I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow rate
at atmospheric pressure, not the flow rate at the compressor's maximum
working pressure. Can someone confirm this for me?

Many thanks,

Chris

Christopher Tidy wrote:
I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow rate
at atmospheric pressure, not the flow rate at the compressor's maximum
working pressure. Can someone confirm this for me?
The volume is measured in SCFM (Standard Cubic Feet per Minute). The
"standard" means referenced to standard temperature and pressure. But,
the reading is only useful when the tank pressure is known. Single
stage pumps lose a LOT of efficiency at higher pressures, that's why
el-cheapo manufacturers give the SCFM rating at 40 PSI tank pressure.
You want the rating at 90 PSI tank pressure. A crummy punp will lose
half its volumetric efficiency at 90 PSI. A good single-stage punp
will still lose 25% or so, going from 40 to 90 PSI.

If they didn't do this, ratings would be almost impossible to compare.
One cubic foot of sea-level air would be compressed to 1/7th of a cubic
foot at 90 PSI, which is 7 atmospheres of absolute pressure. That, of
course, is neglecting any temperature effects.

Jon

If you only want to draw 40 psi you can get xx cfm. If you want to
maintain 125 psi you can only get x cfm.I have not seen one rated at
atmospheric pressure.most quote a lower pressure of 80 or 40psi then at
max working pressure.

Christopher Tidy wrote:
I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow rate
at atmospheric pressure, not the flow rate at the compressor's maximum
working pressure. Can someone confirm this for me?

Many thanks,

Chris

Christopher Tidy wrote:

I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow rate
at atmospheric pressure, not the flow rate at the compressor's maximum
working pressure. Can someone confirm this for me?

Oh, god not again. We beat this to death and then some awhile ago, much much
flames with mucho heat but very little light. I read thousands of lines of
postings and didn't understand what the gurus were saying. Typical specs are
given at 40psi and again at 90psi, but the way I read it none of this matters
somehow. It doesn't really matter about the theory, a decent 5hp 2-stage
compressor can keep up with a die grinder, that's all I really care about.

GWE

Grant Erwin wrote:
Christopher Tidy wrote:

I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow
rate at atmospheric pressure, not the flow rate at the compressor's
maximum working pressure. Can someone confirm this for me?


Oh, god not again. We beat this to death and then some awhile ago, much
much flames with mucho heat but very little light. I read thousands of
lines of postings and didn't understand what the gurus were saying.
Typical specs are given at 40psi and again at 90psi, but the way I read
it none of this matters
somehow. It doesn't really matter about the theory, a decent 5hp 2-stage
compressor can keep up with a die grinder, that's all I really care about.

Well actually I was looking at this second hand compressor:

http://www.mythic-beasts.com/~cdt22/compressor.jpg

I know it's old, but I love projects. The whole thing is 28" long and
has a motor rated at 0.75 hp at 1440 rpm. There are two small air
receivers underneath you can't see. Basically I was wondering what kind
of tools I could run off it. A few things I'm interested in are paint
spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the
fire service) and inflating tyres. At a rough guess I reckon this
machine probably has a flow rate of 6-8 CFM at atmospheric pressure. Any
thoughts?

Best wishes,

Chris

"Christopher Tidy" wrote in message
...
snip-----

The whole thing is 28" long and
has a motor rated at 0.75 hp at 1440 rpm. There are two small air
receivers underneath you can't see. Basically I was wondering what kind
of tools I could run off it. A few things I'm interested in are paint
spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the
fire service) and inflating tyres. At a rough guess I reckon this
machine probably has a flow rate of 6-8 CFM at atmospheric pressure. Any
thoughts?

Best wishes,

Chris


My honest opinion? It isn't worth your effort. 3/4 of a HP won't
produce enough air on a continual basis to do much for you. What Grant
said. I have a two stage 5 horse Quincy, rated 19+ CFM @ 175 PSI and it is
just adequate for running a small blast cabinet. It will run most air
tools with ease, that's not an issue. Small compressors aren't up to
much. Been there, done that. That's why I own the 5 horse Quincy. Unless
you have no prospects of a larger compressor, I'd pass on it and invest in
something that will serve you better in the long term.

