water in air compressor tank?

[Travis Conner]

Well i'm pretty sure I asked about this already, but I've looked through 100 of my posts and can't find it. Anyways, people on here were saying that the heat from the compressor causes the hot air to condense and make water at the bottom of the tank. Well I've noticed I can drain the water out, not even let the pump turn on, then drain more out the next day. What gives?

[Travis Conner]

I guess this is why they say to empty it out completely.

[Jim Becker]

...and this is why I have an auto-drain on my compressor. :) But yes, moisture and a compressor go hand in hand which is why you need to care for them by regularly draining that water out of the tank. You can easily replace the generally "uncomfortable to use" OEM stop-cock with 1/4" NPT fittings and a ball valve to make that easier to do manually.

[Tom M King]

They almost all come with a drain that is really too small to work very well. Even on my large ones, I increased the size as much as possible, by drilling and rethreading to a larger size.

[andy bessette]

One way to deal with it is to install an intercooler. Photos show how I installed mine.

https://i.postimg.cc/zfxDtVQq/compressor-1.jpg (https://postimages.org/)
https://i.postimg.cc/x820qMJy/compressor-2.jpg (https://postimages.org/)
https://i.postimg.cc/9f95Tzjx/compressor-3.jpg (https://postimages.org/)
https://i.postimg.cc/nhs6bMMH/compressor-4.jpg (https://postimages.org/)

[Thomas Canfield]

The intercooler should only improve the performance of the compressor, but the total amount of water "condensed" during all the compression stages will remain the same. It does require draining the little knockout filter that was installed if there is any pocket in the line between stages.

[andy bessette]

The intercooler should only improve the performance of the compressor...

It removes the water before it gets to the tank.

[Richard Coers]

It removes the water before it gets to the tank.
I'm sure it won't remove it all, if even half. No way for that small area to cool the air enough. You need a remote chiller if you want really dry air, not an air to air mounted on the compressor. But of course what works in the desert won't work in Florida.

[Bill Dufour]

It removes the water before it gets to the tank.

It only cools the air and water it does not remove it at all. You have to run the cooled air through a filter to remove any air. As air heats up it can hold more moisture. This humid air can pass though a water removing filter unless the air temperature is below the dew point. The dew point changes with pressure and temperature. As a tank cools overnight the pressure drops and the water cools below the dew point and it condenses on the tank walls and runs down to the drain. Fancy air systems cool the air in a little refrigerator before the water seperating filters
IT is a good idea to run a short length of pipe or tube from the drain fitting down to the valve. This allows more water to collect outside the tank so the bottom is not full of water. It also moves the valve out to where it is easier to actually use it.
Bill D

[andy bessette]

As the air is cooled moisture drops out.

[Andy D Jones]

To the OP's question, one possible reason you got more water out the next day is that the previous time you drained was shortly after the compressor had filled the tank with warm/hot compressed air. You drained only the condensate that was present at that time. The warm air still held more moisture that had not condensed. So once that tank full of warm, moist air cooled, it condensed some more water, which you drained later.

The intercooler mentioned condenses moisture that can be captured before it enters the tank, so that less moisture is left to condense in the tank. The total moisture condensed will be the same, but the amount condensed in the tank will be less. The intercooler also increases the effective air capacity of the tank, by storing only pre-cooled air. The already cooled air does not lose the energy from cooling in the air-lines before it reaches the air tool.

However, since the specific intercooler setup shown dumps the extracted heat back into the compressor cooling air, it may be robbing some of the potential efficiency, since the compressed air is probably heated more by the hotter compressor. The air exiting the intercooler will still be cooler (and drier) than without the intercooler, but not by as much.

Finally, technically (and trivially), intercoolers are employed between compression stages. Coolers after the last compression stage are technically called after-coolers. However, the common use of the term intercooler on turbocharged internal combustion engines (where the final stage of compression actually occurs in the cylinder) has caused the term to be misapplied in common use.

-- Andy - Arlington TX

[Tom M King]

I did a quick hookup of my refrigerated dryer, and put it downstream of the tank. It has an automatic drain, but I never had one on the tank that would last more than a couple of years. I don't get any moisture in the hose, but still need to drain the tank. The dryer is rated for almost double the air that the compressor produces.

