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.

Ned Simmons writes:

CFM X psi for a compressible gas is not analogous to volts
X amps.

Actually, it is analogous:

CFMC amps
pressureC volts
power = CFM * pressure : volts * amps.

Richard J Kinch wrote:

Grant Erwin writes:

CFM does not equal mass flow

No, this is wrong again. CFM *does* equal mass flow.

I think you don't understand what "CFM" and "SCFM" mean.

One CFM means a cubic foot of air at 1 atm pressure ("free" air), flowing
per minute. Or the equivalent mass of air at any other pressure. It does
NOT mean one cubic foot of air at any other pressure.

I'm obviously not the only one who's "wrong", Richard. Actually, I'm relieved
because now that I understand what you mean by CFM it makes everything you
say make sense. I took it to just be a three letter acronym for "cubic feet
per minute" which could be at any pressure. I did this because my whole adult
life I've seen compressors specified this way. If you try a quick google for
compressor cfm psi
you will instantly see what I mean - everyone specifies their compressors
at a cfm @ a psi.

So it's all a matter of a simple misunderstanding. In the circles you travel
in, CFM has some specialized meaning. To the rest of the world, it simply
means cubic feet per minute.

So what do you call simple cubic feet per minute as a volumetric gas flow?

Grant Erwin

Better watch it, Richard. Gary Hallenbeck was the Western Hemisphere's
application engineer for I-R for a lot of years. As far as I am concerned
he speaks with the voice of infinite knowledge about the air compressor
industry.

Looks like I-R (like Quincy and the other major manufacturers) don't know
about your special meaning for CFM either.

Grant

Richard J Kinch wrote:

Gary Hallenbeck writes:


A cubic foot VOLUME is a cubic foot VOLUME regardless of
pressure, temperature or phase of the moon. A STANDARD cubic foot, on
the other hand is a specific mass of the gas in question. Which is
why a 600 cfm compressor can have a 1/3 hp motor and a 10 cfm
compressor can require a 500 hp motor. The 600 cfm compressor being a
cooling fan producing flow @ an inch or two of water and the 10 cfm
compressor producing flow @ 1000 psig. Obviously the 10 cfm @1000 psig
results in a much larger SCFM than the 600cfm @ inches of H20.


Again, you are misunderstanding.

"SCFM" and "CFM" are the SAME THING, except that the "S" prefix indicates
the input air for the system is specified to be 68 deg F and 36 percent
relative humidity, while "CFM" without the "S" prefix just *does not*
specify what the free air temperature and humidity are. Thus performance
of a system in "CFM" could be "better", "about the same", or "worse" than
in SCFM, depending on the temp and humidity of the ambient environment.

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.

In article , Richard J Kinch
says...

CFM in compressors refers to the flow rate OF THE
INPUT AIR AT ATMOSPHERIC PRESSURE.

In your world it does. In general though,
cfm means what the dimensions imply: a
cubic foot of any material that passes by
a point in a minute. It could be water,
air, compressed gas, and it can be at any
pressure the user specifies.

The folks who inhabit this ng have a broad
knowledge base and will use *specific* terms
when suited. Otherwise a generic term like
'cfm' will be seen as an open-ended term.

Your insistance that cfm means delivery
of a specific gas at a specific pressure
makes about as much sense as saying that
a 'cupfull' ALWAYS means one cupfull of
flour in a baking recipe. Nobody is going
to agree with either of those approaches.

Besides, the original question that was posed
was about GAS REGULATORS. Not COMPRESSORS.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

In article , Richard J Kinch
says...

One CFM means a cubic foot of air at 1 atm pressure ("free" air), flowing
per minute.

This may be true in some disciplines, but in general
this is NOT a true statement. Because the inhabitants
of this ng have such a diverse background they will
not want to agree with that *specific* definition.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

I guess you just read that site, eh Richard?

Roger

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.

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.

In article ,
says...
Ned Simmons writes:

CFM X psi for a compressible gas is not analogous to volts
X amps.

Actually, it is analogous:

CFMC amps
pressureC volts
power = CFM * pressure : volts * amps.


Ya think? If that's the case there should be a general
conversion factor such that cfm X psi X constant = watts.
What would that factor be?