Harold

Harold and Susan Vordos wrote:
"Christopher Tidy" wrote in message
...
snip-----


The whole thing is 28" long and
has a motor rated at 0.75 hp at 1440 rpm. There are two small air
receivers underneath you can't see. Basically I was wondering what kind
of tools I could run off it. A few things I'm interested in are paint
spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the
fire service) and inflating tyres. At a rough guess I reckon this
machine probably has a flow rate of 6-8 CFM at atmospheric pressure. Any
thoughts?

Best wishes,

Chris

My honest opinion? It isn't worth your effort. 3/4 of a HP won't
produce enough air on a continual basis to do much for you. What Grant
said. I have a two stage 5 horse Quincy, rated 19+ CFM @ 175 PSI and it is
just adequate for running a small blast cabinet. It will run most air
tools with ease, that's not an issue. Small compressors aren't up to
much. Been there, done that. That's why I own the 5 horse Quincy. Unless
you have no prospects of a larger compressor, I'd pass on it and invest in
something that will serve you better in the long term.

Thanks for the opinions. It looks like this compressor would be a bit
limiting, but I'm still tempted by it. I can't afford a 5 hp compressor,
and although I have the space for it now, I'm likely to be moving and
won't in the future.

I probably should have mentioned that when I talked about spraying I
didn't mean cars or trucks, I meant small stuff which I've built.
Probably I wouldn't want to spray things larger than 18" across. I can
manage without the die grinder but the Cengar saw would be nice. I
checked the Cengar website and the saw needs 5 3/4 CFM at 75-90 psi.
Perhaps I might just manage it with this compressor, or am I pushing my
luck? I figure that inflating tyres and blowing away debris would be
fine with a little compressor like this.

I was also looking at a Broomwade compressor, which is of a similar size
but I believe it is two-stage:

http://www.mythic-beasts.com/~cdt22/compressor3.jpg

I know these machines are projects in themselves, but I don't mind that.
I like their "old school" appearance and construction.

Best wishes,

Chris

"Christopher Tidy" wrote in message
...
I checked the Cengar website and the saw needs 5 3/4 CFM at 75-90 psi.
Perhaps I might just manage it with this compressor, or am I pushing my
luck? I figure that inflating tyres and blowing away debris would be fine
with a little compressor like this.



I question the CFM required to run your saw. Many manufacturers do not rate
the CFM at 100% usage so you get a poor indication of air volume needed to
run the tool.
Either way anything less than a 3 HP compressor will allow very intermittent
use of the tools you mention. I have a Porter Cable 7 HP, ( yeah right,
really about 3 HP), compressor that pretty much does all I need it to do. It
is rated at 9+ CFM at 90 PSI.. I had a real 2 HP compressor for years that
really was inadequate for much air tool use.
If you need a limited amount of air, a larger tank will get you by with a
small compressor. Keep in mind that once the tank has run down, recovery
time is very poor so you will not have air when you probably will want it!
Greg.

"Christopher Tidy" wrote in message
...
snip------
Thanks for the opinions. It looks like this compressor would be a bit
limiting, but I'm still tempted by it. I can't afford a 5 hp compressor,
and although I have the space for it now, I'm likely to be moving and
won't in the future.

I probably should have mentioned that when I talked about spraying I
didn't mean cars or trucks, I meant small stuff which I've built.
Probably I wouldn't want to spray things larger than 18" across. I can
manage without the die grinder but the Cengar saw would be nice. I
checked the Cengar website and the saw needs 5 3/4 CFM at 75-90 psi.
Perhaps I might just manage it with this compressor, or am I pushing my
luck? I figure that inflating tyres and blowing away debris would be
fine with a little compressor like this.

I was also looking at a Broomwade compressor, which is of a similar size
but I believe it is two-stage:

http://www.mythic-beasts.com/~cdt22/compressor3.jpg

I know these machines are projects in themselves, but I don't mind that.
I like their "old school" appearance and construction.

