On site it is often said that the vacuum will not come up or that vacuuming is too slow, and the first reaction is usually that the pump has a fault. But the amount of gas that has to be removed before the system reaches a working vacuum of -0.08 ~ -0.0999 MPa is set first by the volume of the tank and the pipework, while the pumping speed only decides how fast the gas is removed. The same X-100 vacuum pump with a JRP-500 roots blower fitted on tanks of different volumes takes different waiting times, and that is normal.
Vacuuming removes a total quantity of gas, and the volume sets that total
At the start of a vacuum run the tank and the pipework are filled with air at close to atmospheric pressure. Taking that air down to the working vacuum means cutting a large part of the total gas volume in the chamber. That total is proportional to the volume being evacuated and to the starting pressure, and the volume is fixed by the machine layout: the dimensions of the ZYD-100 are 1200×1400×1800 mm, the ZYD-200 is 1800×1600×2100 mm and the ZYD-300 is 2200×1800×2200 mm. The larger the tank and the pipework, the more gas has to be removed at the same pumping speed, and the waiting time is longer in proportion.
This is why the judgement that the pumping speed is sufficient answers only the rate of gas removal and not the total quantity of gas. The flow rate quoted when the machine is selected (6000 L/h for the ZYD-100, 12000 L/h for the ZYD-200, 18000 L/h for the ZYD-300) describes the capacity for continuous oil treatment, which is not the same thing as the time taken to draw the tank down to the working vacuum in one pass. The two have to be looked at separately.
The pumping speed only decides how fast the gas is removed
The pumping speed is the amount of gas removed per unit time. The time needed to reach a given vacuum level depends on the ratio of the total gas volume to be removed to the pumping speed. Doubling the pumping speed halves the time; but if the volume grows by the same factor at the same time, that gain is cancelled out. Two-stage pumping follows the same rule: the X-100 vacuum pump works as the front stage and the JRP-500 roots blower as the rear stage, pushing the ultimate vacuum down to ≤5 Pa. The boosting capability of the roots blower depends on the pressure difference supplied by the front stage, so the effective pumping speed of the set is not the simple sum of the two pumps.
The more important part is the second half of the vacuum run. Once the air has been drawn off, what remains in the chamber is mainly water vapour and gas released from the oil. Pumping on from that point, the pressure in the chamber settles at an equilibrium position set by the oil temperature and the water content of the oil, and it cannot be pulled lower however much extra time is allowed. -0.08 ~ -0.0999 MPa is the working range in normal operation, and the time taken to reach it is set jointly by the volume and the gas load, with the second half of the run governed mainly by the load.
Pipework, joints and residual liquid change that time
The tank is not the only volume; the way it is piped counts as well. The longer the pipe run, the more elbows and the looser the joints, the larger the effective volume and the longer the vacuum time. Any flange or seal that is not properly tightened shows up on the vacuum side as a slow leak. It both lets the gas that has been removed flow back in and drags the effective pumping speed down, so the time is extended instead of the vacuum level settling at some position.
There is one more volume source that is often overlooked: the residual oil in the tank and the liquid held in the pipework. The residual liquid keeps vaporising during pumping, which amounts to adding gas to the chamber while it is being evacuated. This load is more noticeable at higher temperatures, so the vacuum time of the same machine in summer and in winter is not the same.
How to budget the vacuum time on site
Split the time into two parts. The first is set by the volume and the pumping speed, the second by the water in the oil. The first part can be checked against the volume and the sealing condition by closing the valve and holding the pressure: shut off the pump inlet, wait a while and watch how far the pressure rises. A fast rise means there is a leak on the vacuum side, a slow rise means the volume and the seals are both sound. The second part can only be traded against time and temperature: the higher the temperature, the faster the released water vaporises, and the more water the oil holds, the longer this part takes.
| What you see on site | The more likely explanation | How to confirm it |
|---|---|---|
| The same pump and the same oil, but the vacuum time is clearly longer | The pipework arrangement has changed and the effective volume has grown | Check the length of the pipe run, the number of joints and how tight they are |
| The first part is normal but the second part stops improving for a long time | The water vapour load in the chamber is too high | Check the initial water content of the oil and whether the oil temperature is stable at the set value |
| The pressure rises quickly after the valve is closed | There is a leak point on the vacuum side | Check the flanges, the seals and the valve seats section by section |
| The pressure rises very slowly after the valve is closed and the vacuuming is still slow | The volume is too large or too much liquid is held in the pipework | Check the liquid in the pipework and the residual oil that has not been drained |
| The time can be reproduced after switching to the same batch of oil with the same settings | The time is set by the volume and the load and has nothing to do with the condition of the pump | Record the time together with the oil temperature and the initial water content |
- Record three figures on every run: the start and end time of the vacuum step, the oil temperature and the initial water content. Without one of them the next run cannot be compared.
- At acceptance and during maintenance, treat the vacuum time as a repeatable record rather than an impression that the machine pumps slowly.
- Different models and different tank sizes have different time baselines, so a baseline for the machine in front of you has to be established again after a machine is changed.
- To judge whether the pump is abnormal, look at whether the ultimate vacuum holds at ≤5 Pa; that is more reliable than looking only at how fast the working vacuum is reached.
If vacuuming is slow, put the volume and the gas load into the calculation first; the pumping speed is only one of the divisors, and the rest is the time budget.
