I came across this on hard air magazine: https://hardairmagazine.com/ham-columns/why-not-pre-dry-high-pressure-air-when-filling-your-pcp/. He talks about filtering the input air vs. the output. It seems logical, and less expensive, because you are not dealing with the high-pressure side. Does anyone have any experience that can be shared with this approach? Pros and cons? Anybody doing this now, and how effective is it?
Filtering the Input vs. the Output of the Compressor!
- Thread starter GerryR
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AS was explained to me : the high pressure is what squeezes the water from the high pressure air into the filter medium . A rather simple explanation but it gives you a picture . I have not researched this any further .
I think you would need a DE humidifier ?
I think you would need a DE humidifier ?
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I do that, but in a different way. I feed my compressor with a nitrogen cylinder. Have not seen a drop of moisture since I started using the nitrogen.
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I filter both sides of the input/output. I filter the input side because its easy and i had parts laying around but the output side is where you'll get the majority of the moisture out of the air. For what most pcp’s cost and the amount of maintenance these guns need regardless of how dry and clean the air is i think spending the money for a HP oil/water filter its worth the investment. Theres a comprehensive post around here explaining the importance and mechanics of moisture in HP air, I’ll find it and link if for you, definitely worth the read.
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this is the best way for dry air . you can also use your compressor with H2o filters to fill a tank then fill you gun . which is a very common way .
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I remember watching or reading somewhere that filtering the input side is very ineffective. The air basically just rapidly passes through the media, isn’t compressed where the moisture would be more easily forced out so little if any drying occurs.
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This seems contrary to the results posted at the link I referred to, but none of the conditions under which they operated are listed, so we don't know the RH, temperature or anything about the setup. I was hoping someone tried it and had tangible results; there's a lot of hearsay on the web!
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It's unlikely that filtering intake air will remove 100% of the moisture unless one spends more than what the compressor costs on a high grade system. Outgoing moisture is created by adiabatic heating (pressurizing air) & condensation which will happen even if ingoing air is filtered. For our purposes a decent filter & good media in that filter should keep the outgoing, pressurized air dry enough to keep our guns safe.
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Here is a link that shows what moisture in compressed air in an airgun can do. For those that do not believe a PCP should have dry air.
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I think everyone knows that a PCP airgun needs dry air; my original question was "is an input dryer as effective as the article referenced seems to infer?" Still looking for some experienced input.
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I have no experience but my guess is the effectiveness would be a function of the design of the pre filter. With a big enough pre filter with good dessicant I see no reason it would not help. But I doubt it can eliminate the need to filter the compressed air too. Filtering the inlet seems like a way for your compressor to see air more like winter air than humid summer air where I live. Winter air still needs filtering on the outlet of the compressor.
Another drawback is you have to use a dessicant. I get most of the moisture out of the compressed air through precipitation as it cools moving through the lines and then first filter. I vent my YH every 5 minutes of a tank fill and in the summer a lot of moisture escapes. I have dessicant too but it very rarely traps enough moisture to need replacement. If you filter the inlet I think you can count on buying more dessicant.
Another drawback is you have to use a dessicant. I get most of the moisture out of the compressed air through precipitation as it cools moving through the lines and then first filter. I vent my YH every 5 minutes of a tank fill and in the summer a lot of moisture escapes. I have dessicant too but it very rarely traps enough moisture to need replacement. If you filter the inlet I think you can count on buying more dessicant.
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There sure are a lot of myths out there . . . and I'm afraid this article might be adding to the problem as it is not the full story . . .
In short, the correct answer is "Yes, one can adequately dry the air to meet our needs with a desiccant filter used before compression - however, the filter that does this will not be the same as a filter used post compression as the air charge is seen in very different conditions."
Let me be clear that I have not built or used such a system - I am a Shoebox user and thus treat my air before it enters the Shoebox, which is at about 7 bar of pressure. This condition is so ideal for filtering that it would be crazy for me to do it anywhere else. That said, I do know how I would go about doing it.
The next big thing to understand is that the often heard statement that "desiccant media does not work as well at lower pressures and needs high pressure to work properly" is not factually correct. What is correct is that a given volume of desiccant media that would be optimal for high pressure drying would not be able to dry the same mass of air to the same level at low pressure vs. high pressure.
Let me explain:
First, desiccants do not have water molecules squeezed into them, they draw water molecules out of the air stream into their structure via adsorbtion. This difference is important because it points to the fact that matters - the amount of water molecules that can be adsorbed by a given volume of desiccant media (assuming it is not saturated for the conditions it faces) by a given mass of air will be a function of the dwell time that the air charge is exposed to the media.
Also, note that I said "mass of air" and not "volume of air" - we are working with air under very different pressure states, but in the end, we are working with the same amount (or mass) of air.