Best wishes,

Chris


I can't argue with that, Chris. As long as you don't mind using your tools
on a limited basis, there's no reason why you can't get by. You could
even sand blast with it, but not for a very long time. It's all in what you
expect. If you can get it for the right price and dedicate some time to
making it work well, using it will quickly tell you if you made the right
decision, or not. On thing is absolute------you can't have too much air at
your disposal, but you certainly can find yourself needing more than you
have. Only you will know if you're happy, or not. Good luck,
regardless of your decision. Let us hear from you and how things turned
out.

Harold

Grant Erwin wrote:
Christopher Tidy wrote:

I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow
rate at atmospheric pressure, not the flow rate at the compressor's
maximum working pressure. Can someone confirm this for me?


Oh, god not again. We beat this to death and then some awhile ago, much
much flames with mucho heat but very little light. I read thousands of
lines of postings and didn't understand what the gurus were saying.
Typical specs are given at 40psi and again at 90psi, but the way I read
it none of this matters
somehow. It doesn't really matter about the theory, a decent 5hp 2-stage
compressor can keep up with a die grinder, that's all I really care about.

Well actually my thoughts were more pragmatic too. I was looking at this
second hand compressor:

http://www.mythic-beasts.com/~cdt22/compressor.jpg
http://www.mythic-beasts.com/~cdt22/compressor2.jpg

I know it's old, but I love projects. The whole thing is 28" long and
has a motor rated at 0.75 hp at 1440 rpm. There are two small air
receivers underneath you can't see. Basically I was wondering what kind
of tools I could run off it. A few things I'm interested in are paint
spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the
fire service) and inflating tyres. At a rough guess I reckon this
machine has a flow rate of 6-8 CFM at atmospheric pressure. Any thoughts?

Best wishes,

Chris

On Wed, 8 Jun 2005 03:57:38 +0000 (UTC), Christopher Tidy
wrote:


somehow. It doesn't really matter about the theory, a decent 5hp 2-stage
compressor can keep up with a die grinder, that's all I really care about.

Well actually my thoughts were more pragmatic too. I was looking at this
second hand compressor:

http://www.mythic-beasts.com/~cdt22/compressor.jpg
http://www.mythic-beasts.com/~cdt22/compressor2.jpg

I know it's old, but I love projects. The whole thing is 28" long and
has a motor rated at 0.75 hp at 1440 rpm. There are two small air
receivers underneath you can't see. Basically I was wondering what kind
of tools I could run off it. A few things I'm interested in are paint
spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the
fire service) and inflating tyres. At a rough guess I reckon this
machine has a flow rate of 6-8 CFM at atmospheric pressure. Any thoughts?

Best wishes,

Chris
My old 1/2 HP gives me 2CFM @ 80-100 PSI and with 10 gal. storage I
can use a die grinder but not continuously. With minimal storage, the
one you are looking at would be cycling very rapidly and put out about
3 CFM of usable air. By adding storage capacity it might be of some
use.
Gerry :-)}
London, Canada

Christopher Tidy wrote in
:




You can forget the spray painting and die grinder with 6-8 CFM, and
probably the saw too. Just about any die grinder will eat more like 10 CFM
(for a good one) an elcheapo may pull as much as 14-16 CFM.


--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

http://www.machines-cnc.net:81/

Christopher Tidy wrote:
Grant Erwin wrote:

Christopher Tidy wrote:

I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow
rate at atmospheric pressure, not the flow rate at the compressor's
maximum working pressure. Can someone confirm this for me?



Oh, god not again. We beat this to death and then some awhile ago,
much much flames with mucho heat but very little light. I read
thousands of lines of postings and didn't understand what the gurus
were saying. Typical specs are given at 40psi and again at 90psi, but
the way I read it none of this matters
somehow. It doesn't really matter about the theory, a decent 5hp
2-stage compressor can keep up with a die grinder, that's all I really
care about.


Well actually my thoughts were more pragmatic too. I was looking at this
second hand compressor:

http://www.mythic-beasts.com/~cdt22/compressor.jpg
http://www.mythic-beasts.com/~cdt22/compressor2.jpg

I know it's old, but I love projects. The whole thing is 28" long and
has a motor rated at 0.75 hp at 1440 rpm. There are two small air
receivers underneath you can't see. Basically I was wondering what kind
of tools I could run off it. A few things I'm interested in are paint
spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used by the
fire service) and inflating tyres. At a rough guess I reckon this
machine has a flow rate of 6-8 CFM at atmospheric pressure. Any thoughts?

