I've seen that most inline filters on the high-pressure side use something called molecular sieve 13x. I assume this is the best material for this purpose? What would happen if silica beads were used instead?
Dessicant
- Thread starter Golden Slumbers
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They have to be dried often. The molecular sieves are usually clay pellets iirc. I just put a big dessicant dryer in the INTAKE of my compressor before the air filter and now subject my compressor to at leas 10 less moisture according to the water drain.
How I have my compressor currently.
Silica>air filter> compressor >condenser water trap> carbon/molecular sieve cartridge dryer >output hose.
It's working well. I need to bake the contents of the molecular sieve cartridge soon. I have to dry my silica gel every 4th compressor run where I'm at. 250 degrees for an hour does it in a toaster oven on foil.
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Molecular sieve absorbs more water at lower humidity levels than silica,
the end result is much drier air.
the end result is much drier air.
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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.
There's a lot of information on moisture removal methods in old threads here, it just takes time to find and read it,. While the principals of moisture removal will be the same there's numerous variables to consider the will be particular for individual needs. Got to determine your variables 1st before determining how to address them. Each filtering media has it's own properties thus removes moisture differently.
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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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Wow, thank you for that informative response! So I assume you have a separate compressor that feeds the input to your Shoebox? I'm curious about your setup.
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Yes, the original "Shoebox" compressor was a marvelous bit of kit created by Tom Kayne, and it sold for a number of years as the only real viable alternative to the very expensive dive compressors - the only other powered compressors that were available when the Shoebox came out were manual three stage hand pumps that were modified to be driven by a motor and pulleys.
Tom was an amazing guy who has done a lot of fantastic engineering work over the years. Anyways, he was into paintball and some air gunning at the time and wanted a way to get the compressed air they needed. He came up with the very practical solution of using a shop compressor to function as the first stage of what amounts to a three stage compressor system, with his Shoebox providing the second and third stage. Some people call the Shoebox a "booster pump" because it raises the air pressure up from the shop compressor level, but this not factually correct - booster pumps use the compressed air itself as the power source to raise the air pressure further (thus only a small portion of the shop air ends up as high pressure air - roughly 90% of it is vented in the boosting process - while the Shoebox does not "waste" and shop air in use). Interestingly, the Shoebox can be used without a shop compressor feeding it, but it runs about 7 -8 times slower that way, and it gives up the ability to easily dry the air before compression.
The system is incredibly robust, but is slower due to the sizing of the components. It takes roughly 4 times as long a run time for the same fill that a CS-4 would do, if I recall correctly. But they last a long time - mine is eleven years old with well over 300 hours on it and is still like new. A little while ago I figured that if all that air were used for ~30 FPE pellet shots, it would have supported about 180,000 shots so far, and may last many more decades if I can keep it running (some of that air was vented for maintenance and modifications of guns).
Some of us Shoebox owners take offense at those that call the cheap Chinese compressors of today shoebox compressors (based on size, as Tom did with his), mostly because there is no comparison in their build quality. I find mine to be a true set it and forget it compressor, with the slow fill being a feature and not a drawback - no part of my air path gets over 85 degrees F as it runs, and it is all air cooled. I compress while mowing the lawn or watching sports on TV.
Anyways, thats enough typing for today. Here is what my set up looks like, not counting the shop compressor in the garage (it feeds my workshop through a manifold system).
You are welcome. I think the best way to differentiate them is by capitalizing Tom Kaye's version (as a proper noun, I guess) and not capitalize the class of compressors . . . personally I'm not that invested in the differentiation but lots of other Shoebox owners are - I think because the quality of design execution is so much different (and maybe also because the early cheap crap from China is what drove Tom to shut down his company because he ended up losing money just to sell off his inventory).
Like in many things, in HPA we get to pick two of the three attributes : Cheap, Fast, and Good (in this case, typically meaning reliable over the long term). The Shoebox was the first one (and maybe the only one) that offered the first and third item, but as a fully USA made product it could not compete with what came from China, even it was more reliable - most of the current crop of Chinese shoebox compressors are the first and second one. Some of the GX pumps look to be offering more of the third; time will tell.
I think one of the keys is that it is very hard on the machine to get to 4500 psi with just two stages, and that is what most of the cheap units try to do - doing so takes compression ratios of 17:1 on the two stages, and that is diesel engine territory there; three stages takes under 7:1 to get there, thus it is much easier to do so robustly . . . so if you want a two stage unit to last, don't run it that high - only use it for lower direct fills of gun
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