Air compressor tank draining and rust

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  #1  
Old 11-24-08, 10:14 AM
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Air compressor tank draining and rust

I recently got my first compressor (Craftsman 19541, horizontal tank). I want it to last a good long time, and I especially don't want the tank to explode. I drain it after each use but it seems to me that It's probably impossible to get every last bit of water out of it. How dry does it have to be to keep it from rusting?

Here are two things I've observed, and I'd like to hear your thoughts on them:

1. Once after using it I drained it completely (or so I thought, see #2 below) and then closed the drain valve. About a week later I opened the valve and was surprised that some air came out. Could this just be because the tank was colder when I closed the valve after draining than it was when I reopened it a week later? I then moved the compressor with the valve open and some water came out. The water was brownish but after the water dried it left an oily residue so I'm hoping that the brown color was either from compressor oil or some kind of oily anti-corrosion coating on the inside of the tank (is there such a thing?) rather than rust. Could any significant damage have occurred from having a little pressure and water in the tank for a week or so?

2. Sometimes when the compressor is draining, the air flow slows down to almost nothing, then there's a popping sound and the air flow speeds up again. This process can repeat several times. The only explanation I can think of is that when air is flowing quickly the valve gets cold enough for the water going through the valve to freeze, then when the flow slows down the ice melts and the valve clears. Does this make sense? Is this a normal thing that all compressors do?

Thanks!
 
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  #2  
Old 11-24-08, 03:13 PM
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First, the popping sound you hear is sediment being forced through the small hole in the drain plug. Place your hand under it and it will sting you quite nicely. But it is normal. At least you are getting rid of it. What I do is close the valve when the pressure is reduced and open it back up. It will dislodge the solid particles and blow them on out as it should. Above all, drain it daily. The rust colored water, as far as I can tell, is normal, as all my compressors have done it from day one. I also leave the drain open when storing it so any residual water can find its way out should it get lost in the tank . Fluctuations in temperature can cause pressure to build or reduce and it is, also, normal.
 
  #3  
Old 11-25-08, 07:30 AM
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it's normal

You can't completely stop rust in a tank. They are actually considered to be "wet" tanks. Just drain it like chandler said and it'll last forever. My small compressor's drain is always open when not in use. The big one only gets partially drained about once a week, because blowing down a big tank is a chore and a half. And it's 35 years old.
 
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Old 11-25-08, 01:45 PM
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Not sure where I found them, but there exists a purge valve you can install on your drain that is also hooked to a live line, such as the pitot tube to the switch. When the compressor turns off, about 3 seconds later this valve spits out accumulated water at the valve. Granted, it may not get it all if there is alot, but it does a "per cycle" purging. I have very little water in my large compressor since I installed it.
 
  #5  
Old 12-16-08, 01:46 AM
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Rust fighting oil for inside tank

I have a 2hp 8 gallon central pneumatic compressor.
I drain it from the lever valve on top as well as the plug at bottom. I haven't gotten any water out of the bottom valve as of yet but notice some rust where the connector snaps onto the hose.

Someone mentioned to me that rustoleum sells an oil that you can pour into the tank then pressurized the tank then release the air and it helps to reduce rust in the tank.

Does anyone know of this product or anything similar?
 
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Old 12-16-08, 02:53 AM
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Welcome to the forums macbqe!

You should always drain the tank from the bottom. That allows the air pressure in the tank to force any water out the bottom.
 
  #7  
Old 12-16-08, 08:58 AM
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Originally Posted by macbqe55 View Post
Someone mentioned to me that rustoleum sells an oil that you can pour into the tank then pressurized the tank then release the air and it helps to reduce rust in the tank.

Does anyone know of this product or anything similar?
If you pump is oil lubed, you'll have oil in the tank anyway Adding more oil to the tank will only contribute to gumming up your lines, filters and tools. Don't lose any sleep over a little tank rust, they are designed to take it.
 
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Old 12-16-08, 07:51 PM
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And if you have excessive oil in your tank you can forget painting with it, unless you install a post filter.
 