Ned Simmons

In article , Ned Simmons
says...

conversion factor such that cfm X psi X constant = watts.
What would that factor be?

Hmm. This might actually work out, the energy
stored in a pressure vessel is (IIRC) 1/2 P(sq) x V
so if you imagine an amount of air at a given
pressure passing a point in a pipe, then
that would be a time rate change of energy.

I think I would have to get ahold of my copy
of Haliday and Resnick at work, with the units
conversion tables at the back, to give the
constant though.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

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.

You're obviously right, Ned. I believe it's something like 4 cfm @ 90 psi
equals 1000 watts based on what you can get out of a 1.5 hp compressor.
I have also sadly lost my copy of Halliday and Resnick, so I am not inclined
to do all the physics calculations to get a better answer. Obviously, the
temperature and barometric pressure will play a part too.

Grant Erwin

Ned Simmons wrote:

Ned Simmons writes:

CFM X psi for a compressible gas is not analogous to volts
X amps.

Actually, it is analogous:

CFMC amps
pressureC volts
power = CFM * pressure : volts * amps.

Ya think? If that's the case there should be a general
conversion factor such that cfm X psi X constant = watts.
What would that factor be?

On Tue, 23 Dec 2003 15:01:54 -0600, "Tim Williams"
wrote:

"jim rozen" wrote in message
...
The regulator is a feedback controlled valve. ... it does not get
perceptibly hot.

Indeed because expansion is occuring inside the regulator,
under certain conditions, they may start to chill preceptably.

So where does the heat go? I'm going to guess that compressive systems
such as this act as heat pumps, thus all the heat goes back to the
compressor, while cooling occurs at the tools and regulator. Eh?

Tim

It can be a tricky one to grasp at first. This is an adiabatic expansion. No
external work is being done and no heat is being added to, or taken from, the
gas. The reason it cools down is because it is doing work on _itself_ during
the expansion.

Mark Rand
RTFM

In article , Grant Erwin says...

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

Ha ha, not only that, but there are about a thousand
other uses for the term 'cubic feet per minute' that
have nothing whatsoever to do, with air compressors!

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

In article , Grant Erwin says...

You're obviously right, Ned. I believe it's something like 4 cfm @ 90 psi
equals 1000 watts based on what you can get out of a 1.5 hp compressor.

That's going to be close, as 1 hp is 750 watts.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

In article , Mark Rand says...

It can be a tricky one to grasp at first. This is an adiabatic expansion. No
external work is being done and no heat is being added to, or taken from, the
gas. The reason it cools down is because it is doing work on _itself_ during
the expansion.

But in a real world regulator, it will not be quite a true
adiabatic expansion. Some heat will come in from the outside,
so it will want to be isothermal too.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

Ned Simmons wrote:
In article ,
says...
Yes, I started that thread too. The reason that this has come back
is that I was never convinced the last time. Let me ask YOU, Ned, to
answer my question? I'll repeat it:

Postulate a big air tank pressurized to 180 psi, with a long (long
enough so the air has time to cool to ambient) pipe to an ideal
regulator which regulates the pressure down to 90 psi. The
regulator's output is a pipe of the same size which is connected to
a constant load of such a size as to make the cfm going into the
regulator measured to be 10 cfm @ 180 psi. What cfm will come out of
the regulator at 90 psi?


If we assume that the air behaves as an ideal gas and the
pressures are absolute, then there'll be 20 CFM @ 90 psia
flowing out of the regulator. I don't think there's ever
been any question about that. But that doesn't mean that
there isn't a loss in the potential of that air to do work.

CFM X psi for a compressible gas is not analogous to volts
X amps.

Ned Simmons

Whenever you let a confined gas expand there has to be a loss in its
potential to do work. Consider a pneumatic cylinder, it has more potential
energy in a more compressed or closed state. Is there any difference between
doubling the enclosed volume of a cylinder and expanding compressed air
through a regulator? That cylinder could do a certain amount of work
expanding, f x d. You would end up with double the amount of air at half the
psi, yet work was accomplished expanding the cylinder so there must be a
loss of potential energy in the compressed air. That's my "hand waving"
argument.