Air at 4500 psi is at ~300 bar, or 300 times atmospheric pressure. This means that the volume of the air being drawn into the compressor (setting aside the water vapor content for a moment) is 300 times larger than the volume of the air leaving the compressor (at least once it cools back down to ambient). What logically follows from this is that the air stream flowing through a given cross sectional area (for example, a 5mm diameter orifice) will flow 300 times faster before compression than after compression (setting aside the temporary impact of heating of compression). That leads to a vastly different dwell time within the filter media.
The other big difference is that the air charge entering the compressor will contain VASTLY more water vapor than the air charge leaving the compressor - most likely at least 40 times as much, but potentially 200 times as much. The vast majority of that will condense to water during compression, and that is the stuff that is vented out as liquid as we vent during compression.
Just stopping the compressor from showing vented water is not enough - we also need to get the rest of the water vapor that will condense out later when the air charge cools down after compression. This is where I think the linked article falls short - their "test" is whether or not the compressor vents water, not whether or not the air is adequately dried (they assume it must be if nothing is venting, but that is not always true). To me, that filter shown in the photo in the article appears to be filter that one would use on the output of a shop compressor - I do not believe it to be anywher near big enough to get to the dwell times we need. I dostrongly suggest reading the post linked above in reply #5, as well as two earlier ones that I mention in that linked post to better understand this (#34 and #35 in that same linked post, along with the linked one which is #111).
So with all that, how would I go about it? I would want a very long and moderately wide media filter on the input side - I think about 4 foot of 1.5" PVC pipe would fully do the job, most likely for multiple fills too - although one would have to test it out to know. In fact, something like this clear 2.5" x 36" pipe would be even better as we could see the media in it (https://www.amazon.com/POWERTEC-Compatible-Collection-Woodworking-70176W/dp/B0DSHWWSHN/ ) . Of course, it would take a lot of media to fill, but with Silica Gel it could be reused. I'd be sure to make sure it valves on both ends that could be closed to keep ambient moisture from getting to it when not in use. But it should do the job. Maybe it would not need to be the full 36" long, but testing would show for sure.
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When filling tanks I filter 3 places not including the compressor's purge system.
1. Regular household compressor to 150 psi. The compressor tank is the 1st filter.
2. Compressor output into 2 QT of silica desiccant at 80 psi, 2nd filter
3. Regulated output to 3 psi feeds into a CS4. (CS4 as sort of a coalescing filter)
4. Output of CS4 into a small about 1.5" by 8:" HP filter with molecular sieves.
5. To the tank.
Number 4 probably isn't needed as I never seen even 1 drop of water come out of the purge valve after filling a 9L from empty in 2 cycles.
1. Regular household compressor to 150 psi. The compressor tank is the 1st filter.
2. Compressor output into 2 QT of silica desiccant at 80 psi, 2nd filter
3. Regulated output to 3 psi feeds into a CS4. (CS4 as sort of a coalescing filter)
4. Output of CS4 into a small about 1.5" by 8:" HP filter with molecular sieves.
5. To the tank.
Number 4 probably isn't needed as I never seen even 1 drop of water come out of the purge valve after filling a 9L from empty in 2 cycles.
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Silica gel on input is pointless. A perfect setup wont even get within 100 degrees of the dewpoint needed.
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In early days I wanted to save some money and made a pre-filter with colour changing granules.
The problem with media filtering the input side is ... you cannot drain it, you need to replace that soaked media more often.
And ones I toke my gun apart, toke me enormous time and effort to clean from rust.
Right that day I ordered a orange dual cylinder mechanical - oil and moisture removal filter.
That was years ago.
You cannot imagine how much moisture it is capable of separate no matter summer or winter.
Ones you see it you will regret earlier investing into mimics.
The problem with media filtering the input side is ... you cannot drain it, you need to replace that soaked media more often.
And ones I toke my gun apart, toke me enormous time and effort to clean from rust.
Right that day I ordered a orange dual cylinder mechanical - oil and moisture removal filter.
That was years ago.
You cannot imagine how much moisture it is capable of separate no matter summer or winter.
Ones you see it you will regret earlier investing into mimics.
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Looking at the amount of dessicant they’re using it is similar to mine on the input side. I have an Air Venturi 4500 and there is a filter on the HP side that is integral, i have an Alpha filter behind that. Only the bleed screw on the compressor’s integral filter ever blows off moisture. In my opinion the intake filter does little to nothing but gives me some peace of mind that I’m not sucking dust into the compressor, the first HP filter does most the work, the Alpha filter takes care of any moisture that made it past the first two water traps and dessicant.
You can try using just an intake side filter but if you’re blowing off water from the bleed screw you will know for certain it isn't removing enough moisture to be effective for our applications.
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Filtering the input is 0.3% as effective as filtering the outlet. The only thing an input filter is good for is filtering out particulate to protect the interior of the compressor.
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