Observations of a new owner of a used Shoebox

[SciGuy]

Only if there was enough moisture to actually condense and puddle in the tank.  I know my shop compressor that is hooked to the Shoebox takes several Shoebox runs before it has accumulated enough moisture that I can bleed off and see the water.   

[Rescue35]

I don't think it would help. The SCBA valves I've had off have a tube that extends several inches into the cylinder.

[PakProtector]

I have thought to remove the tube from a tank's valve so that when inverted it would allow the condensed water to blow out. I figgered that was for post-shoebox acquisition. IMO, what's required is a bleed-able condenser on the HP side of the shoebox. Chill it below room temp and keep the stuff going into the tank as dry as is possible. Once drier than saturation at 4500 psi, it only gets further from sat as pressure drops. At a given temperature, you can only have so much water vapour ina set volume. Reduce that volume, and you reduce the amount of vapour that can occupy it...so squeeze it by a factor of 300 and either you have to get rid of liquid, or you have to have air that is 1/3% saturation level.
cheers,
Douglas

[lloyd-ss]

But will all this said, it seems that people are generally getting by with either a desiccant canister between the shop air compressor and the Shoebox, or, no desiccant at all (SciGuy). 
I know that I am too often guilty of trying to fix something that isn't really broken, LOL.  Maybe this is one of those times?

[canyonbiker]

But will all this said, it seems that people are generally getting by with either a desiccant canister between the shop air compressor and the Shoebox, or, no desiccant at all (SciGuy). 
I know that I am too often guilty of trying to fix something that isn't really broken, LOL.  Maybe this is one of those times?

Before I bought some silica gel on eBay I made a desiccant filter from 1" schedule 80 PVC pipe with threaded removeable ends and used "Mimi cat litter" from Walmart. Mimi litter is silica gel and I found this out after doing some research on the "net". It was 4 pounds for $4.00 which is a heck of a deal. I don't know if it can be regenerated but at that low price i would just throw the old away when it became saturated. The PVC pipe can be made any length you want and i made mine 24" long then I used a green scouring pad ( Scotch brite pad ) in each end as a filter to keep the silica gel from getting out of the pipe. I put this in between my compressor and the shoebox.

[badredbird]

i use a dessicant filter with no problems   key would be maintenance and change them out id gather

[lloyd-ss]

i use a dessicant filter with no problems   key would be maintenance and change them out id gather

I think so too.  whenever I bleed anything during humid weather I always get some mist, and I get minor rust inside some of the smaller fittings.  Maybe the cooling from the venturi effect contributes to localized condensation.
Lloyd-ss

[Norm_m]

I've had my Shoebox for several years and I have a filter/moisture/oil trap between my oilless shop compressor and my desiccant filter and as soon as I see the desiccant beads change color into the oven they go.

What surprised me the most was one time during the summer when I was changing the O rings in both cylinders was that on the high pressure cylinder the spring on the check valve was all rusty and damp. So even with moisture traps and a pretty good size desiccant filter during the humid summer time there was still a moisture issue. As would be expected there was no issues with the low pressure cylinder.

I have never seen any evidence of a moisture problem (rust, pitting or anything like that) whenever I have taken any of my PCPs apart to work on them but IMO even with a cooling fan that I installed on my Shoebox and a desiccant filter on the input air line there is moisture being generated.

Norm

[rsterne]

I'm wondering if you could convert a 13CI (or larger) tank into a dessicant filter for the output side of the ShoeBox.... I'm thinking of the standard 3000 psi version, if you are going to 4500 you would need a suitable tank, of course.... The idea would be to make a fitting that went into the neck of the tank with a dip tube containing a filter material (eg. scotchbrite?) going to the bottom of the tank.... then a shorter outlet tube, again with a filter, for an outlet.... The tank would be filled with silica-gel.... Both inlet and outlet would terminate in Foster fittings, one male, one female, outside the tank.... The air would enter through the long tube, pass through the dessicant, and the exit through the short tube.... The filters are to prevent the silica-gel from getting out of the tank....

Drying the air coming out of the ShoeBox would remove any remaining moisture before you put it into your tank or gun.... It would be a "line of last defence", and you would still run a dessicant filter between your shop compressor and the ShoeBox and make sure it stays blue.... However, if you disassembled and emptied out the HP filter after, say, 10 fills, you would know what is going on....

Then you could let the rest of us know.... *LOL*....

Bob

[Norm_m]

Yes some kind of dryer on the discharged would be nice but being 4500 PSI does make it a little interesting. Now I'm depending on guys like yourself and Lloyd to come up with an idea! 

Norm

[lloyd-ss]

I like the high pressure outlet desiccant filter idea, and forcing it through a path is good.  The standard Disco fill adapter assy has a filter disc built into it, and the discs are cheap.  Maybe they could be used as a final filter.  If my calculations are close to correct, it would take about an hour for 20 cuin of HPA to make it thru the filter at the output rate of the shoebox.  So maybe one air exchange per hour thru the canister. That would give plenty of time for the moisture to be absorbed, but I'd want to check the absorbancy vs temperature for the silica gel in case hot or cold was better or worse.  Only problem might occur when the line is bled, if that puts a burst of air thru the canister and disturbs everything.
Lloyd-ss.   