Even on my small compressors, I changed to the largest ball valve I could get to work.

[Malcolm McLeod]

It only cools the air and water it does not remove it at all. You have to run the cooled air through a filter to remove any air. As air heats up it can hold more moisture. This humid air can pass though a water removing filter unless the air temperature is below the dew point. The dew point changes with pressure and temperature. As a tank cools overnight the pressure drops and the water cools below the dew point and it condenses on the tank walls and runs down to the drain. Fancy air systems cool the air in a little refrigerator before the water seperating filters
IT is a good idea to run a short length of pipe or tube from the drain fitting down to the valve. This allows more water to collect outside the tank so the bottom is not full of water. It also moves the valve out to where it is easier to actually use it.
Bill D


To the OP's question, one possible reason you got more water out the next day is that the previous time you drained was shortly after the compressor had filled the tank with warm/hot compressed air. You drained only the condensate that was present at that time. The warm air still held more moisture that had not condensed. So once that tank full of warm, moist air cooled, it condensed some more water, which you drained later.

The intercooler mentioned condenses moisture that can be captured before it enters the tank, so that less moisture is left to condense in the tank. The total moisture condensed will be the same, but the amount condensed in the tank will be less. The intercooler also increases the effective air capacity of the tank, by storing only pre-cooled air. The already cooled air does not lose the energy from cooling in the air-lines before it reaches the air tool.

However, since the specific intercooler setup shown dumps the extracted heat back into the compressor cooling air, it may be robbing some of the potential efficiency, since the compressed air is probably heated more by the hotter compressor. The air exiting the intercooler will still be cooler (and drier) than without the intercooler, but not by as much.

Finally, technically (and trivially), intercoolers are employed between compression stages. Coolers after the last compression stage are technically called after-coolers. However, the common use of the term intercooler on turbocharged internal combustion engines (where the final stage of compression actually occurs in the cylinder) has caused the term to be misapplied in common use.

-- Andy - Arlington TX

These knowledgeable men can play with my air anytime! :cool::D:cool:

[Bill Dufour]

One way to deal with it is to install an intercooler. Photos show how I installed mine.

https://i.postimg.cc/zfxDtVQq/compressor-1.jpg (https://postimages.org/)
https://i.postimg.cc/x820qMJy/compressor-2.jpg (https://postimages.org/)
https://i.postimg.cc/9f95Tzjx/compressor-3.jpg (https://postimages.org/)
https://i.postimg.cc/nhs6bMMH/compressor-4.jpg (https://postimages.org/)

Please tell me the welded pipe is only structural and not under pressure!
Bil lD

[andy bessette]

Please tell me the welded pipe is only structural and not under pressure!
Bil lD

Absolutely; only structural support for the cooler. I used what was on hand when I fabricated the supports.


...Coolers after the last compression stage are technically called after-coolers...

Correct. This should be called an after-cooler.

[John Stankus]

As the air is cooled moisture drops out.

Yes, but not all of it. The amount will drop to the vapor pressure of water at that temperature.

So let's do a thought experiment. At room temp (~72F ~22C) the vapor pressure of water is ~20 torr, so at 50% Relative Humidity the partial pressure of water vapor is 10 torr. We compress that from 14.7 psi (1 atm, 760 torr) to 120 psi, or increase the pressure by a factor of 8X.

In our compressed gas now the partial pressure of water should be 8 x 10 torr or 80 torr, but this may be higher than the vapor pressure of water so what is in excess of the vapor pressure will condense to liquid. At about 47C (~117F) is where the vapor pressure exceeds 80 torr. So if the compressed gas is cooler than 117F water will condense.

I don't know how much the compressed air heats up as its compressed (my calculation of adiabatic heat rise was quite a bit, but it is not an adiabatic system), nor do I know the efficiency of an intercooler (especially an air/air heat exchanger), but if we make the supposition that we can get the compressed air down to 90 F (32C) where the vapor pressure of water is about 36 torr. About 44 torr/80 torr or about 55% of the water will condense and hopefully be removed by the filter, meaning 45% will remain. So this 36 torr of water vapor is now in your tank. It will continue cooling with time to get to room temperature where the vapor pressure is about 20 torr. About 16 torr of the water will condense and end up in the bottom of your tank. For my 25 gallon tank on my compressor that would translate into about 1.5 mL of water(granted it is a bit less than the 5.5 mL from the 60 torr without the intercooler and filter), but it will still accumulate.