On Thu, 25 Dec 2003 11:07:49 -0800, Grant Erwin
wrote:

You're obviously right, Ned. I believe it's something like 4 cfm @ 90 psi
equals 1000 watts based on what you can get out of a 1.5 hp compressor.
I have also sadly lost my copy of Halliday and Resnick, so I am not inclined
to do all the physics calculations to get a better answer. Obviously, the
temperature and barometric pressure will play a part too.

Grant Erwin

Nearer 250 Watts of useful power. A _lot_ of the power going into the
compressor ends up heating the air. Think of what can be done with a 500 Watt
electric angle grinder and then think of what a fearsome beast a 1000 watt die
grinder would be!

Mark Rand
RTFM

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.

The best bet with an HVLP gun is to use a turbine or regenerative blower
like the ones Spencer makes (vortex blowers) The moisture is generally not a
problem under reasonable humidity conditions. I have an Accuspray gun and
run it both ways. Using a compressor it will keep a 5 HP 3 phase compressor
running more than 50% of the time and you end up with a lot of moisture to
handle.





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 think Ned Simmons is on the right track in this thread. Yes, you can get a
higher volume of air at lower pressure but it is not efficient to do so with
a regulator. Compressing air just to decompress it without recovering any
energy is extremely wasteful. Applications using compressed air that need to
move large volumes of air use the venturi principle, not regulators. Do you
see how the transformer analogy does not apply?

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.

In article ,
says...
In article , Ned Simmons
says...

conversion factor such that cfm X psi X constant = watts.
What would that factor be?

Hmm. This might actually work out, the energy
stored in a pressure vessel is (IIRC) 1/2 P(sq) x V
so if you imagine an amount of air at a given
pressure passing a point in a pipe, then
that would be a time rate change of energy.


If your formula is right (it sounds plausible to me), take
a tank where V=1 and P=1.

(P^2 X V)/2 = 1/2

Expand the gas so P=1/2 and V=2 and plug back into the
formula and you get 1/4. In other words, no constant
factor, no analogy, and expansion thru a regulator is
irreversible and wasteful of energy.

Ned Simmons

In article UtKGb.406579$655.95206612
@news4.srv.hcvlny.cv.net,
says...



CFM X psi for a compressible gas is not analogous to volts
X amps.

Ned Simmons

Whenever you let a confined gas expand there has to be a loss in its
potential to do work. Consider a pneumatic cylinder, it has more potential
energy in a more compressed or closed state. Is there any difference between
doubling the enclosed volume of a cylinder and expanding compressed air
through a regulator? That cylinder could do a certain amount of work
expanding, f x d. You would end up with double the amount of air at half the
psi, yet work was accomplished expanding the cylinder so there must be a
loss of potential energy in the compressed air. That's my "hand waving"
argument.


Works for me. That's pretty much the same argument I made
in the old post I pointed to with the Google link. Here it
is again.

http://groups.google.com/groups?
q=g:thl672785172d&dq=&hl=en&lr=&ie=UTF-8&oe=UTF-8
&selm=MPG.189e9353cdce4f36989893%40news.rcn.com

Ned Simmons

On Tue, 23 Dec 2003 16:34:25 -0600, Richard J Kinch wrote:
Gary Coffman writes:

Mathematically,
this has the same appearance as a dissipative resistance, but there
is *no dissipation*. Energy not used is simply retained in the tank.

Impossible.

We agree that mass flow (CFM) is conserved (equal) on either side of the
regulator.

We know: Power = CFM * pressure.

The regulator imposes: pressure (out) pressure (in).

Thus, since CFM is equal on both sides of the regulator, but pressure
decreases, there is a loss of power in the output compared to the input.

Power is not a conservative quantity. Energy is, and the energy not
released is retained in the tank. In other words, power = energy/time.
The regulator effectively changes (lengthens) the time over which a
given amount of energy is released from the tank. That does show
up as reduced output power. But power isn't what's conserved,
energy is, and it is retained in the tank until it is released.

This ends up as heat as a direct consequence of the restriction that
creates turbulence and lowers the pressure. This waste heat is mostly
added to the output flow, even though the output may be at a cooler
temperature due to expansion. Some of the power lost is hissing noise
that radiates away and also eventually becomes heat. None of this is
"visible" to the source, so it is not "simply retained in the tank".

Those parasitic losses are relatively negligible in a well designed system.