  #9  
Old 01-14-11, 01:40 PM
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Eliminating water all together in the tank

I live near Galveston Bay and we have a lot of humidity most of the year. While I agree that the tanks are designed for some moisture inside, it still bugs me, always has. I started looking at desiccant units to get dry air for some of my tools. Then it occurred to me to use a desiccant tank on the intake side of the air compressor. This can just be a piece of pipe with screens to hold the beads. The pipe needs to be larger than the pipe feeding into the intake of the compressor. The silica gel beads have quiet a bit of space between them and they are quite uniform, but it still needs sufficient flow area to not reduce the intake too much. An inline humidity indicator would show when the desiccant unit needs to be removed and dried in the oven. Have any of you ever heard of such an arrangement?
 
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Old 01-14-11, 05:15 PM
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Fly_sailor, welcome to the forums!! It isn't so much the humidity in the air itself (certainly a factor, though), but the actual compression of the air that causes the humidity to be squeezed out. Even if you use the desiccant on intake (not a bad idea, I might add), you will still have moisture inside the tank it will need purging occasionally.
Hey, design one that will screw into a 3/4" IPF on the intake with slide out changeable desiccant pods (one for using, one for drying) and I'll split the profits with you
 
  #11  
Old 01-14-11, 06:28 PM
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Installing desiccant at the compressor inlet will not do what you hope it will.
As chandler said, it is the act of compression that sets up the moisture to be removed after the compressor, not before it.
What causes moisture to condense in the air tank is the same principal that makes refrigeration work.
You add heat to a condenseable gas then remove the heat to change it to a liquid state which will happen in the air tank.
The air tank will remove most of the moisture, an inline filer will remove what doesn't condense in the tank.

You really should not loose any sleep over residual moisture in an air receiver tank......They are designed to be wet.
 
  #12  
Old 01-15-11, 06:46 AM
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i also live just north of Galveston-with the constant humidity.
i drain my tanks about weekly.- i also ran a 20' 3/4" galvanized pipe across the shop ceiling-& put my filter drier at the other end. the long pipe dissipates the heat from the compressed air when in constant use/to help the water condense in the pipe so the drier can catch it.
if the air is still hot when it reaches the drier-it will not have the moisture removed.
& it will really show up in the tool in use-especially if you try to spray paint.
 
  #13  
Old 01-15-11, 08:18 AM
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Relative Humidity

The trick is the vapor pressure of the water inside the tank is the same as the vapor pressure outside the tank once the temperature inside the tank returns to ambient.
Effectively, the air in the tank can hold the same amount of moisture as the equivalent volume of air outside the tank regardless of the pressure inside the tank.
The typical air compressor, for home use anyway, builds 120 PSI which is 8 atmospheres (9 atmospheres absolute) If the desiccant removes 89% of the moisture from the incoming air stream, there will not be enough moisture inside the tank to condense. There might still be enough to condense momentarily when it goes through a nozzle and chills down.
But you made me investigate this a bit further. The relative humidity coming off the silica gel needs to be about 10% at 95 degrees ambient. At that level, the silica gel can only absorb about 8% of its mass as water. So it requires 12 times as much silica gel mass as the mass of water to be removed between heating the gel to remove the captured moisture.
For my little 3.7 CFM air compressor that works out to removing .5 pound of moisture per hour of operation. If I wanted to run the little beast for 8 hours then I have to remove 4 pounds and would need 48 pounds of gel.
So thanks for getting me off my butt on this. It's an interesting idea, I suppose, but not worth the trouble.
Still you can't keep an old retired mechanical engineer from trying to invent something new.
Have a good one, ya'll.
 
  #14  
Old 01-16-11, 05:27 AM
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Thumbs up Hey, interesting numbers!

Not sure if anyone would bother but a homemade aid to moisture removal would be to add a coil of tubing inline between the compressor head and air tank.
As long as this line was properly sized this would reduce the temperature of the discharged air and keep the receiver tank temps lower causing more moisture to condense there.

Really though, moisture in compressed air is a fact of life.
Unless someone was prepared to bite the bullet and install a refrigerated air dryer you could reduce moisture problems with an extra filter.

I pretty much eliminated moisture problems when I bought a new hose and connected a desiccant dryer to the beginning of it.
When I did final sanding and painting I would use this new hose and filter and regularly change the desiccant.
 
