Hi Grant,
I really didn't want to get involved in this whole discussion, so I
should just bite my tongue and leave it here, but... just a couple of
little things:
In those bazillions of specs the CFM is generally the displacement of
the pump, which we all know is a far cry from the amount of air that it
actually delivers. Free air delivery (FAD) is a spec that comes closer
to what you actually get out of it, but it very often isn't quoted
because it doesn't look nearly as good on the brochure.
I don't know what cleandryair are selling either, but I have to disagree
with you - I think that the page is useful in alerting a potential buyer
to the need to consider more than just the CFM@psi on the brochure when
they are deciding on a unit.
Roger
Grant Erwin wrote:
The cleandryair page claims that the ONLY meaning of CFM is as the input
volume of an air compressor. Thus to them 16 cfm @ 90 psi would have no
meaning. As this is in direct conflict with the way many if not all
industrial
air compressors are specified (if you don't believe me just google on:
compressor cfm psi
and see all the bazillions of specs in this format, it only takes a second)
I find their attempt to limit this terminology to be unhelpful. That page
does little to shed light and in my opinion is designed to steer people to
their way of thinking and thus their product line whatever that is.
There *are* valid thermodynamic concerns here, but they aren't relevant to
my original question. It is true that it is inefficient to compress air to
a high pressure (with lots of heat) and then remove the heat and moisture
to get clean dry air. Obviously you are spending electrical $$$ to dump
heat
into some heat exchanger. This is a fact. What I would find a more helpful
fact is the information how to get clean dry air without doing the above.
I have always understood that the way to go is to get a 2-stage compressor
which can run a high volume of air at a high pressure (like 180 psi, gaged,
or 180 psig) and then pipe that air to a refrigerated air dryer and then
filter the moisture and oil drops out of the air. I think Richard Kinch's
point is that you will get a lot more energy out of that same compressor
if you use it hot and wet. Very true, but you can't spray-paint a car with
hot wet air, nor can you run it into a plasma cutter, nor is all that
moisture good for your air tools.
Way back when I first posted the question, I was wondering about using a
"regular" air compressor to generate air for HVLP painting. As I knew for
sure you couldn't get the kind of volume out of my compressor that a HVLP
gun needs, I wondered if you could "transform" it to a much lower pressure
but much higher volume. I now believe enough light has been shed on that
issue
to say yes you can get higher volume by regulating to a lower pressure as
long as the temperature doesn't drop.
I try not to get embroiled in argumentative discourse on this NG but I seem
to have "fallen from grace" this time. I can only say that I believe all
parties have been respectful, and anyone who has taken the time to read all
of this material and to consider each poster's comments deliberately,
will have
learned a considerable amount.
Grant Erwin
Kirkland, Washington
Roger Head wrote:
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.