Shoebox, desiccant, dew point and low pressure usage calculations
I was pondering how Shoebox owners are able to avoid seeing water in their tanks despite using only a drier between the 1st stage compressor and the Shoebox. Some report no water in their tanks, while others see water at the Shoebox output. Both are apparently possible, but how can this be?
There are a pair of on-line dew point calculators that allow a nice exercise in understanding how dry air needs to be at 1 atm to be non-condensing at 4500 PSI. Used together, they convinced me that it IS possible to get sufficiently dry air with a drier between a 1st stage compressor and shoebox. However, it is also easy to screw up by using the drying media too long. It looks like the media needs to be changed WAY before the indicator shows the media is saturated. Only the initial portion of the capacity is of use if you want non-condensing air at 4500 PSI.
The below is a bit convoluted, but convinced me that those who claim zero air in their tanks with a shoebox setup aren’t completely nuts.
I’d still add a high pressure, drier to actually do the job. The calculators also show how doing it at high pressure takes advantage of more media absorbing capacity.
How dry does air at 1 atmosphere have to be so it is dry enough to be in a 4500 PSI tank?
Let’s assume we have a tank that is filled to 4500 PSI. We want the water content in that air low enough to not condense at any temperature the tank will see. We will also assume 100% relative humidity is acceptable in a tank. Let’s also choose a lowest temperature of 55 F.
Using the calculator at http://www.howelllabs.com/resources/dew-point-conversion-calculator/
We put in a known dew point of 55F, 4500 PSI and calculate what the dew point would be at 1 atm (14.7 psi)
The resultant dew point is -44.9F at 1 atmosphere. Now we need to convert that dew point into a relative humidity%
Visit http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html
Put in room temperature of 72F and a dew point of -44.9F
The result is 0.53% RH at room temperature. Our 1 atm, room temperature air needs to be dried down to 0.53% RH.
Can we achieve that level of dryness at 1 atm using desiccant?
Now, look at where 0.53% RH. is on the absorption curve for various drying media….
from https://www.sorbentsystems.com/desiccants_charts.html
0.53% RH is at the extreme left end of the curve for all the desiccants. It’s tough to make it actually happen before the desiccant runs out of capacity to absorb that low. If our goal is a tank that won’t condense until 55F at 4500 PSI, we aren’t likely to get air dry enough at 1 atm via desiccant. The desiccant runs out of absorbing capacity too soon. So, no, we probably can’t get dry enough at 1 atm.
How about desiccant in a dryer between 1st stage & a Showbox?
We redo the first calculator using 94.7 as our “new” pressure (that’s 80 PSI + 14.7 PSI atmosphere)
That calculates our “new” dew point of -23.2 F
The 2nd calculator puts that at 1.8% RH at 72 F. That’s gives us almost 4 times more leeway, but still not a huge fraction of total capacity before maxing out the desiccant. Yes, it can work, but the desiccant needs to be replaced early, perhaps at 1/10 the time before an indicator shows the media is exhausted. For instance, a cobalt chloride indicator won’t change color until reaching over 50% RH. That is way above the 1.8% RH we need.
Yes, you can do it, but change the media super early.
What pressure does molecular sieve need to be at to use most of its absorptive capacity?
Let’s see what pressure we need for molecular sieve to load up to nearly the knee of its absorption curve. Looking at the graph, we see that is at about 20% RH.
Using the 2nd calculator we find 72F with 20% RH corresponds to a dew point of 28.7 F
Then, iterative work in the 1st calculator (still stipulating 55F dew point and 4500 PSI in tank) shows us that a pressure of at least 1590 PSI is needed to use most of absorption capacity of molecular sieve.
Molecular sieve needs to be at a pressure of 1590 PSI or higher if you want to use most of its capacity AND yield non-condensing conditions in a 4500 PSI tank with a 55F dew point.
What if we are slopping and assume the tank will never go below 72F?
Well you’d be a fool, but If we assume 72F as the 4500 PSI dew point, you only need to have -35.5F dew point at 1 ATM. That level works out to 0.92% RH. That is plausible to attain with low pressure desiccant, but again you quickly exhaust the media.
