What is the real Double stages vacuum oil purifier?

We see there are many oil purifier factories, there are many double stages vacuum oil purifier, but if you read them in detail you will find there are much different from each factory machines, why? And what is the real one of “double stages vacuum oil purifier”?

Double stages means as following:

Firstly, for the vacuum pump system, one vacuum pump is not enough, because the vacuum pump working vacuum is limited, the ultimate vacuum value usually -0.095MPa, it is not enough for removing the trace water and gas from oil, so we need use roots pump (booster pump), which is more speed than vacuum pump, the ultimate vacuum, value can reach to -0.099MPa, so the better result in removing trace water from oil.

Secondly, Double stages also means the double vacuum separating, we know the bigger vacuum separator, the better working result. If we spray oil two time in vacuum separator, so you can remove gas and water completely than one time. So some factory do the “two time spray” in one vertical tank, also some do it in two horizontal tank.