When a site discusses degassing performance, the attention is almost entirely on the vacuum gauge: where the pointer stands, and whether the working vacuum counts as acceptable. But the oil in the vacuum chamber is a variable in its own right. It decides how much space the oil mist can occupy inside the chamber, and how much degassing time each batch actually gets.
The oil level decides how far the mist can spread
The basic action of vacuum degassing is to heat the oil to a certain temperature and spray it into a fine mist, so that the oil and the gas still held in it can separate under low pressure. The sprayed mist needs some travel inside the chamber to complete the separation of the two phases. The chamber volume is fixed, so the higher the level, the smaller the space left below the nozzle for the mist to spread; the mist is submerged by the incoming oil before it has dispersed, and the degassing is completed in a state where the two phases have not separated.
The opposite does not work either. The mist has to fall back under its own weight, and with too low a level the fall is too short. The droplets return to the surface still carrying a large amount of dissolved gas, and the sprayed oil mixes with the oil in the bottom of the tank, which amounts to collecting the oil that has not been degassed.
The working vacuum of the ZYD series is -0.08 ~ -0.0999 MPa, and the ultimate vacuum is ≤5 Pa. These two figures describe the negative pressure the system can reach, but for them to act on the oil the oil has to have entered the vacuum zone as a mist. With the level wrong, the best vacuum in the world achieves nothing.
The oil level and the inlet rate hold each other back
In a vacuum chamber in operation the level is not a fixed value. The inlet raises it and the outlet lowers it, and the difference between the two decides how fast the level moves. If the site changes the throughput setting, what actually changes is the balance across the whole chain.
- The inlet exceeds the outlet: the level keeps rising, the space for atomisation is compressed step by step, and the degassing result slides down with it.
- The outlet exceeds the inlet: the level keeps falling and there is plenty of room in the chamber, but the pump may start to lose suction.
- When the two match, the level settles at a position, and only then is the degassing efficiency the real capability of this machine.
The oil itself also affects this balance. An oil of high viscosity moves slowly in the chamber, so the level reading lags behind the actual inlet. An oil with a lot of water forms foam once it has vaporised, and the volume taken by the foam layer pushes the level up, even though the real quantity of oil in the tank has not increased.
| What is seen on site | The more likely cause | How to confirm it |
|---|---|---|
| The level reading is high and the gas content after degassing is still high | The space for the mist to spread has been compressed | Reduce the inlet rate and watch how the level and the gas content move |
| The level reading jumps up and down and there is a lot of foam in the chamber | The foam layer takes up space and the level sensor is reading the foam | Check whether the defoaming action comes in in time |
| The level rises slowly while the outlet oil temperature is normal | The inlet and outlet quantities do not match | Compare the present throughput setting with the actual running time |
| The level is low and the pump loses suction or makes noise | The outlet quantity is larger than the inlet quantity | Stop the outlet and watch whether the level comes back up |
Where level detection sits in the control chain
Precisely because the level has a direct effect on the degassing result, it is not only a number displayed on the panel. On models with infrared level detection the sensing element sends the level signal into the control circuit, and the machine refuses to keep running when the level falls below the lower limit or rises above the upper one. Together with infrared foam detection and automatic defoaming, a foam layer pushing the level up is dealt with before the level ever goes out of range.
The position of the level sensing element matters just as much. Fitted too close to the oil inlet, the sprayed mist interferes with the reading directly; fitted too close to the oil outlet, the dip in the surface caused by the drawn-off oil is counted in. On site both show up as a level display that does not agree with the real quantity of oil, and the control system is making its decisions from that reading.
How to judge on site whether the level is right
There is no need to open the chamber. A first judgement can be made from a few observable things: first, whether the inlet and outlet flow rates are steady over time and the level stays essentially still during the run; second, whether the vacuum can hold the working range, because a vacuum that will not come up while the level is still rising usually means the space for atomisation has been squeezed out; third, whether the gas content of the degassed oil holds at ≤0.01% (GB/T 423).
Taken together these three are more reliable than any single reading. If the level is stable, the vacuum is stable and the gas content still will not come down, then the level is not the cause and the search goes back to the oil temperature, the state of the flash evaporation and the vacuum system itself.
The oil level in a vacuum chamber is not there so that more oil can be loaded; it is the working space left for the mist, and a chamber filled too full cannot be degassed properly however steady the negative pressure.
