In order to understand water in air, you have to first understand that water, which we all know is H2O, can exist in three phases: solid, liquid, and gas. We can pretty much ignore solid in our use, although for filtration purposes it is simply the frozen form of liquid. Gaseous water is individual H2O molecules, and liquid water is multiple molecules bound together via hydrogen bonds. This happens when the ability of air to “hold” any more individual water molecules is exceeded, and this limit is a function of temperature and pressure – it is really a limit on energy states of the water molecules. When this limit is exceeded, the water will begin to condense out into a liquid state.
When this condensation happens, it starts as tiny droplets of liquid water suspended in the air charge, much like a fog – it is liquid water at this point, but the droplets have not encountered enough other tiny droplets to become large enough to fall out of the air via gravity. That is why foggy conditions require relatively still air – as the air begins to move more, the droplets connect and fall out faster (or they contact something and stick to it). So if we basically have three different things to “trap via filtration – liquid water that has fallen out of the air stream, small water droplets in the air stream, and the remaining gaseous water vapor that has not condensed out, but could condense out later if the temperature drops or if the pressure rises.
As for tools for active filtration, we have three types:
- Coalescing (remove contaminants by getting them to stick together (coalesce) and drop of out of the airflow - works great for removing water/oil mist in air flow, especially when combined with coolers to reduce the air charge temp and drive more condensation),
- Absorptive (uses physical media to directly capture contaminants, like the tampon type - requires the contaminants to be "big enough" to be captured, thus does not work for vaporized contaminants as it will not trap individual molecules of gaseous water, which are too small),
- Adsorptive (uses a media to adsorb contaminants directly, such as desiccant or charcoal, that capture gaseous contaminants directly via adsorption).
Do note that we can use chillers to further improve the process of coalescing and absorbing – reducing that air charge to at or below the temperature at which it will be used allows us do less drying with adsorptive (or desiccant based) filters, which is a good thing as it reduces either the load on the media or the amount of media we need to use to get the job done. This is why I like the idea of putting a coalescing filter in a bucket of ice water for our use – chill the air further before it gets to a desiccant filter, which should be in the path.
Absorptive filters, like the like cotton pads, simply do not pull vapor out of the air stream (notice that paper towels and napkins don’t get “wet” on humid days), but they will pull the drops suspended in air out of the air stream. This is a viable option as a step in the filtration process, and can even be used without a coalescing filter if the media is large enough, but it is not sufficient by itself if the air charge temperature is still above the final usage temperature, or if the pressure will continue to rise because these changes will lead to further condensation later.
Desiccants work by adsorbing individual water molecules into their structure. The media is typically beads, and at the molecular level these beads are quite porous. As air flows around them, the water vapor is drawn into the bead and bonds to the chemical structure of the material. Such media will even pull water vapor out of ambient air, like silica beads in a safe – so it is important to make sure that the airflow path to the media be closed off when not in use, or the media will loose its ability to work faster than planned. Most desiccant materials get “overwhelmed” by liquid water as it is too much to handle at once. Such media can become overheated and even burst when it encounters liquid water (meaning the beads burst, not that the filter housing). This can result in throwing of fine fragments of abrasive dust into the air stream, which is not good for tank or Airgun valves.
Both absorptive and adsorptive filters are going to use media, and thus must be sized properly. Both will benefit from the use of a coalescing filter before them, as it will reduce the load on these filters – and having either a coalescing or absorptive filter in front of an adsorptive (desiccant) filter is important as they “do not play well” with liquid water at all.
A well-designed system for air guns will always use an adsorptive filter and should use at least one of the other two before the air charge passes through the desiccant to reduce the chance of liquid water hitting the beads.