On a lot of sites two-stage is understood as one more pump, and therefore better performance. What really decides whether two-stage makes sense is not the number of pumps, but the fact that the gas removal process itself has a physical boundary determined by the vapour pressure of the oil. Single-stage degassing runs into that boundary and stops there, and the job of the second stage is to push that boundary a little further down.
Where single-stage degassing stops
Once the oil is heated, both the oil molecules and the water molecules enter the gas phase. As long as oil vapour and water vapour are present in the vacuum chamber, each of them contributes a partial pressure; when the pumping speed of the pump is no longer sufficient to carry these partial pressures away, the pressure in the chamber stabilises at some value and does not go any deeper. That stable pressure is determined by the oil temperature at the time: the higher the temperature, the higher the floor.
A single-stage system therefore faces a dilemma: raise the temperature and the dehydration is more effective, but the vacuum becomes shallower; lower it and the vacuum can be a little deeper, but the water will not vaporise. The ultimate vacuum figure is markedly lower than the working vacuum precisely because the no-load, dry, oil-free conditions exclude the vapour pressure factor; as soon as oil enters, this boundary appears at once.
What each of the two stages does
The first stage carries the main load: it faces a mixed gas consisting mainly of water vapour and oil vapour, with a large flow and a high liquid content, and it needs a high pumping speed to move a large amount of material away. The second stage takes over what the first stage could not press down further, which is mainly the dissolved gas left in the oil together with a small amount of non-condensable gas; what it needs is a lower pressure environment, not a larger flow.
Once the two tasks are separated, the fore pump can remove a large amount of water vapour efficiently, and the back pump works under low pressure and a small flow with a clean gas, reaching a lower ultimate vacuum. For the ZYD-100, figures such as an ultimate vacuum of ≤5 Pa and a gas content of ≤0.01% (GB/T 423) depend on the back stage removing the remainder of the gas left by the front stage.
Where the cost lies
One more stage means one more set of pumps, one more set of seals and pipework, one more set of parts to maintain, and more power consumption and floor space. The total power of the ZYD-100 is 72 + 5 kW, the overall dimensions are 1200×1400×1800 mm and the weight of the machine is approx. 720 kg; these figures already include the volume and the energy cost that the two-stage structure brings with it.
The difference in the maintenance load is just as clear: the more pump sets there are, the more points there are at which the pump oil, the seals and the cleaning and the servicing have to be dealt with. Using only half of what a two-stage machine can do, and using the two-stage machine on a duty that genuinely needs it, give very different returns.
There is another misreading to avoid: two-stage does not mean the same batch of oil is processed twice, it means that one treatment is split into a front and a back section. The time saved comes from here and the price paid comes from the extra equipment; the two cannot cancel each other out and should not be added up together.
When a two-stage machine is worth having
The basis for the decision is the distance between the starting condition and the target value, not the grade of the equipment.
| Item of comparison | Single-stage option | Two-stage option |
|---|---|---|
| Working vacuum | TYA series: -0.08 MPa | ZYD-100: -0.08 ~ -0.0999 MPa |
| Ultimate vacuum | Limited by the single pump set and the sealing | ≤5 Pa |
| Suitable starting condition | Water and gas content not high, dehydration as the main objective | High gas content, a gas content of ≤0.01% required |
| Energy and floor space | Low | High |
| Maintenance points | Few | Pump sets, seals and pipework multiply |
- The incoming oil already has a high gas content and the target sits at the ≤0.01% gas content level: a two-stage machine makes sense.
- The main objective is dehydration and the oil at the start is fairly clean: the single-stage option offers better value for money.
- The power capacity and the maintenance conditions on site are tight: start with the single stage and leave the margin to the temperature and the time.
- Both a filtration accuracy of ≤1 μm and a cleanliness of NAS 1638 class 5 are required: the whole chain has to work together, and adding a stage of vacuum on its own does not solve it.
What the second stage buys is not a higher speed but a lower pressure floor; whether it is worth buying depends on how far your oil still is from the target.