Drying Compressor Air

This long, detailed discussion covers the whys and hows of drying out the air that flows through your compressed-air lines to power tools and spray equipment. April 6, 2007

I am setting up a new air compressor in my one man shop. It is a 5hp Quincy, rated at 16 cfm at 90 psi. My main use for the machine is to run an Accuspray HVLP conversion gun. I will also use it for my pin and brad guns.

How are you filtering air on your compressors? I am starting with a basic 5 micron filter/regulator that is supposed to remove particles and water. I planned on mounting this right at the outlet on the tank. Obviously I want to have the air clean and dry enough for spraying, but I am not sure of how much filtration I need. Are there any smaller type desiccant dryers that I should be using? Also, should there be a coalescing filter in the loop to catch any potential oil from the pump, or this is overkill?

I don’t spray daily, more like every couple of weeks, if the usage makes a difference as far as how far to go with the filtration.

Forum Responses
(Cabinetmaking Forum)
From contributor L:
Yes, use the oil filter too. You have the placement of the water filter wrong, though. You need to place it farther from the compressor. It works best when the air is cooler, not straight out of the tank. Place the oil filter after the water filter. I don't use a desiccant dryer and I don't have problems finishing. I'm in the NE USA, not the most humid place, but at times it gets there.

From the original questioner:
I was thinking of adding a second filter further down the line, as there are times where I may be drawing air right near the tank, therefore needing the filtration right near the tank initially. Thanks, and I am in the NE part of the USA as well, and it can get humid in the summer.

From contributor F:

Contributor L is right. Putting a filter right at the compressor doesn't do anything. These filters only catch liquid water and when you place the filter right at the compressor, the water in the compressed air at that point is mostly vapor and will pass right through the filter. Some tips…

Run metal plumbing which will help cool the warm compressed air to help the water vapor condense into liquid water. Copper is a good way to go because it will not rust and introduce pipe scale into your system.

If possible, run the main plumbing lines on the outside of the building to further help cool the compressed air.

Run the plumbing at about 1/4" per foot gradient and have it angled to drain back towards the compressor and away from your filter stations.

Install drip legs complete with drain cocks nearby and upstream from your filter location/locations. Install a drip leg just downstream from the compressor itself to keep as much of the system’s liquid water as possible from getting into and rusting your compressor’s air tank.

As contributor L said, install a coalescing oil filter\filters downstream from the water trap\traps.

There are a lot of desiccant dryers on the market. You would want to have these located as close as possible to the actual point of use in the spray finishing operation to avoid having the air cool further after being dried and resulting in more water condensing and mixing with the paint. They make some fairly inexpensive "point of use" desiccant driers. These have a very small capacity and would become saturated quickly if the air is very wet or the use time very lengthy. They are usually sold as for a one time use, but you can easily replace the desiccant yourself and reuse them. Saturated desiccant can also be dried in an oven or other source of heat and then reused.

From contributor D:
The easiest solution for you would be to mount your water removal filters as previously suggested. Then connect a coalescing filter with couplers so you can move it from one hose coupler to the next as needed. Norgren makes a Excelon series filter 1/2" P/N F74V-4A-MB available from Dixon Valve & Coupling. They also carry replacement elements. We use the same filter to purge electronic panels. With continuous usage, we usually get over a year out of the element. The filter runs about $250.

From contributor G:
Run air lines outdoors in the freezing winter?

From contributor F:
Freezing conditions are one good reason to run the plumbing at a gradient so as to drain. I suppose there are some climate conditions that would cause problems due to extreme cold (Siberia?). The air as it leaves the compressor is in a heated state, so all things should equalize in most conditions. It freezes where I live and I have not had trouble. My exterior plumbing is 3/4" copper pipe. It would take a lot of moisture to plug that when you consider that 1/4" air hose and air ports deliver enough CFM to do the job.

From contributor G:
How cold and for how long does it get in your area? I am in northern Iowa and it can stay below zero here for weeks at a time. I think those temps would freeze up an airline. My air compressor supplier suggested against exterior airlines, so I have never tried it.

