On many sites the centrifugal oil purifier and the vacuum oil purifier are treated as older and newer versions of the same thing. They are not. The vacuum route deals with water and gas, the centrifugal route deals with solid particles, and the two have almost no overlap in their target contaminants. Once this is separated, the selection does not go astray.
What centrifugal separation does
A centrifugal oil purifier makes the oil rotate under a high-speed rotor, and the solid particles that are denser than the oil are thrown against the outer wall of the separation chamber by the centrifugal force, while the oil flows out from the centre. The key is the centrifugal acceleration. It is proportional to the square of the rotational speed: double the speed and the centrifugal acceleration is four times what it was. That is the physical reason why a difference of one factor in speed produces such a large difference in the separation result. Whether fine particles can be thrown out depends on whether the centrifugal acceleration can overcome the other forces acting on them, including gravity, the flow resistance and the viscous drag of the oil. The higher the viscosity, the greater the viscous drag on the particle, and the harder the separation becomes at the same speed.
What a higher speed intercepts
- Higher speed: finer particles become capable of being thrown out, and the upper limit of the cleanliness of the separated oil rises.
- Higher speed: at the same time it brings higher energy consumption, more noise and vibration, and stricter requirements on the dynamic balancing of the rotor.
- Rising viscosity: the viscosity directly weakens the separation effect, so a high-viscosity oil should be heated to reduce its viscosity before it enters the centrifugal machine.
Why waste oil containing metal swarf suits this route
The contaminants in gear oil, hydraulic oil and waste oil are typically metal swarf and oxidation products. Metal swarf has a density far higher than that of the oil, a regular shape and real weight, and in a centrifugal field it belongs to the class of contaminant that is easiest to throw out. Once it has been intercepted, the pump, the valves and the mating surfaces inside the machine are no longer worn by it. In the other direction, if these oils are fed straight into elements with a very fine accuracy, the metal filings form a dense cake on the surface of the medium, the differential pressure rises quickly, and both the consumable cost and the running cost go up noticeably.
The functional difference from the vacuum degassing route
| Comparison | Centrifugal LX-100 / LX-200 | Vacuum degassing route |
|---|---|---|
| Main target | Solid particles, metal swarf | Dissolved water, gas |
| Separation principle | The centrifugal acceleration produced by a high-speed rotor | Vaporisation and condensation separation under negative pressure |
| Sensitivity to the viscosity of the oil | High, the oil has to be heated when the viscosity is high | Relatively low, the influence of the temperature is within the temperature control range |
| Consumables | Mainly the removal of sludge, with little consumption of the medium | Mainly the consumption of filter elements and sealing parts |
| Typical application | Clarification of waste oil, on-line purification on a continuous line | Dehydration and degassing of insulating oil |
If the oil contains a large amount of solid swarf and the water content also has to be brought down very low, this is not a choice between two alternatives. The usual approach is to remove the solids first by centrifugal separation or by a plate and frame filter press, so that the water load entering the vacuum stage is reduced and the vacuum stage can degas the oil properly. The value of putting the two machines in series comes precisely from that division of labour.
The centrifugal machine throws the solids out of the oil and the vacuum machine draws the water and the gas out of it; the two are not alternatives to each other.