Compressed air and air drying

[Kerrin Galvin]

Hi guys,
I went down this road a couple of years back. My compressor is fully enclosed. When I brought it, from a beauty salon of all places, the first run & open the drain took quite awhile to drain all the water!

After watching many YouTube videos I brought a 16 tube oil cooler, it’s rated to 300 psi. I take the air straight of the compressor & via flexible tubing to the cooler which is mounted to the outside of, but stood off, the enclosure by about 2”, then spent more on the AN8 fittings than the cooler. This is followed up at the cooler outlet by a water trap, from there back to the tank via the inlet non return.
When the compressor starts the drain on the water trap is open until the air pressure rises a few psi, blowing out any remaining water. Once the tank pressure is a set pressure & the compressor shuts down, it depressure via the depressuring valve, the last few PSI see the water trap drain open & the water dump.
It works very well. We’re I live we have high humidity pretty often & since installing the water in the tank has just about dropped to zero. I have since added a fan to see if that helps & it does… surprise surprise!
I ran it one day doing some bead blasting for an extend time, the compressor outlet at the cooler was around 90 C outlet from the cooler around 30C, given that the day temp was around 25 -26C & my shop faces the sun I figured that was pretty good!
Cheers Kerrin

 

[GreenTwin]

"Hobby" is a relative term.
My workshop equipment is dual-use.
I use it for small hobby work, and also full sized engine restoration.
The full sized work can require a significant amount of air.
My oil burner uses compressed air too, as a siphon nozzle, and so another air use.
.

 

[Steamchick]

Thanks Green Twin.
As I don't use air tools much - Maybe just a couple of times a year0 that use is tiny by comparison. But I do run small model engines on compressed air - and test safety valves etc. at near tank pressures - but not a lot of air used. Sometimes I inflate a tyre... or use a spray paint gun, but my machines don't use compressed air.
So I didn't figure large amounts of compressed air could be used in a model making shop.
In my teens, I worked part-time in a machine shop - re-bores, crank re-grinds, lathe work, head re-working, etc. on car & truck engines, and compressors of all sizes. We used the compressor for air maybe a couple of times a month? - and blew-down at the end of shift/use. Often just for a small tool dresser/grinder rather than a huge air job.
However, I do think your oil burner will use quite a lot by comparison! - So I guess your air receiver has a few pints of condensate by the end of a shift?
Woking some sums... just for curiosity :

If the compressor is working to 6 bar, and compressor air is at 90C, the air is carrying at maximum 0.08kg/cu.m of water. This is more than 100% humid air at 40C . But when it cools in the receiver, maybe to 50C? - it cannot hold so much water, - hence the condensate - as it can only hold around 0.015kg/cu.m of water.
When expanded back to atmospheric pressure - in the nozzle of the oil burner - it will then have a dewpoint of about 15C . I think this means humidity of just less than 20%? = quite dry.
On th same basis, a dentist uses an air blast to dry teeth when sticking caps to freshly ground dentine. - Dries the tooth almost instantly!
But then I don't think you care much about the wetness of the air going into your furnace, just the amount of water being drained from your air receiver at the end of a session.
Similarly, I reckon Kerrin will have dry air with a dewpoint around zero C. 0.004Kg/cu.m of water, which may be something like 5% humidity?
I guess if anyone wanted drier air than that, you could get a domestic freezer, set below freezing point, and blow the air into a receiver inside the freezer, which would progressively fill with snow and ice, but the air from that would be super-dry?
K2

 

[timo_gross]

Just an automatic drain on your tank with no dryer will mean you have air at 100% humidity at that room temperature/pressure. That assumes you have used things to cool your air and remove moisture. You will likely get condensate at your tool etc because the air will cool as it expands. For some applications, that does not matter. For others, it is the kiss of death.

I store dry air vs drying post storage because that increases the tank life and limits the amount of rusting on black iron. The negative is that I dry air that might be used for a process that does not need dry air. However, the large regenerative desiccant dryer I have far exceeds the capacity of a normal 10 HP or less air compressor and costs almost nothing to run. The negative is that it takes up the floor space of a vertical air compressor by the time you include the regenerative parts, piping, etc. Most companies would not look at a used system and most hobbyists can not afford the space. I mount mine on a stand above the compressor. If you can, it pays to look for deals on Ebay, Craigslist, etc because they can go pretty cheap. Mine was significantly less than a cheap refrigerated dryer and like already mentioned, costs almost nothing to run.

Yes, the automatic draining just prevents water from sitting in the tank for long term and it is a convenience. I doubt it does change the water content in the air. Compared to periodically manual draining.
I never saw a Desicant Air dryer in Taiwan, they usually add a refrigerant cooler to the systems.
A big electricity hungry cooler is doing the water removal before it goes into the distribution. In warm and humid areas (Tropical climate) I guess it saves some energy to put the buffer tank in front of the cooler.
Do you regenerate the Desicant somehow and how often is that done?

 

[SpringHollow]

The desiccant dryer has a heater tower and blower which blows hot air through the desiccant to restore the desiccant. It is so large I only hit the regeneration button once every year or two. Without the Franzinator etc, I would likely have to regenerate on a frequent basis.

 

[Lloyd-ss]

Darn, we've got some clever and resourceful people around here. But @The Ignoble Troll , I can't quite picture where the piston and vanes fit into the system and whether it removes water while filling the tank or using air from the tank. It seems like both of the DIY designs shown in this thread will work best when the air flowing thru the device is at a fairly high velocity.(?) @SpringHollow I can see how the F-er works by rapidly expanding and thus cooling the air coming out of the skinny inner pipe.... so long as the velocity is high enough.

I have another variant on the desiccant method. It is a molecular sieve (a bit of a misnomer), in size 4A. It looks almost like desiccant beads but each bead has thousands of holes in it just the right size to trap a molecule of water (the size 4A). Unfortunately, the air must linger several seconds inside a long tube filled with the molecular sieve beads for the water to become trapped. It's not very useful for shop air but I use it for drying the high pressure air for my other hobby. The little compressor I have compresses atmospheric air to 4,000 psi (280 bar) but the actual volume output at 4,000 psi is very low and the sieve is on the output side so the air has time to linger in the long tube for almost a minute before passing along into the storage tank. There is a lot of summer humidity and the water will literally spray out of the small diameter HP lines that are used unless the air is adequately dried. The molecular sieve can be regenerated by baking, similar to many desiccants.

 

[The Ignoble Troll]

Darn, we've got some clever and resourceful people around here. But @The Ignoble Troll , I can't quite picture where the piston and vanes fit into the system and whether it removes water while filling the tank or using air from the tank.

The tank is fed from the drain port on the compressor, not the air compressors quick connect, so the water from the compressor is continuously expelled. The air/water get injected down a steel pipe to the piston.

The piston lifts and drops as needed, so that it always forms a minimal circular orrifice against the bottom of the tank.

This cools the air and blows the air through the water, where the sides of the tank cool it further, to below room temp by a few degrees.

It only works well with water already in it, based on how fast the dessicant changes colour.

There is a manual valve that lets water be pushed out the same tube that in came in through, for drainage.

Air exciting the tank for the air system is run past the cold tube so that it acts as a cold trap, which further reduces relative humidity.