Important info on air compressor water removal

[SniperX]

Hi all, 

I'd like to share some key info that may not be common knowledge regarding HP air filtration. 

It seems most of the posts I've seen on here regarding filtering best practices are missing something very important....need for a PMV! 

Key Point: Most of the moisture removal in a proper filtration system is actually ensured by a PMV (Pressure Maintaining Valve), also called a Back Pressure Regulator. 

Gap to the common practice: It seems most are relying solely on desiccants enclosed in a Tuxing style filter cartridge to remove water. While the desiccants are great at removing moisture, without a PMV, they quickly become saturated and loose effectiveness. 

What is a PMV? Its a back pressure regulator that goes on the end of your filtration system. Its job is to ensure that no air leaves the filter (like a Tuxing / gold tube) until it reaches the set pressure of the PMV (Typically 2,000 psi). At this pressure, the air acts like a compressed sponge and is simply not capable of holding the water in suspension. Based on information from www.scubaengineer.com, the PMV alone ensures removal of 99.3% of the water!!!  Here's the link if you'd like to read for yourself: https://scubaengineer.com/high_pressure_compressor_filtrat.htm

This is a big deal because it means only 0.7% of the de-humidification load then falls on the desiccant, increasing overall effectiveness and service life of the desiccant media. 

A few key points: 

1) This really only comes to play when you're filling either an empty tank or empty gun. If the pressure vessel you're topping off is already above 2k psi, then the moisture in the air will naturally collect at the bottom of your filtration system. Take note of that. The filter should be configured in such a way that the output hose is at the top. 

2) On the high end compressor options designed for breathable air, the PMV is already built in (Airetex compressors, Bauer, etc). However, if you use the popular Yong Heng with a Tuxing style filter, you'll want to add the PMV to the output side of the Tuxing filter. 

3) PMVs are available currently for about $75. However, you'll want to make sure you get quality stainless fittings rated for high pressure. I bought mine from McMaster. The Tuxing output threads are 1/8 BSPP and the threads on most PMVs are 1/4 NPT.

Below are a few pics of my setup.

Hope this helps!

 

[bambuino]

Thank you for putting this out there.

 

[dizzums]

ive heard that somewhere before .. the filter does add some back pressure, but topping bottles, its generally over 2k psi anyway ... havent had any issues without one in that respect ...

 

[sleebus.jones]

The article you linked to was an excellent read. Great find!

 

[biohazardman]

This info has come up many times here on the forum. My Yong Heng has a water separator onboard and I use a string of two now three filters on the output.

I run my compressor in the basement at 55-65% humidity and 65-70 deg.

I purge the system while running every five minutes and change out YH filter #1 every twenty.

Like dizzums I haven't had any problems, in three years, of running my compressor thus far without a PMV so not likely I will be installing one any time soon. Opened up my tank and changed the valve, after two years, and found no moisture inside nor any indication that there ever was any. I have also disassembled a couple of my guns including my Impact, after three years and there was no sign of any moisture having ever been there.

Although in theory this might be required in actual practice, as you have pointed out, it is not always the case.

As we are not divers and as we do not empty our tanks and very seldom fill from empty this has little bearing on airgun compressor use.

Although it might be nice to have a PMV, once again, we do not use our tanks like divers so basically do not need a PMV.

https://www.airgunnation.com/topic/experience-based-opinions-on-yong-heng-vendors-request/#post-1039607

 

[fe7565]

A loosely related note: buy quality high-pressure liquid-filled gauges. I went cheap once for my big tank and the liquid from the (Chinese) gauge leaked into the air lines over several months. When I opened the lines it was a mess inside and wonder how much of that went inside the rifle(s). 

 

[Saltlake58]

I've been struggling to understand the math behind humidity and how compressors and air dryers work. Honestly, all I can say is that through observation, after 3 years of filling my tanks with just the fabric / tampon style filters, I've had zero water problems in the tanks. I blew a valve on my oldest tank, so did an inspection. No sign of water. Purposely emptied my newer tank that was just filled on a humid day, no water sign at all. Even used a borescope camera to check for water and nothing.

To me it is all very confusing. I get the desire for dry air in my gun, and all the banter about this or that that you need to do to keep water out of your tanks, but in the end observation shows me it's not much of a problem.

I do purge my compressor every 5 minutes or so to make sure I get max water out of the system, and I do change the YH filter when it becomes discolored, but that's about it.

Actually, this debate highlights why I don't bother with desiccant. I don't know what I'm doing with it, and there are just too many opinions about how it's got to be done. Even threads disagreeing on how long to leave desiccant in before changing. No one really seems to know. 

If someone out there could do some experiments that show via observation when water becomes a problem and share the results, I'd be very grateful. All I can say is that my experiments show virtually no problem.

