DIY Filter towers

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oxyhacker:
There's a shop not to far from here that was still using, as recently as 10 years ago (and maybe still is, I haven't been there since then) a huge old 1950's 2400 psi compressor, a ScubaAmp added in the '60s to bring it up to 3000 psi, and a filter consisting of a 63 c.f. aluminum tank completely filled with carbon. The tank has a nicely made special fitting in place of the valve, which has a dip tube that goes all the way to the bottom so the air had to go the full depth of the carbon, however there was no dessicant, no baffles to prevent channeling, or anything to control how tightly the carbon was packed - is suspect the carbon actually blows about whenever the air flows. And since the tank was mounted upright, no way to drain it. The owner assured me it was a factory job, which he'd bought with the compressor, and boasted about how much money he'd saved over the years by not having to buy expensive cartridges.
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Ah yes, the not so good "old days" :11:

Tobin
 
I've been trying to download some industry books on metals research but I need a subscription to get in. Stalled temporarily. Hacker raised the issue of cold working aluminum alloy and the possible pitfalls for purposes of DIY canisters made from that metal. I am inclined to think that machining without excess heat may cause slight surface hardening of alloys but not much else. There could be small stresses and a potential over long term for corrosion cracks. I don't think that temper of the object would be changed. Hence, I had my DIY filters anodized and otherwise have never worried about it. However, opinions have been voiced so when an issue materializes on SB it doesn't stay dormant forever.
 
pescador775:
I've been trying to download some industry books on metals research but I need a subscription to get in. Stalled temporarily. Hacker raised the issue of cold working aluminum alloy and the possible pitfalls for purposes of DIY canisters made from that metal. I am inclined to think that machining without excess heat may cause slight surface hardening of alloys but not much else. There could be small stresses and a potential over long term for corrosion cracks. I don't think that temper of the object would be changed. Hence, I had my DIY filters anodized and otherwise have never worried about it. However, opinions have been voiced so when an issue materializes on SB it doesn't stay dormant forever.
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Normal metal removal (metal cutting, not forming) should not "cold work" aluminum appreciably. Aluminum is an excellent thermal conductor. That means the heat generated in the cutting zone will be transported away in both in the chips and into the balance of the work piece. I would not expect any temper changes from typical "good practices" metal removal.

I suspect that reduction of stress risers from good design, i.e. no sharp reentrant corners and good surface finish is likley far more important than the effect of thin Type II anodizing.

Tobin
 
Hello All,

It sounds as though the dive shop owner Vance describes went to the same school as the guy I worked for. Also, the long dip tube is what I envisioned when I mentioned a valve with two ports. Tobin points out that a tank stuffed with a medium like cotton etc. has no way to prevent channeling-something I never considered and was probably not considered by the "engineer" who put this filter together. We never measured the moisture in the filtered air so I can not speak with any authority on it’s dryness other than just from the experience of breathing it. There was a crude moisture trap near the compressor that separated some of the moisture and oil prior to going to the filter. The filter element (with handy strings for easy removal) was designed to absorb moisture. To me this made sense at the time. I will also say that when changing the element out, the top section was indeed very dry. Tobin correctly points out that there is a long list of reasons why this set up is a bad idea, but never-the-less, this filter remained in use for years and as far as I can tell did a good job. Again, I am not proposing that anyone go back to this system, I only bring it us as a matter of curiosity.

couv
 
A couple of points:
I have a filter made from accumulators from Groban (great folks, if you are in chicago take a look at their warehouse, its about a city block square and filled 20' high with stuff). They are about 22 in tall, 3 wide. They were a PIA to clean--I ran them through the diswasher twice (when my wife wasn't home) then simple greened and scubed. I have 4 of them, they were about $40 each. Someone in the compressor business, who opinion I respect has said that SNAP RING accumulators have failed when used in this way. It hasn't happend to me, but I cringe when the pressure gets high. I'm going to get rid of them.
I have a lathe, but am not confident in my ability to machine threads in pressure vessels. So I was thinking of say 2" stainless tube, with an O ringed plug, either drilled and cross bolted, or using tie rods: This has the advantage of being an easy machining project, I have a drill press and lathe.
Also I really want to pump to 4500, lets call it 5000 for a little extra margin.
So opinions: Will say 2" schedual 80 304 stainless, with an O ring plug, and say 2 grade 8, 1/2 inch cross bolts work.
Also, I take it using pipe threads is a bad idea?, how about using pipe threaded caps to hold an oringed plug?
 
116fire:
A couple of points:
I have a filter made from accumulators from Groban (great folks, if you are in chicago take a look at their warehouse, its about a city block square and filled 20' high with stuff). They are about 22 in tall, 3 wide. They were a PIA to clean--I ran them through the diswasher twice (when my wife wasn't home) then simple greened and scubed. I have 4 of them, they were about $40 each. Someone in the compressor business, who opinion I respect has said that SNAP RING accumulators have failed when used in this way. It hasn't happend to me, but I cringe when the pressure gets high. I'm going to get rid of them.
I have a lathe, but am not confident in my ability to machine threads in pressure vessels. So I was thinking of say 2" stainless tube, with an O ringed plug, either drilled and cross bolted, or using tie rods: This has the advantage of being an easy machining project, I have a drill press and lathe.
Also I really want to pump to 4500, lets call it 5000 for a little extra margin.
So opinions: Will say 2" schedual 80 304 stainless, with an O ring plug, and say 2 grade 8, 1/2 inch cross bolts work.
Also, I take it using pipe threads is a bad idea?, how about using pipe threaded caps to hold an oringed plug?
Click to expand...

I have not run the numbers on your basic tubing, but let me say that any threading on a pressure vessel can be problematic.

Sharp corners create stress risers. So does a poor surface finish, if you are "tearing" the metal, you can leave lots of small imperfections that can lead to failures.

Threads should have a radius at the root, and require very rigid set ups. SS will work harden and that can make multipass threading hard to do. You want to take deep cuts to penetrate the work hardened layer, but often the set up and power available allows only shallow cuts.

In short without the right gear I'd avoid any large diameter SS threading.

Tobin
 
I've seen cross bolted DIY filters and haven't heard of a problem. Do you really mean 1/2 inch, grade 8? Man, that is stout. About schedule 80 pipe. I dunno, this "schedule" business is arcane. However, if you can find some 2 3/8 or 2 7/8 OD sched 80, 304 stainless it should be OK. I mean, the wall thickness varies from .22-.28 inch. That should be strong enough for this design. The aircraft hydraulic accumulators are made from some kind of magnetic stainless but the side walls aren't that thick. There are books which describe the pressure ratings of 80S (stainless). Also, one could estimate the rating by looking up the tensile strength and calculating the pressure with Barlow's formula. There is a modified formula available for calculations involving large tube like this. In fact, there are calculators on the net which can do most of the work.
 
I did the calc for 304 SS using Barlow's formula
Barlow's Formula calculator, Texas Pipe and Supply Co., Inc.
and the yield number for the 2 7/8 inch OD, Sched 80 is 7600 psi. The burst is much higher. So, even with internal or external thread this approach should be reasonably safe. If the "chasing" (cutting) method is to be used, one might look for an alloy which shows lower work hardening. Keep an eye on the other specs like tensile strength.
Pesky
 
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