I know I'm a bit late with the input but I noticed some answers I feel are outdated and some that have very good information and need to be collated.
Your basic question is why is a vacuum system more efficient in removal of water?
First of all we need to understand what we are trying to achieve.
It has become a widely accepted practice in industry to maintain mechanical oils below 200ppm. >200ppm is regarded as free water and this will reduce component life.
What are the options?
1) to remove free water in large quantities a centrifuge (eg Alfa Laval) is a useful tool, also a coalescing unit would achieve this as well. (Please note centrifuges and coalesces will only deal with free water. This equates to water levels >200ppm.)
2) for removing emulsified and dissolved water you can’t beat the vacuum system (Hydac, Pall ect) and this will dry the oil to < 50ppm. The problem with the vacuum units is they can’t handle high volumes of free water, they get bogd down. As long as the free water remains as droplets through the oil its OK but when it comes into the vacuum chamber in litre volumes it struggles. Ideally if you had a coalescing unit first with a vacuum dehydrator second you couldn’t beat it.
3) there are packages (CC Jensen of Denmark produce a Desorber that can handle high volumes of water and keep the lubricant below 200ppm) on the market that can remove both free water and dissolved/emulsified water and guaranteed to reduce the moisture level below 200ppm
Ultimately one of the suggestions posted is crucial and that is to eliminate the problem first, always. The reality is that you will always come up against problems you can’t fix immediately and production must always come first so my suggestion is buy a system that is portable and can handle your worst, so that the equipment is always protected and you have time to plan repairs on the cause.
I hope this helps someone.