I am a physical chemist who teaches Thermodynamics, a Chemical Engineer probably could get a little better estimate than my back of envelope.(heat exchanger efficiencies and filter efficiencies and all that)

John (who sort of thinks there might be an exam question here for next Fall:rolleyes:)

[andy bessette]

John--very interesting.

[Malcolm McLeod]

Yes, but not all of it. The amount will drop to the vapor pressure of water at that temperature.

So let's do a thought experiment. At room temp (~72F ~22C) the vapor pressure of water is ~20 torr, so at 50% Relative Humidity the partial pressure of water vapor is 10 torr. We compress that from 14.7 psi (1 atm, 760 torr) to 120 psi, or increase the pressure by a factor of 8X.

In our compressed gas now the partial pressure of water should be 8 x 10 torr or 80 torr, but this may be higher than the vapor pressure of water so what is in excess of the vapor pressure will condense to liquid. At about 47C (~117F) is where the vapor pressure exceeds 80 torr. So if the compressed gas is cooler than 117F water will condense.

I don't know how much the compressed air heats up as its compressed (my calculation of adiabatic heat rise was quite a bit, but it is not an adiabatic system), nor do I know the efficiency of an intercooler (especially an air/air heat exchanger), but if we make the supposition that we can get the compressed air down to 90 F (32C) where the vapor pressure of water is about 36 torr. About 44 torr/80 torr or about 55% of the water will condense and hopefully be removed by the filter, meaning 45% will remain. So this 36 torr of water vapor is now in your tank. It will continue cooling with time to get to room temperature where the vapor pressure is about 20 torr. About 16 torr of the water will condense and end up in the bottom of your tank. For my 25 gallon tank on my compressor that would translate into about 1.5 mL of water(granted it is a bit less than the 5.5 mL from the 60 torr without the intercooler and filter), but it will still accumulate.

I am a physical chemist who teaches Thermodynamics, a Chemical Engineer probably could get a little better estimate than my back of envelope.(heat exchanger efficiencies and filter efficiencies and all that)

John (who sort of thinks there might be an exam question here for next Fall:rolleyes:)

Okay. Okay. ... You can play with my air too.;)

[roger wiegand]

I'm curious what the Bad Thing is that happens form having some water in the tank? Based on the above discussion there will always be enough moisture to cause rusting if that's going to happen.

My 60 gal compressor is down on the lower level of the barn, not particularly accessible. I run the water out of it every 2-3 months in the winter, probably monthly in the summer. I get a quart or so at a time. The water has some oil and is never rusty in appearance. The compressor runs maybe once a day when I'm not using it (I will, someday, track down the leak!), a couple times a day when I actively use it for running nailers and blowing stuff off. I'm using it more now that I have the nifty little Grex 2" sander that I've started using for off-lathe sanding.

I bought the compressor in 1986 and have used it this way since new, so if it's going to have a bad effect on longevity it probably better hurry up. Am I missing something or is the water a more application-specific problem, eg with spraying finishes, which I've never done.

[Malcolm McLeod]

I'm curious what the Bad Thing is that happens ...

In my experience, it is generally a process concern: your Grex may not sand well; spray oil finish is ugly; powdered bulk solids conveying may stop conveying. The air delivery volume may be impacted. Or, longer term the air receiver/piping system corrodes..?

[Travis Conner]

I also moved my water separator so it was had 100ft of hose to go through and put a small whip hose to the hose reel. Someone on here said the hot air won't get caught by the separator. Hasn't hardly caught a drop. I keep the compressor in another room so now it has about 100ft of air line routing up 15ft to the ceiling and over to the hose reel connected to the wall. Maybe routing the hose vertical catches most of the water, idk, but i'm a bit irritated that my little $15 craftsman water separator works better than this $175 pneumatic plus separator. I didn't think the craftsman would handle the inlet pressure so I replaced it for safety reasons. Hopefully moving it away from the compressor will make a difference, we will see.

