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Confusion!! re: Air filters, coalescing filters, desiccant dryers, water separators

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  • Confusion!! re: Air filters, coalescing filters, desiccant dryers, water separators

    I just installed my first true air system in the shop. The idea is to use it primarily for misting coolants, a future MQL system and a hydropneumatic cylinder on an auto feed mechanism. I previously had a small 7gal., 1HP unit that would run continually to keep up with a simple KoolMist nozzle @ 60psi. I didn't use it often because of the duty cycle and the noise.

    Now the compressor is in a separate room, well sound-proofed, and is a 60gal. vertical unit with 10.2 SCFM @ 90psi. I expect it to be sufficient for my needs. After putting in all the black pipe, regulator/filters, etc. I have begun to second guess myself. I dismissed early on adding a 3-in-1 refrigerated air dryer unit. The cost was more than I paid for the compressor itself, and I couldn't swallow the figure. I thought I would have the two air outlets for coolant with a simple air filter (5 micron) and add a dessicant dryer to the third outlet. This would eventually serve the air/hydraulic power feed mechanism.

    Being new to all this and previously using NO filters at all, things were sounding the same and looking overkill or duplicate duty: units like water separators sounds awfully similar to air filters sounds awfully similar to desiccant dryers. All remove moisture in one form or another. Now I'm wondering what is truly necessary, what is overkill and what is acceptable but (un)advised.

    So, first to give a succinct overview of the system as it is now. The 60gal. tank and compressor is in an unheated garage. The piping system runs through a wall into a heated/air conditioned basement shop. There are numerous drip legs, and everything is plumbed to let gravity lead condensate to them. Each air outlet has at least one riser leading to it. Other than the compressor's input air filter, the only air treatment equipment is at the outlet. Each outlet has a regulator/filter combo unit. Specifically, that unit is McMaster number 4910K14, a quality Norgren unit.

    I am now considering adding a water separator to the main line downstream of the 60 gal. tank but before it enters the basement. Main Line piping is 3/4NPT size. Here are some questions I have at the moment:

    *will the separator unit not be effective unless an after-cooler brings the newly compressed air temperature down upstream? i.e. does a separator remain able to effectively remove moisture regardless of incoming air temperature? I realize temperature is related to dew point, but I'm looking for practical answers to this one.

    *bad idea to place a separator in an unheated garage due to the possibility of freezing temperatures and potential blockage (ice) in winter? ...remember there is mostly warmer air from the compressor running through it.

    *with an oil-lubricated, piston compressor: that is the only real entry of oil into the system I can account for. Will a desiccant dryer be significantly damaged by any oil mist to necessitate a coalescing filter upstream of it? I understand that oil will coat the desiccant and not allow it to function… but is there really enough oil entering the system in the first place---plus making it through a 5 micron air filter before reaching the desiccant unit? I mean, is it likely to just reduce the period between desiccant depletion a minor amount--or can it truly cause a problem?

    *desiccant air dryer is even being considered because of the air/hydraulic cylinder. do desiccant dryers need an after-filter to make sure particles of desiccant aren't carried into the cylinder and damage it? Or is that complete overkill except in very demanding, sensitive equipment---which a drill press feed is certainly not.

    *better, then, to not even use a desiccant dryer and just use a fine coalescing filter after the 5 micron air filter?

    *leave everything as it is now---only air treatment being the Norgren filter/regulator unit. Oil mist, moisture vapor---buncha hoo-ha!

    I have some questions pertaining to MQL systems specifically, but I think I will leave this thread alone now. That's enough questions so far! Haha. Thanks for the thoughts, suggestions and perspective.
    Last edited by Arthur.Marks; 07-10-2012, 09:01 PM.

  • #2
    Probably not the best answer for your particular questions, but I'd like to provide these thoughts.

    IMHO a desiccant dryer isn't necessary unless you are painting or using a plasma cutter. Air tools can survive some moisture in the air as long as they are properly lubricated before and after use. After use may be the most important to prevent corrosion between uses during storage. My air tools are well over 30 years old and still work fine using just a separator/filter, but I admit, I don't use them every day.

    Personally, I don't like inline oilers at an outlet or servicing all the lines. I guess they are OK at an outlet if you plan to dedicate an air hose and that outlet strictly to air tool use, but once the hose in contaminated with oil it can't be used for painting nor a plasma cutter. The small oilers that some people put at the air tool make the thing unwieldy and get in the way more often than not. My preference is to oil the tool directly.