Gary

Grant Erwin writes:

If you try a quick google for
compressor cfm psi
you will instantly see what I mean - everyone specifies their
compressors at a cfm @ a psi.

Right. That's exactly what I have been saying. CFM must always have an
associated pressure to indicate performance. The mass of air is
nevertheless always referenced to the density and pressure of free air
(1 atm), not the stated psi.

For example, my compressor will deliver 6 CFM at 90 psi. That means it
will inhale 6 cubic feet of free air, and push it out at 90 psi.

So it's all a matter of a simple misunderstanding. In the circles you
travel in, CFM has some specialized meaning. To the rest of the world,
it simply means cubic feet per minute.

No, there is no misunderstanding. CFM with respect to air compressors
only means what I have been describing. It has nothing to do with the
"circles" I travel in.

I would recommend _Machinery's Handbook_ on this subject if you are
still unconvinced.

So what do you call simple cubic feet per minute as a volumetric gas
flow?

Cubic feet per minute.

Grant Erwin writes:

Looks like I-R (like Quincy and the other major manufacturers) don't know
about your special meaning for CFM either.

The statement I was quoting, "Obviously the 10 cfm @1000 psig results in a
much larger SCFM than the 600cfm @ inches of H20." is in error, whoever
said it. The first item respresents 10 cfm of free air, the second 600 cfm
of free air, and the latter is much larger. The meaning of "SCFM" vs
"CFM" has nothing to do with the pressure differences being asserted.

On Wed, 24 Dec 2003 19:50:30 -0600, Richard J Kinch wrote:
jim rozen writes:
Cubic feet per minute is only a time
rate of volume.

No, no, no, no, no. CFM in compressors refers to the flow rate OF THE
INPUT AIR AT ATMOSPHERIC PRESSURE.

That's right. When you see a compressor rated at say 10 CFM @ 90 PSI,
that means 10 CFM of *inlet* air compressed to 90 PSI. It does NOT mean
10 cubic feet of 90 PSI air. This should make sense when you notice that
a compressor may have several fairly similar CFM ratings given at several
different pressures. That's because the only differences are due to
differences in the pumping *efficiency* of the compressor at different
delivery pressures. It is still inhaling roughly the same amount of air per
stroke at the same strokes per minute, so it still has roughly the same
CFM rating whether that's given at a delivery pressure of 45 PSI, 90 PSI,
or 140 PSI. It just takes longer to initially pump up the tank to the higher
delivery pressures.

Gary

jim rozen writes:

In your world it does. In general though,
cfm means what the dimensions imply: a
cubic foot of any material that passes by
a point in a minute

CFM *does* refer to a plain old cubic foot per minute, but at the INLET,
not the OUTPUT. There's no other "world" involved, or lack of generality.

Richard,

I DO understand what CFM and SCFM mean. I worked with compressor
specification and air system analysis ,around the worl, for 25 years.
Please explain upon what authority you base your contention about CFM.
For one thing your definition of "Standard Conditions", which is what
the S stands for, does not match anything I have ever seen anywhere in
the world and believe me I have seen about every definition of SCFM
there is. A CFM is just what it says it is. One Cubic Foot per
Minute at any temperature, pressure or humidity. It can be used to
define VOLUME flow for any gas, but does not necessarily have any
relation to MASS flow (except in an accidental context). Common
compressor terms include ACFM, ICFM, SCFM, etc,etc.. ACFM is "actual
cfm which refers to whatever the compressor produces at discharge
conditions(say 125 psi , 300 deg F and 100% rh for instance). ICFM
is "inlet cfm" which is flow at whatever conditions happen to be at
inlet (say 12.7 psi , 20 deg F and 14% rh) , SCFM is the MASS flow at
standard conditions which, in the USA are generally 14.696 psig, 60
deg F and 0% relative humidity. In Europe it is standardized as 1 bar
pressure 0 deg C and 0% relative humidity or alternately 1 bar,25 deg
C and 0% rh, the primary requirement being to use the same convention
at all times. Incidentally in the USA, 379 STANDARD cubic feet of
air weigh 28.79 lbm and contain one mole of air. SCFM seldom has any
relationship to conditions at the inlet of the compressor since 0% rh
is virtually unobtainable in nature. Inlet pressure, temperature and
rh have very large effects on system performance and can result in
changes of several hundred horsepower on medium to large compressors.
Gary Hallenbeck
Senior Technical Specialist (Ret.)
Western Region
Ingersoll-Rand Air Compressors

On Wed, 24 Dec 2003 19:32:33 -0600, Richard J Kinch
wrote:

Grant Erwin writes:

CFM does not equal mass flow

No, this is wrong again. CFM *does* equal mass flow.