[rsterne]

Not to mention an hour of ShoeBox time just to fill the canister?.... Make you want to leave the air in it and bleed after it....

Bob

[sperho]

Little late to this thread, but if you want to dry air, 4A molecular sieves will do a better job than silica.  I use it almost exclusively at work for drying chemicals and gases. It is more expensive than silica, but can be regenerated in an oven. For gas applications, I pack a tube first with a relatively thin layer of indicating drier (usually Drierite), then fill the rest of the tube (atmospheric side) with mol sieves.  The Drierite is there to tell me when the mol sieves are saturated.  Seal the tube when not in use to prevent loading during storage. The capacity, sorption rate, performance at elevated temperatures, performance at lower RH values are all better with mol sieves than with silica.  You can buy it from pretty much any laboratory supply house.  I use VWR. There are probably cheaper sources out there, but I don't shop around too much when it comes to buying work supplies.

I have no idea how much it would help for air gunning, but I thought I would at least put this option out there.

[rks1949]

But will all this said, it seems that people are generally getting by with either a desiccant canister between the shop air compressor and the Shoebox, or, no desiccant at all (SciGuy). 
I know that I am too often guilty of trying to fix something that isn't really broken, LOL.  Maybe this is one of those times?

Before I bought some silica gel on eBay I made a desiccant filter from 1" schedule 80 PVC pipe with threaded removeable ends and used "Mimi cat litter" from Walmart. Mimi litter is silica gel and I found this out after doing some research on the "net". It was 4 pounds for $4.00 which is a heck of a deal. I don't know if it can be regenerated but at that low price i would just throw the old away when it became saturated. The PVC pipe can be made any length you want and i made mine 24" long then I used a green scouring pad ( Scotch brite pad ) in each end as a filter to keep the silica gel from getting out of the pipe. I put this in between my compressor and the shoebox.

Have you got any pictures of the filter you made? This sounds like a great idea! Ron

[lloyd-ss]

Sperho,
Where in the system do you think the molecular sieve would function best: at the low pressure input, or the high pressure output?
Thanks,
Lloyd

[sperho]

Sperho,
Where in the system do you think the molecular sieve would function best: at the low pressure input, or the high pressure output?
Thanks,
Lloyd

Good question.  Let me do a bit of literature research to see what sorption data I can find...


edit:  It's complicated.  I dug through a bunch of abstracts (didn't want to fully abuse my access to the scientific literature...) and found that sorption capacity can increase at elevated pressures, but at some point, according to one article that I found, the desorption rate starts to increase at around 60 bar.  It was unclear from the abstract how the rates at up to 200 bar compared to sorption capacity at 1 bar.  I found another paper that showed that below 1 bar, zeolites (molecular sieves) win compared to silica as far as capacity goes, but they did not reveal any information in the abstract as to what the rates were.  So, no conclusion at this time.  When I get back from a trip that I have to take for work next week, I can ping some of my chemical engineer friends (I'm a chemist) to see if they have any detailed knowledge to share...

[lloyd-ss]

Sperho,
Thanks for digging deeper on the research on this, and it sounds like you have access to good resource info & personnel.
Thanks,
Lloyd

[rsterne]

Generally speaking, you can remove much more moisture on the outlet side of a compressor.... When packing air to 200-300 times it's original density, even really dry air will end up at 100% humidity.... so best to dry it after compression....

Bob

[sperho]

Generally speaking, you are correct.  For refrigerated, condensing/dew point driers, absolutely, you are right.  However, for sorptive systems, one must take into account the adsorption isotherm of the sorbent.  I think that you will still do a better job drying at high pressure, but I took the Lloyd's question as specific to mol sieves.  According to the literature, there appears to be an unexpected inversion in the isotherm at pressures above 850 psi on mol sieves.  The question is whether or not it's just a gentle peaking in the curve or a cliff.  I suspect that it's a gentle maxima and that it never comes back down to ambient capacities.  I'd be willing to bet big bucks that the high pressure side with either silica OR mol sieves is way better than on the intake side, though.  In the absence of personal data (or data supplied by one of you guys), I was just poking around in the literature for confirmation...  In the absence of further research, I would recommend that if you put a dessicator on the intake side, use mol sieves.  If you put a dessicator on the high pressure side, use silica or mol sieves, they'll both probably do a sufficient job and a better job than on the intake side.

[lloyd-ss]

I finally had a chance to look up a little info on the  molecular sieves, and they are pretty interesting, and not at all what I was expecting.
It appears that they might function best if the air flow thru them is very slow, which would be the case on the high pressure outlet.
It still seems like you could just put a bunch of the 4A beads int he bottom of your airtank and let the beads absorb the water while the tanks were setting idle. And by the way the water is trapped in the molecular sieve beads, it seems that the beads themselves stay dry on their surface, so the tank would not really have any moisture trapped in it.
Did I get it somewhat right? If they could just be dumped into your tank that certainly would be simple!
Lloyd