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Originally Posted by Gordon Gray
I can sweat copper......but just didn't have a warm fuzzy feeling about it with the pressure blasting takes.
I thought the same but then found a technical article from "Yorkshire" which is a brand of copper pipe and fittings. It shows, amongst other things, the maximum safe working pressure of various sizes of copper pipe at various temperatures.

I am not sure if copper or steel would be better than plastic for a drying loop (although my gut says it should be as it should dissipate heat more quickly than plastic. I am sure the Prof will have a more informed answer than just my gut feeling). When I (eventually) come to my own compressed air, I was planning on John Guest nylon pipe and fittings for distribution and copper for a drying loop.

Obviously aluminium is an option although the air rated stuff seems to be plastic coated which would insulate it. Stainless steel would also be an option if one wanted to spend a ton of cash.

Link to technical article: https://yorkshirecopper.com/wp-content/uploads/Technical-Guide-full-issue-02_14_Really2.pdf

John

Last edited by George Kaplan; 05/12/22 7:17 am. Reason: Link added
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I worked with a lot of compressed air systems, black pipe is the norm, tools air is usually delivered by at least 3/4" bore pipe, this should be clipped with a 1 in 40 fall for Horizontal runs with a water trap drain at the end point, regulators are unreliable, use the biggest practical physical size and keep a spare. large supergrid transformers have high volume silica dessicant breathers, maybe you know some electrical distribution folks that may have what you need.


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cyborg's dryer loop is a work of art......like all his work.

Mine's what you'd expect from a cobbler.

[Linked Image]

[Linked Image]

[Linked Image]

Note the galvanized nipple rusting ( first photo) .......but I'm on the East Coast USA.......so we have plenty of humidly here.

I can run my blaster all day and have to drain a "little" water out of the loop when I'm finished.

BUT.......I don't paint with this set up......just another example of a dryer loop.

Gordon

Last edited by Gordon Gray; 05/12/22 12:37 pm.

Gordon Gray in NC, USA........my son says.... "Everybody is stupid about something"
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NASA study of the effects of various drugs on spider web construction.
The LSD influenced one strikes me as Found Art.

[Linked Image]


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Originally Posted by Hugh Jörgen
NASA study of the effects of various drugs on spider web construction.
I'm at a loss for words.

I've given quite a bit of thought to this (i.e. water extraction, not controlled substances). Unfortunately, much of it starting at 3 am last night...

Issues of heat transfer, thermal conductivity, and laminar and turbulent flow are involved, so the general problem of removing heat from moving air is complex. However, I have the specific problem of painting motorcycle parts, which simplifies things considerably. For reasons I'll explain, this problem is different than that of painting a car, or Gordon's operation of a blasting booth for several hours.

No matter how hot the air was when my compressor last filled the tank yesterday, it sat overnight so it's now at ambient. Let's say the compressor is presently at the full 120 psi and the air inside the tank is now at 70 ℉. If I opened the petcock at the base of the tank some liquid water would be expelled which, since it's a closed system, means the 70 ℉ air inside is at 100% humidity.

Using values I measured, it would take 1¾ minutes at 11 cfm for the pressure to drop to 80 psi, at which point the compressor would kick on. However, the gun requires just under 5 cfm, so that would be just over 3¾ minutes. For those initial 3¾ minutes, running that humid 70 ℉ air through external coils/pipes of any kind that also are at 70 ℉ would extract no water whatever. Unlike a car or blasting cabinet, it's quite possible painting a batch of motorcycle parts wouldn't take any longer than 3¾ minutes of continuous operation of the spray gun, in which case an external set of coils/pipes would extract no water from the humid air.[*]

[*]Mixing units, air at 70 ℉ holds ~20 g of water per cu.meter. A 60 gallon tank is 0.227 cu.meter, which at 120 psi is equivalent to ~27 cu.meters, so that's a total of ~540 grams. However, the amount of water in 70 ℉ air flowing at 5 cfm for 5 minutes (25 cu.ft. = 0.71 cu.meter) is 14 grams = 14 mL. Ideally, that's how much water I would like to extract from the air prior to it reaching the desiccant.

