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Dessicant Compressed Air Filters

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I can get you started:

  • Virtually all air here on Earth has a certain amount of moisture content, variously described as relative humidity value (RH%), humidity ratio (lbs of moisture/lbs of dry air), Dew Point (temperature at which the air will condense on surfaces)...
  • This is the air that enters your compressor, so the compressed air still has moisture in it.  As the air is run through the compressor the air compresses, but the water doesn't.  This results in a higher proportion of water, by volume, in the compressed air stream, which often can cause problems for downstream equipment
  • The two main methods of moisture removal from air are: (1) lowering the air temperature until the moisture condenses out, (2) use of a desiccant material that will absorb the moisture out of the air.
  • Lowering the air temperature is usually accomplished with a direct expansion cooling process using an industrial refrigerant (Dx cooling).  A heat exchanger between the compressed air and the cooled refrigerant extracts heat from the air and condenses the moisture in it on the coil surface of the exchanger.  As far as I know this is the most common and effective industrial method of removing moisture from compressed air.
  • I am not that familiar with desiccant compressed air dryers, but like conventional desiccant air dryers they appear to use a silica gel or activated alumina material that has a high affinity for moisture to extract that moisture from the air.
  • While a desiccant dryer can theoretically remove more moisture from air, in practice there are a couple of drawbacks:
    • the desiccant must be regenerated on a fairly regular basis by either being heated to drive off the moisture or having an already dry air stream run over it (of course if you had the latter you wouldn't need the dryer in the first place)
    • Over time the desiccant loses effectiveness and must be replaced (often due to dust, dirt, debris...being pulled into the system and coating the media)
    • the desiccant unit and associated pre-filtration, put a significant pressure drop on the system, making the compressor work harder for the same throughput
  • In my experience most industrial compressed air systems use refrigerated air dryers as a first stage for removing moisture, and only add  a desiccant system for extreme moisture removal afterwards.
  • If you only need a small amount of moisture removal on a home system, it can sometimes be accomplished by running the compressed air in steel or copper piping and letting the moisture condense on the interior of the pipe.  A substantial drip leg for each drop can help remove this moisture before it gets to your terminal equipment.  Of course these drip legs need to be purged regularly, as should your air compressor tank, for the same reason.
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Thanks for the tutorial.  This is for a home shop with, occasional use, so the refrigerated system is not practical.   The question is then:  It seems unlikely that all desiccant type systems are created equal - so how should I evaluate the functional quality of the available systems being considered ?

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Usually compressed air lines are run with a downhill slope with a T fitting and valve at the lowest point. Water that condenses on the inside of the line drains to the T and is released or carefully blown out through the valve.  The T also allows any debris to fall into the down pipe and helps keep it from going into the machinery. There are cartridge type moisture removers that can be added in line close to the point of use that help if you still have a moisture problem. 

Many home type machines (that I am familiar with) can tolerate a small bit of moisture. You can drive yourself crazy trying to chase decimal points. Ask the machine manufacturer about the recommended moisture content allowed for the incoming air to the machine. For instance, I was told plasma cutters for instance should have a low moisture content for best operation.

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Depending on your air application, you may want to install a filter to remove oil to the line.

Moisture, oil, and debris removal is important if for instance you are using the air for spray painting, more so if it is a customers high end vehicle. (grin) It is easier to eliminate a problem rather than to go back and try to fix it.

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Knots, what kind of equipment will you be running with your compressed air? Some equipment types are much fussier than others when it comes to air quality.

As an example of a simple setup for cleaning and drying my compressed air, I am currently using three modular units to process the air for my plasma cutter. These are mounted on a bracket that is attached to my plasma cutter cart, and include:

  • a general-purpose filter.
  • a coalescing filter.
  • a dryer that uses silica gel desiccant beads.

The two filter units help remove dust, water, oil, etc., while the dryer unit absorbs water vapor

The coalescing filter and desiccant dryer units are typically sold as a "cleaner/dryer" pair. The filter units have drains for purging liquids that collect in them, and they contain small replaceable filter cartridges. My coalescing filter unit has a flow-resistance indicator on top that shows when the filter cartridge becomes dirty enough to require changing. 

Supplying clean, dry air to a plasma cutter will help extend the life of consumables like electrodes and nozzles, and spray-painting equipment also likes clean, dry air. There is an outlet screen in the dryer unit to prevent particles of the silica gel beads from being transported downstream, although for what I would call "micro polishing" of the air, one could install an additional general-purpose filter after the dryer unit to catch any microscopic bits of the silica gel that might have gotten through the screen.

Generally speaking, the more you filter and process the air, the more flow resistance you will tend to have, but for many operations it's worth working the compressor a bit harder in order to get the high-quality air that many applications want. Whatever equipment you are using, it's a good idea to have a pressure gauge (and for some equipment a flow gauge) AFTER the filter and dryer units, so that you know what pressure (or flow) you are actually getting at the equipment while it's in operation. In the case of my plasma cutter, there is a built-in pressure gauge in the plasma cutter chassis, so that I can monitor the pressure during operation.

The desiccant beads that I use are an "indicating" type of silica gel. The beads change from blue to pink as they absorb water. The manufacturer of the dryer should be able to tell you what size/type of desiccant is recommended for their particular dryer unit. The dryers are often sold with one or two small "starter" bags of desiccant beads that you could match and probably find much cheaper from some third-party source.

If you buy a bulk quantity of silica gel desiccant beads, the price goes way down per unit of weight! The last batch I bought was large enough to fill a five-gallon bucket. I don't recall the price or weight offhand, but it was cheap enough that I usually just replace the used desiccant beads with new beads when the color has changed. That doesn't mean that I throw away the used beads; I put them in a separate jug for possible later rejuvenation by heat. Check with the desiccant manufacturer to see what recommendations they might have for rejuvenating the desiccant, since the beads and/or the color indicator additive can be damaged if done improperly, and there can be some limitations on rejuvenation cycles.

As to other ways of drying air, as mentioned by Latticino, there are refrigeration units, which are often used in commercial/industrial high-duty-cycle operations. There are also self-regenerating desiccant dryer units. However, for my small home shop neither of those would be within my budget, not to mention taking up valuable space, and I think that the filter-and-desiccant approach is working quite well for me. I don't mind changing the desiccant once in a while...it only take a couple of minutes.

Oh, and don't forget that your compressor has an inlet dust filter of some kind, which should be maintained properly to make your compressor last longer, and at the same time, it will make your downstream filters (which tend to be more expensive) last a LOT longer.

Al (Steamboat)

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