These are approximations, but I think adequate to illustrate several things about using desiccants.
1. You probably are not going to get air dry enough at 1 atm using desiccant if you aim for 55F safe tanks.
2. You CAN get it dry enough at a 80 PSI intermediary drier, but the desiccant media needs to be replaced very frequently – far before the indicator shows the desiccant is saturated.
3. Molecular sieve needs to see about 1590 PSI or higher to use of the majority of its absorptive capacity.
4. The higher the pressure seen by the desiccant, the better
5. If you (foolishly) accept 72F as the lowest temperature your tank will ever see, it is plausible to dry air sufficiently at 1 ATM pressure. However, you will have little margin for lower temperatures AND you must use very fresh desiccant. Thus, it is possible for a hand pump to get dry enough air to avoid visible water in a gun, but only for a limited number of fills.
6. Low pressure air driers are quickly overwhelmed achieving the super low RH% needed. Stop soon enough and you see no water out of a shoebox. Use the media too long and you get water.
7. At the very low RH values we are trying to achieve, molecular sieve is a better water absorber than silica gel. We can see that by comparing their RH vs capacity curves at the low end of the RH vs absorption capacity curve.
Note, this all completely ignores the even bigger water removal possible if one compresses, cools & physically separates condensate, but a shoebox doesn’t have a separator anyways.
in summary, it Is possible to avoid water in a 4500 PSI tank with lower pressure desiccation, but very easy to screw up by using desiccant media too long. If you’re doing 80-100 PSI pressure drying, one probably needs to be swapping out the media by 1/5 to 1/10 the time it takes for the indicator to start changing color. If you are trying to dry at 1 ATM, the desiccant is going to be ineffective in very short order.
All Replies
The volume of water is small relative to the size of the tank or reservoir. That small amount of liquid promotes corrosion. Damp is enough. Doesn’t have to be sloshing about for water to promote corrosion. It’s tough to observe because It became liquid due to compression to a the high pressure. When you empty the tank for inspection, the pressure drops and that evanescent water changes phase back to gas. Remember, it was water vapor at 1 atm. It condensed to liquid phase under pressure. To actually see it, you would need to view it while still under pressure. Only when you get to very large amounts of liquid, will enough remain in the tank to be seen.
Just how little water is permitted in grade E air? That would be dry enough for SCUBA? Its about 67 mg or 1.5 drop of water in a 80 CF tank. More than that is considered too wet. You probably won’t be able to see that volume of water once you depressurize and open a tank. Only when you get to really gross amounts of water will some be visible.
Now this sounds like a snake oil salesman. If you can’t see it, or feel it (or even prove it’s in there) what does it matter? When I took 6 different guns and 3 different tanks apart to inspect after using a F8 (with only a home made silica filter between the 2 compressors) I did not see any water, I did not feel and water and I saw NO EVIDENCE of any damage at all. So does it really matter if there is any water (that you are unable so see, feel or prove is in there)? If the air (which contains this water unable to be seen) is pumped in, isn’t it then shot out?
Why is moisture removal so important? What if some moisture makes it into the air tank or air reservoir on the gun?
Liquid water + high pressure, available oxygen promotes oxidation of steel and aluminum tanks. This can be anywhere on the walls of the tank or reservoir. Also, anywhere, dissimilar metals are in contact with each other and water, there is galvanic corrosion. This is particularly important at valve thread and valve components. Oxidation continues until the oxygen and water are consumed. In a steel tank, you’ll see brown rust. In aluminum cylinders, the oxides are white or nearly invisible if in thin layers. Areas of pitting can erode the tank enough to cause containment failure. Also, corrosion along metal grain boundaries may weaken a vessel rapidly in a small area.
The problems occur inside a high pressure vessel. We can’t see it happening an may not discover it until leaks, stuck valves, or something more catastrophic occurs. What’s worse, because it is under 4500 PSI, corrosion happens at a higher rate than we are familiar with at ambient pressure.
Frequent inspection and testing isn’t practical, so the solution is prevention by keeping water concentration very low in high pressure vessels. SCBA standards call for air to be so dry that a dew point of -65F is achieved. My calculations above were quite a bit more lenient.