 

[Eaglebeak]

1. I think that you are a little confused about what the article is actually trying to say. A PMV doesn't remove anything. It only maintains a minimum pressure to allow the separators to work at their optimum range. Molecular sieve is most efficient at pressures over 1800 PSI. It will still work at pressures below that but not as efficiently. Topping up at 2000 PSI or higher makes a PMV completely redundant. The example above of how much water can accumulate in a 3000 PSI fill from empty is an extreme example. 45 degrees C is well over 110 F and his tropical air example is probably assuming 100% relative humidity. 
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[rcs9250]

Biohazardman, 

Not using our compressed air for diving is a moot point, dry air in our guns system is quite important on repeatable performance.

For example, in the aviation world there is oxygen and then there!s aviators oxygen. By analysis of the oxygen, they are identical, but aviators oxygen is dryer by specification. Why? Because at the altitude aircraft can fly, their systems can be exposed to very cold, below freezing temps, if there!s moisture, not even liquid water but moisture (as in humidity) it can freeze components together, like poppets, that will then perform incorrectly or not at all. 

Most air gunners aren!t concerned with freezing but all that extra water /moisture and who knows what else is what we’re trying to avoid.

Moisture in compressed air is just the result of physics never sleeping.

 

[SniperX]

Biohazardman,

I'm not trying to sell PMVs here!

I'm simply trying to share accurate information so our fellow airgunners are well informed and can feel confident in the performance of the fill equipment they spend their hard earned money on. 

I made clear that the function of the PMV is naturally present if the operating pressure is at or higher than the PMV set pressure (2k psi). 

Lets say you had to replace a gauge on your tank and the re-fill starts from 0 psi: 

With PMV -> You'll fill your tank with dry air while maximizing the effectiveness of your desiccant and maximizing its usable life. 

Without PMV -> Until the system pressure reaches approx 2k, you'll waste your time doing 5 minute purges as the water is suspended in the air. It will flow directly into your tank. From 2k to 4200psi, you'll be sending dry air into your tank. Every subsequent fill from say 3k to 4200 will continue to add dry air, reducing the overall moisture content in your pressure vessel. Lets assume the time your Yong Heng takes to pump your tank from 0psi to 2000 psi is an hour, your desiccant media will be soaking in water for that duration. Versus, the time the Yong Heng takes to bring just the Tuxing up to 2k psi, lets call it 2 minutes. If you had a PMV, your desiccant would be doing 1/30th the work. I'm sure you can see value a PMV brings to the expense of maintaining functional Tuxing filter. 

I would suspect most of us willing to spend several thousand dollars on our airguns, tanks, and fill equipment would be willing to spend the extra few bucks on a PMV to ensure their equipment is capable of providing dry air throughout the whole range of fill possibilities (including filling an empty tank). If not, at least they'll be able to benefit from understanding the physics a little more.

 

[biohazardman]

Yes, I understand your argument and did not say the PMV would not werq, or be a good idea, but I have had the same media filter in my system for over a year now. Yes, it is beginning to show its age but my second YH filter has not become damp yet so it is still werqing. The cost of replacing the YH cotton filter is very low as when I remove them I set them aside to dry and use over again. 

At $75 and another $40 or $50 for a check valve for the other end this may be something to do but as said not a requirement if one does not have the $ and their media filters are lasting for a decent amount of time.

For people with higher than 55 to 65% humidity, like I have, this may be a very good idea and save them some money on media cartridges as well.

This would also be a good idea for those with the 12V water-less, oil-less compressors as it would save the compressor from a couple minutes of werq and thus extend the life of the compressor.

I should have put these cases for the PMV in with the cases against them in my original post. But I spaced it.

Thanks for the post it is well written and informative.

 

[Saltlake58]

So, carrying on the conversation - on the Yong Heng compressor I have, it has two bleed valves. It appears to me that the bleed valve on the right (facing the compressor) is part of an apparatus acts as a PMV. It's a place where water collects as it precipitates from the compressed air. Bleeding this apparatus or component removes the water from the compressor just as a PMV would.

Compressed air flows through the gauge block then out the fill hose. As the air is compressed, water precipitates out and fills the reservoir above the bleed screw.

I believe that acts as a primary PMV leaving very little water behind for desiccants or other filters.

Also, a couple things don't ring with me. Need clarification.

The amount of water shown is for a 80cf tank to 207 bar and I assume at 100% humidity. That's about 3050 PSI. It's also stated that this is only an issue under 2000 psi. so, it's important to understand how much water is in the air compressed up to 2000 PSI, as the air over that shouldn't play into the equation of how much water is compressed. Do we know how much water is in the sub-2000 PSI air? I suspect that sub 2000 PSI air is less than 1/3 of the tank capacity, but am not sure on that one. If it's 1/3 of the capacity (IE maybe 25cf) then the amount of water precipitated is more like 50CC instead of the 149 CC shown. And if humidity is anything less than 100%, that'd also figure into reducing the water precipitated out. Living in the desert, I rarely fill in anything over 30% humidity.

