In an oil purifier the foam is not something that is distributed evenly; it has clear paths of entry. For a machine that treats oil with water in it or oil with a high gas content, whether the foam can be stopped before it reaches the vacuum chamber decides directly whether the degassing efficiency of the whole machine is reduced.
Three sources, three different characters
The first type is entrained air. A loose joint on the return line, a pipe end reaching below the liquid surface and drawing in air, or the negative pressure at the tank opening sucking in air all roll air into the oil. The interfacial film of these bubbles is thin and most of them can coalesce and rise; their character is that they arrive fast and in quantity.
The second type is supersaturated water vapour. Once the oil is heated to a certain temperature, the solubility of the water falls and a local supersaturation appears, and the fine droplets that separate out are stabilised into foam in the oil. This is the most common type, because the aim of the dehydration is in itself to drive the water out, and the foaming is almost a necessary side reaction.
The third type is emulsification. The surface active components and the oxidation products in the oil form an interfacial film on the surface of the droplets, so the foam is more stable and the defoaming is slower, and in the short time it does not look as if it is subsiding.
What happens when the foam enters the vacuum chamber
At the moment the pressure drops sharply, the gas in the foam expands violently and the volume is multiplied several times over. The effective volume of the vacuum chamber is filled with the gas, the pumping speed is wasted and the ultimate vacuum cannot be reached. This is the direct reason why the reading on the gauge clearly becomes shallower when there is foam.
The more troublesome effect is the entrainment of gas and liquid. The droplets produced when the foam bursts are carried with the gas flow into the vacuum chamber, and part of the oil mist is drawn off by the vacuum pump, so the pump oil is diluted by the oil and the gas and the emulsification and the carbonisation speed up. The end of this chain is a pump oil in a worse condition and a further fall in the ultimate vacuum, and it is a process that reinforces itself.
When the foam covers the heating surface it also causes uneven local heating, so that the temperature control reading deviates from the actual oil temperature.
Two routes for removing the foam
The physical route relies on mechanical bubble breaking and physical separation. Baffles, meshes and swirl structures change the gas flow pattern, the impact of the liquid phase breaks the bubbles up, and the centrifugal separation uses the density difference to separate the gas from the liquid. The advantage of these arrangements is that they introduce no foreign substance into the oil.
The chemical route means adding a defoaming or de-emulsifying agent. It works quickly, but the price is that a new component is introduced into the oil; for the oils that are sensitive to the impurities, such as the insulating oil, the effect has to be assessed first and it cannot be assumed to be acceptable.
Where the detection is fitted decides whether it is of any use
The point at which the infrared foam detection with the automatic defoaming is connected is critical: it has to be fitted in the buffer or the separation section before the vacuum chamber, and the defoaming action is triggered when a rising foam level is detected. If it is fitted after the vacuum chamber, the foam has long since expanded and the protection no longer means anything.
| Source of the foam | Character | Consequence of entering the vacuum chamber | First measure |
|---|---|---|---|
| Entrained air | Large quantity, arrives fast, merges easily | The volume is occupied, the pumping speed is wasted | The sealing of the return line and the position of the air inlet |
| Supersaturated water vapour | Varies with the temperature and the water content | As above, with oil mist entrained | A reasonable temperature control setting, separation in the buffer section |
| Emulsification | Stable interfacial film, slow defoaming | A continuous supply of foam, an unstable vacuum | Break the emulsion first, assess an agent if necessary |
- The defoaming section has to be before the vacuum chamber, it cannot be fitted behind it.
- The foam comes and goes and the vacuum gauge becomes shallower at the same time: check the return line for the draw-in of air first.
- A large amount of foam with a high water content: lower the temperature control limit and reduce the foaming from the vaporisation.
- A defoaming agent used with insulating oil always needs a compatibility assessment first.
My conclusion is that the foam is not something a filter can deal with; it has to be stopped before it reaches the vacuum chamber, otherwise every step after that pays for it.