From contributor N:
Air won't freeze. If it is below 0 outside, the intake air will be dry because all the moisture has frozen out of it, i.e. snow. A compressor won't add water to the air; it can only compress the water vapor already in the air. If you put in the drip-legs that contributor F suggested, you'll have no problems. I've seen compressors sitting outside with only a corrugated roof for protection.

From contributor F:
The climate where I have my exterior compressed air plumbing is mild compared to Iowa. It does freeze here. I was warned about the possibility of frozen airlines but that was only if I allowed a low spot in the plumbing where liquid water could collect or else failed to run the plumbing on grade so as to drain down to the drip legs.

As I see it, it would take a lot of liquid water to plug the 3/4" ID of the copper pipe with ice to the point where you had less than 1/4" ID of flow, which is usually sufficient. With my system and climate, I doubt I ever get more than 1/32" ice blockage on the bottom of the pipe. And as pointed out, the air is usually fairly dry, anyway, at those extremely cold temperatures.

I think if you are going to do a new compressed air plumbing installation and it is possible to run the main lines outside of the building, it is worth it, because drier air is better for your equipment and especially for finishing. I doubt I would re-do an existing system that way unless I was in a climate that was causing major problems due to moisture in the compressed air.

From contributor G:
My concern is that the moisture in the air would freeze up in layers over time during long cold spells. My intake is plumbed to the outside of the building as recommended by my compressor supplier to keep noise down (witch it does) and also to draw cleaner air. There must be moisture in that cold air because the moisture traps still fill up surprisingly fast in the winter. The main problem is summertime humid air which must be dried. An outdoor line wouldn't cool in the summer when it is most necessary. Your system should be keeping the air dry in the summer, so it for sure will in the winter, making an outdoor airline unnecessary. I do have sloped lines, drip legs, auto tank drain and point of use filter/regulators. My 10hp compressor also has the factory installed after-cooler which just reroutes finned airline behind the flywheel/pulley/fan to cool the air before entering the tank. Even with all this I think my next step will have to be a refrigerated dryer.

From contributor F:
Well, you must have air conditioning if exterior air plumbing would not be cooler in your shop in summertime. My shop is definitely hotter inside than outside in summertime. Also, my plumbing is up under the eave and mostly on the shady sides of the building. As to it being meaningless in winter to have exterior plumbing, as you know, there is still moisture in the air in winter and most shops are heated inside during winter, so piping outside the building would be cooler, thus causing more of the water vapor to become liquid before passing through water traps.

Anyway, this type of system was recommended to me by a compressor supply company and it makes a lot of sense to me. Even from the standpoint of not having to negotiate interior obstacles while running the main lines was appealing to me for running my plumbing.

The main thing I wanted to point out to the original poster was, whether plumbing indoors or outdoors, try to have at least 40 feet of metal piping between the compressor and any water trap. Water in vapor form passes right through traps, so the air needs to have time to cool first.

My supplier told me that some shops with limited square footage will run their air through 50 feet of copper tubing that has been coiled inside of a 50 gallon drum so as to chill the air before it reaches any drying equipment.

From the original questioner:
Thanks for all the help. You have provided some excellent tips that I will be putting to use. My compressor is on the second floor in my building, and my spray area is mainly on the first floor, but occasionally I would like to spray upstairs too. My shop is small, a 3 car garage, and I put the air compressor upstairs to save floor space, and reduce noise on the main floor, where 90 percent of my work happens.

I think I will run a copper loop on the ceiling of the first floor, and drop a leg on a wall for the water to drain to. Off of the wall leg, I will tee off to a filter set for my sprayer. This will hopefully be the point of driest air. I can come back up through the second floor for air use upstairs, which I originally planned to take right off the tank. This will also give me dry air compared to taking it off the tank.

From contributor F:
You just happened to post at a time when it is all fresh in my mind from having completed my own new system after I started having finishing problems due to water and oil in my compressed air.