So, getting away from all the math, has anyone filled an 80cf tank from zero, followed good practice and blead the YH every few minutes, then degassed the tank and observed how much water actually got in, then compared to a PMV setup? I think that observation could put this whole conversation to bed.

I have filled a 16cf (98CI) tank, and immediately emptied it just to see what was inside when I got done. The tank was dry. Obviously, the 80cf tank is bigger, so if there is an issue, it'd show better in the 80CF tank.

Anyone done that with an 80cf tank?

 

[SniperX]

Saltlake, 

Quick preface: For the purpose of this conversation, AIR = Oxygen 21%, Nitrogen 70%, and small amounts of other constituents (including water vapor). Air can be compressed. Water can not. There are multiple factors which impact AIR's ability to hold water (ex: pressure and temperature). In our context, pressure is the main variable. As pressure increases, AIR's ability to hold water decreases. At a pressure of approximately 2,000psi and above, possible water vapor in AIR is less than or equal to 1%. 

In response to your statement: "Bleeding this apparatus or component removes the water from the compressor just as a PMV would." The "bleed valves" are just that. They have no impact on pressure. Their job is to be the first low point in the system to collect the water that "falls out of the air." The PMV on the other hand is just a regulator that forces air to back-up in the system until its set pressure (2k psi) is reached, then it allows the air to flow out. The PMV has no water collection or draining function.

The job of the PMV is to cause pressure to increase to 2k psi as quickly as possible so that the bleed valves can collect and drain the water.

In the most common use case (a top-off), since the pressure is likely 3k or higher already, the water will collect in the bleed valve for draining. The PMV really only comes into play when the pressure vessel pressure is less than 2k psi. 

In response to your point on collected water volume: Water present in the AIR is generally indicated via RH and Dewpoint. In a tropic region, both RH and Dewpoint are very high and thus the water "held" in the AIR is also high. The text you're referring to illustrates the total moisture collected during the fill of a single scuba tank (the water present in 80 cubic feet of tropical air). Its purpose is to illustrate how important it is to control the moisture and that the PMV is the single most important part responsible for dehumidification. By itself (no other filters or desiccants) it reduces possible 100%RH to less than 1%RH. 

You make a good point wanting to see "real world" comparisons of water collected with and without PMV. I'd like to see some physical data also. Im just not willing to tempt fate with my YH by running it with back to back full tank fills, lol. Until then, I'll go with the science. 

As Biohazard man says, if you're mostly topping off guns and tanks which have more than 2k psi and you manage the collected water by purging often, the science says you'll be fine. 

 

[Saltlake58]

SniperX - Sadly, I am in the position to test by observation as of yesterday, About a month ago, I purchased a brand new Wingman 74cf tank. My poor compressor slaved away filling said tank over 4 evenings. It's been in the high 90's here and I fill in a shed outside. It shuts down when the motor gets too hot. I pump the water out into the grass and keep refilling the tank so the head temp stays at about 56C but it will shut down when the motor gets too hot. Humidity was low at about 25% to 30%, One night filling was during rain, so more humid than normal. I did bleed often. Yesterday I tried to fill my gun off the tank for the first time. Tank was empty.

O-Ring between the tank and valve blew out the side. Probably glad I wasn't there when it happened. Bits of the O-Ring were sticking out the side. It probably blew and made a hell of a noise. It happened sometime in the past couple days.

So, even though the perfect seal is broken there should be some sign of water if I had a problem. I'll try to get out and see today and let you know.

Anyone know the size of the O-Ring that goes on a 74cf Wingman Tank using a 7/8 Jubilee valve from PA? I've contacted them but it's the weekend, so don't expect a response till tomorrow.

I'll let you know.

Also, when I fill it again, maybe we can extrapolate the volume of air below 2000 psi and above. If the YH fills at a uniform rate, I can time how much time it takes to get to 2000 PSI and how much time it takes to get from 2000 to full. to give us an idea of the air volume at 2000 vs full. It always seems that last 500 PSI is a bear and takes longer. Never timed it though to see and make sure.

I'll try to get out later this AM and check the tank for water.

 

[oldsparky]

A work around that I used before having my current compressor was to install a valve at the filter inlet, let the compressor gauge get up above 3000 psi before opening. But like was said above, on a normal top off our tank has about 200 bar so we don't really need anything because air won't start filling our tank till our compressor exceeds tank pressure. 



But you have a great point! Filtration is better at higher pressure.

