After the vacuum chamber has been pulled down to its ultimate vacuum, there is one step that is easily overlooked: whether to break the vacuum, that is to release the negative pressure in the chamber, before the oil is charged. The step looks like nothing more than opening a valve, but it decides two things at once — whether the chamber holds air when the oil goes in, and in which state a sealing defect is easier to find.
What the venting step actually does
The purpose of pulling the vacuum is to remove the gas from the chamber so that the oil can degas under low pressure. The ultimate vacuum of the ZYD-100 is ≤5 Pa and the working vacuum is -0.08 ~ -0.0999 MPa; at that point very little gas is left in the chamber. If the venting valve is opened, the pressure in the chamber returns to atmospheric and the vessel is under positive pressure again. If the oil is charged directly, the chamber stays under negative pressure and the air is trapped inside the system.
The difference between the two states is not in the pressure value itself but in which way the gas moves. With positive pressure inside the chamber, a sealing defect lets air leak outwards and a continuous flow crosses the defect, which makes it easy to detect. Under negative pressure the air is drawn in from outside, and the same defect becomes an inlet for air.
The cost of each approach
With venting first, the chamber returns to atmospheric pressure and the ZH screw oil pump feeds the oil into it. As the level rises, the air in the chamber is pushed towards the top and the exhaust side. That air then has to be removed by the vacuum section, which is an extra load. The benefit is an opportunity to check the sealing: after venting, hold the chamber at positive pressure, and a fast pressure drop points to a leak, with no oil in the machine yet, which makes the repair clean.
Charging directly without venting keeps the chamber under negative pressure, so little air enters while the oil is filling, and the oil is at low pressure from the first moment it enters the chamber. For a machine that runs continuously, stopping to vent and pulling the vacuum again throws away the vacuum already established, and the throughput lost in that period is wasted. The cost is that sealing defects do not show up before the oil is charged and can only be inferred from how fast the vacuum is pulled and from the final gas content figure.
| Approach | State in the chamber | What it gives you | What it costs |
|---|---|---|---|
| Vent first, then charge | Back to atmospheric pressure | A leak can be found by a positive pressure hold before any oil goes in | The air in the chamber has to be pumped out again by the vacuum section |
| Charge directly under vacuum | Held at -0.08 ~ -0.0999 MPa | The oil degasses effectively from the moment it enters | Leaks are harder to expose before charging |
When venting first makes sense
- The first vacuum pull after a new machine is installed, or a trial run after the pipework has been opened and reassembled: the sealing has not yet been verified with oil, so vent and do a positive pressure hold to stop the leak before the oil goes in.
- The previous run was abnormal: the vacuum level will not rise and the gas content stays outside ≤0.01% (GB/T 423). To decide whether it is a leak or a process problem, venting and holding the pressure narrows the question down to the sealing of the chamber.
- The chamber has to be serviced, cleaned or opened for inspection: the pressure must be released first, otherwise the oil vapour in the chamber comes out under pressure the moment the door is opened.
- Normal continuous production: there is no need to vent in the middle. Interrupting the vacuum and rebuilding it wastes the throughput of that period.
What to look at afterwards
Whether to vent is a process step; the result is what counts in the end. The ZYD-100 is specified at ≤3 ppm water content (GB/T 7600), ≥65 kV breakdown voltage (GB/T 507) and NAS class 5 cleanliness (NAS 1638). If a large amount of air is carried in with the oil, all of these figures move in the wrong direction, not only the gas content. In reverse, if the positive pressure hold after venting finds no leak and the gas content is still high after charging, the problem is more likely in the oil itself or in the temperature conditions than in the sealing of the chamber.
Keeping the action and the judgement apart is convenient: record the time of the venting and the vacuum reading at that moment, then record the gas content after degassing. Put the two sets of data side by side and you can tell what this step actually changed.
Venting is neither a superfluous step nor a mandatory one; it simply brings the question of whether air enters the chamber out in the open before the oil does.
