When you see something like "3.2CFM at 60PSI", where are the cubic feet
measured?

Does that mean that the compressor is putting 3.2 CFM of air at 5x
atmospheric pressure into the tank (and hence that you'll get 16 CFM of
air out the hose, once it's relaxed back to atmospheric), or does that
mean that you will be able to inflate 3.2 one-cubic-foot balloons per
minute with the air that comes out of the hose?

I assume that it's the latter, but I want to check.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Does that mean that the compressor is putting 3.2 CFM of air at 5x
atmospheric pressure into the tank (and hence that you'll get 16 CFM of
air out the hose, once it's relaxed back to atmospheric), or does that
mean that you will be able to inflate 3.2 one-cubic-foot balloons per
minute with the air that comes out of the hose?

I assume that it's the latter, but I want to check.

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

When a compressor pumps one "CFM" (cubic foot per minute), that means
the intake port inhales one cubic foot of "free air" (air at
atmospheric pressure, which is 0 psig) per minute. (Note: A CFM does
not mean in any sense the compressed volume.)

Randy

Tim Wescott wrote:
When you see something like "3.2CFM at 60PSI", where are the cubic feet
measured?

Does that mean that the compressor is putting 3.2 CFM of air at 5x
atmospheric pressure into the tank (and hence that you'll get 16 CFM of
air out the hose, once it's relaxed back to atmospheric), or does that
mean that you will be able to inflate 3.2 one-cubic-foot balloons per
minute with the air that comes out of the hose?

I assume that it's the latter, but I want to check.


All depends on what company made it...
If it's a Sears then 3.2 CFM is what it would produce wired for 440
turning about 10K rpms and measured just before it blows apart from the
stress...

If it's a good unit then it should mean the airflow at the exhaust port
of the compressor head prior to any fittings, with the compressor
running at specific rpms and usually measured with NO air inlet filtration.

Same method is used for a water pump except you have to add in any
incoming pressure/vacuum as well.

--
Steve W.

"Steve W." wrote in message
...
Tim Wescott wrote:
When you see something like "3.2CFM at 60PSI", where are the cubic feet
measured?

Does that mean that the compressor is putting 3.2 CFM of air at 5x
atmospheric pressure into the tank (and hence that you'll get 16 CFM of
air out the hose, once it's relaxed back to atmospheric), or does that
mean that you will be able to inflate 3.2 one-cubic-foot balloons per
minute with the air that comes out of the hose?

I assume that it's the latter, but I want to check.


All depends on what company made it...
If it's a Sears then 3.2 CFM is what it would produce wired for 440
turning about 10K rpms and measured just before it blows apart from the
stress...

If it's a good unit then it should mean the airflow at the exhaust port of
the compressor head prior to any fittings, with the compressor running at
specific rpms and usually measured with NO air inlet filtration.

Same method is used for a water pump except you have to add in any
incoming pressure/vacuum as well.

--
Steve W.


Not correct. The capacity of an air compressor is measured at the compressor
inlet with the compressor running at a specified speed and an open discharge
port.


Centrifugal water pump capacity varies with the pressure difference between
the inlet and outlet so a centrifugal pump is specified by a set of graphs
( pump curves) that show the flow rate along the X axis and pressure
differance along the Y or vertical axis. They vay with the speed of the
impeller. Positive didplacement pumps such as lobe pumps or "Mono"pumps will
deliver a flow rate porportional to impeller speed into any pressure
difference provided the drive has sufficient horsepower. Of course the
maximum pressure is limited by the pressure rating of the system.

Grumpy wrote:
"Steve W." wrote in message
...
Tim Wescott wrote:
When you see something like "3.2CFM at 60PSI", where are the cubic feet
measured?

Does that mean that the compressor is putting 3.2 CFM of air at 5x
atmospheric pressure into the tank (and hence that you'll get 16 CFM of
air out the hose, once it's relaxed back to atmospheric), or does that
mean that you will be able to inflate 3.2 one-cubic-foot balloons per
minute with the air that comes out of the hose?

I assume that it's the latter, but I want to check.

All depends on what company made it...
If it's a Sears then 3.2 CFM is what it would produce wired for 440
turning about 10K rpms and measured just before it blows apart from the
stress...

If it's a good unit then it should mean the airflow at the exhaust port of
the compressor head prior to any fittings, with the compressor running at
specific rpms and usually measured with NO air inlet filtration.

Same method is used for a water pump except you have to add in any
incoming pressure/vacuum as well.

--
Steve W.


Not correct. The capacity of an air compressor is measured at the compressor
inlet with the compressor running at a specified speed and an open discharge
port.

