GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: lloyd-ss on February 02, 2013, 12:35:17 PM
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I bought a used early model Shoebox from a guy to help him buy a new Freedom 8 Shoebox. Nice of me, LOL.
He told me that it only pumped to 2200 and needed a rebuild and supplied me with a rebuild kit and a new hardened piston kit. Cool. Reputable guy, and I wanted to see what made them tick anyway.
I watched the rebuild videos that are on the Shoebox site and was impressed with the simple elegance of the design, and the re-buildability. My hat is off to Tom Kaye. He filled a serious void in the paintball and airgun world.
Took it apart. Cool.... the main dynamic sealing o-ring on the high pressure piston came out in 2 pieces. The obvious cause of the 2200 psi limit. I think that means that the low pressure piston can build up to 2200 psi if needed and the second stage does the rest. There was a bunch of gray grease residue inside the passages and the check valves. The lithium grease is white, so what was with all the gray?
While I had the cylinders out I machined some cooling grooves .06 wide x .06 deep spaced .10 apart at the hot end of the cylinders (poor-man's freedom 8, ha, ha). When cutting the grooves I realized that the cylinders are probably made with a special alloy with high silicon (sand) content (maybe ??) for wear resistance. They cut kind of "crunchy" rather than gummy. Anybody old enough to remember the Chevy Vega with the aluminum block without cast iron sleeves?
I took my time and did everything as I was supposed to and put it all back together. Hooked it all up, connected to a small scuba tank. Turned it on (wow, noisy!) , dead headed against the tank yoke pressure gauge. Drat!! Nothing!! Not even a needle pulse on the gauge. Now, I am a pretty handy guy and ought to be able to handle this, but it was getting very late and I figured "What did I miss?" Turned it off. Hmmmm.... what's that hiss. OK, duh. The bleed valve closes when screwed out, not in. Voila! It works. Time for bed.
Next morning. Hooked it up again and working like a charm. But the little oil-less pancake compressor said on the label 150 psi and it was pumping to that, but it's reg would only take the output to about 100 psi. So what does any PCP person do with a low regulator? Yup, a couple of washers under the spring got the output to the advertised 150 psi. Seriously, though, don't touch that unless you are aware of the safety ramifications. Boosting the input for the Shoebox to 150 psi made a noticeable decrease in the time it took in "minutes per 100 psi." I also aimed a small blower at the hot end of the cylinders and honestly, they never got too hot to touch.
One other thing I noticed was that in operation, the top cylinder was sliding back and forth in the manifold block by a tiny amount with each stroke. Hmmm. the source of all the gray in the grease? I removed the snap ring that secured the cylinder, and made a couple of shims from a soda can (.007 thick) to take up the slack, and now have zero movement in the top cylinder.
I have already topped off three tanks, one to 4500 (carbon fiber) and two to 3500 (steel). Other than the noise, I love this thing. But I do have to say that after the pancake compressor cuts off, the Shoebox seems really quiet, LOL.
So thanks to everyone who has said they love their Shoebox. You made me take the plunge. The only problem is that I really like the guys (and girls!) at the dive shop where I got my refills. Now what?
Lloyd-ss
PS, Desiccant filters on the input? What do you use? The air is dry now, but our summers are really humid. Thanks!
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You should find that increasing the input pressure to 150 psi should make the ShoeBox shut off before 4500 psi, as the opposite occurs.... If you run one without a shop compressor, they can (supposedly, according to Tom) build all the way to ~6500 psi.... It's because of the geometry of the off switch springing, apparently.... Did you change the spring on the switch to get the full 4500, or was there enough adjustment?....
I run 25' of coiled air hose (for cooling/condensing) between my shop compressor and a water/oil trap and then a 1 gallon tank (running at the recommended 85 psi) that I can drain (never had a drop of liquid water in either the trap or tank).... I then have two (recently added the second) silica-gel dessicant filters between the pony tank and the ShoeBox.... When I was running only one, if the gel was pink, I would get a fine mist from the bleed screw on the ShoeBox (which is at the bottom of the tank block, and I think acts as a bit of a moisture trap).... If the gel was blue, I never saw that....
I was baking the gel to reactiviate it, and what used to be 2-3 fills gradually got down to one, and even at that the gel was turning pink near the end of the fill, with the accompanying blast of mist from the bleeder.... so I guess it has a "life" or rather loses some ability to be dried over time... That is why I added the second dessicant filter, to catch anything that makes it through the first one....
