In a two-stage vacuum oil purifier the extraction is shared by two pumps: the X-100 vacuum pump in front and the JRP-500 roots blower behind it. The pipe between the two is not a plain connection. It also does a job that is easily overlooked: it traps the oil mist the first pump carries over, so that the gas reaching the roots blower is as clean as possible. This section is usually called the interstage condensation stage.
What happens when the two are connected directly
The first pump removes not only gas but also the fine oil mist carried along with the flow. Once that mist enters the roots blower it settles on the rotor and on the inner wall of the pump casing, and three consequences follow: the pump oil is emulsified and its lubricating film is destroyed; the rotor and the bearings wear faster; the ultimate vacuum capability of the pump drops. What the site sees is a machine that cannot reach the ultimate vacuum ≤5 Pa of its rating, and a fault that is hard to trace back to the mist during maintenance, because the pump itself has not failed.
The interstage condensation stage exists to remove that mist between the two stages. The mixed gas is cooled as it enters this section, the mist condenses into droplets as the temperature falls, the droplets separate from the gas, the liquid runs away through the drain, and only the gas phase continues to the roots blower. The lower the temperature of the gas before it enters the next stage, the lower its saturated vapour pressure, and the larger the share of residual oil vapour that the condensate carries down. The working conditions of the roots blower are better for it.
This section sets the final vacuum level
The working vacuum of the ZYD series is -0.08 ~ -0.0999 MPa and the ultimate vacuum is ≤5 Pa. The first figure is the extraction capability during normal running, the second is the best level the machine can reach. A two-stage arrangement gets close to the second because the first pump compresses the gas to a certain pressure and the roots blower continues the extraction at that pressure. Once the interstage condensation stage fails, gas contaminated with oil mist enters the roots blower, its boosting capability is held back, the ultimate vacuum no longer meets its figure, and the working vacuum also sits below the normal value.
| Item | Value |
|---|---|
| Ultimate vacuum | ≤5 Pa |
| Working vacuum | -0.08 ~ -0.0999 MPa |
| First stage extraction | X-100 vacuum pump |
| Second stage boosting | JRP-500 roots blower |
| Gas content in oil | ≤0.01% (GB/T 423) |
It is worth noting that the failure of the condensation stage is a gradual process. It does not stop the machine at once, and the alarm does not necessarily act. The machine simply drifts away from its rated performance. So when the ultimate vacuum does not meet its figure and both pumps themselves are normal, the gas path should be followed rather than the pump oil replaced and the vacuum pulled down again and again.
How the site checks whether this section is working
- Look at the drain of the interstage condensation stage: after a period of running there should be condensate coming away. The amount is not large, but it should be continuous and steady.
- Look at the vacuum pump oil: if the oil drawn off is clearly pale, cloudy or emulsified, the mist has already bypassed the condensation stage.
- Check whether the ultimate vacuum holds: on a new machine, or one that has just been serviced, the no-load extraction should come close to the ≤5 Pa level.
- Check that the drain is clear: once the drain line is blocked, condensate collects inside the section and the condensation stage becomes a chamber that holds pressure.
The first of these is the one most easily missed. If the interstage condensation stage drains nothing, the machine still pulls down to a working vacuum of -0.08 ~ -0.0999 MPa and the operator panel shows nothing unusual, yet the roots blower is already being consumed slowly by the mist. By the time the ultimate vacuum clearly falls short, that build-up has been running for a while. Putting the drain into the routine shift check is much easier than taking the pump apart after the trouble appears.
The section between the two stages of a two-stage vacuum is not there to add a step; it is there so that the roots blower receives clean gas.
