The figures on the vacuum gauge of a vacuum oil purifier are the readings that are most often looked at on site, and also the ones most often misread. The same machine reaches a deep negative pressure at no load and becomes clearly shallower at full load; these are not the same figure, the first is the capability of the equipment, the second is the working point of the equipment on the oil circuit. Confusing the two leads to errors in the selection, in the acceptance test and in the daily interpretation.
What each of the two readings describes
The ultimate vacuum is the best state the system can reach under ideal conditions: with the oil circuit clean, the medium dry, the pipework free of leaks, the vacuum pump in good condition and no large amount of oil and water vapour in the chamber, it is the lowest pressure the complete system can be drawn down to. It reflects the combined capability of the pumping speed of the pump, the sealing and the cleanliness of the pipework, and it is the figure used to compare different models when the equipment is selected.
The working vacuum is on the other hand the reading of the equipment under real operating conditions: the value that settles on the vacuum gauge while the oil is being heated, is flowing through the elements and there is a mixture of water vapour and oil vapour in the vacuum chamber. For the ZYD-100 two-stage vacuum oil purifier the working vacuum range is -0.08 ~ -0.0999 MPa and the ultimate vacuum is ≤5 Pa. The two figures are far apart, because under working conditions there is always water vapour and oil vapour in the vacuum chamber, and the partial pressures they form in the chamber constitute a floor that cannot be pressed below.
Why "the deeper the vacuum, the better" is not accurate
First, the working vacuum is constrained by the vapour pressure of the oil. The higher the temperature, the higher the saturated vapour pressure of the oil and the water, and the shallower the negative pressure that can be reached on the oil circuit; raising the temperature to obtain a deeper reading on the gauge often only shifts the burden onto the pumping speed of the vacuum pump and the life of the pump oil.
Second, oil with a high gas content and heavy contamination foams and atomises more easily under deep vacuum. A large volume of gas entering the vacuum chamber occupies the pumping speed, and the oil mist drawn away by the pump dilutes the pump oil and speeds up its emulsification and carbonisation, so the equipment becomes less durable rather than more.
Third, the altitude affects the vacuum level. The same equipment at a high-altitude site reaches a shallower working vacuum, because the fall in atmospheric pressure directly raises the floor for the absolute pressure, and data measured at sea level cannot therefore be applied directly to a site on a plateau.
When each of the two figures is used
The ultimate vacuum is used for comparison and for acceptance: when the equipment arrives, the ultimate vacuum should be retested under the specified conditions to confirm that the pump sets, the sealing and the pipework have no assembly defects. The working vacuum is used for the daily reading: how deep the negative pressure is during operation reflects the degassing load of that batch of oil, the water content of the incoming oil, and whether the system has a leak.
Recording the two readings separately and using them separately is what prevents the mistaken conclusion that the machine is sound because the no-load value can be reached, and it also avoids taking the normal shallowing at full load for a fault and going through a fault-finding routine on it.
There is one more useful dividing line in the fault-finding: a deterioration of the ultimate vacuum calls for a check of the manufacture and the assembly, a deterioration of the working vacuum calls for a check of the oil and of the operation. Keeping the two paths apart is much faster than trying the same knob again and again.
| Reading | What it describes | Measurement condition | ZYD-100 reference value | Typical use |
|---|---|---|---|---|
| Ultimate vacuum | The upper limit of the system capability | No load, dry, clean | ≤5 Pa | Comparison when selecting, acceptance on delivery |
| Working vacuum | The actual working point on the oil circuit | During the heating and the oil feed | -0.08 ~ -0.0999 MPa | Monitoring during operation, fault diagnosis |
- The reading on the gauge suddenly becomes shallower: usually the water or the gas content of the incoming oil has risen, air has entered the vacuum system, or the temperature setting has been turned up.
- The reading on the gauge does not reach the range: check the condition of the pump oil, the sealing and the pipe joints first, and then the water content of the incoming oil and the temperature setting.
- The no-load value can be reached but the running value cannot: this is normal, and the equipment should not be judged faulty on that basis.
The ultimate vacuum is where the machine came from and the working vacuum is its daily life; the two figures have to be recorded separately and used separately.