The first rebuild at 75 hours was just the inner (high pressure side) 1/8" ID white O-ring on the small cylinder, and all I did was swap it with the outer ("low" pressure side) white O-ring.... At 150 hours I replaced both the white O-rings on the small cylinder.... I'm now at about 230 hours and it's still ticking right along....
Bob
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Good posts!
I love my guys at the dive shop too, but being self-reliant with your own little compressor is appealing. :)
YM
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Bob,
Thanks for the thoughts on the desiccant and dryer. I will do something, for sure.
I still haven't wrapped my head around how the high pressure shut-off gizmo works. The previous owner might have tweaked it cause he is a hands-on guy.
Lloyd-ss
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On the Shoebox Compressor website in the FAQ area Tom says not to go over 100 PSI on the shop compressor. I asked Tom about that and he said going above 100 PSI would do exactly as Bob said that it would lower the shutoff set point.
I plan on posting some pictures of the updates I have done to my Shoebox and now it runs like a charm and it is now very quiet. Removed the original shutoff unit and installed an electric pressure switch, which is adjustable to any set point you want and removing the machanical shutoff did away with the major noise of the shutoff arm hitting the spring extension on the toggle switch.
BTW Lloyd glad to hear that someone else learned the same lesson about the bleed valve being wide open when screwed in. LOL
Norm
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So all that clakity-clackity arm does is turn the machine off at max pressure?
And the only problem with running too much input pressure is that it lowers the max pressure cutoff point?
Seeing that I am using the compressor to fill tanks, and with most of them I can't go to 4500 anyway, is there any reason for me not to run the higher input air pressure? (I guess what I really should do is check out the info on the Shoebox compressor site.)
Thanks for the feedback.
Lloyd-ss
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If you are achieving 4500 psi fills, I can see no reason to back down the input pressure.... and yes, the clackity-clack arm changes angle as the pressure builds and eventually shuts off the manual switch.... For some reason, the input pressure is part of the equation....
Bob
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I bought a used early model Shoebox from a guy to help him buy a new Freedom 8 Shoebox. Nice of me, LOL.
He told me that it only pumped to 2200 and needed a rebuild and supplied me with a rebuild kit and a new hardened piston kit. Cool. Reputable guy, and I wanted to see what made them tick anyway.
I watched the rebuild videos that are on the Shoebox site and was impressed with the simple elegance of the design, and the re-buildability. My hat is off to Tom Kaye. He filled a serious void in the paintball and airgun world.
Took it apart. Cool.... the main dynamic sealing o-ring on the high pressure piston came out in 2 pieces. The obvious cause of the 2200 psi limit. I think that means that the low pressure piston can build up to 2200 psi if needed and the second stage does the rest. There was a bunch of gray grease residue inside the passages and the check valves. The lithium grease is white, so what was with all the gray?
While I had the cylinders out I machined some cooling grooves .06 wide x .06 deep spaced .10 apart at the hot end of the cylinders (poor-man's freedom 8, ha, ha). When cutting the grooves I realized that the cylinders are probably made with a special alloy with high silicon (sand) content (maybe ??) for wear resistance. They cut kind of "crunchy" rather than gummy. Anybody old enough to remember the Chevy Vega with the aluminum block without cast iron sleeves?
I took my time and did everything as I was supposed to and put it all back together. Hooked it all up, connected to a small scuba tank. Turned it on (wow, noisy!) , dead headed against the tank yoke pressure gauge. Drat!! Nothing!! Not even a needle pulse on the gauge. Now, I am a pretty handy guy and ought to be able to handle this, but it was getting very late and I figured "What did I miss?" Turned it off. Hmmmm.... what's that hiss. OK, duh. The bleed valve closes when screwed out, not in. Voila! It works. Time for bed.
Next morning. Hooked it up again and working like a charm. But the little oil-less pancake compressor said on the label 150 psi and it was pumping to that, but it's reg would only take the output to about 100 psi. So what does any PCP person do with a low regulator? Yup, a couple of washers under the spring got the output to the advertised 150 psi. Seriously, though, don't touch that unless you are aware of the safety ramifications. Boosting the input for the Shoebox to 150 psi made a noticeable decrease in the time it took in "minutes per 100 psi." I also aimed a small blower at the hot end of the cylinders and honestly, they never got too hot to touch.
