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April 2001

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 21 Apr 2001 10:13:17 +0300
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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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