But, let's say I need to spray for longer than 3¾ minutes. In my measurements at 11 cfm, after 1¾ minutes the pressure in the tank had dropped to 80 psi, at which point the compressor kicked on. After a total of 6 minutes the pressure only had dropped a further 10 psi to 70 psi. This means that had I extracted just 5 cfm, after 6 minutes the pressure only would have dropped to 75 psi. So, the compressor would have been on for just 6–3¾ = 2¼ minutes, adding 200 ℉ air to the large quantity of 70 ℉ already in the tank. Without doing the calculation, it's clear the addition of this relatively small amount of hot air only would result in the average temperature of the air in the tank rising by a fairly small amount. Although, even if it were to, say, 80 ℉, the additional temperature would allow the air to hold somewhat more water. However, it only would be that additional water that could be extracted by coils/pipes at 70 ℉. And then, only if the coils/pipes were long enough, because a heat exchanger working on such a small temperature difference is inefficient.

Again, the situation would be different for long, continuous operation of a compressor. After some number of minutes all of the air in the tank would be at a high temperature, holding lots of water, so external coils/pipes would be useful.

Returning to my problem, even at 70 ℉, air that has 100% humidity holds quite a bit of water. However, the only way to eliminate that water with coils/pipes is if they are at a lower temperature. Dropping the temperature to 40 ℉ would eliminate ⅔ of the moisture. Harbor Freight sells a 22 cfm "air dryer" (AKA refrigerator) for $500 that cools to 36 ℉. However, aside from the cost and size of the unit, that's a lot to spend to still leave ⅓ of the moisture in the line for the desiccant to deal with.

Anyway, at 3 am I concluded that for my purposes I need to design a system that only needs to extract ~14 mL of water from air flowing at 5 cfm for 5 minutes, not one to extract water from higher air volumes flowing for longer times. A coil in a bucket of ice water comes to mind.

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Originally Posted by Magnetoman
Harbor Freight sells a 22 cfm "air dryer" (AKA refrigerator) for $500 that cools to 36 ℉. However, aside from the cost and size of the unit, that's a lot to spend to still leave ⅓ of the moisture in the line for the desiccant to deal with.
But, if you look at it from the other perspective, it reduces the amount of water to deal with by 2/3.

Originally Posted by Magnetoman
A coil in a bucket of ice water comes to mind.
As I was reading your post I was imagining something similar.

Also, you could make it dual purpose so that when you are not painting you could put to another use.



John

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Gordon, you are far too kind. I got the plans from a fellow in the UK who is an amazing painter. Your setup certainly looks more professional than my hopper fed cabinet!

I usually just grab a 2K rattle can instead of the gun. I’m lazy and don’t like sloshing around in thinners. Last time I used a gun, it was to paint a Suburban. It was the manual for the plasma cutter that finally instilled enough fear in me to add the additional plumbing and dryer….. although based on MM’s calculations, I need to run it through the beer fridge.

MM… there must be loads of distillation units that are thrown on the scrap heap at the local university. I recall missing out on an autoclave that was the size of a small submarine. I was about 12 at the time and it still haunts me.

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Originally Posted by George Kaplan
[quote=Gordon Gray]I can sweat copper......but just didn't have a warm fuzzy feeling about it with the pressure blasting takes.
Originally Posted by gavin
I thought the same but then found a technical article from "Yorkshire" which is a brand of copper pipe and fittings. It shows, amongst other things, the maximum safe working pressure of various sizes of copper pipe at various temperatures.
yes
even the thinnest grade of 3/4 " soft copper pipe ( type M in the usa )
and 95% tin solder are good to roughly 300 psi @ 250 F.

with the limits more in the soldered joint .
as Drawn copper and thicker wall grades of pipe have much higher working pressures .
( 50/50 solder is only good to 80 psi at 250F.)
... "silver" lead free solder compounds ( alternatives to 95/5 lead free )
flow at about 40F. lower than 95/5 and closer to the older 50/50 solder working conditions
but with the higher-strength rating of 95/5

A soldered joint develops strength along its capillary length

if you choose to silver solder or braze the joints , where temperatures are higher then 750 F. Degrees
thinking this is a better way to go , it gets more complicated .
the braze fillet is important , as it transfers the strength and then some capillary action adds to sealing the joint
and .. the copper psi rating defaults to
the lower softer annealed strength
for the copper fittings thickness... because
Brazing temps are also copper annealing Temps . ( may not be a problem , but something to be aware of )

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I didn’t give the copper much thought as far as the pressure goes. It is used here in some shops which suggests to me that it passes muster with the code police. Although that is an assumption on my part. After re- plumbing my house I eventually became fairly proficient at sweating copper fittings. Cleanliness is next to godliness…. that and decent flux. In any event, it’s fairly far down the list of sources for possible explosions. The air tank is another matter…. it still gives me the willies.

