Vacuum degassing is far more difficult than vacuum drying, and the reason lies in the way water is dissolved in the oil. Heating drives part of the water out, but to separate it completely the oil and the water also have to be brought into intimate contact. Several approaches have been developed around that objective; three-dimensional thin-film flash evaporation is one of the more thoroughly proven ones.
The basic idea: make the contact area as large as possible
Ordinary evaporation relies on bubbles: the liquid is heated and water vapour rises out of the oil. The problem with that approach is that the water inside a bubble has to complete the whole rise before it can escape, and the bubbles readily coalesce and grow, which reduces the separation efficiency instead of improving it. Three-dimensional flash evaporation takes a different route: the oil is first atomised, and the droplets then fall onto the reactor and spread out into a thin oil film.
The film has two clear advantages. First, the surface area of the film is far larger than that of droplets of the same volume, so the mass transfer area between the gas phase and the liquid phase is opened up considerably. Second, the thin layer shortens the path along which water escapes from the oil. The flash evaporation section of the ZYD-100 consists of an atomiser, a three-dimensional flash evaporator, a flash reactor, an oil collection tank, a defoamer, a sight glass and a level gauge. Inside the reactor, several Raschig rings are stacked layer upon layer, so the oil mist sprayed onto them is repeatedly thinned and broken up again, forming a large film.
Temperature and vacuum are a pair, not two independent knobs
Raising the temperature does two things: it lowers the solubility of water in the oil, and it raises the evaporation rate so that the vacuum can carry the water vapour away. The vacuum level, on the other hand, reduces the partial pressure of the water vapour far enough for water to vaporise at a relatively low temperature. When the two are matched, deep dehydration can be achieved at a comparatively low temperature, which is where the term low-temperature vacuum comes from. The constant temperature range of the ZYD-100 is 20–80 °C, and it is precisely that range that leaves room for matching the two.
A common misconception about temperature
The claim that the hotter the oil, the faster the degassing only holds for the first part of the range. The marginal return of heating further falls away quickly while the cost rises clearly: insulating oil held at a high temperature oxidises faster, the acid value goes up and the colour deepens, and the treated oil ends up worse than it was before treatment. Bringing the upper temperature limit down to 70 °C where the water content is high is what brings the average temperature down.
| Operating condition | Temperature control tendency | Vacuum control tendency | What to watch |
|---|---|---|---|
| Before new oil is put into service | On the higher side, to speed up degassing | Above -0.08 MPa | The amount of water drawn off and the fluctuation of the vacuum level |
| Old oil with a high water content | Upper limit brought down to 70 °C | Keep it stable | The amount of foam and whether water is carried over |
| Continuous processing of insulating oil | Kept as low as possible | Stability comes first | The colour of the oil and the change in the acid value |
Flash evaporation raises the separation efficiency; it does not change the separation conditions. If the temperature and the vacuum level are not matched, no atomising structure can deliver what it promises.