Heatless compressed air dryer

The pressurized air from a standard air compressor is fine for most uses. But some applications like plasma cutting call for low-humidity air and the hardware available to facilitate this can cost a bundle. [Roland] and his cohorts at TX/RX Labs (a Houston, Texas Hackerspace) just built this air drying system.

It works using a desiccant; a substance that sequesters moister. It’s the stuff in those little packets you find in shoe boxes and the like with a warning that you shouldn’t eat it. The image above shows two chambers which house the desiccant. Only one is used at a time, so that as it’s ability to remove moisture drops, the system can switch over to the other chamber. There’s even an automatic recharging system built in that uses a portion of the dried air to remove the humidity from the unused desiccant chamber.

There’s a functional diagram at the link above. It’s resolution is low enough that the text is almost unreadable but we’ve asked [Roland] if he can repost the image. This seems like a build in which other hackerspaces will be interested.

24 thoughts on “Heatless compressed air dryer

      1. Not so.

        I designed control systems for commercial ones of these made by Airmate SMC about 40 years ago, quite real, no heating.

        These are usually preceded by a water jacket (ono) cooler and mist coalescer to remove the lumps. :)

        A bleed of dried air from the active side is normally used to back flush the wet side to ambient.

        The exit air still has 100% RH and a refrigerative drier is required to get it any lower.

  1. Nice! Neat idea, though I would have been tempted to modify a fire extinguisher to house a huge pile of desiccant; I think the valve arrangement is already ideal for it (pipe in via the charge stem, draw out from the hose output.) They’ve even got a pressure gauge on ‘em already :)

    With a big enough pile and two of them, just set it up so that the desiccant can be removed and put in an oven (you obviously need the rechargeable kind of desiccant.)

  2. At least they didn’t use PVC! Desiccant dryers are pretty good but unlike refrigerated ones, they do have a service life before they fail by, well, not working. Also, compressors can generate A LOT of water by volume. Far more than you would think.

    Correct me if I am wrong though but I believe you can dry out the desiccant and reuse it simply by putting it outside under a low humidity sun or under an oven?

      1. Houston is pretty much a swamp with good solar exposure. It can win arguments with Florida over the definition of “high humidity sun”.

        In other news, YAY Tx/Rx! I’ve seen the device in question, it’s mounted to a dolly and is intended for use with the space’s plasma cutter. It was quite an interesting hack to watch come together.

    1. Desiccant can be either rechargeable or non, and indicator or non.

      The rechargeable stuff usually requires a fair amount of heat. You might be thinking of odor-absorbing zeolite material, which does get recharged by leaving out in strong sun (UV and heat help release the stored-up odors.)

      Ideal would have been a sight glass where the air last leaves, so you could see the color change in the desiccant – that would have eliminated the need for the switchover controller and whatnot.

      As to the volume of moisture – there are moisture traps that should be installed down-line from the compressor, preferably after a heat exchanger or length of long metal pipe (you could fake it with an oil cooler for a car, or a set of refrigerator coils), which will get rid of the worst of the moisture. The higher the pressure and the more you cool the air before it hits the moisture trap, the drier the air will be. Saves your tank, too, if you do it before the tank.

  3. clever idea to have the large diameter tubes to reduce the air velocity (to let the water fall out of the air) to do a pre-water separation before hitting the final desiccant stage.

  4. Desiccant dryers are great. We used a set at my old work. The things were the size of a semi trailer turned upright. To be fair, we had 6x300HP air compressors feeding the system. The bloody thing had 10″ steel air lines at 120psi, it fed a 500,000 square foot aluminum foundry.

