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ShavingMaker
02-05-2003, 09:15 AM
Some recent threads asked about how to get rid of the moisture in compressed air lines. (See "Air drier in the home shop system", General, Nov02)
Think about getting rid of it BEFORE it enters the compressor instead. Dealing with moisture under low pressure is a lot easier than when it is under high pressure, although you will still need to use the normal high pressure devices.
The secret is to cool the incoming air to a point below the DEWPOINT.
For example, last night in New Ulm, MN it was 3F with relative humidity of 82%, dewpoint of Zero degrees, due to go down to -4F overnight with snow flurries. When the temp went down from 3F to Zero, the humidity started to condense and freeze into snow flurries. Now that's colder than many areas, but it demonstrates the point. On a hot and humid day in the summer at 89F with a dewpoint of 75F, you would only need to cool the incoming air to 75F to get the water to drop out of the air!

The solution to many of the water problems in air lines can be lessened greatly by sending the incoming air through a cooler before it gets to the compressor.

Running the air through a radiator that circulates ground water would work. Running the air through an underground 6" plastic pipe with holes in the bottom for drainage would work also. The dessicant-in-conduit idea that Forrest Addy came up with in an earlier thread would work fine, and without the danger of high pressure. In fact, you could check a company that makes a commercial dessicant dryer at www.hbcotes.com (http://www.hbcotes.com) (no endorsement, just an idea). Then there is my idea of running the incoming air through the block walls of my shop, but then I may have a moisture problem in the walls to deal with!

OK, now I'm ready for the comments.

Forrest Addy
02-05-2003, 01:32 PM
You're talking my language. Dessicant on the compressor intake makes sense. The lower air pressure and the consequent water vapor partial pressure means it takes a much larger dessicant cannister to extract moisture from the same weight of air flow.

I suggest you make your own dessicant cannisters from 4" OD copper, EMT electrical conduit, or exhaust tubing. It's gotta be metal, not plastic because you have to heat them to bake out the water. Make three or four just long enough to bake in the kitchen oven so you can keep them in rotation. The pipe and fittings are affordable but the dessicant can run to $60 maybe more.

Charge them with the dessicant (used for drying flowers) you get from the craft store. Put screens in each end to retain the dessicant. Add a low restriction fitting that hooks up to your compressor. Install a line filter to keep dessicant dust from the compressor intake. Add a bull's eye window in the cannister near the compressor intake so you can monitor the indicator beads that change color when they saturate.

Bake them overnight in a 300 degree oven to dry out the moisture. If you make four you can have a couple ready, one in service and one in the oven. This makes for an efficient rotation.

Hey, it also works as in intake muffler.

[This message has been edited by Forrest Addy (edited 02-05-2003).]

Uncle Dunc
02-05-2003, 03:00 PM
The only problem I see with the ground water cooled condenser, aside from the normal plumbing hassles, is that as the relative humidity drops, the dew point drops. When the moist room air first enters the condenser, it gives up lots of water to the condenser surfaces, but as soon as the relative humidity drops enough that the dew point is lower than the temperature of the condenser surfaces, no more condensation.

A cross flow design would help some, so the coldest water is meeting the driest air, but my suspicion is that for most of the year in most humid climates, ground water isn't going to be cold enough to really dry the air.

As a pre-dryer, to reduce the load on the post-compressor dryer or FA's dessicant approach, it makes a lot of sense, but I don't think it could do the job alone.

Thrud
02-06-2003, 12:55 AM
Forrest

Good idea about the 4" copper pipe and dessicant. I would silver solder the copper though, the oven heat could melt plumbing solder. Dessicant can be difficult to find, but it is not expensive - last time I bought some it came in 340lb. kegs - don't remember the cost...

darryl
02-06-2003, 03:53 AM
Driveway deicer is dessicant, and where I live, it's cheap.

ShavingMaker
02-06-2003, 09:19 AM
The web site I mentioned above for "hbcotes" has a diagram of their dryer. Really simple. Dessicant is in a wheel form that rotates. Damp air is pulled through one part of the pie, and heated air (120-140C or 250F) is blown back through a smaller section of the pie to dry out the dessicant. It is a continuous process and the rotating action makes sure all of it gets dried evenly.
Using a heat gun to dry out the dessicant in a home built version would probably work as well or better than heating up a whole oven. Easier on the wife too!

Oh yeah, uncooked rice works well as a dessicant too. Great for putting in a paper bag and throwing in with cameras, etc. to keep them dry. Put it in a sock, heat it in the microwave for about 2 minutes and you have a good heatpak for the aching muscles too. You can even eat the stuff!

[This message has been edited by ShavingMaker (edited 02-06-2003).]

chief
02-07-2003, 05:22 PM
The dessicant/airdryer should be placed on the discharge side of the compressor. Moisture will still be present after compressing the air.This is stanard industry practice.The heated air helps to drive the moisture off of the dessicant while catching moisture. IF placed on the inlet the dessicant will quickly become wet and useless
unless it has a reheater system, the broken down dessicant and also be carried into the compressor and destroying the unit.
I would also stay away from copper tubing and aluminum cylinders, good grade WOG piping and a cleaned, hydro tested, discarded,oxygen bottle from a welding outfit will make a safe dessicant tower.
Also drain your reciever periodically