They are both called vacuum oil purifiers, and a single-stage and a two-stage machine do not look very different from the outside: both have a vacuum tank, a heater, a pump set and a filter. The real difference lies in how the gas path runs inside, and it directly decides how dirty an oil the machine can handle and how far the degassing can be taken.
The structural difference between single-stage and two-stage
A single-stage structure has only one vacuum separation chamber, where the oil mist completes the gas-liquid separation in a single pass: the gas is drawn off and the oil returns. The first stage carries almost the whole water vapour load, so once the incoming oil has a high water content the gas volume in the chamber rises sharply, the separation efficiency falls noticeably, and a deeper vacuum is needed to compensate.
A two-stage structure splits the separation into two steps: the first stage removes most of the water and the gas, and the second stage deals with the residual water vapour at a high vacuum. The benefit of this division is that the working conditions of each stage are kept within the range it is good at: the first stage does not have to carry the whole load, and the second stage works on a gas phase that is already very clean, so the depth of degassing is more controllable.
The pump set decides how deep the vacuum goes
The separation efficiency is ultimately limited by the vacuum capability. The ZYD-100 uses a rotary vane vacuum pump X-63 in combination with a Roots pump JRP-500; the Roots pump acts as a booster, compressing the gas extracted by the first stage and passing it into the vane pump. The two pumps work in series, which is what holds the working vacuum steady at -0.08 ~ -0.0999 MPa and the ultimate vacuum at ≤5 Pa. A common mistake is to treat the pressure reached as the only sign of how good a machine is; what matters more is whether that vacuum can be held over time.
Where the difference shows
- Degree of dehydration: after treatment in the ZYD-100 the water content is ≤3 ppm (GB/T 7600), and a single-stage machine often cannot hold this under the same conditions.
- Control of the gas content: after treatment the gas content is ≤0.01% (GB/T 423), a level that is a hard requirement in applications sensitive to gas.
- Resistance to shock: when old oil with a high water content is fed into the machine, a two-stage structure does not become unstable because of a momentary surge in the gas volume.
- The cost: a more complex pump set, an installed power of 72+5 kW, and a greater footprint and manufacturing cost than a single-stage machine. What that extra cost buys is the difference in the ability to meet the indicators.
How to decide how many stages to use
If the source of the oil is well defined, mostly new oil, and the throughput is modest, a single-stage structure is attractive on cost, with the disadvantage that the margin for deep dehydration and degassing is limited. If the condition of the oil is complicated, the water content is high, and old oil also has to be revolved, the margin that the extra stage provides is a real gain. The more useful question during selection is: when the dirtiest batch of oil enters the machine, can it still hold the indicators down?
Two stages do not mean twice the effect. If the vacuum capability, the degree of atomisation and the heating temperature do not match each other, staging is only a decoration.