I think you don't understand what "CFM" and "SCFM" mean.

One CFM means a cubic foot of air at 1 atm pressure ("free" air), flowing
per minute. Or the equivalent mass of air at any other pressure. It does
NOT mean one cubic foot of air at any other pressure.

The "S" prefixed simply specifies the input free air is understood to be at
68 deg F and 36 percent relative humidity, to simplify the variations of
system performance (usually, but not always, slight) due to those
variables.

Roger Head writes:

I guess you just read that site, eh Richard?

I read it, yes, at least the parts about CFMs. Why?

Gary Coffman writes:

Power is not a conservative quantity. Energy is, and the energy not
released is retained in the tank. In other words, power = energy/time.

Your statement is contradictory, although I think I understand what you
mean. Power can be converted to heat and thus not "conserved". I
suppose what I should say is that the power going into the regulator
either comes out the other side, or is lost as heat.

This ends up as heat as a direct consequence of the restriction that
creates turbulence and lowers the pressure. This waste heat is mostly
added to the output flow, even though the output may be at a cooler
temperature due to expansion. Some of the power lost is hissing noise
that radiates away and also eventually becomes heat. None of this is
"visible" to the source, so it is not "simply retained in the tank".

Those parasitic losses are relatively negligible in a well designed
system.

I don't know what you mean by "parasitic" here. Regulators are
inherently inefficient devices, and part of the input power is always
wasted (ultimately) as waste heat by them.

Think of the case where you have a "perfect" regulator lowering the
input pressure to ambient pressure (0 psig). For example, imagine a
pressurized tank that you instantly crack open in half. Or for a better
imaginary example, imagine that you have a compressed tank, and you can
magically make the tank disappear instantly, leaving the mass of
compressed air suddenly unconfined. Where did the power go? You get a
noisy bang, and a lot of turbulence, which ultimately degrades into heat
equal to all the energy previously stored in that compressed air.

Similar puzzle: shorting a perfect capacitor with a perfect conductor.
Where does the energy go?

On Wed, 24 Dec 2003 19:57:22 -0600, 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.

Contrary to what this basically informative, but somewhat flawed
website tells you, Flow is not necessarally measured at the inlet of
the compressor(in fact infrequently so except in the case of
centrifugal compressors which are usually rated in ICFM.) Every
specification I have ever seen requires quotation in SCFM. And from
your own website reference,which you have misquoted comes"By itself,
CFM tells you nothing about the weight of the air that is inside the
cube or the work that is available from the air. The first letter, "S"
or "I" or "A", ties the volume to a set of conditions that will define
the air in the cube and in doing so, define the available work."
Notice that the S ties the VOLUME to a specific set of conditions.


They then proceed to define S correctly and I and A in somewhat
misleading terms.

Gary H.

Roger Head writes:

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.

That should always be called "displacement CFM", which is indeed a bogus
spec.

If a commercial product has a sticker claiming CFM at some psi, it had
better deliver that at the output fitting, unless otherwise set forth as
the theoretical displacement or some other nonsense. That the cheap
imports have misrepresented such standards of performance, doesn't nullify
the clear meaning of the specification.

Hi Roger,

If you read my reply to Richard, I think you may see some of the
discomfort that Grant is expressing. The site is essentially correct,
but contains some definite technical mistakes. I spent 25 years
working for Ingersoll-Rand in the air compressor business so I have a
certain expertise in this area.
1. Compressor capacity is not determined on the inlet side,
except for Centrifugal units which are usually constant mass flow
machines and dependent on inlet conditions to a very large degree.
Most systems are rated in SCFM required. In specifying a centrifugal
machine it will be rated in delivered SCFM for specific inlet
conditions.
2. ACFM is a term I have only heard used in rating very small,
cheap compressors and is usually similar to "Free Air CFM" or "CFM
displacement" These terms are used to make the compressor look
comparable to a more expensive unit which is rated in CFM at a
specific pressure.
3 SCFM is the only way of comparing or rating compressor
delivery that gives a standard reference. In the compressor game
salesmen will use all sorts of numbers to rate efficiencies and flows
to make their machine look better on paper. SCFM or lbm or other mass
flow per horsepower eliminates the tom foolery and gives an apples to
apples comparison.

