The first vacuum pull-down on a new unit takes a long time, and it is often reported as a fault. The first pull-down works against a system full of air rather than against a machine that has already been run in, and while the system moves from atmospheric pressure towards vacuum the pumping speed keeps falling. Separating those two points explains why the first pull-down is always slower than the ones after it.
Slow is the process, not reaching the target is the problem
A vacuum pull-down has two stages. In the first, the air in the pipework, in the oil and gas separation section and in the pump cavities is drawn away step by step. In this stage the pumping speed falls clearly: the pumping speed is given relative to the pump inlet pressure, and the lower the inlet pressure, the less air is removed per unit of time. Drawing from -0.02 MPa down to -0.08 MPa and continuing from -0.08 MPa down to the ultimate vacuum are two sections of the journey that take completely different times.
The working vacuum of the ZYD-100 is -0.08 ~ -0.0999 MPa and the ultimate vacuum is ≤5 Pa. The working vacuum is the level reached in normal running, and the ultimate vacuum is the best this machine can do. The time it takes to go from atmospheric pressure to the ultimate vacuum is an inherent feature of this arrangement, not a fault.
The time taken in the second stage is governed more by the performance of the vacuum pump itself. The extraction of the ZYD-100 is carried out in two stages, the X-100 vacuum pump and the JRP-500 roots blower, with the first stage compressing the gas to a certain pressure and the second continuing the extraction at that pressure. The point of the two stages in series is that the second stage works at a lower pressure and takes over the part of the gas that the first stage could not draw in.
Where the air actually sits
The air faced by the first pull-down is not only in the oil tank. It is spread across the whole system:
- The space inside the oil tank and the separation section, whose volume is set by the tank dimensions. The dimensions of the ZYD-100 are 1200×1400×1800 mm.
- The inside of the pipework that connects the sections, where elbows, valves and flanges each take up part of the volume; the longer the pipework, the larger this item.
- The vacuum pump cavity and the roots blower cavity themselves, especially the part above the pump oil that is not filled with oil.
- The small clearances on the sealing faces and the places where the valves do not close fully, and these are the only places where the pull-down can get slower as time goes on.
The first three are fixed and belong to the one-off cost: once they have been drawn off, very little air is left in the system, and every later pull-down faces the same small remainder, which is why the later ones are much faster. The fourth is different, it gets worse with time, and that is the point that tells you whether a slow first pull-down is abnormal.
How the first pull-down should be done
On sequence: first confirm that every valve that should be closed is closed and that the pipework connections and flanges are tight, then start the X-100 vacuum pump. Bring the JRP-500 roots blower in only once the working vacuum has settled, so that the two stages each work in their own range. Record two times over the whole run: how long to reach -0.08 MPa, and how long to go from -0.08 MPa to the ultimate vacuum. Write those two figures together with the ambient temperature of the day into the machine log; they are the baseline against which the later pull-downs are judged.
There is one more thing to watch: the pump oil level in the vacuum pump. With too little pump oil there is no proper oil seal at the top of the pump cavity, gas passes across the cavity, the pull-down stalls at a pressure it cannot pass, and at the same time the noise and the temperature rise of the pump are both higher than they should be.
When slow is abnormal
| What is seen | Likely cause | How to confirm |
|---|---|---|
| Pressure stays at one value and will not fall | A leak in the pipework or at a flange | Isolate section by section, hold the pressure and find the leak |
| Pressure rises again after a stage has been reached | Oil carried into the separation or condensation section | Check that the drain of the condensation section is clear |
| Pump noise high and temperature rise fast | Pump oil too low or gas passing across the pump cavity | Check the pump oil level and the condition of the oil |
| First one slow, later ones normal | Residual air in the system, a normal part of the process | Use the two recorded times as the baseline |
Put this table together with the observations above and the question of a slow first pull-down has a clear basis for judgement: slow but eventually reaching the target, with the working vacuum and the ultimate vacuum both stable, is the process; slow and not reaching the target is a problem, and the table gives the order to work through.
A slow first pull-down is mostly the system clearing its air; slow and not reaching the target is when the machine has a problem.