    A simple water separator/filter is OK for air tools, as long as it's emptied regularly. The same goes for the compressor tank. Water accumulation there humidifies the air to some extent. The filter aspect is important, over time rust flakes, or other contaminants will form and may enter your air tools. A filter at the compressor won't do much to prevent that but the ones you've installed at each outlet will. You may have to experiment with the water separator location and it may even vary with season because the air temperature will change with distance from the compressor, cooling as it gets farther away and allowing moisture to condense in the lines. Your drips will be helpful and may give you an idea of the best location for the separator after some use. I find that the drip for the outlet farthest from my compressor seldom has any condensation while the ones closer do. In winter when the humidity is low none seem to accumulate any moisture.

    I hope this is helpful.


    • #3

      I retired after 42 years as a commercial/ industrial plumber. I am not an engineer, but I have some opinions on air systems I would like to share.

      I like to see an air filter on the intake of the air compressor. If you have an air conditioned space and don't mind over sizing the intake pipe, you could source your intake air from the conditioned space to help reduce moisture intake right off the bat. It doesn't matter where you remove moisture from a system.

      The mantra of pneumatic systems is CLEAN, DRY OIL FREE AIR. Your use determines what level of those three conditions you need. If you have moisture traps all over your system that are removing that much moisture it sounds like you need to address the moisture levels pretty seriously.

      I like to use a oversize coalescing filter right out of the compressor storage tank, the larger size helps reduce pressure drop, and will remove both oil and water. If you have a very high moisture content, The addition of a cooling coil upstream of the coalescing filter would also help. The filter removes liquids... it doesn't know oil from water. Downstream of this would also be the place for a regenerating desiccant dryer as well. I like the desiccant systems, as they will give you very dry air that a refrigerated system cannot match.

      If you need DRY air, but not a lot of it, the point of use small desiccant dryers are far more economical than the refrigerated dryers. If you need a lot of dry air I would look into a regenerating desiccant fair dryer which will deliver large amounts of DRY air. Try to get a refrigerated dryer to give you air with a dew point of minus 60 degrees... good luck, the desiccant systems win every time. Other than an automatic drain on the tank, all treatments will be downstream of the main coalescing filter. If you install a regenerating system, it will more than likely have it's own coalescing system.

      You would likely get more particulates into your air stream from the piping system than the compressor. Reserve your micron filters for systems that require them. If you filter the inlet air, where else would the particles come from? If the air is dry you will not get any rust in the system, another good reason for dry air.

      The piping system could be flushed with a cleaning solution if you are getting a lot of junk in the air, but like I said, particulates most likely won't be a problem.

      If you need a down stream regulator, and you have a lot of moisture in your system, you may benefit from a desiccant dryers immediately down stream of the pressure reducing valve. The expansion of the air in the PRV causes the air temperature to drop, and if the moisture level in the system has a dew point below the above the resulting temperature you will wring out more sensible moisture. This added filter would not be required you have a central dryer.

      Most systems are not this sensitive. The only place I use desiccant dryers is to feed my plasma cutter, and I use small disposable dryers at that (Harbor Freight).

      If your cylinders require lubrication, I would place the oiler as close to the cylinder as possible. Oil will not likely benefit your control valves. I would not add oil unless the manufacturer of the cylinders recommends it.
      ARS W9PCS

      Esto Vigilans

      Remember, just because you can doesn't mean you should...
      but you may have to


      • #4
        These are very helpful and informative responses! To further clarify, the feed cylinder is spec'd for clean, dry air only. Lubrication is not required nor recommended. The drip legs I installed were informed directly by this diagram: TP Tools, airline piping diagram. Someone on this list posted the link in the past, and it seemed the most succinct and understandable to me at the time. The system has only been fully installed for a few days now. So far, I have not found any real quantity of water collection in the drip legs. The only point in the system that collects any noticeable water thus far is to be expected---the 60 gal. central tank.

        Most of my concern is tempered by my previous experience with a much smaller compressor. With continued use, water particles in the airstream were very noticeable while using an air gun. Other than the blow-off gun, I have no plans on using any true air "tools." The system is primarily for coolant use on my lathe and mill. It will also find use on my drill press for the automatic feed cylinder unit and possibly a vise or other workholding device. Here is an explanation of the feed unit:

        The workholding devices would be, for example, a heinrich 33 air vise or 1-AC air collet fixture---neither of which require air line lubrication to my knowledge.
        Last edited by Arthur.Marks; 07-11-2012, 02:41 AM.