Best wishes,

Chris


I'll say that some compressed air is a lot better than none. When I was
a kid I had a little corn-popper compressor with about a 4 gallon tank.
I could knock 5 wheel nuts off if I loosened them all first, but if I
took three all the way off there wan't pressure enough to break the last
ones loose.. but it would air up a tire just as good as a big one..

John

JohnM wrote:
Christopher Tidy wrote:

Grant Erwin wrote:

Christopher Tidy wrote:

I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow
rate at atmospheric pressure, not the flow rate at the compressor's
maximum working pressure. Can someone confirm this for me?




Oh, god not again. We beat this to death and then some awhile ago,
much much flames with mucho heat but very little light. I read
thousands of lines of postings and didn't understand what the gurus
were saying. Typical specs are given at 40psi and again at 90psi, but
the way I read it none of this matters
somehow. It doesn't really matter about the theory, a decent 5hp
2-stage compressor can keep up with a die grinder, that's all I
really care about.



Well actually my thoughts were more pragmatic too. I was looking at
this second hand compressor:

http://www.mythic-beasts.com/~cdt22/compressor.jpg
http://www.mythic-beasts.com/~cdt22/compressor2.jpg

I know it's old, but I love projects. The whole thing is 28" long and
has a motor rated at 0.75 hp at 1440 rpm. There are two small air
receivers underneath you can't see. Basically I was wondering what
kind of tools I could run off it. A few things I'm interested in are
paint spraying, a die grinder, Cengar saw (a.k.a. shark saw, as used
by the fire service) and inflating tyres. At a rough guess I reckon
this machine has a flow rate of 6-8 CFM at atmospheric pressure. Any
thoughts?

Best wishes,

Chris


I'll say that some compressed air is a lot better than none. When I was
a kid I had a little corn-popper compressor with about a 4 gallon tank.
I could knock 5 wheel nuts off if I loosened them all first, but if I
took three all the way off there wan't pressure enough to break the last
ones loose.. but it would air up a tire just as good as a big one..

Thanks for another opinion, John. I'm still pondering the 3/4 hp machine
but I suspect I not going to buy it. The guy wants more than I thought,
although I'm still tempted as it looks like a nicely built machine.

By the way, can anyone summarise briefly explain the advantage of a
two-stage compressor? Do you get better performance at higher pressures?

Best wishes,

Chris

"Christopher Tidy" wrote in message
...
snip------

By the way, can anyone summarise briefly explain the advantage of a
two-stage compressor? Do you get better performance at higher pressures?

Best wishes,

Chris

Unless you have need for the higher pressures they generate, I don't think
there are any other benefits. In my mind, it's another way to increase
storage capacity. Regardless of the size of the tank, you have more air
stored for use, so you can get more from a smaller tank. That means you
can use air longer before the compressor kicks in, but that then becomes the
limiting factor. At that point, you're limited to the capacity of the
compressor, the chief reason to not buy one that is undersized for your
needs.

Let us know how it goes, and if your decision was good for you.

Harold

On Sat, 11 Jun 2005 23:00:26 -0400, JohnM wrote:




I'll say that some compressed air is a lot better than none. When I was
a kid I had a little corn-popper compressor with about a 4 gallon tank.
I could knock 5 wheel nuts off if I loosened them all first, but if I
took three all the way off there wan't pressure enough to break the last
ones loose.. but it would air up a tire just as good as a big one..

John
Or clean out the lint trap on the boss's automatic washer.
Gerry :-)}
London, Canada

Christopher Tidy writes:

I've been looking at a few second air compressors. I've always
understood the volume flow rate quoted in CFM to refer to the flow rate
at atmospheric pressure, not the flow rate at the compressor's maximum
working pressure. Can someone confirm this for me?

"CFM" refers to the cubic feet of free air per minute, regardless of the
delivery pressure. It does not mean the compressed volume delivered. See
my page, "Evaluating True Horsepower and CFM Ratings of Air Compressors":

http://www.truetex.com/aircompressors.htm