These three items lead me to believe that the website is either well
intentioned and poorly informed or that there is a particular economic
slant to the information presented.

Gary Hallenbeck

On Thu, 25 Dec 2003 14:10:44 GMT, 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.

Gary

Sorry, but you are mistaken. Most compressors are rated in delivered
flow. Centrifugal units are frequently rated in inlet cfm. Somewhere
in this thread I gave the example of a compressor rated at 10 cfm and
a very high pressure. Specifially I will use a high pressure unit
that existed at Mare Island Naval Shipyard in Vallejo CA, It produced
15 CFM at 4500 psi, but it's inlet capacity was about 500 cfm at 14.7
psi. Think of it this way. If you have an air tool that requires 4.5
cfm at 90 psi, what size compressor do you need to run it? Remember
it needs 4.5 cfm at 90 psi. Convert 4.5 cfm at 90 psi and 100% rh to
cfm at inlet conditions and I think you will find you have a much
larger number.
Gary H
On Fri, 26 Dec 2003 01:41:01 -0500, Gary Coffman
wrote:

On Wed, 24 Dec 2003 19:50:30 -0600, Richard J Kinch wrote:
jim rozen writes:
Cubic feet per minute is only a time
rate of volume.

No, no, no, no, no. CFM in compressors refers to the flow rate OF THE
INPUT AIR AT ATMOSPHERIC PRESSURE.

That's right. When you see a compressor rated at say 10 CFM @ 90 PSI,
that means 10 CFM of *inlet* air compressed to 90 PSI. It does NOT mean
10 cubic feet of 90 PSI air. This should make sense when you notice that
a compressor may have several fairly similar CFM ratings given at several
different pressures. That's because the only differences are due to
differences in the pumping *efficiency* of the compressor at different
delivery pressures. It is still inhaling roughly the same amount of air per
stroke at the same strokes per minute, so it still has roughly the same
CFM rating whether that's given at a delivery pressure of 45 PSI, 90 PSI,
or 140 PSI. It just takes longer to initially pump up the tank to the higher
delivery pressures.

Gary

In article , Richard J Kinch
says...

Gary Coffman writes:

Power is not a conservative quantity. Energy is, and the energy not
released is retained in the tank. In other words, power = energy/time.

Your statement is contradictory,

Actually not at all. It is exactly, and technically
completely correct. The conservations laws do indeed
talk about *energy* and not the time rate of energy
delivery.

although I think I understand what you
mean. Power can be converted to heat and thus not "conserved". I
suppose what I should say is that the power going into the regulator
either comes out the other side, or is lost as heat.

I suggest you read up an introductory thermodynamics book.
First off, gary's comments were that power (time rate
of energy delivery) is *not* conserved. What you mean to
say above is that "*energy* can be converted into heat..."
which is of course true, but in doing so it IS conserved
as heat is a form of energy. If you do all the bookkeeping
all the numbers add up. Mechanical work, heat, kinetic
energy, stored potential energy, all sum to a constant.

Similar puzzle: shorting a perfect capacitor with a perfect conductor.
Where does the energy go?

The only place it can go: radiation.

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================

On Mon, 22 Dec 2003 07:07:55 -0800, Grant Erwin
wrote:

I have been thinking about the issue of model equivalency between an air
regulator and a transformer again.

I'm tuning in late here, sorry. I skimmed thru the postings and see
some confustion: the terms CFM and SCFM have been used somewhat
interchangably which may be causing some of the confusion.

SCFM, as was asserted a couple of times, is a measure of mass flow ;
whatever the actual flow rate is, it's normalized to what that amount
of air (at standtard temp) would be at atmospheric pressure.

