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Old 01-05-2009, 01:22 PM
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DIY compressed air refrigerator

I had an idea on how to make a low budget compressed air refrigerator but I'm not sure that I totally understand how it works and if the parts I was considering using would handle the task.

I have an old refrigerator in my garage that I keep drinks and other stuff in. What I was considering doing was plumbing in an air inlet and outlet into the side of the refrigerator and on the inside soldering several oil coolers in series that sat in the bottom of the fridge. Thereby running my compressed air through the oil coolers before it reaches whatever I'm working on. But I have a few questions from those more knowledgeable than I. Would this help to reduce moisture in my lines? Could the oil coolers withstand the psi? Since an oil cooler isn't used in a pressurized system in its normal function, would say a turbo intercooler handle the task better. Or possibly an old a/c condenser from a house unit small enough to fit in the fridge.

I realize this is about as redneck as it gets but I need a cheap way to cool this air, and spending $500 when I could spend $50 is not in the economics right now. What do you guys think?

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Old 01-05-2009, 01:56 PM
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In theory it would work, those oil coolers should handle a buck and half psi no problem. However I am sure your fridge is WAY too small a cooler to cool the large amount of air (and especially water) going thru a compressed air system. A dedicated air cooler will use much more power to cool a given air stream than a home refrigerator can supply. Think about it - you are not only cooling air which admittedly isn't a big heat load. However you are also condensing 'steam' by converting water vapor to liquid. This phase change operation consumes more heat/# than for any other element or compound known.
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Old 01-05-2009, 02:17 PM
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Unless you need REALLY dry air then one or two of those coolers with a fan mounted behind them should do the trick, at least for painting or sandblasting. The trick would be to run the air through this cooler as soon as it leaves the compressor then through a properly plumbed pipe system with a good water separator at the discharge end.
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Old 01-05-2009, 02:29 PM
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I don't use huge volumes of air very often. The main reason I would like to do something else is due to the fact that I have two good quality driers and my bead blaster clogs after 3 minutes of use and I get condensation bubbling out of the fittings. I drain the tank after every use and have a copper line that I run around the room with drops and watertraps. I shut off the sections of tubing that I'm not using when I am working with air. What gives? Maybe its just the constant humidity where I live.
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Old 01-05-2009, 05:04 PM
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Use the frig. to make ice. Then put the oil coolers all piped together and all the ice cubes in a tub, that will give you about 4-5 hours of dry air. when finished start making ice again.

We painted a car one afternoon by using that method, 50 lb ice, tub, and last the water trap. I still have the copper coil saving it for the next time.
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Old 01-05-2009, 05:49 PM
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Right before I retired I spec'd out a refrigerated air dryer for our instrumentation shop. I bought a packaged unit from Graingers for about $500. I was really surprised at how well it worked and how much water it removed from previously thought dry air.

Vince
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Old 01-05-2009, 06:22 PM
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HF sells a decent "hobby" unit $371 shipped to my door

http://www.harborfreight.com/cpi/cta...emnumber=40211



3-in-1 air dryer system features modes for pre-refrigeration, evaporation, air and moisture separation modes to remove up to 90% of the moisture from your compressed air lines.

Accommodates compressors up to 21.6 CFM, 140 PSI max
Lowers the dew point to 36F

Recommended operating pressure: 100 PSI. Not for medical use. Operates between 35.6-140F. Uses R-134A refrigerant. 115 V input; Air inlet: 1/2"-14 NPT
Weight: 73 lbs.
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Old 01-06-2009, 06:50 AM
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[QUOTE=DaSouthWon]I had an idea on how to make a low budget compressed air refrigerator but I'm not sure that I totally understand how it works and if the parts I was considering using would handle the task./QUOTE]

I think The terms used were "low budget" and not spend $500.
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Old 01-06-2009, 09:22 AM
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Please excuse my ignorance, but how exactly is the water extracted from the line after cooling. I understand that cooler air carries less humidity, but if the air enters the cooler warm does cooling it does it change from liquid form to a vapor or is there another mechanical means of extraction. My assumption is that air hose length would be rather critical to the effectivness. I don't really care for any HF tool over $20 based sheerly on investment/benefit/risk. Especially with the amount of mving parts contained in their air cooler.
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Old 01-06-2009, 10:51 AM
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Basically the warmer the air the more moisture will remain in vapor form and in vapor form it will simply pass right through most separators only to condense in the hose and as the air exits the hose and expands inside whatever tool is being used at the time. The trick is to have as much water vapor as possible condense into liquid form before entering the separator so that it can be extracted instead of simply passing through as a vapor, this is why mounting the separator close to the tank renders it almost useless. Most mechanical separators work by centrifugal force from spinning the air by having it flow across vanes which causes the liquid water to be collected in the reservoir where it can be drained later. A properly plumbed piping system will also have a long vertical drop with the air to the separator taken off about 18" or so from the bottom, this 18" under the take off point will act to collect water that condenses on the sides of the pipe and drains down but it also removes water droplets that are still suspended in the moving air. This occurs because of the laws of physics that dictate that moving mass (in this case the water) will resist making a sharp turn thus as the air/water travels downward through the pipe the much lighter air will exit at the lower resistance at the take off point while the heavier water will tend, because of inertia, to continue on in a straight line into the collection reservoir which is the bottom 18" of the vertical pipe. What all this means is that water vapor is extremely hard to deal with while liquid water is fairly easily removed so it is very important to condense as much water vapor as possible BEFORE any device intended to remove it.
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Old 01-06-2009, 12:11 PM
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Wow, thanks oldred, that helps me understand a lot. Just promise me there is not going to be a test tomorrow. Ok, if I understand correctly, I would idealy put a seperator as close as possible to this hypothetical refrigerated air drier. Then, have another that the cooled air passes through, since cooler air carries more liquid form water than it would after its had a chance to get warm again in the air hose.

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Old 01-06-2009, 02:26 PM
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Actually if the separator is located at the exit point of the cooling device that should be sufficient however another separator located at the end of the plumbing before the air enters the hose is not a bad idea, usually not necessary but it certainly can't hurt. This of course is assuming you will even have a plumbing system after the cooler, and if the cooler does it's job the added plumbing may not even be necessary.
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Old 01-24-2009, 07:44 AM
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re

I have been using that HF dryer on the output of a industrial 5 hp compressor[IOW not a HF or sears 5 HP] for 5 years with dry air and no worries. For the delivered price it is hard to beat.

The hotter you run your compressor the more water you get, so when you beat the snot out of a small compressor, you get a lot of water.

SLope your lines back to the compressor

The oil coolers with a fan will work until the air gets hot enough to hold water again.
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Old 01-24-2009, 08:38 AM
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Quote:
Originally Posted by kgus
The hotter you run your compressor the more water you get, so when you beat the snot out of a small compressor, you get a lot of water.
I always thought the amount of water from a compressor was due to the out side relative humidity, high humidty more water, lo humidty less water, did not know compressors made water.
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Old 01-24-2009, 09:07 AM
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The total amount of water is of course directly related to humidity however for practical purposes the hotter the compressor runs the more water you will have to deal with in the air line. When the compressor is running cooler the water will tend to collect in the compressor tank but the hotter it gets the more water that will be carried into the lines as water vapor. In a sense both of you guys are right.
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