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April 2001

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From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 21 Apr 2001 22:46:02 EDT
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So Brian,

You're saying that it isn't possible to put a air compressed air system in 
place, that will provide one with air that doesn't contain the kind of 
moisture, that requires you to drain from traps every day...so my plant 
engineer is right, we'll always have water in our lines no matter what kind 
of air drier we have...that is, unless we want to spend the money that you 
say we have to spend to acheive that...

Seems pretty discouraging to me...and to all the vendors that make air driers 
that claim they can provide their customers with clean, dry, shop air...they 
are lying to us.

Brian, I'm not talking about clean-room air, just air that is free enough 
from moisture that you don't have to drain traps everyday...I don't think 
that's too much to ask...

-Steve Gregory-

<< Steve
 
 What you are saying is half true. A lot depends on the humidity of the
 air being compressed. It is clear that if you have, say, 20 g/m3 of
 water vapour in air (c. 80% RH) at 25°C, adiabatic compression to, say,
 10 bars will give you 200 g/m3 as a first approximation. As the
 temperature will rise during compression, the RH will not change a great
 deal. As the air cools again down to ambient, condensation will occur
 and roughly 150 g of water will condense out per m3 of compressed air. A
 lot of this will occur in the reservoir, which needs regular purging,
 but the reservoir is usually at an elevated temperature, so a lot will
 occur in the lines and will find its way into your filters.
 Notwithstanding, ALL the air you are using will be quasi-saturated.
 
 So, what to do? Let it be said, the only way to have cleanish air is to
 eliminate the impurities, such as moisture, oil, dirt, etc. This usually
 requires a total re-installation, especially if you have iron pipes for
 distributing the air. These will have rusted to hell. The compressor
 should be an oil-less type (e.g. a Roots blower), to prevent an oil mist
 from entering everywhere, the reservoir should have an automatic water
 purge, the outlet of the reservoir should be fitted with a 5 micrometre
 filter, followed by alternating cryogenic condensers, operating at -25°C
 (one defrosting off circuit while the other is working on circuit),
 followed by a large, properly maintained, silica gel filter. That way,
 you will get reasonably clean, dry air. If you want better, you will
 have to resort to a liquid air reservoir. For the air lines themselves,
 I recommend certified high-pressure non-halogenated plastic pipe.
 
 Now all this costs money, especially in maintenance. Most compressed air
 systems receive minimal maintenance, if any at all. Big mistake! Your
 problem, Steve, will be persuading the powers-that-be that something
 that cost a few thousand 25 years ago and nothing since, except a a few
 kWH of electricity/day and a few litres of oil per year is suddenly
 going to cost a small fortune in incremental capital costs and a
 quasi-infinite increase in incremental operating costs... Ball in your
 court :-(
 
 Brian
 
 
 
 "Stephen R. Gregory" wrote:
 >
 > Hi All,
 >
 > I've become involved with our facility compressed air. We seem (in my
 > opinion) to have an excessive amount of moisture in our air lines. There is
 > an air dryer installed downstream from the compressor, but the water traps
 > that are installed at various locations on the production floor need to be
 > drained regularly (sometimes daily).
 >
 > My take on this is that the air drier that we have either isn't functioning
 > properly, or that it doesn't have the capacity that we need for the demand 
of
 > air that we require.
 >
 > I'm being told by our "Plant Engineer" that there isn't anything out there
 > that will eliminate the moisture from our compressed air lines, he says 
we're
 > always going to have water in our compressed air lines...I beg to differ 
with
 > his opinion.
 >
 > Would any of you please post something that says that it is possible to 
have
 > clean, dry, compressed air on your production floor? This is air that is
 > supplied for our automation, and everything else.....
 >
 > I know that this is possible, I've worked at too many other companies where
 > one of the last things I had to worry about was the quality of our 
compressed
 > air, but I have a new challenge now...I see water in the traps throughout 
the
 > production floor every single day that need to be drained daily (and other
 > things that I don't want to get into), but yet I'm told our air drier is
 > functioning properly, and that it just doesn't get any better....that it's
 > just physics.
 >
 > I'm about to go crazy...please help me.
 >
 > -Steve Gregory- >>

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