Filter for air compressor? | NC Woodworker

I've got an 80 gallon tank with a 5hp two stage compressor that sits inside the shop in its own 'closet' that is vented. I don't have any water issues, and I only drain the tank one time a year...or less. I have not seen any oil either, and I do check the level in the compressor occasionally and have not seen any need to add in the last ten years. My compressor is likely overkill for the use I have but I never cut it off, just sits there ready for me... and the use that I give it doesn't put much of a demand on it. I don't have any kind of filter or drier. When I painted cars with it I did, but that use is very different than what I do now.

Now the way my luck runs, just as soon as I post this it will start blowing oil. To a large degree, the amount of water and oil in your shop air compressor's stream will depend upon whether you are drawing air while the air is hot (as in the compressor is running or has recently run) or after the air in the tank has had time to cool down to ambient before use (say, an hour, or more, after running). With large air compressor tanks and light use you are commonly just drawing off cold air and by then most of the excess water, and virtually all the oil aerosol (for oiled compressors), have condensed and settled to the bottom of the tank, greatly reducing the risk of discharging either in your air stream. But if you are using the air hot or while the compressor is running, especially with smaller compressors, then water condensation and oil aerosol become a real risk in your air stream and filtering for these becomes especially important if such may harm your finish or intended use. Of course, it is always a good idea to consider filtering the air if there is any chance of water or oil fouling your finishing, etc., even if only to play it safe since redoing a finish can be a real headache after the fact.

How often you need to replace your filter really depends upon how much air you are using and how quickly your usage fouls the filter. The most common inline filter is just absorbent paper, felt discs, or fiber that the air passes through first while others may include silica gel desiccants and a single felt disc, or two, to capture oil droplets. There is no need to bleed off the air in your tank at the end of the day, it is perfectly ok to simply leave the tank charged with air, so that may extend your filter life. Keep in mind that the chief application of the inline filters is typically spray finishing (such as with an HVLP gun) where air stream contamination is a great concern and their air use relative to run time is often very high, hence the recommendation to replace weekly when heavily used. If your use is much less and your application less demanding then you may well find that you can go much longer than a week between changes. These inline filters are best inserted in the line just a few feet before the end of your air hose (I use a 2-3ft section of hose after mine when using the filter) so that they may also capture any moisture that has condensed in your run of air hose.

If you do not already have a combined water and oil trap on the outlet of your air compressor then you may wish to add one to capture more of the liquid water and oil ahead of your inline filter. These traps only capture water that is actually liquid while under pressure and exiting the tank, so they do not capture inline condensation nor condensation that may form at the exhaust end due to the sudden pressure drop, but they do a good job at capturing any liquid water and oil exiting from your compressor tank, reducing fouling of your air hoses to a considerable degree. This can also help to extend the life of your inline air filter since they reduce the water and oil reaching your inline filter. Oh, I know I have way more compressor than I need now, but the building it's in has very low humidity as it's climate controlled inside... so I'm sure that is why it has very little moisture when drained. When it was outside (when I used to do paint and body work) I would have to drain it every few days, and I had to have a water separator and filter. I have not seen the need as yet for it here in my current use. I'm guessing here, but I'd say it runs about three hours a week at the very most, so it's not running very much. I've thought about downsizing, but I hate to spend money on something when what I have is working. And besides, I've still got my mechanic tools, and while I hope I never have to do big work again, I would if the need arises on my own stuff. The compressor is one I got from Sears in , and the oil I've used in it is some I got when I bought the unit that Sears put out. I can't remember the exact weight though. I know that some years ago I had to change a seal on the unit for a leak, and replaced the oil in it then, but it's never actually used any oil outside of the leak it had. I check that about every other month now, and it's always the same place every time.

I had another compressor that was an Ingersol-Rand that was a two cylinder single stage, made more noise than a thrashing machine and finally blew apart. I had to add oil to it very often and the thing had to have a filter and oil separator, also sat outside in a shed. I'm a firm believer in two stage units, and so far this one has been bullet proof, and watch it fail when I go to the shop this weekend... 5hp,2 stage,30 y.o. Here.

Between the compressor pump and the tank we have a water cooled "expansion" tank.That eliminates 98% of moisture and mayonnaise (oil/water mix from pump)from ever reaching the tank...vastly lessoning burden on filtration down stream of tank.The water cooling put it over the top and only gets turned on during long cycles.

The idea of an expansion tank is hardy new,Sailor Bealle(sp) ,along with other compressor co's routinely had them mounted back even before WWII.Marketing and generally cheapened products saw them disappear.These were medium sized compressors' BTW...less than 5HP.So don't think it was industrial.Quite the opposite,it's the smaller compressor which needs it more.Google old Sailor Bealle images.They're darn cool looking,finned aluminum(vg examples of US casting capabilities,bar none),that will put a smile on anyone's face.

We have other systems in place as well.But the expansion tank is the brains in the matter.The use of a typical"fruitjar" separator is all but useless unless it's mounted WELL downstream of the heat which is still present right at the tank.

If you can't wrap your head around the e tank,at least get a coil of soft copper mounted right after the tank,with your fruit jar after...and at the top.Air in at bttm,runs through the still like coil..up,then fruit jar.Put the coil on a QC,and you can throw it in a cooler of ice if painting HUGE projects.Even heard one shop running a small coil,after the tank,into the storage tank of his shop toilet.Which does work. Sorry Greg,wasn't referring to anyone particular.I work daily with the air coming out of compressors...for well over 40 years.It's the expansion tank in my case that is about 30" long which got water cooler...I think you're talking about the compressor pump being cooled?

On a general note...the notion is that air(in an e tank),can travel faster,and can be redirected faster than the water and funk.This allows for part of the separation.the other part is when a gas is traveling in a pipe then suddenly hits a much larger cavity...some of the moisture falls out.It gets further development when changing temperature.

Further study;if anyone gets a chance,carefully bust open one of those orange disposable bulb type filters that you'd use on a spray gun.Try wrapping your brain (you'll get the joke,once in there) around how these simple filters work their magic.I just did this 6 or so months ago.Wish I'd done it 20 years ago.