Not tech enough to copy links. In the PCP forum there's a thread, Moisture in PCP airguns. Page 2, post #35, explains things in good detail about compressed air moisture removal.
I'm by no means an expert and can only speak in layman's terms. When researching and getting into PCPs last year my take on dessicants was;
Silica beads are better at absorbing larger water molecules out of moving air as it passes around them. And moleculer seave works better at absorbing smaller molecules out of the still air around them.
There's a lot of information on moisture removal methods in old threads here, it just takes time to find and read it, and determine what applies best in your particular situation.
The post you reference is one of mine, so thanks - there are actually two that kind of go together, #34 and 35. Here is the direct link to #34:
https://www.airgunnation.com/threads/moisture-in-pcp-airguns.1321274/page-2#post-1839752 You are not quite right on your characterization of molecular sieve and silica beads - which is common - as is the misinterpretation of the chart shown in post #3 above . . .
Both forms of media will adsorb water vapor (which is individual water molecules evaporated in air) within their "operational window" equally well. Providing that neither type is saturated for the conditions at hand, molecular sieve will adsorb "more" water vapor out of a given air charge than silica gel will, in terms of the resulting relative humidity / dew point - which means that molecular sieve can get the air "drier" if both are working fully (not saturated).
When it comes to "liquid water" - meaning multiple water molecules bound together via hydrogen bonds, which for air that starts out with no liquid water in it (only vapor) can only happen once the dew point gets close to 100% somewhere in the volume of air at whatever pressure it is at (most likely adjacent to a wall for air under pressure) - neither works well. In fact neither plays well with liquid water, and the presence of it usually will result in a fracturing of the bead which can throw off abrasive dust (which is why it is good to prevent liquid water from getting to your media).
As for the chart in post #3 by
@rgb1 , it is important to remember that we are dealing with air that has been compressed - the relative humidity of the ambient air that went into the compressor is almost completely irrelevant. That is because unless that ambient air has a RH of under 1% at normal room temperatures - which you won't find anywhere on earth, except maybe in Antarctica in the middle of winter (without adding any humidity) - the the air coming out of a compressor is going to be at 100% humidity regardless of if it was at 80% or 10% going into it (thus the water that vents out of the compressor - as the excess has condensed leaving only that which the air can continue to carry as a vapor). So the only part of that graph that is relevant to any filtering of
compressed air - even in cases like mine, where I filter at ~100 psi before feeding my Shoebox compressor - is only at the
far right hand side. What that says, contrary to what most think when looking at the whole graph, is that silica gel will hold more water than molecular sieve does - almost twice as much. Of course what I said above still applies - working molecular sieve can "dry" the air more than silica gel can. But silica gel can adsorb more total water vapor per gram of media than molecular sieve will.
The key to all of this is that the media can't be saturated, and honestly that is where silica gel as an advantage in that it is very easy to source indicating media with silica than with molecular sieve - plus it takes longer to get saturated as it can adsorb more vapor.
But silica gel will only dry air to an ambient dew point of about -45 degrees F, where molecular sieve will drive it down to about -70 or so. At 4500 psi at 70F or higher, there is not going to be any really noticeable difference between the two levels, but there will be a small amount of water vapor remaining with silica gel that could condense as temperatures drop below that level, and properly functioning molecular sieve would prevent that.
What is also true is that we are talking about a very small amount of potential difference in water vapor at these pressures - slow compressors that don't heat the air much (like the old Shoebox) can't pass much vapor as the air charge has cooled so much anyways. But fast ones like Yong Hengs will pass a lot more water vapor that will condense later as the air charge cools. So that factor has to be considered in your choice and sizing of desiccant filtration.
Personally, I find it easy to manage water vapor with the right use of desiccant filtration, so I see no reason not to do so. Like
@6gun I use both types of media to feed my Shoebox, and probably would use both on the output of any other compressor if I had one that required me to filter there. The other great thing about silica gel is that it is easy to recharge over and over, and it visually lets you know when it is time to do so (doubly so in my case, as 100psi filter housings allow the use of polycarbonate so we can see the media without removal). This lets the molecular sieve that sees the air stream after the silica gel last
much longer, as it does not hold as much water, typically can't be recharged at under 1000 degrees F, and is difficult to source with an indicator - meaning the only way to know if it is still working is to weigh the media, and once it has gained about 15% of its weight it is probably about time to replace it. And for clarity, that silica gel before the molecular sieve will be adsorbing probably close to 95% of the potential water vapor that the molecular sieve would see on its own (or more, depending on air charge temperature), so the impact on its usefull life is massive - 20 times longer for a given amount of media. That is why I use a huge 1.5 pound silica media filter of in front of a tiny 2 ounce molecular sieve filter (again, at only ~100 psi in my case). With that size ratio, both media types are working for me for multiple years before needing recharged or changed . . .
Best of luck in all your compressing efforts!
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