Compressors??

[redtrucker610]

I'm new to PCP, just getting my .25 Synrod for myself for Christmas. I will be using a Hill pump for a while until I can save up for a compressor and a tank. For compressors what would be a good option for my situation and personal use? I shoot in my back yard so  I would shoot pcp more often if I had a powered air source. I would like to keep my cost as low as possible. I would ether have to keep the compressor in my shed or a spare room in the house. Living here in southern Tennessee it can get very humid. I do have a 3.5 hp air compressor with a 15 gallon tank in the shed for occasionally filling tires or running air ratchets or my impact wrench. Compressor is rated at 6.8 CF per minute at 90 psi with a maximum 125 psi output. From reading here and other forums my chooses seem to be a shoebox, alteros, Alpha 90, Omega super charger or maybe the new one from air venturi. From what I have seen the shoebox max with a guppie tank seem like the cheapest route but the shoebox would have to stay in the shed.  I'm guessing because of our high humidity a quite stand alone unit I could run in the house would be a better option? What do the more experienced pcp shooters here think? Looking to come up with a figure on how much I need to save up for to take the plunge.

[Back_Roads]

 I have a shoebox set up in the basement with an old oiless compressor , and is barely audible upstairs.

[Booger]

I bought a used shoebox max ($425) and I top off my great white once a month in the winter and 2X a month in the summer. About 2 hours running each time. I usually let my tanks drop to about 3500 psi. I have heard the F10 Shoebox is about 3X faster, but that is about $1000.

[ShawninIL]

I have the Omega Air Charger(Alpha 90) and love it. 

[benj397]

Well, I have a predecessor to the air charger.  Glad I bought it and would definitely recommend one.

The air charger doesn't need a feeding compressor and is thus portable.  Can run off a generator if needed.

[ezman604]

I'm on the West coast of TN. LOL
Humidity...it's an issue. I started with a Shoebox and a 3-1/2HP oilless feeder compressor. I ran it in my shop because of the noise. If I had to work while it was running I had to don ear protectors. And I ran 3 separate moisture traps/dryers between the two compressors. It still didn't remove all of the moisture.
I decided to splurge and got myself the Omega Super Charger from Jim at Precision AirGuns. And couldn't be happier. It has an adjustable moisture purge system and takes a 20amp 115v circuit. I have it sitting in my Man Cave and can run it while on the phone or watching TV. It is pretty quick also. The cost was a bit of a sting but I got over it. I have about 72 hours of run time on it now.
And Todd at AoA gives most excellent customer service support for them!!!
I recommend it for sure.
To make certain I have no moisture, I'm also running a room dehumidifier at 45% moisture.

[guykuo]

I would suggest having an air drying filter on the output of whatever compressor you buy, unless the compressor already includes a high side filter. It is much harder to adequately dry air to the point it will not condense any moisture when the air will under go compression to 4500 psi after the filter.

That’s a mouth full. I will try to explain….

When you compress air, the water vapor that is already in the air is not destroyed by the compression.

Dew point is the temperature at which water vapor will condense. Dew point goes lower if you reduce water content. Dew point goes higher if you increase pressure.

Assuming your tanks will not see lower than 55F, you can calculate how low an atmospheric pressure air dryer has to drop relative humidity for that same air to undergo compression to 4500 and still have a dew point below 55F.

Also, on well designed compressors, a means of mechanically separating condensed moisture from the high pressure output is provided. This “water and oil separator” is what you read about needing to be bled periodically during use of a compressor. The separator removes condensed water and lets the high pressure air output to omit a large portion of the water content.The resultant output is however at 100% RH for the pressure and temperature of the separator, not actually as dry as desired. It is still well above the content to keep dew point below 55F. On the plus side, a separator has infinite capacity to remove water down to 100% RH. A separator dramatically reduces the water load a subsequent drying filter has to absorb.

If a compressor lacks a separator, the condensed water must be absorbed by the high side filter. You can get away without a separator, but your high pressure drying media will get used up faster.

What about an atmospheric pressure air dryer or interstage air drying filter? Won’t those work?
Yes, they are better than nothing, but physics make it very difficult for them to remove water content low enough to make the subsequent 4500 PSI compressed air still have a dew point less than 55F. Remember, compression raises the dew point.

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.

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 w w w.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 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 by an atmospheric pressure dryer.


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 w w w.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.

This graph also shows why molecular sieve is a better choice than silica gel when one wishes to achieve a low RH%. At low RH% we are trying to achieve, molecular sieve has a much higher capacity than silica gel. It isn't until one reaches higher, undesirable humidity end points does silica gel have more capacity. Another advantage of chemical sieve is that it continues to be effective even if hydrocarbon (oil) is present.

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.

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.

Can one determine if RH% in a tank or air cylinder is low enough. Yes, if you sample the air and have it analyzed by a lab. There are also high pressure water content visual indicators made by Bauer that are used in the diving world. Both are expenses I doubt air gunners will undertake.

Can you just open the tank and look inside for liquid water? No. that won’t work. The water content which has been condensed is going to be liquid at pressure. When you drop the pressure, it gets a chance to become vapor again unless very large amounts are present.

One may not like the fact that a high side pressure is much more effective, but disliking the idea and not wanting the cost of a high pressure dryer don’t change the physics. We don’t want water vapor in our equipment because it promotes corrosion. Corrosion of 4500 PSI components does not improve performance or safety.

[redtrucker610]

So with all that said, if you have a high side drier is it necessary to still have driers and filters on the low side if using a shoebox? What about the more expensive stand alone compressors like the omega super charger. If the compressor has a purge system does it still need a high side drier?

[guykuo]

The high side drying filter is best thought of as a very limited capacity scrubber for the last bit of water vapor. It can go to super dry, but has very small total capacity.
One is needed to get to the super dry conditions desired, but making a high side drying filter do ALL the work would mean it would be used up within a very short time. Think tens of minutes rather than hours.

The Omega's purge system is a mechanical water separator. That has unlimited capacity but can only achieve 100% RH% at best. It can take out the majority of water, but simply cannot wring out lower. You still need a high side filter after that if you want to keep RH% going into your tank low enough to not condense as the air cools.

A pre-drier or intermediate drier IS still desirable in addition to a high side filter on a Shoebox. Why? The Shoebox completely lacks a mechanical water separator. Everything that goes into the compressor, comes out the air output. Water isn't destroyed by the compression process. You just end up presenting it all to your high side filter (or your hapless equipment). The high side filter has to absorb all of it and we already know it has limited capacity. Hence, a low side or intermediate dryer helps by reducing the total water load present at the output. A low or intermediate dryer helps and is better than nothing. You are best off with a combination of systems that does both...

1. Remove the bulk of the water content with a high capacity absorber or water separator (gets content down low enough for limited high side capacity to last long enough)
2. Complete the final drying to a low RH% with a high affinity absorber at high pressure (gets RH% at HIGH pressure down to prevent corrosion or condensation)