[Bill Dufour]

My five Hp Quincy two stage compressor increases the high pressure cylinder exterior temperature about 20-40 degrees above ambient after it is warmed up. This has a few feet of intercooler tubing between cylinders. The 60 Gallon tank can get 10-15 degrees warmer then ambient but, never too hot to touch.
Bill D

[Bill Dufour]

I also moved my water separator so it was had 100ft of hose to go through and put a small whip hose to the hose reel. Someone on here said the hot air won't get caught by the separator. Hasn't hardly caught a drop. I keep the compressor in another room so now it has about 100ft of air line routing up 15ft to the ceiling and over to the hose reel connected to the wall. Maybe routing the hose vertical catches most of the water, idk, but i'm a bit irritated that my little $15 craftsman water separator works better than this $175 pneumatic plus separator. I didn't think the craftsman would handle the inlet pressure so I replaced it for safety reasons. Hopefully moving it away from the compressor will make a difference, we will see.


Hose does not do much to cool the air. Metal pipe would have a huge effect. A radiator is just a bunch of metal pipe in a small space.
Never use PVC for compressed gasses, it can shatter.
Bill D

[Bill Dufour]

I'm curious what the Bad Thing is that happens form having some water in the tank? Based on the above discussion there will always be enough moisture to cause rusting if that's going to happen.

My 60 gal compressor is down on the lower level of the barn, not particularly accessible. I run the water out of it every 2-3 months in the winter, probably monthly in the summer. I get a quart or so at a time. The water has some oil and is never rusty in appearance. The compressor runs maybe once a day when I'm not using it (I will, someday, track down the leak!), a couple times a day when I actively use it for running nailers and blowing stuff off. I'm using it more now that I have the nifty little Grex 2" sander that I've started using for off-lathe sanding.

I bought the compressor in 1986 and have used it this way since new, so if it's going to have a bad effect on longevity it probably better hurry up. Am I missing something or is the water a more application-specific problem, eg with spraying finishes, which I've never done.


You can add a float type automatic drain or a pressure cycle type. Or better yet is to add a electric drain valve with a timer. I have mine powered from the motor contactor so it only drains when the compressor is running. It auto drains every time the power is turned on and every 10-15 minutes of continuous running.
Bill D.
PS: people have said the Harbor Freight. pressure differential one is okay and cheap..

[roger wiegand]

In my experience, it is generally a process concern: your Grex may not sand well; spray oil finish is ugly; powdered bulk solids conveying may stop conveying. The air delivery volume may be impacted. Or, longer term the air receiver/piping system corrodes..?

If I'm reading this thread correctly it sounds like most of the time the air in a compressor tank is pretty well saturated with water vapor, no matter what you do in terms of draining it. I can totally see the virtue of a dryer that would remove some/all of that water from the air as it leaves the tank for your intended use. But what harm is a puddle in the bottom of the tank doing? It doesn't sound as if it significantly increases the moisture content of the compressed air leaving the tank as that moisture is constantly regenerated each time the compressor cycles and is sufficiently high that the air is saturated to the point where water is condensing out. The puddle isn't going to increase the moisture content at that point.

[Charles Taylor]

But what harm is a puddle in the bottom of the tank doing?

Rusting. When the tank wall is sufficiently corroded, pinhole leaks develop, or in rarer instances, the tank fails in more spectacular ways.

[John K Jordan]

Well i'm pretty sure I asked about this already, but I've looked through 100 of my posts and can't find it. Anyways, people on here were saying that the heat from the compressor causes the hot air to condense and make water at the bottom of the tank. Well I've noticed I can drain the water out, not even let the pump turn on, then drain more out the next day. What gives?

Travis,

I didn't read all the messages so all this and better may have been mentioned in this long thread. The air heated from the act of compression can hold moisture, depending on the humidity at the compressor intake. As the air cools, the moisture will condense. Water condensing from moist, warm air in the tank will stay in the tank. My compressor is a 60 gal 5-hp iR and the manual recommends draining every day. An automatic drain on a timer or triggered by on shuttoff can save a lot of crawling on the floor.

