Compressed air dryer question

I bought a used compressed air dryer for $10 last weekend. Tried it and found that it was working, since the outgoing air was cold and the
release valve was functioning. My question is about capacity. This dryer uses about 264 watts of power, according to my wattmeter (kill-a-watt). Is that enough for a 3 honest HP compressor. Thanks.
i
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1 HP/hour == 745.7 watt/hour
But, um... how many watts does it take to dry air?? I dunno.
LLoyd
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True air drying reqs an air-conditioning type compressor, I believe. 246 watts seems kind of low, but is possible. Might not use a compressor. Cold outgoing air may not be a sure-fire test, what with the joule effect of expanding gases'n'shit....
Proly the unit should be rated ito cfm at some psi..
The real test is if any condensate is produced. ---------------------------- Mr. P.V.'d formerly Droll Troll

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On Wed, 14 Sep 2005 13:00:33 -0400, Proctologically Violated©®

What my dryer does is it cools the passing air to be ice cold. I suppose that it causes condensation of excess water vapor. There is a separator and timer driven drain.

I will check the nameplate again... I will spend more time on it, I need to plumb it properly.
i

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Air dryers refrigerate the air, knock out the moisture, then heat exchange the outgoing air with the incoming air to precool it and minimise the energy needed to run it. Outgoing air should be cooler than incoming air but not COLD as this would indicate that the heat exchanger was probably fouled with old oil from a previous compressor. It might be able to clean it with hot water and detergent if this is a problem..
Tom

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Lloyd E. Sponenburgh writes:

What's per-hours got to do with it?

This is a thermodynamic problem with various factors involved.
Practical small units require about 20 watts per cfm.
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Richard J Kinch wrote:

It depends on how humid the air is going into the compressor. Around here (St. Louis) it can get pretty wet at times, and the compressor tank drain can really dump a lot of water from a short run of the compressor. I have a small unit that doesn't have a CFM rating, but I think it is good for maybe up to 5 CFM or so. It has a 1/8 Hp refrigerator compressor on it, and a Freon codenser coil identical to what would be on a small dehumidifier. It does an excellent job with a 3-5 CFM continuous load on the compressor.
Jon
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Jon Elson writes:

No, not if it reaches the dewpoint, which is almost always the case. The compressed air is at 100 pct RH. That's why you have liquid accumulating in the tank.
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Ignoramus25888 writes:

Could be. The smallest sizes are designed for 6 or 10 cfm, and 3 HP would be about 10 or 12 cfm. My little Hankison unit runs at about 200 watts and is rated for 10 cfm. I've measured the relative humidity of the output air at 10 percent, with hot 100 percent moist air input.
Cold outgoing air is not a good performance indicator. Many designs efficiently reheat the air with the hot side of the heat pump.
To properly test performance, you must measure the relative humidity of the output air. Get a wet-bulb/dry-bulb psychrometer and measure the relative humidity of the output air by directing a stream of exhausted compressed air over the bulbs of the thermometers. Or improvise your own wet-bulb and dry-bulb thermometers and use my psychrometric chart:
http://www.truetex.com/psychrometric_chart.htm
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Thanks. I have a thermometer with a built in psychrometer. I will check it out. Should be easy. A good idea.
i
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