CFM is actual flow, more analagous to current in an electric circuit.
For temperature the same, V1 CFM at P1 pressure would expand to 2V1
CFM at 1/2 P1 -- Boyle's gas law. In that sense ( constant
temperature or isothermal expansion), a regulator acts like a
transformer.

However, an accurate model would have to take thermodynamics into
account: air cools as it expands so if there is flow then there is
also temperature difference from P1 to P2 unless additional energy is
added downstream to warm the air back up. The equations describing
this for air can be found in Machinery's Handbook.

Because air on the low side is cooler than air on the high side, the
low-side air will have correspondingly less volume than it would have
had if there were no temperature change.

Air flow meters: it's not hard to kludge a fairly decent airflow
meter using hot wire aenomometry. There are probably many website
about it. One can use bead thermistors, or even small incandescant
bulbs with holes punched in the evelopes, though the latter tends to
be a bit fragile. Calibrate by holding it out a car window on a
stick long enough to get away from the car body and driving at various
speeds on a calm day -- or spinning it with a variable-speed motor in
the lab if you can conjure some sliprings.

On Mon, 22 Dec 2003 07:07:55 -0800, Grant Erwin
wrote:

I have been thinking about the issue of model equivalency between an air
regulator and a transformer again.

Hot wire aenomometers: check out
http://www.efunda.com/designstandard...res_theory.cfm

A simple filament driven by constant current (monitor voltage) would
be a start. I've used automotive taillight bulbs with the envelope
carefully broken away. Smaller bulbs are more sensitive, also more
fragile.

A bridge setup using two bulbs, one in the flow and one shielded from
flow, both driven by the same constant current, provides some
compensation for ambient temperature.

Both of these schemes assume constant resistance of the filament which
isn't so, but may be close enough for foolin' around -- particularly
with a calibrated device used at about the same ambient temp most of
the time. Ya don't run the filaments anywhere near red hot, just
warm. Delta -T may be small enough that delta R with delta-T may be
negligable compared to effects of air velocity.

For higher accuracy, consider small resistors (maybe surface mount)
with thermistor beads glued to them in close thermal contact. Use
fine wires or traces so as not to heat-sink them. Drive both
resistors with current controlled by opamps controlled by the
respective thermistors so both resistors are held at the same
temperature. The voltage difference on the resistors would then be a
function of air velocity -- perhaps about proportional to the square
root of velocity. This should make a pretty decent instrument,
though a bit slower than just a filament.

The mass flow meters in some fuel-injection systems work by hot-wire
aenomometry, as Bosch. (At least they used to)

Honeywell made a device called the "microbridge" which was essentially
a silicon hot-wire aenomometer bridge on a chip. Those things were so
fast they'd respond to sound waves. We used them with small
orifices to measure small pressures; if someone was talking loudly
near the duct I'd see audio on the pressure signal!

On Fri, 26 Dec 2003 12:18:34 -0600, Don Foreman
wrote:

Hot wire aenomometers: check out
http://www.efunda.com/designstandard...res_theory.cfm

Some of this stuff is starting to come back to me.

A direct-heated aenomometer actually depends on the element having
some tempco of resistance w.r.t. temperature. Bead thermistors work
well but tungsten works OK also, with tempco of about 0.45%/degC.

As air movement removes heat, temp drops, resistance drops and
voltage drops (with current drive). This produces an error signal
which can be amplified to raise the current until it nearly makes up
the power lost to the wind with only a small temp change necessary to
produce the error signal necessary to re-establish equilibrium.

Note that such a device is a mass flow meter so it would measure SCFM
rather than CFM Given velocity (hence CFM) of air at higer
pressure would produce higher reading on the anomometer.

On Fri, 26 Dec 2003 07:52:20 GMT, Gary Hallenbeck
wrote:


Gary

Sorry, but you are mistaken. Most compressors are rated in delivered
flow.

Several web sites make it clear that SCFM connotes weight of air
delivered. 1 CFM is defined as a cubic foot of air if it were at
standard condx -- of which there are several definitions.

An efficient two-stage pump's delivery in what they call ACFM varies
only slightly from 100 PSI to 175 PSI delivery pressure. Further,
the rated capacity at rated pressure is not much less than piston
displacement. See
http://www.abacamerican.com/Bel%20Aire%20electric.htm
for examples.

Some say CFM, others say ACFM, it's a mess!