Hi Grant,

Now my head is spinning, and I don't think it's the brandy sauce on the
Xmas pudding!

Initially I just nit-picked on a statement that CFM is a measure of mass
flow, so I posted a site ref to demonstrate the difference in the
implications of SCFM and just CFM.

Richard came back with his knickers in a twist, then again a few minutes
later acknowledging the validity of the ref site. I assumed that he had
only just got around to reading it, and now understood things.

In another post that you made a few minutes before this one that I am
replying to, you also appeared to agree with me that Richard had the
wrong end of the stick. But now you are upset about the ref site, and I
don't understand why - I'm not arguing with you, just puzzled.

What they're saying is that the measure of a compressor system is the
number of SCFM (mass of air) that it can deliver at some specified
outlet pressure (with the assumption that the outlet temperature is at
the agreed standard value, etc etc), because that is a measure of the
work that can be done by that air. Now unfortunately most compressors
aren't operating in a STP enviroment, so what they are saying is that if
you want a certain amount of work to be performed by your compressed air
then you need to look at the inlet conditions (ambient temp, pressure,
humidity, effects of inlet filters and manifolds etc etc) to determine
what capacity compressor you will need to look for. Remember that the
capacity written on the data sheet (probably as CFM @ xxx psi)
implicitly assumes that the ambient conditions are at STP.

So if we want a compressed air system to do a certain amount of work,
and we will be using it in STP ambient conditions, then (ignoring sales
hype, and a bunch of other inefficiencies, etc) we should be able to
select a suitable unit by looking at the straight data-sheet specs. But
if we want to do that same amount of work on top of a mountain, then we
will have to select a unit that has a greater specified capacity. And so
on, if other conditions vary from STP.

Please tell me what it is that you don't like about cleandryair's page.

Regards,

Roger


Grant Erwin wrote:

Yes, this page says what you say it says. So who the heck is
cleandryair.com and why should I believe them over what is obvious, what
is common practice in the air compressor industry, and what is taught in
every engineering school?

I suspect this page is trying to redefine some terminology to give
themselves
some kind of business advantage. I have no problem with this, but don't
expect
me to believe it.

Grant Erwin

Richard J Kinch wrote:

Roger Head writes:


http://www.cleandryair.com/scfm_vs__icfm_vs__acfm.htm



This page correctly explains that CFM, SCFM, ACFM, etc., ALL refer to
"the volume of air that is compressed each minute and it is measured
on the _inlet_ side of the compressor." The "S" or "A" prefixes
simply further specify the temp and humidity of this inlet air.