  #15  
Old 02-19-11, 05:43 PM
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Thanks fly-sailor. You did a great job of talking me out of pre-drying with silica. ALMOST ! Unfortunately, I recently purchased 7 lbs of gel and, in spite of the physics, I will press on with my plan. Would be interested in your "calculations" based on my configuration. I'll be sucking air thru a 3" pipe filled with silica (and a viewport to see when it's time to recharge). Instead of garage air (95 and often 80 % humid here in central Florida) I'll be feeding the thing from a 2" hose directly out of my home A/C handler. That air should be 70 degrees and < 50% rh.
 
  #16  
Old 02-20-11, 06:49 AM
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correction

My last reply stated the inside house temp. The output a/c (and input compressor temp) is more like 48 degrees and < 20% rh, Quite a change in dew point.
In a hundred searches I've never seen a suggestion to feed your compressor with filtered, cooled a/c air. Simple, cheap, low maint. Can't be anything but good for the compressor life too. Obviously I'm missing something!

The industry has hugely expensive AFTER cooling devices... but I have to wonder....
Isn't it cheaper to cool ambiant air to a very low dew point before compressing than cool extremely hot/wet air later ??? Either me or the whole industry has got it backwards. OK, I'll bet against me in this.
 
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Old 02-21-11, 02:47 PM
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spend a buck, save a penny

Maybe if you cycle your compressor once a day, it would not add up to much to feed it conditioned air. But what if it runs 8 hours a day - that's basically the cost of running 2 compressors. Besides, there's more to air prep than just filtering out the water. Oil vapor and other contaminants have to go too. So you'd still need the coolers, traps and filters on the pressure side.
 
  #18  
Old 02-21-11, 08:11 PM
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bucks spent....

The wife at home all day, running the a/c, has already spent that buck. So I guess I can't count that anyway. For sure on the post processing. I've not mentioned my after receiver stuff: 2" sloped black pipe runs with drain legs to an initial water trap to a small water cooled pipe to another water trap/reg to an inline dessiccant trap near the tools (as needed). A few bucks there... but if I don't get 20 years out if it I'll be very pissed (and old).
 
  #19  
Old 02-22-11, 06:59 AM
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20 years

Has probably more to do with the initial build quality. Maintanance does not hurt, but there's only so much you can do with everything being value engineered these days. I don't know what you have, but most consumer compressors today are thrown away after 5 or 6 years.

You setup sounds good. 2" pipe Make some air
 
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Old 02-22-11, 11:26 AM
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Pre-drying air

Trying to remove moisture from the air before it enters the compressor is an exercise in inefficiency.
When a compressor raises the pressure of the air it also raises the temperature.
An added bonus of this process is that it also raises the dew point temperature of the air which makes moisture condense at a much higher rate than air at room temperature.

For any effort you make to try to remove moisture at room temperature you will get many times the effect if you do it after it leaves the compressor.
This is why other than basic filtration air is not pre-treated in industry.
 
  #21  
Old 03-03-11, 03:59 PM
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a more compact solution ?

I assume that a signifigant amount of cooling in an air system is by thermostatic expansion... first from the compressor to the reciever then again when the 1/4" line opens to a larger drip line pipe.
So here's another idea I had but never built. Why not continue that process a few more times. For example, feed the usual air output into a 1 or 2" copper pipe, several feet long with a condesate drain at the bottom. Around the top 1/3 of that pipe exit the air into a short 1/4" (1/8 ?) tube (re-compression!) and sent it to another identical condensing pipe. Repeat this 1 or 2+ more times and the air should be cool and ready for a final water trap.
My guess is that the cooling effect will be greater than the heating that occurs as the air re-compresses each time in the small connecting tubes. Of course I could be on the wrong side of the laws, of physics, once agin.
The goal, you ask? in a limited space situation eliminate the need for long cooling pipe runs and multiple spaced drip drains.
 
  #22  
Old 06-03-12, 02:03 AM
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Horizontial Tank

Level the tank so the water drains. My craftsman, 25 lbs is not level so the water inside stays at one end. By leveling it first and keeping the valve open at night seems to help a little, i would think. lol

Still has moisture in though. I don't think the moisture will harm the tools so much, but im more worried about it mixing in the paint.

Im going to try this*********** to make it a little easier. It is a pain to go under it and drain it.
 

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