One other thing I noticed was that in operation, the top cylinder was sliding back and forth in the manifold block by a tiny amount with each stroke. Hmmm. the source of all the gray in the grease? I removed the snap ring that secured the cylinder, and made a couple of shims from a soda can (.007 thick) to take up the slack, and now have zero movement in the top cylinder.
I have already topped off three tanks, one to 4500 (carbon fiber) and two to 3500 (steel). Other than the noise, I love this thing. But I do have to say that after the pancake compressor cuts off, the Shoebox seems really quiet, LOL.
So thanks to everyone who has said they love their Shoebox. You made me take the plunge. The only problem is that I really like the guys (and girls!) at the dive shop where I got my refills. Now what?
Lloyd-ss
PS, Desiccant filters on the input? What do you use? The air is dry now, but our summers are really humid. Thanks!
Lloyd I fly electric model airplanes as well as shoot pcp rifles and I used a heat sink made for a 25mm electric motor to slide over the cylinders to help with the cooling. I had to resize them a little (squeeze them down to a smaller internal size ) to get them to fit tight to the cylinders and they work great!! I am sending a picture so that you can see them.
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Nice idea with the heat sinks! Getting rid of some heat has to be a good thing.
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I found out through Tom's help that it is a good idea to use the hose bleeder instead of the shoebox bleeder, becaise behind the shoeboxbleeder in the block is a very small spring ballbearing and orings that can get a piece of trash or other debris behind it. Then you have to tack the block completely out to clean it in most cases. I was able at Tom's suggestion blow air back through the out put to close it. Anyway that is a PITA and Tom said to just use the bleeder on the hose if you have one. I finally had to replace the block. I now have the F8 and I haven't touched the bleeder valve on the SB except for installing it.
Just my two cents and IMHO I think a very good idea.
Bob
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that shoe box is something else , i just run mine at 90 psi have about 260 hours on it and it runs fine , have the hardened steel rods , am very good about lubing it, was told never to use the green lithium causes probs , probably will wait till something happens like it doesnt fill to rebuild, i do have a question tho for some of the guys more knowledgeable than me, i have tanks but i never fill them i always direct fill the gun , i do use a fan and a dessicant and water trap in lines the questionis this am i putting more stress on the shoebox compressor by direct filling gun s or less another words should i be direct filling guns or would it be better on machine to fill my tanks any opinions on this``````````````````/
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I don't know if the ShoeBox will care, but filling a tank and then filling your gun from that should result in drier air going into the gun.... with the chance of moisture collecting in the tank, of course.... The drier the air going into the ShoeBox the better, but boosting 90 psi air to 3000 will still change 3% relative humidity into 100%.... On the other hand, expanding the air from the tank into the gun will effectively lower the dew point, insuring no liquid water in the gun....
Bob
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thanks hadnt thought about that angle
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I was looking around online and you can buy desiccant beads in bulk...... a gallon of beads for $25 to $75 ....... Could you take the valve out of your regular airgun-only scuba tank and fill it a quarter to a third full with the desiccant beads, and then just use it that way?
I wonder if there is a down side to that?
Of course you would have to empty the beads out the next time you took the tank in for its annual visual inspection.
Lloyd-ss
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I don't know how that would work, probably fine.... You would want to make sure the beads didn't become saturated, I can envision them trapping moisture against the tank wall and causing more corrosion?....
Bob
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I'm in Bama and have been running a SB for 2 years now. I fill a Guppy tank and use that to fill my guns. Last week I completely stripped my AR2079a HPA down to do a little valve tweaking and relube everything with Krytox. I have never used desiccant with the SB. I fill in my air conditioned house, but in the summer even in AC the air is pretty humid. After 2 years of filling my rifle from the tank there was not a hint of corrosion. None!!!! Everything was in perfect condition.
After seeing this I no longer have any concern about not using a desiccant with my SB.
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I'm in Bama and have been running a SB for 2 years now. I fill a Guppy tank and use that to fill my guns. Last week I completely stripped my AR2079a HPA down to do a little valve tweaking and relube everything with Krytox. I have never used desiccant with the SB. I fill in my air conditioned house, but in the summer even in AC the air is pretty humid. After 2 years of filling my rifle from the tank there was not a hint of corrosion. None!!!! Everything was in perfect condition.
After seeing this I no longer have any concern about not using a desiccant with my SB.
Mike,
Have you by any chance removed the valve from your guppy tank and seen if any moisture is in there?
Thanks,
Lloyd-ss
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Hi Lloys, this is off topic. I was reading you poster and noticed at the bottom Central, VA. I'm in Richmond and have not found man AGer in the area. Started with AG about 5 month ago and have a Xs46u ram from mike and a Disco.
