water in air compressor tank?

[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)

[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)

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.

air_comp_ctrls_IMG_20150124.jpg

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?