Moisture condensing in the pipes/lines outside the tank can either drain away from the tank or drain back into the tank, depending on the slope of the lines. The amount condensing in the lines can depend on how much you use the compressor, the ambient temperature, and the type of lines (i.e. copper vs plastic). Some of the water draining back towards the tank can be caught in a gravity trap to be drained manually, reducing the amount that ends up in the tank. Some installation experts recommend running at least 25' of line length between the compressor and the dryer - some recommend sloping it away from the tank and some recommend sloping it towards the tank, presumably to be removed by daily draining.

That water that drains away from the tank can get blown down the line towards the tools and such but can be removed by a gravity trap, a water separator, an in-line dryer or condensing cooler. I use a trap made from a piece of brass tubing with a valve on the bottom, a water separator and a desiccant dryer before the regulator and the manifold which distributes air around the shop.

432885

JKJ

[Travis Conner]

I can't view that image. Maybe I shouldn't route the air line straight up like a did. That's something new I heard to keep more water from flowing down the lines, but doing it that way is basically sending it back into the tank, which I don't like either.

[Jim Becker]

I can't view that image.

Click on the "Donate" button at the top of the page and become a contributor. $6 a year. That will enable viewing of images, having access to private messages and access to the Classifieds, among other benefits.

Jim
Forum Moderator

[Travis Conner]

How come I can see the pic in post number 4?

[John C Bush]

Automatic tank drain valves are ~$130, need 120VAC and are a simple solution to keep condensate out of the tank without having to remember to do it manually. Mine opens every 45 min.-usually just as I open the door to my compressor room-the loud air blast scaring the Yahoo out of me. A vertical line with a loop below the tank exit and a drain valve manages extra water if you are using a lot of air, and a separator/filter at your tol connection should be an easy fix.

[Bill Dufour]

Automatic tank drain valves are ~$130, need 120VAC and are a simple solution to keep condensate out of the tank without having to remember to do it manually. Mine opens every 45 min.-usually just as I open the door to my compressor room-the loud air blast scaring the Yahoo out of me. A vertical line with a loop below the tank exit and a drain valve manages extra water if you are using a lot of air, and a separator/filter at your tol connection should be an easy fix.
I bought one from the bay for around $30 delivered. It was used American made. It came with a muffler which does not do anything. Took me a year to realize I could run the low pressure discharge line uphill to the discharge point only the high pressure side has to slope down towards the valve.
I have no idea how they do this in space ships with fuel pumps.
Bill D

[Jim Dwight]

I learned the hard way to open the little drain valve on the tank of my little Senco more often. When you don't, the tank rusts and it gets in the lines and your compresser does't work. I know there are automatic valves and possibly other ways to keep the tank from rusting but the easiest thing for me is to just remember to open the valve after I use the compressor. Only takes a minute.

[John K Jordan]

How come I can see the pic in post number 4?

You might be able to see photos that are hosted on another web site instead of uploaded to SMC. I don't know, just guessing.

The management here encourages uploading photos because if the external web site goes away someday the photos are lost and the usefulness of the message and continuity of the thread are lost.

BTW, the almost negligible cost of becoming a Contributor has other good benefits besides viewing uploaded photos. I use the monthly recurring contribution method so I never have to think about it.
https://sawmillcreek.org/payments.php

JKJ

[Travis Conner]

My Quincy has a ball valve built in already, so really I just kick it open with my foot while holding my ears and wait until until it's not white anymore coming out. Takes about 15 seconds of draining and I do it just about every time I'm around the compressor. Most never do anything and still seem to get 30 years out the tank. I have a buddy who has one that probably from like 1985 but he drains his completely after every use, but he also doesn't use his. I use mine every couple days probably.

[Paul Haus]

This is 'somewhat' related here. When I first got my 60 gal compressor, it had the usual POS valve on the bottom of it. Hard to get to and not the easiest thing to drain. I spent some time reading and looking for info. I read about the automatic drains, but also found negatives on them. I read about some that had issues with rust in the tank from standing water. I came up with what I think is the better solution: a 1/4" elbow, about a 4" piece of pipe, a ball valve and another elbow.
Elbow in the bottom of the tank then the pipe, next the ball valve and finally the elbow to point it down towards the floor. The ball valve makes it easy to reach and drain and the piece of pipe acts as a water collection point. Any water in the tank settles in the pipe, away from the tank helping to prevent/reduce the chance of rust in the tank.
I don't know if there's a 'good' automatic drain valve, most I read up on seemed to have a number of complaints about not working correctly. My solution was the low cost hopefully most effective alternative.

