The bottom of the vacuum chamber is the part of an oil purifier that is least often seen and most easily forgotten. When a run finishes, a thin layer is left at the bottom of the tank: the water phase that has separated out of the oil, together with the solid particles that came in with it. In most cases this deposit does not affect the degassing result of the run it forms in, but it does not stay where it is. The next filling, heating and atomisation lift it again, and it returns to the system on that run.
Two sources of the deposit: the water phase and solid particles
The first source is water. When insulating oil is atomised in the vacuum chamber its boiling point drops sharply and the water is drawn off as vapour; once the amount dissolved in the oil exceeds the saturation value at that temperature, the surplus separates out as droplets. The higher the temperature and the more water the incoming oil holds, the more water is released. These droplets are heavier than the oil, so they fall to the lowest point of the tank and collect in a layer that cannot be flushed out through the oil inlet, because it is not in the pipework at all but in the bottom of the tank.
The second source is solid particles. The particles carried in by the inlet pipework, the debris shed by the upstream filter section and the particles worn off the seals all find a resting place at the bottom of the tank. The fine filter stage of the ZYD-100 is fitted as 5 elements of 150-500-5 in parallel, and the pipe run in front of those elements is where the particles collect. Once the pipework carries larger particles they settle on the bottom of the pipe first, are then swept into the tank by the oil flow, and finally build up a layer on the tank floor.
Three paths by which the deposit reaches the next run
The first path is resuspension. When the next run starts the temperature in the tank rises and the pressure falls, and the water-bearing deposit on the floor is lifted by the oil flow and the atomising gas. It turns into a suspended state and disperses into the oil. Water vaporises before the oil does, so the fine water droplets that are lifted up quickly turn into vapour in the vacuum, and the gas content measured on that run comes out higher than the one before, while the water content need not improve at the same time: part of the water has been drawn off and part of it has gone back into the oil.
The second path is the circulation back to the inlet. With the machine circulating, the deposit lifted from the floor goes back to the inlet side with the oil flow and enters the filter sections in front of it. The coarse elements are commonly 150-250-50 and 110-250-50 and the second stage is 110-400-20; the dirt holding capacity of those sections is slowly used up by the particles that circulate again and again, which shows as the differential pressure rising faster than it would on a new element.
The third path is the drain. When the oil is drained from the tank during maintenance, the deposit goes out with it. If the drain line runs into a return tank or a collecting tank, that batch of oil carrying water and sludge contaminates the downstream side, and when it comes back into the system it is the next source of particles. So before draining, confirm where that batch of oil is going.
How the state of the tank floor is judged on site
The most direct evidence appears when the oil is drained: if a grey-brown or dull film is left at the bottom of the container instead of a clear layer, the deposit has already entered the oil path. The second sign is appearance. A deposit containing water left in the oil for long enough makes the oil look cloudy, which is not the same as the normal darkening that oil shows after filtration; cloudiness means emulsification. The third sign comes from the data: if the same batch of oil is degassed twice in a row and the gas content of the second run is clearly higher than the first while the breakdown voltage does not improve with it, the water left in the tank from the previous run can be suspected of having come back.
| What you see on site | The more likely explanation | How to confirm it |
|---|---|---|
| The drained oil leaves a grey-brown film at the bottom of the container | The deposit from the tank floor goes out with the oil | Let the oil stand after draining and look at the separation and the sediment |
| The same batch is degassed twice and the gas content of the second run is higher | The water left in the tank by the previous run has been lifted again | Compare the water content and the gas content recorded on the two runs |
| The differential pressure of a newly fitted coarse element rises clearly faster | Particles from the tank floor keep entering the upstream filter section | Record the rate at which the differential pressure rises before and after the element change |
| Water still runs down the tank wall after the oil reaches the set temperature | There is a lot of water on the tank floor and separation is still going on | Check the initial water content and whether the temperature setting is inside the 45–80 °C range |
When and how to clean the tank
- Do the draining and the tank floor check as one job every time the fine elements are changed, rather than treating the drain as a separate action.
- When the deposit is confirmed to contain solid particles, clean the upstream filter sections together with the tank; cleaning the vacuum chamber alone does not remove the source of the particles.
- Rebuild the baseline for the vacuuming time after the first run that follows a clean, because the free space in the tank and the amount of residual liquid have both changed.
- Record the quantity of oil drained and the state of the deposit after every clean, as the reference for the next assessment.
The layer on the bottom of the vacuum chamber does not disappear on its own; it waits for the next batch of oil to carry it back. The deposit is the bill left by the previous run, and the next run pays it.