Keep in mind that what you propose to do as far as the footage is concerned does not give you drier air. What it does is cool the air so that the content of water vapor becomes liquid water. So right before the first trap, you hopefully have cool but wet air. Then, after you go through a water trap and perhaps further downstream a desiccant drier, you have the drier air.

P.S. After you understand about how cooling the air turns the water vapor into liquid water which can be caught by traps, it also should make sense about the importance of having any desiccant driers as close as possible to the point of use in a finishing operation. You see, if you run 50 feet of compressed air plumbing and then go into a water trap… that’s fine. But if you then install a desiccant drier right downstream from the water trap and coalescing oil filter but have a 50 hose between the desiccant drier and the point of use in spray finishing… it is possible for the air to chill even further in that fifty feet of hose and bring more liquid water into your finish.

Desiccant driers also differ from standard water traps/filters in that they do remove water vapor. Apparently they don’t remove 100% or it wouldn't matter if you ran a lot of hose between the drier and the point of use.

Does anyone understand why the water vapor that can condense into liquid water after further cooling due to excess air line downstream from a desiccant drier will not affect a finish if there is no excess air line beyond the desiccant drier?

From contributor B:
For 25+ years, before retirement I was an industrial compressed air engineer. Reading the responses to your question I felt compelled to help, hopefully.

1. Particulate filtration: Weekly clean the inlet/suction filter of the compressor. Yours will have small foam or felt pads. Don't wash them, just knock/blow out the dust. Unless you have long runs of steel pipe after the compressor, you will not need any further particulate filtration for your machine tools. If there are long runs of old steel pipe, add filters as close to the points of use as possible.

2. Oil filtration: Bearing in mind that any oil carryover from the compressor will be in vapor form. Oil removal filtration should be positioned after your cooling/drying station.

3. Water removal: All compressors produce water vapor that downstream condenses as it cools, turning to liquid. However, even if this is drained off at an auto-drain filter, there will still be water vapor entrained in the air. This vapor condenses instantly at the point of air use as the pressure drops, e.g. at an air blowgun, inside your air tools (bad for 'em) and at paint spray guns where the water vapor mists with the paint, affecting what otherwise would be a superior paint finish.

Think twice before you decide to buy a desiccant dryer, as regenerative types will consume about 15% of the air produced by the compressor. All you should need for woodworking, including fine finish painting, is a refrigerated dryer. Make certain that the cfm rating is at least that of the compressor. Also make certain that it is located near to the compressor in the coolest but frost free and dust-free area, otherwise the performance will be affected. Keep it clean, especially the condenser.

A refrigerated dryer will typically condense and automatically drain off all water to a dewpoint of 4 degrees centigrade (39o f). Search Google to find out about pressure and atmospheric dewpoint.

From contributor G:
Good info, contributor B. Does the compressor of a refrigerated air dryer run constantly whether air is being used at the moment or not?

From contributor B:
Good energy-minded question. Essentially, a fridge dryer needs a couple of minutes running to get down to operating temperature before passing compressed air through it. A low cfm volume unit would run all the time except when the air compressor is electrically switched off. We are talking dryers that have very low energy small/fractional horsepower motors, same as a domestic refrigerator.

A Thermal Mass Dryer (TMD) is a variation of the fridge dryer. Instead of the direct tube-in-tube type evaporator where the air is chilled by the refrigerant through the tube wall, the refrigerant in a TMD unit cools an integral tank of antifreeze (Glycol usually) which acts as a heat sink. The air then passes through a series of tubes or a tube coil within that chilled antifreeze, hence the heat from the air is adsorbed by the antifreeze.

The operating energy benefit of this type is significant where there are large horsepower motors involved with frequent and sometimes prolonged zero-flow of compressed air. The fridge unit simply stop/starts via a thermal sensor switch activated by the antifreeze temperature and sometimes by a parallel operating RH (relative humidity) switch in the downstream compressed air pipework.

There is, as usual, a price to pay in that TMDs are not so efficient despite what the manufacturer salesman may say. The chilled air temperature usually has a big swing margin, producing an average guaranteed dewpoint of only 10o C instead of 4o C. I doubt if you could obtain a TMD for such a small application as discussed here.