Just figure I say hello.
daniel
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Mike,
Have you by any chance removed the valve from your guppy tank and seen if any moisture is in there?
Thanks,
Lloyd-ss
I'm due for a visual inspection this month. I'll try to remember to post what I find out then. I expect that there is some moisture in the Guppy, but NONE of that appears to make it to the rifle.
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I am asking this because I don't have a clue. :D
If a tank were inverted and left it overnight and in the morning open the valve a little would any accumulated moisture blow out?
I don't have a shoebox and get refills at the fire dept. I am just curious. ;D
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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.
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I don't think it would help. The SCBA valves I've had off have a tube that extends several inches into the cylinder.
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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
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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?
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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.
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i use a dessicant filter with no problems key would be maintenance and change them out id gather
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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
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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
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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
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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! ;D ;D
Norm
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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.
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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
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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.
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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
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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
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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...
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Sperho,
Thanks for digging deeper on the research on this, and it sounds like you have access to good resource info & personnel.
Thanks,
Lloyd
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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
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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.
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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
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Putting some in the tank would work. I don't know how long it would take to equilibrate, but shouldn't take too long I would think. Putting them in some kind of pouch to keep them from rolling around would be advised. They are friable and you wouldn't want the dust making it to the gun or having any ill effects on air channels in the plumbing. I would use the round beads instead of the pellets - the pellets are typically a bit more dusty out of the container. Oh, and you are correct, they don't get mushy and sticky - they stay rigid and shouldn't cake up in use.
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Hey Lloyd, I finally saw your post here on the Shoebox. Had mine for about 3 years now. Guess it is an early model like yours. Last week I needed to top off one of my 4500 cf tanks. It was down to 3200 psi. I hooked it up to my Shoebox and turned everything on. Well it shut off at 3500 psi. This is the third time in two years that this has happened. So furtunatly I had bought a couple kits when it happened the first time and had fresh spare parts.
I reubuilt the top and bottom cylinders and adjusted the chain tension. Hooked everything back up and away she pumped. Got my 4400 psi top off on my tank and I am happy now.. :) I have two 3000 psi tanks that I use in the shop for testing and keep them both topped off as well. I have to say that the Shoebox has saved me lots of mony over the last three years. No 70 mile round trip scuba shop runs now to fill them any more.
I also use two external fans blowing on the cylinders. I normally run the compressor around 6-7 hours to top off my big tanks and the cylinders never get more than luke warm to touch.
The grey stuff you find in your cylinders is a combination of moisture and Lythium grease. I found the same thing you refered to in mine each time I rebuilt my cylinders. You also may notice that the inner most oring is the one that gets the hotest, It discolors more thatn the outer o'rings.
One very important thing is using the water absorbing crystals or dryer setup. I ues two inline and keep my pancake compressor lower than my Shoebox .
I would think you would be very happy with yours since you do allot of testing and need plenty of HP air around the shop.
Knowing you Lloyd you will probably come up with a better Shoebox sooner or later.. :)
Gene
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I only have a few hours operating time after the rebuild, but so far, so good. Plenty happy. I definitely want to get some moisture control in place, maybe the silica gel on the input and molecular sieve on the output.
Still in the process of moving and setting my shop back up.... and finding everything that I boxed up. Wow, I'll be glad when I get settled back in.
Lloyd-ss
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Didn't identify a true expert in high pressure drying, but if I run into an Air Products expert onsite at my workplace, I'll corner him to find an answer. I did, however, do some more digging in the open literature and aside from the one paper that indicated a fall-off in the adsorption capacity at >60 bar, I found no other papers corroborating this. I also looked in to commercial systems for drying relatively high pressure air (1 system up to 6,500 psi) and they mostly used mol sieves. I'm 99% confident that the best "cheap" drier will be a good bed of packed sieves at the high pressure side of the compressor. Mol sieves for 2 reasons: (1) they can achieve a lower dew point than silica and (2) they retain their absorption at much higher temps than silica, so you don't have too worry too much about cooling the dryer in the event it warms up if it is positioned near the outlet. Downside if you want to regenerate: mol sieves take about 525F to regenerate. Probably several hours at that temp at RT. So, if you don't have access to furnaces like I do, you can probably get buy with an oven at full bore. Might be beneficial to do a self clean before regeration so that the headspace isn't littered with organics from all that cheese that fell off the pizza during baking!