Reminds me of the shop lackey who was tasked with draining the water out of the tank. The rather large compressor was in a fairly confined space. He crawled in there… basically upside down and unscrewed a big plug from the bottom of the tank. Unfortunately he had failed to bleed the air out of the tank first.

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Hi All,
I wonder if some type of cyclone would be able to centrifuge the moisture from the compressor air ?

Since this topic has veered off into so many rabbit holes and not a nut turned on the MM Vincent cry
I am offering my solution !!!

I entrust my motorcycle paint jobs to a local to me custom bike painter, His work has won awards around the World
His workshop and paint booth as far as I have seen does not contain any very complex air conditioning /handling equipment deemed essential in Arizona
Attached pic of Ger Conlon when he visited the late Bob McKay and painted the tank/seat unit at Bob s place
also a tank portrait of the late great Phil Lynott (Thin Lizzy)

John

46118136_10156873961256340_7019543387208417280_n.jpg 340614_281897718488492_1193629233_o.jpg
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Originally Posted by chaterlea25
His workshop and paint booth as far as I have seen does not contain any very complex air conditioning /handling equipment deemed essential in Arizona
I guess thats because Ireland has such an arid climate. laughing

John

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Originally Posted by chaterlea25
I wonder if some type of cyclone would be able to centrifuge the moisture from the compressor air ?
H2O has mass 18, N2 is 28 and.02 is 32, so there's considerable difference in the mass of water and "air." At this very moment Iranian centrifuges are separating U-235 from the almost-equal-weight U-238 so water from air, as opposed to blood from a turnip, would be a piece of cake. If I had one of their 90,000 rpm centrifuges.

Originally Posted by chaterlea25
this topic has veered off into so many rabbit holes and not a nut turned on the MM Vincent
It's not the destination, it's the journey...

I've been tied up with other things today, but what I next need to do is calculate the length of tubing of several diameters (i.e. ¼" and ⅜") required to completely cool air flowing at 5 cfm with ice or, ideally, dry ice. Since dry ice is at –109 ℉ it would remove all the moisture, not just a lot of it. Google claims most Walmart stores carry dry ice, so if the one nearest me does, I'll base my coil design on it.

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The critical part of your proposed dry ice air cooler is not on the "ice" side of the coil but internally where the air flows.
The heat transfer coefficient internally is low and would benefit from either a turbulator--- such as a spiral of metal pushed down the tube--or preferably from a heat transfer viewpoint--a proper secondary surface internally--where the secondary surface is metallurgically attached to the bore of the tube.
This is from digging deep in my memory from when I was a heat transfer development engineer in the automotive industry.
Ah-- happy daze!

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Originally Posted by Tridentman
The critical part of your proposed dry ice air cooler is not on the "ice" side of the coil but internally where the air flows.
The heat transfer coefficient internally is low and would benefit from either a turbulator--- such as a spiral of metal pushed down the tube--or preferably from a heat transfer viewpoint--a proper secondary surface internally--where the secondary surface is metallurgically attached to the bore of the tube.!
Would something along the lines of an automatic transmission radiator plumbed into the line work, or are they low pressure devices/ unsuitable for pneumatic applications?
Earthmoving equipment such as skid-steer loaders has quite large oil coolers for the hydraulic pump and drive motors.

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TM put his finger on the issue, and is why I don't have a solution as yet.

Looking at the extremes, a 1"-dia. tube would flow air without restriction, but only the air next to the walls would be chilled, so the transfer of heat out of the flowing airstream would be slow unless the air were turbulent. For smooth flow it would require a very long length of 1" tubing to chill the entire air stream. At the other extreme, a small diameter tube would have all of the air close to the walls so it only would require a relatively short length of tubing, but because of the small diameter that short length might pose too much resistance to flow. The Goldilocks diameter and length is somewhere between those extremes, but it is by no means simple to determine.

As for repurposing a cooler from some other device, and taking the Trident's oil cooler as an example, it was designed to cool the oil flowing inside using air flowing outside. Oil has a high thermal conductivity so it transfers heat fairly efficiently from throughout the fluid to the walls of the cooler, and the outside air then carries it away. That isn't the situation I have so, while a repurposed cooler might work, it only would do so out of luck, not out of design.

