The efficiency of the vacuum degassing step depends on the pressure state the oil is in when it enters the tank. The usual site practice is to pump the oil into the vacuum tank first and then start the vacuum pump to draw down the pressure; that order causes no visible problem when the machine has just been started and the tank still holds air, because the vacuum pump pulls the pressure down eventually either way. But it splits the degassing into two stages: the first stage removes the air from the tank, and only the second stage actually deals with the gas and water dissolved in the oil. If the changeover is handled badly, the time the oil spends at atmospheric pressure is wasted.
Why filling the oil first lengthens the degassing time
Oil is stable at atmospheric pressure, with the dissolved gas and water held in the oil phase. Only when the pressure is brought low enough does the boiling point of the oil fall, and only then can water and gas leave the oil phase. The order should therefore be to draw the vacuum first and establish the negative pressure, then let the oil in, so that it is atomised under low pressure as soon as it enters the tank and its dissolved components are drawn off. If the oil goes in first, the air in the tank has to be displaced before the tank pressure can come down; during that period the oil inlet valve is either closed and the tank runs idle, or open with the oil entering at a pressure close to atmospheric. Both options push the effective degassing time further back.
The oil feed rate has to keep up with the pumping speed
The pumping speed of the vacuum pump is limited. If the oil entering the tank exceeds the amount of gas the pump can take off under negative pressure, the pressure builds up, the working vacuum drifts away from -0.08 MPa towards the shallower end of the scale and stops there. Feed the oil too slowly, and the pump runs at low load for long periods, which is not efficient either. The site judgement is to watch whether the tank pressure gauge reading is stable: if it holds around -0.08 MPa under load, the oil feed rate and the pumping speed are broadly matched; if the reading keeps drifting towards the shallower vacuum, the oil feed is probably too large, or the resistance from a rising filter element differential pressure is holding the pumping speed back.
Ultimate vacuum and working vacuum are not the same thing
The ultimate vacuum on the nameplate, ≤5 Pa, is the limit the machine reaches under ideal conditions with no oil and no load, and it reflects the upper limit of the pump set; the working vacuum of -0.08 ~ -0.0999 MPa is the range during loaded running and reflects the operating point in actual degassing. The two differ by several orders of magnitude. This is not a limitation of the machine: while it runs with oil, water vapour and gas are continuously fed into the vacuum system, the exhaust rate can never catch up with the intake rate, and the pressure therefore settles at a point well away from the ultimate value. Judging a machine that is currently processing oil against its ultimate vacuum will always give the wrong answer.
| Process sequence | Tank pressure state | Observed on site | Result |
|---|---|---|---|
| Vacuum first, then oil | Negative pressure established, approx. -0.08 MPa | The oil inlet runs smoothly, the oil surface does not churn | The oil begins degassing as soon as it enters the tank |
| Vacuum and oil together | Negative pressure still being established | The oil surface churns, the oil inlet feels restricted | Gas intake exceeds the pumping speed, efficiency is reduced |
| Oil first, then vacuum | Close to atmospheric pressure | Pressure builds up in the tank, the pump load is heavy at the moment of start-up | Air is forced into the oil, one extra evacuation stage is consumed |
The sequence to confirm on site
- Confirm the tank has reached the working vacuum range before opening the oil inlet valve
- Adjust the oil inlet valve opening from small to large, watching it together with the tank pressure reading
- Only when the loaded reading holds steady in the working range are the oil feed rate and the pumping speed matched
- On shutdown, close the oil inlet valve before stopping the pump, so that the tank stays under negative pressure
Drawing the vacuum before filling the oil is not a matter of procedural preference, it is a question of which pressure stage the degassing happens in; with the order right the oil starts working as soon as it enters the tank, with the order wrong the same tank of oil takes one extra evacuation stage.