Can you take apart your equipment and really verify there isn’t any corrosion happening? Yes, but you must inspect under magnification all the components. Did you check that all the threads are not being dulled or misshapen by subtle corrosion? Look at pictures used for training tank inspectors. I would miss a lot of things that would condemn a tank. Sure, corrosion signs are seeable, but most of us aren’t going to look inside our tanks with sufficient detail and expertise. The time span between hydro tests assumes that the tank has been filled with dry air. Get water condensation inside the tank and you are shortening its safe use lifespan.
Are moisture levels from a Shoebox with no filters any more than hand pumping?
Probably, yes. An unfiltered shoebox has greater potential to deliver excess water into your tank or gun than a hand pump. On first glance, the two may seem equivalent, but there are important differences. Hand pumps work even slower than a shoebox, pump in short episodes, include a water trap, and cool between pumping episodes. The slow rate of air delivery means the hand pump gives the compressed air a chance to cool and condense out water. That water gets caught in the internals of the pump and its water trap. The water trap is very small in capacity, but you are probably only filling a gun with a hand pump. You don’t process out enough water to overflow the trap before bleeding.
The combination of cooling, slow enough delivery, and water trap means a hand pump is a little less likely to deliver hot, humid air that immediately condenses in your air reservoir. It is likely near ready to condense, unless you used a drying filter, but if you open your reservoir without dropping the temperature, you probably won’t see condensed water. However, cool your air reservoir by going out on a cold day. You can bet you are dropping the temperature below the dew point. When was the last time you opened your gun’s air reservoir out in the field on a cold day to look for moisture?
In contrast, an unfiltered Shoebox runs continually, delivers air with less cooling, and lacks a water trap. The output air isn’t as hot as from a 3-6 CFM compressor, but it is still warmer than ambient or a hand pump. Water content is not destroyed by compression. Without a means of removal, what goes into the compressor, comes out the other end. Only, now it is under a lot more pressure and the dew point is going to be higher than you want. Without a water trap or dryer, any compression process will increase the concentration of water in the air. The Shoebox is no exception. It doesn’t destroy water. Where do you think it goes. The question is whether the compressed air still be low enough in water concentration to avoid condensation in your equipment. That’s very hard to do achieve without something to separate out condensed water (post compression and cooling) or absorb the water.
For instance, currently where I am, it is 69 degrees with a RH of 55%. That is a dewpoint of 52.16 at 1 atm. If you take that air and compress it with an unfiltered shoebox to 4500 PSI, the humidity is so hight the dew point would be a whopping 249F. In a dry place like Vegas, it is now 98F with RH 8%. Dew point is 26.7F. Post compression that is still a dew point of 196F! Assume a lower compressed pressure like 3,000 PSI and the dew point is still 178F. Even in a dry environment, compressing air to PCP or tank pressures, without any means of water removal, creates a water concentration high enough to condense at room temperature.
Compressors don’t destroy water molecules. Even with dry, desert air, compressing to 4500 PSI will concentrate moisture. That water needs to be separated or absorbed. The only other place it can go is into your tank or air reservoir. Even if you fail to see it on inspection, it has to be there. The molecules weren’t destroyed or removed. Ironically, a simple hand pump achieves some water removal while an unfiltered Shoebox doesn’t have a water removal mechanism. Even a water separator on the output line would help, but you may as well add a full featured filter.
The situation is dramatically improved by adding an good sized, advanced drying filter after the Shoebox. The drying filter helps in several ways. You increase air cooling by virtue of the filter’s large metal chamber, add time for condensation to occur, provide an ad hoc water trap and bleeder, and drive humidity below the compressed air’s dew point via a molecular sieve drying mechanism.
Why is moisture removal so important? What if some moisture makes it into the air tank or air reservoir on the gun? Are moisture levels from a shoebox with no filters any more than hand pumping?
Good analysis. Thanks!
-x356b
- You must be logged in to reply to this topic.
Thanks guykuo for the detailed information. A lot of the points that you had made a lot sense.
My thoughts are similar to what BigTinBoat said above. Also, if you can’t see the moisture then how would you even know if your air has moisture in it?