As an aside, I've watched a lot of youtube videos on a variety of technical subjects, some good and some bad. However, videos on getting water out of compressed air are by far the most inaccurate of any subject I've watched so far. I've seen maybe a half-dozen of those videos and all of those show major misunderstandings of the problem as well as the solution. That any actually manage to get water out of the air is by luck, not by design.

Air on the inside being cooled by water on the outside isn't the same as water on the inside being cooled by air on the outside. Like the ski resort filled with girls hunting for husbands and husbands hunting for girls, the situation isn't as symmetric as it might seem at first glance. If I put a 50 ft. coil of ⅜" Cu in a bucket filled with dry ice/acetone I'm pretty sure it would cool the air, but I'd like to design something based on at least a modicum of engineering input.

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Originally Posted by Magnetoman
...... Since dry ice is at –109 ℉ it would remove all the moisture, not just a lot of it...........
It will also freeze that water and eventually restrict or block air flow.

But it would probably take longer than 3-1/2 minutes.

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Originally Posted by Magnetoman
Looking at the extremes, a 1"-dia. tube would flow air without restriction, but only the air next to the walls would be chilled, so the transfer of heat out of the flowing airstream would be slow unless the air were turbulent. For smooth flow it would require a very long length of 1" tubing to chill the entire air stream. At the other extreme, a small diameter tube would have all of the air close to the walls so it only would require a relatively short length of tubing, but because of the small diameter that short length might pose too much resistance to flow. The Goldilocks diameter and length is somewhere between those extremes, but it is by no means simple to determine.
It's even worse than that. Unless you can induce turbulence there will be a still air layer at the pipe walls with increasing velocity laminar flow as you move away from the walls.

Tridentman's turbulator induces turbulence, which disrupts the still air layer and allows more air to come into contact with the conductive walls at a lower temperature. Aluminium has high thermal conductivity, as does copper. Plastics, not so much.
Actually, black iron will be particularly bad for the purpose (but not as bad as plastic) because the rough surface is particularly good at forming a still air layer.

Copper may be best, because the thermal conductivity should be better than aluminium oxide, and a polished surface is more slippery and should have a thinner still air layer.
Can the flow bench be repurposed to experimenting with the relative heat loss capabilities of the candidate tubing?

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Just to lob another fact into the situation---you mention the Trident oil cooler.
Just FYI that cooler has an extended secondary surface on the inside of the tubes as well as fins on the outside of the tubes.
Of course you still face the eternal quandary of the heat exchanger designer---the trade off between increased performance due to the extended surface and the pressure loss that this incurs.
HTH

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Originally Posted by Stuart Kirk
Originally Posted by Magnetoman
...... Since dry ice is at –109 ℉ it would remove all the moisture, not just a lot of it...........
It will also freeze that water and eventually restrict or block air flow.

But it would probably take longer than 3-1/2 minutes.
Wonder what the effect of stopping and starting gas flow would be on that time?


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There's quite a good article on removing air from compressed air lines on the MIG welding forum, See This Link.

The article more or less describes what others have already mentioned and this guy uses copper pipe with various drops and traps to catch the water. A filter regulator is fitted in the air supply before the oil and particle filter.

The author claims this setup is suitable for DIY use and could be enhanced for more professional use by adding a refrigerated drier after the compressor, and additional filters down to 0.1 microns in size, but how that is achieved isn't mentioned.

Worth a read if only to give you some ideas.

Last edited by gunner; 05/13/22 1:33 pm.

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They do sell after coolers. Some relatively inexpensive. Mount it between the compressor and tank along with a water trap.

I’m not turning wrenches either…. I’m getting ready to bore holes in the walls to install AC lines before Mother Nature give us another shellacking.

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This is all such fun, and there is a parallel here with the life of the poor orchestral Horn player, one of the key brass players in any orchestra. ("The French horn: the ill wind that nobody blows good..")

The problem the horn player has is that a typical French Horn has around 12ft of tubing (typically, brass) through which the warm, heavily humid air expelled under pressure from her lungs has to travel before it meets bell end of the horn where the music emerges. As the warm air travels through the tubing, it meets the relative chill of the air-conditioned atmosphere of Carnegie Hall. That's a lot of distillation.