        • #5
          I worked on a breatheable (?) air system long ago, and it went something like this: Oil free compressor (oil in the bottom end but none in the top) to a refridgerant drier to filter/water seperator with automatic drain (I forget the micron size) to a one micron oil coalescing filter. Refridgerant drier was preferred over dessicant bed because of the amount of air and the level of humidity. Bad thing is, they installed the damned thing in an unconrolled environment, so it froze up when the temp dropped below freezing. Good thing is, that didn't happen very often.

          Some of that might be overkill, since you are not looking for breatheable air, and this was 20 years ago and my remembery is not so good, so take it all with a grain of salt.
          Definition: Racecar - a device that turns money into noise.


          • #6
            Not a educated person in this field but i read the water seperator needs to b as close to the end youll b using the air at cause the farther the air went the more it will condensate. And i have mine in a seperate boxed in room also and temperture change will cause more condensation. Sorry if wrong,but thats what i read.


            • #7
              Do yourself a huge favor and install a cooler between the compressor outlet and the tank. It can be as simple as a coil of copper tubing, but it must slope downhill towards the tank. The idea is to get the air back to ambient temperature before it enters the tank, and almost all moisture will drop out in the tank instead of your air lines. If the air isn't completely cooled by the time it leaves the tank, you'll have moisture condensing out in the lines or your point of use.


              • #8
                Air dryers

                As you will see, I am a desiccant fan boy.

                If you can get a real good deal on a refrigerated system, no more than $200, it might be something to consider, but as you can see here:


                the new regenerative desiccant systems are about 25% more than a new refrigerated system, but the operating costs are the problem. The refrigerated system would profit from the addition of the coalescing filters I mentioned previously. Much higher with the refrigerated systems. The dew point of -40 degrees is far superior to the best that a refrigerated dryer can do. If your demands are low enough, the systems like this are more than adequate:


                this one also filters down to .1 micron, which is nuts for industrial systems piped with steel pipe.

                Here is another interesting article:

                I like the idea that jdunmyer included regarding the extra cooling coil. Make sure you include an auto drain on the tank if you use the cooling system as the water will end up in the tank... it's got to come out somewhere in any event. Drains like these handle large amounts of water.

                As far as locating dryers near the point of use, and considering that you are not using any conventional air tools which need a lot of air, removing the moisture from the system before it enters the piping system is the most reliable method. Once the moisture is out it does not reappear. I still vote for relocating your intake to the office ceiling or a closet. A final filter just before your equipment might be in order if your system is dirty, but don't use a smaller filter size than required to hold the pressure drop to a minimum. Static pressure varies greatly from dynamic measurements. Make sure that your filter delivers your minimum of 60 PSI under maximum load if cycle times are critical.

                ARS W9PCS

                Esto Vigilans

                Remember, just because you can doesn't mean you should...
                but you may have to


                • #9
                  ironmonger, I must thank you for informing me of the CamAir CT30 system. That likely is the best fit; but I was completely unaware it existed. What I am picturing is using it like this... As most of my air requirements are not critical---siphoning misting units and MQL unit---I am reasonably sure my current set-up is more than adequate. For pneumatic workholding and machine use, though, I can bring the CamAir CT30 to whichever machine it is needed with standard air hose connections. When it is not needed, it can be easily disconnected from the system and won't suffer needless depletion of the desiccant. That is my thought on it currently.

                  I still find some of the mechanisms in these systems confusing. For example, water separators. I have only found one reference to how a separator removes water from the system vs. a general air filter (which also does) or a coalescing filter (which also does). I understand the difference between the latter two. A general filter is a polypropelyne filter of a certain size structure of "holes" in its composition. A coalescing filter is a different medium but is essentially the same. The medium is more of a fiber so it also absorbs oil, which a general filter will let through as long as the particles fall below a certain size. The coalescing filter material, rather, absorbs the oil very efficiently. BUT what exactly is a water separator? One reference simply explains that it creates a vortex of the air while moving through the separator. The vortex, I guess, forces the heavier water particles out to the edge. Is that right? So a water separator would really only be efficient after something like an aftercooler which by design condenses out a large quantity of water but doesn't remove it. The reason I feel there is more to it is because so many of the water separators are labeled "point of use" for placement. By point of use, your system piping should have already significantly collected the condensed water in the drip leg. Other moisture would largely be vapor and not be affected by a "vortex" type of extraction anyway.
                  Last edited by Arthur.Marks; 07-12-2012, 12:41 PM.