 

[Saltlake58]

I just checked my 74cf tank where the o-ring blew. No sign of water in the tank. It's only been filled once, looks like rapid decompression so would have gotten extremely cold. In my case, I think I'm good with where I'm at on water in the tank. I think the factors differing in my case vs the scenario we all started with is:

• Relative Humidity at 35C, Scenario 100%, me, more like 25%
• Scenario assumed no bleeding, I bleed frequently
• Altitude may have played a very minor difference as the scenario is at sea level, and I'm at 4300 feet.

Good discussion all!

 

[elwoodblues]

I have a solution to moisture in your air DONT use air. On the intake port use pure nitrogen. all of the small compressors YH GX etc all have a SINGLE air inlet port.

I have a nitrogen tank that holds 120 cuft on N2 one cf of nitrogen is the equivelent of 28316. CC so that 1 cu ft can fill my gun about 100 times or 10000 + times over the 120 cf bottle before I need to refill . the local welding supply store charges me $18.00 to exchange my empty bottle for a full 1. I also have a regulator that I set to about 6-10 psi feeding into the compressor intake port they all have fittings that hold a small filter for incoming "AIR" change the fitting mine ( a GX cs2 is 1/2 " npt) most probably are the same. Hey if you have a mig welder you have an inert gas regulator for N2 allready just need the fittings to get from that hose to the intake. Now you have "air" that is as dry as can be. 1 one PPM at a dew point of -70 c

 

[Saltlake58]

I have a solution to moisture in your air DONT use air. On the intake port use pure nitrogen. all of the small compressors YH GX etc all have a SINGLE air inlet port.

I have a nitrogen tank that holds 120 cuft on N2 one cf of nitrogen is the equivelent of 28316. CC so that 1 cu ft can fill my gun about 100 times or 10000 + times over the 120 cf bottle before I need to refill . the local welding supply store charges me $18.00 to exchange my empty bottle for a full 1. I also have a regulator that I set to about 6-10 psi feeding into the compressor intake port they all have fittings that hold a small filter for incoming "AIR" change the fitting mine ( a GX cs2 is 1/2 " npt) most probably are the same. Hey if you have a mig welder you have an inert gas regulator for N2 allready just need the fittings to get from that hose to the intake. Now you have "air" that is as dry as can be. 1 one PPM at a dew point of -70 c

But, you have something the rest of us don't. Knowledge of welding and how it all works. While Nitrogen would be a vastly superior way to fill my guns, I really have no idea how to make it work. I'm no welder, though I've thought about it, but never done it. And, I believe you mentioned something about a 600 pound tank in an earlier post. Wish I could handle it, but nope, not even a possibility. Great to know that there are other places to get "air", but for me, I just don't have the knowledge to pull it off.

Keep the info coming though! Always good to see how others get their shooting fix!

 

[bandg]

I have a solution to moisture in your air DONT use air. On the intake port use pure nitrogen. all of the small compressors YH GX etc all have a SINGLE air inlet port.

I have a nitrogen tank that holds 120 cuft on N2 one cf of nitrogen is the equivelent of 28316. CC so that 1 cu ft can fill my gun about 100 times or 10000 + times over the 120 cf bottle before I need to refill . the local welding supply store charges me $18.00 to exchange my empty bottle for a full 1. I also have a regulator that I set to about 6-10 psi feeding into the compressor intake port they all have fittings that hold a small filter for incoming "AIR" change the fitting mine ( a GX cs2 is 1/2 " npt) most probably are the same. Hey if you have a mig welder you have an inert gas regulator for N2 allready just need the fittings to get from that hose to the intake. Now you have "air" that is as dry as can be. 1 one PPM at a dew point of -70 c

But, you have something the rest of us don't. Knowledge of welding and how it all works. While Nitrogen would be a vastly superior way to fill my guns, I really have no idea how to make it work. I'm no welder, though I've thought about it, but never done it. And, I believe you mentioned something about a 600 pound tank in an earlier post. Wish I could handle it, but nope, not even a possibility. Great to know that there are other places to get "air", but for me, I just don't have the knowledge to pull it off.

Keep the info coming though! Always good to see how others get their shooting fix!

Beyond those concerns, many have found a lack of availability of nitrogen at an affordable price in certain regions. Probably great IF you can get it.

 

[elwoodblues]

I have been to many places in many states and none of them have not had a welding supply/ industrial gas distributor, the equipment needed involves a regulator cga 580 inlet and a gas hose for the outlet. For the tank any 20cf (or larger) will work , google 20cf nitrogen tank remember 28316 cubic centimeters in 1 cubic foot of nitrogen = 100 fills for my marauder. and refills for that small tank are about ten bucks. I will try to post a picture of my set up.