Quite frequently, the horn player has to turn the horn around and around to get the distillate to pool at a point where there is a little spring valve. Activating the valve allows a big glug of viscous spittle to drop to the floor of the Carnegie Hall stage. People of artistic disposition try to look away, but one's eye is kinda drawn to the process.

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Originally Posted by NYBSAGUY
Activating the valve allows a big glug of viscous spittle to drop to the floor of the Carnegie Hall stage.
It turns out my granddaughter took up the French horn several years ago, and she's become quite good with it. Often when she practices at our house the output develops a sort of 'click' as the internal globs of mucous clog passages and then break free under the pressure. When she's unable to clear the horn herself, we take a trip to the garage to blow it into the sink with compressed air. Successive ~1 sec. blasts with each key depressed does the job. However, while you might think more air would be better, we determined early on that much longer than 1 sec. also removes lubricant that is needed for the plungers, causing them to stick.

Originally Posted by gunner
Worth a read if only to give you some ideas.
Thanks for posting the link. However, the most basic question is how long and what diameter do the pipes need to be (with scientific justification of the answer), but the article is like the youtube videos in lacking the answer. Also, as I wrote before, for my purposes I need to reduce the room-temperature air from a large tank to a low temperature, not reduce high-temperature air from a continuously-running compressor to room temperature.

Aside from how to get some water out of the compressed air, there's the fundamental question of how much water needs to be removed. It has been remarkably difficult, and thus far impossible, for me to find quantitative information on the required moisture level in compressed air that is needed in order to avoid the issues with spray paint that occur if the moisture-laden air straight from a compressor is used.

It seems moisture might not cause problems with paint if it is reduced sufficiently such that the air from the spray gun remains above the dew point, so that liquid droplets don't form. However, even if that turns out to be the case, that's an operational definition of "low enough" that only can be tested after the fact, not a quantitative answer that can be used in designing a system. Also, keep in mind that expanding air cools, so even if the remaining moisture level in the line is such that the water is in vapor form as the air enters the gun, it still could condense into droplets as it leaves if the temperature below the dew point.

Continuing to move forward despite not finding essential information, it turns out I have a 25 ft. coil of ¼"-ID refrigerator tubing. From my previous measurements on a ¼" air line I know that it will limit flow to ~9–11 cfm when the compressor is at 80–120 psi. This makes it cheap[*] and easy to do the following measurement. I'll bend the ends of the coil at 90-deg. to keep them above sea level, immerse the coil in a tub of ice water, blow compressed air in one end, and measure the temperature of the air emerging from the other end. What I find from this measurement will help determine my design.

[*]such coils sell for ~$30, that I won't have to spend.

Whether it needs the full 25 ft. to chill the air, or if 10 ft. is sufficient, or if I need to use 7/16" tubing (which I also have) to reduce the flow restriction, it will be easy to install quick-connects and a water trap on such a coil and insert it between the two 25-ft. sections of ⅜" air line.

Including what should be delivered from McMaster-Carr later today, it looks like my system for reducing moisture will consist of, in order,

-- Ice-water-cooled Cu coil of diameter and length TBD.
-- Oil/moisture separator. Filters particles to 3 µm, reduces oil to 5 ppm, and removes "large amounts of water."
-- Air dryer/filter. Silica gel desiccant
-- Pressure regulator. Filters particles to 5 µm. I'll use this to feed the regulator on the gun with 60 psi air.

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add a holding tank ,
time in the tank substitutes for heat transfer efficiency .

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Quote
Aside from how to get some water out of the compressed air, there's the fundamental question of how much water needs to be removed.

Given your desert location with minimal humidity (< 10%?), I'm surprised that there is so much moisture to remove from the airline, compared to where I live in the UK where the current humidity is 72%, but I can appreciate that there are other factors at play such as high temperatures.

As well as removing moisture from the air supply, I would also be paying attention to other issues such as the temperature/humidity in the spray booth, and the type of paint used, all of which can be affected by the environment.

As far as I can fathom, the ideal ambient conditions for spraying are somewhere between 20-30c with humidity between 40-70rha. The air feed to the spray gun should also fit somewhere within these parameters, but with as much moisture removed to avoid droplets in the spray. Ideally, the supply air should be at the same temp/humidity as the ambient air to avoid shocking the paint and cause a reaction.

I don't have a ready answer to the issues at hand but suggest you speak to a local sprayer who might have some ideas to help.

Hope this is of some use.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
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