    Anyway, you usually have two systems, once currently drying the incoming air, and the other in the “regeneration” cycle. Regeneration meaning heating and ventilating the desiccant so that it boils off the absorbed water vapor. And yeah, the systems we had I think had a 10 year life on the desiccant… could be wrong though.
    Amazingly enough this system was more efficient than the refrigerant based dryers we had been using. Dry compressed air is expensive to get…

    1. Their setup is a standard heatless regenerative which uses a portion of the dry air from the drying tower to dry the wet tower. The way it works is the water is adsorbed (don’t confuse with absorbed) by the desiccant, it basically clings to the surface of the desiccant beads. Dry air from the drying tower outlet is fed back down the top of the wet tower and it will dry off the surface of the wet desiccant beads, this is called the purge cycle. The moisture flows out of the bottom of the dryer usually through a muffler. This cycle can be repeated back and forth for many years usually and generally stop working due to a failure in the valve train. However, desiccant beds can also be damaged by saturation of the beads with compressor oil.

      These types of dryers can be expensive to operate due to the fact that they use ~10% of the compressed air volume for the purge cycle. In your 6x300hp setup using heatless regenerative dryers would cost about USD $16 an hour to operate if your electrical cost was USD $0.11 kWh like it is here in Michigan. This turns into about USD $140,000 if you ran it round the clock for a year which is most likely the case in a foundry. Compared to refrigerated these things are way more expensive to operate. The reason that heated regen dryers are used is usually to help keep this cost down. Considering these costs, your refrigerated dryers are much more efficient, it just that the regen provides much dryer air. Your refrigerated dryers not operating properly is the most likely cause for wet air you had experienced.

      Generally these dryers are used where compressed air is subjected to below freezing conditions. Most refrigerated dryers provide air with a pressure dew point of 38ºF where a regenerative dryer will go anywhere from -40 to -100ºF depending on the purge cycle. When air lines run outside in below freezing conditions the will freeze closed if you use something like a refrigerated dryer.

      There are also other options like deliquescent (salt beads that absorb the moisture and use only one tower) and membrane dryers (removal of moisture at the molecular level). Membrane dryers also are used for the generation of Nitrogen.

      In all there is a lot involved in providing quality compressed air to the point of use. Quality air is important as it is the largest factor in the lifetime of tools that run off of it. BTW, I am an air compressor technician by trade. :)

  5. oops, forgot to ask…I wonder how they are getting the 2nd chamber dry? Like I said in my previous post, you got to go into a regeneration cycle by boiling off (for lack of a better term) the water from the desiccant and I don’t see that happening here… Heat is usually necessary to dry the desiccant to remove the water, or I suppose dryer air. In this case they are using the “dry” desiccant chamber to dry the “wet” one?

    Or are they saying they are running ambient air through it to get it down to humidity levels of the shop? Seems kinda…well, interesting.

    I would like to know what kind of humidity % change they are getting with this setup.

  6. This concept is completely valid and even sold commercially. Just do a Google search on “Heatless Desiccant Air Dryers”.

    I recently saw one in use at a TV/Radio station. They used it to provide positive pressure inside of copper pipes that were acting as high power coax cable (pipe inside of a pipe). The air needed to be dry in order to prevent corrosion and maintain signal quality for broadcasting.

  7. My first observation. Quick change large hose couplings, and they appear to be brass or bronze. They seem to be what the hacker space write up called unions, I never thought they where rated for pressures as thisThey don’t come cheap for a hackerspace having a hard time getting money together, I didn’t have that luxury on the small tank truck I drove for an independent oil producer. While not as handy, hammer bar or gas unions should work as well. Hard to tell, but surely they are running the air through a conventional water trap(s) before the desiccant. No doubt using one clear desiccant to dry the other will eventually hit a wall

  8. the desiccant uses adsorption(not a typo, and not to be confused with absorption). the air is piped up one tube through the bottom where the desiccant takes the moisture out before it is blown out the top. at the same time the second tube has it’s valving switched so a small amount of dry air is being blown in the top and vented to atmosphere out the bottom. this combined with gravity drys the desiccant in that tube, and when the first tube gets saturated the valving switches and the tubes reverse rolls. quite common system (in the natural gas industry anyway). nice work on a DIY one. keep up the good work.

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