There is one row in the parameter table that is easily skipped over: the working vacuum of -0.08 ~ -0.0999 MPa. It states a range, not a fixed point, and the ultimate vacuum of ≤5 Pa is on another row and at another order of magnitude. Read the two rows as one, and on site this leads to conclusions of the kind that a reading of 5 Pa on the vacuum gauge means the machine has been working in that state all along, whereas the readings the machine actually runs at are not like that.
The degassing process of a vacuum oil purifier needs the oil to be at a low pressure, so that the water can vaporise and separate out and the gas dissolved in the oil can escape. The lower the pressure, the greater the driving force for the degassing, and the better the degassing efficiency and the final gas content. The problem is that the lower the pressure is pushed, the heavier the load on the pump set, and the more likely the pumping speed is to fall behind when the oil and gas load is high. The working vacuum is therefore not a single value that is better the lower it goes, but a range that can be chosen to suit the operating condition.
What the two ends of the range each govern
-0.08 MPa is the shallow end of this band and -0.0999 MPa is the deep end. The deep end means a lower absolute pressure inside the tank and less residual gas, so the oil coming out is easier to bring within the standard limits for gas content and water content; the price is a lower pumping speed and a vacuum level that may not hold once the oil and gas load becomes large. The shallow end makes lower demands on the pump set and is easier to keep stable when dirty oil with a high contamination level is processed continuously.
Which end to choose therefore depends on two things: the condition of the oil coming in, and what this stage of the process is meant to achieve. For new oil or for deep purification, work towards the deep end; for long continuous runs with a high level of impurities in the incoming oil, the question is whether the vacuum level can be held steady, rather than clinging to one number.
| Reading | What it describes | The state in which it appears |
|---|---|---|
| -0.08 MPa | The shallow end of the working vacuum | Routine degassing operation, with a relatively light load on the pump set |
| -0.0999 MPa | The deep end of the working vacuum | Deep purification, new oil, or conditions with a tight limit on gas content |
| ≤5 Pa | Ultimate vacuum | The limit the pump set can reach when there is no oil in the tank and no oil and gas load |
Why the ultimate vacuum is not a working point
The ultimate vacuum of ≤5 Pa describes the capacity limit of the pump set itself, measured with no oil in the tank and no oil and gas load. It is a reference value for one machine, used to judge how good this vacuum system is, and not a reading that appears for any length of time during processing. In normal degassing there is always oil mist and an oil and gas load in the tank, and the pumping speed, the sealing condition and the condition of the oil all pull the actual reading away from the limit.
Only when the two quantities are taken together is the judgement complete: the ultimate vacuum says what level this system can reach, and the working vacuum says what level the process allows it to sit at over the long term. When judging on site whether the degassing section has a problem, what matters is whether the reading at the tank can hold steady inside the working range, not whether it has touched 5 Pa.
Further factors change these two readings
- Altitude. The higher the altitude, the lower the atmospheric pressure, and the lower the ultimate absolute pressure the same pump set can reach on site; the ultimate vacuum reading of one machine is therefore not the same at different altitudes, and the site altitude has to be stated at the selection stage.
- Ambient temperature. The working ambient temperature range of the machine is -20–45 °C; when the temperature is low the viscosity of the vacuum pump oil rises and the pumping speed falls, so the time needed to reach the same vacuum level gets longer. This point is particularly noticeable on northern sites in winter.
- Condition of the oil. When the incoming oil has a high water content, a high gas content or a high viscosity, the oil and gas load rises, the pumping speed falls behind and the vacuum level settles near the shallow end of the range. This is not a fault in the machine; the operating condition has pushed the working point back.
- Sealing condition. When the vacuum level will not go up and cannot be held steady however it is adjusted, check the sealing and the low points of the pipework first rather than changing the setpoint repeatedly. The setpoint is only a target, and leaks in the pipework and residual liquid can make the target itself unattainable.
The working vacuum is a range, which means that the depth of degassing can be chosen to suit the operating condition; the ultimate vacuum is an upper-limit reference, not a reading the machine will sit at all the time.
