C: viscosity is no longer the problem, oxidation starts to be
As the temperature goes on rising, the viscosity falls, the oil flows more easily and the degassing rate still appears to improve. But a second curve starts to climb: the oxidation rate of the oil at high temperature increases markedly, the acid value rises, resinous oxidation products increase, and the viscosity index and the colour value deteriorate accordingly. Oil that stays in the high temperature region for a long time has lost quality even if all the water has been removed.
The 80 °C is therefore an upper limit imposed by the thermal stability constraint, not a recommended value for the point of best efficiency. The temperature control range of 45–80 °C on the nameplate states the capability boundary; the actual setting should fall at some point inside that range according to the condition of the oil, rather than running along the upper limit.
70 °C: the compromise for oil with a high water content
When there is a lot of water in the oil, the conflict is amplified. More water means a higher temperature is needed to drive the water out, and a higher temperature in turn aggravates oxidation; oil with a high water content is often already well oxidised to begin with, so the two curves push against each other. The compromise is to bring the upper limit down to 70 °C, giving up part of the dehydration rate in exchange for a lower oxidation rate.
It should be noted that the temperature control range of the TYA series is 45–80 °C, with the upper limit brought down to 70 °C where the water content is high; the ambient temperature range is -20–45 °C, and the altitude also affects the vacuum level. In other words, the site conditions and the altitude have to be taken into account when the temperature is set.
Temperature and vacuum efficiency are not a linear relationship
Plotted against the temperature, the degassing efficiency turns out to be a curve that is steep at the front and flat at the back. Over the short distance above 45 °C the viscosity falls noticeably and the efficiency improves quickly; past a certain point the partial pressures of the oil and the water vapour in the gas phase rise rapidly, the pressure in the chamber is held up, the working vacuum becomes shallower, the pumping speed is used up by the oil and the water vapour, and the gain in efficiency falls away quickly.
At the same time the oxidation risk does not stop growing. In other words, once the temperature has passed a certain point, further heating gives a negative return: the degassing is a little faster, but the oil ages a good deal faster.
| Temperature | Physical meaning | Main benefit | Main cost |
|---|---|---|---|
| 45 °C | The lower limit for flow and dehydration | The viscosity is acceptable and the water can separate and vaporise | Below this the pump load is heavy and the degassing is slow |
| 70 °C | The compromise upper limit for a high water content | Balances the dehydration rate against the oxidation rate | The dehydration cycle is relatively longer |
| 80 °C | The upper limit set by the thermal stability and the viscosity | The lowest viscosity and the best flow | Faster oxidation, a rising acid value and a higher vapour load |
- Oil temperature below 45 °C for a long period: deal with the viscosity first, and do not jump to the conclusion that the vacuum system is at fault.
- Water content on the high side: bring the upper limit down to 70 °C and trade time for the quality of the oil.
- The vacuum reading becomes shallower while the water content does not fall: check whether the temperature has been set too high, so that the vapour pressure is holding the vacuum level up.
- The colour value darkens and there is a sour smell: the temperature is too high or the residence time is too long, and the setting has to be brought down and the batches shortened.
The higher the temperature the faster it does not go; it is a curve with a turning point, and past that point further heating only speeds up the ageing.