[David Buchhauser]

This is 'somewhat' related here. When I first got my 60 gal compressor, it had the usual POS valve on the bottom of it. Hard to get to and not the easiest thing to drain. I spent some time reading and looking for info. I read about the automatic drains, but also found negatives on them. I read about some that had issues with rust in the tank from standing water. I came up with what I think is the better solution: a 1/4" elbow, about a 4" piece of pipe, a ball valve and another elbow.
Elbow in the bottom of the tank then the pipe, next the ball valve and finally the elbow to point it down towards the floor. The ball valve makes it easy to reach and drain and the piece of pipe acts as a water collection point. Any water in the tank settles in the pipe, away from the tank helping to prevent/reduce the chance of rust in the tank.
I don't know if there's a 'good' automatic drain valve, most I read up on seemed to have a number of complaints about not working correctly. My solution was the low cost hopefully most effective alternative.

Paul,
I basically did the same thing to my compressor several years ago. 90 degree fitting out of bottom of tank, straight length of 1/8" npt pipe out to side with valve (petcock) at end of pipe. Easy to drain it into a shallow pan. It only took me a little over 30 years to get this done. Previously I would put the shallow pan under the bottom of the tank and lay down on the floor to reach the drain petcock. This is much more convenient. I've had one of the auto-drain timers for about 15 years, but never got around to installing it. I like the manual mode better.
David

[Travis Conner]

Andy let me explain. I can fire up the compressor on a monday and then leave it pressurized, but turned off and still be able to get moisture out each day on Tuesday, Wednedsay, Thursday, and Friday.

[David Buchhauser]

Andy let me explain. I can fire up the compressor on a monday and then leave it pressurized, but turned off and still be able to get moisture out each day on Tuesday, Wednedsay, Thursday, and Friday.


Sounds to me like you might have a water leak.
David

[J.R. Rutter]

I did a quick hookup of my refrigerated dryer, and put it downstream of the tank. It has an automatic drain, but I never had one on the tank that would last more than a couple of years. I don't get any moisture in the hose, but still need to drain the tank. The dryer is rated for almost double the air that the compressor produces.

Why not put the refrigerated dryer ahead of the tank? I get next to no water buildup in my tank with this setup. I'm talking crack the valve annually just to check...

[Andy D Jones]

Andy let me explain. I can fire up the compressor on a monday and then leave it pressurized, but turned off and still be able to get moisture out each day on Tuesday, Wednedsay, Thursday, and Friday.

Interesting... The only thing I can think of is that daily temperature cycles may be causing "dew" (condensation) daily. The amount of condensate drained should reduce over time (days) without powering up the compressor. This could be exasperated by a rough tank interior, which helps keep condensation on the walls with out all of it draining to the bottom, to be expelled when you open the drain periodically. Is your tank vertical or horizontal? This would be more likely to happen in a horizontal tank.

I'm guessing this compressor does not get used a lot, if you can run 5 days without powering it back up, especially if you drain moisture daily too. Or it is a very large tank.

-- Andy - Arlington TX

[Bill Dufour]

My Quincy has a ball valve built in already, so really I just kick it open with my foot while holding my ears and wait until until it's not white anymore coming out. Takes about 15 seconds of draining and I do it just about every time I'm around the compressor. Most never do anything and still seem to get 30 years out the tank. I have a buddy who has one that probably from like 1985 but he drains his completely after every use, but he also doesn't use his. I use mine every couple days probably.


Drill a small hole through the wall near the drain valve. Put a piece of rubber hose over the nipple and hose clamp it in place. Poke it through the wall and set a brick on it to reduce whip. Or a tee to equalize the forces so it doe not whip.. Or jet a piece of pvc pipe into the ground a few feet and drop the hose into that as a muffler.
Bill D