That would be the measurement of SCFM at atmospheric pressure. IE the
amount of air that the compressor
will pass at 14.7 psi.
To get a CFM rating at a specified pressure you have to regulate the
output of the compressor so that it actually develops that
pressure. Then measure the airflow.

The problem is that it all depends on the manufacturer and which way
they actually measure.

SCFM - The amount of air that the compressor will suck in at a set rpm
with open ports at sea level pressure (14.7 psi) at 68 degrees
temperature and 36% RH.
Expressed as only a number IE- 25 SCFM


CFM@ XX PSI- The amount of air the compressor will pump at that pressure
with 14.7 psi at the inlet. This is due to the volumetric efficiency
losses from the increased pressures. Measured at the output of the
compressor to eliminate the calculations of the VE losses inside the
unit. These are the ones you see on most consumer rated units -
3.5 CFM @ 90 psi, 2.8 cfm @ 120 psi It is not a linear table though as
the VE losses vary with temperatures and pressures


ACFM - This is measured in actual use. Take a known volume tank, pump
air into it while measuring the time it takes to get to a set pressure.
Use a nice equation to calculate out what the compressor is actually doing.



"Air gets denser as you compress it, less dense as you heat it. In
order to provide accurate flow measurement to an end user or to
provide an "apples to apples" comparison of flow, that variability
has to be removed.

So a "standard" cubic foot was created based on a
standard pressure and temperature.

Typically being 1 atmosphere and temperature of either 68F or
15C. In other words, 1 scf is the space that 1 cubic foot of air
occupies at atmospheric pressure and a standard temperature. At 90
psig that same cubic foot takes up a lot less space than it did at
atmospheric pressure so 1 cfm @ 90 psi contains 7 scf.(standard cubic
feet)

In real life, cfm is the useful output rate and it is a measure of
exactly how many cubic feet leave the compressor at the rated
pressure in a minute. There is no regard for the density of that
cubic foot or whether or not my compressors cubic foot contains as
much air as your compressor. That's where scfm comes in - it
equalizes the playing field, providing standardized correction to
capacity claims at 110 psig versus those at 90 or 150 psig for
instance.

In reality no one using compressed air to drive a tool really cares
about scfm - we're all concerned about how many cfm at my required
pressure can I get so I know I can drive this air wrench etc. As long
as I get 30 cfm of 90 psi air and my wrench works I don't care how
many scf that air contains."





Centrifugal water pump capacity varies with the pressure difference between
the inlet and outlet so a centrifugal pump is specified by a set of graphs
( pump curves) that show the flow rate along the X axis and pressure
differance along the Y or vertical axis. They vay with the speed of the
impeller. Positive didplacement pumps such as lobe pumps or "Mono"pumps will
deliver a flow rate porportional to impeller speed into any pressure
difference provided the drive has sufficient horsepower. Of course the
maximum pressure is limited by the pressure rating of the system.

And to test for rated flow you connect up the inlet to a known water
source (for fire apparatus you are supposed to draft with no inlet
pressure) Run the pump up to the correct rpm and measure the output
flow. If all is well then you switch to the next rpm/pressure level and
measure again.
It is one of my least favorite things to do. Set up the test tank.
Connect up the suction, put on hearing protection, start pumping water.
Do your initial test. Then run the pump for an hour while testing every
15 minutes. Then move up to the next test. I will say that running some
of the older rigs when your testing at the 250psi level can be
interesting. With the engine doing all the work it does place a lot of
strain on them. The old Detroits REALLY scream then.

We used to measure the old piston rigs the same way.
--
Steve W.

On Tue, 03 Jan 2012 06:32:28 -0500, "Steve W."
wrote:

Grumpy wrote:
"Steve W." wrote in message
...
Tim Wescott wrote:
When you see something like "3.2CFM at 60PSI", where are the cubic feet
measured?

Does that mean that the compressor is putting 3.2 CFM of air at 5x
atmospheric pressure into the tank (and hence that you'll get 16 CFM of
air out the hose, once it's relaxed back to atmospheric), or does that
mean that you will be able to inflate 3.2 one-cubic-foot balloons per
minute with the air that comes out of the hose?

I assume that it's the latter, but I want to check.

All depends on what company made it...
If it's a Sears then 3.2 CFM is what it would produce wired for 440
turning about 10K rpms and measured just before it blows apart from the
stress...

If it's a good unit then it should mean the airflow at the exhaust port of
the compressor head prior to any fittings, with the compressor running at
specific rpms and usually measured with NO air inlet filtration.

Same method is used for a water pump except you have to add in any
incoming pressure/vacuum as well.

--
Steve W.


Not correct. The capacity of an air compressor is measured at the compressor
inlet with the compressor running at a specified speed and an open discharge
port.

That would be the measurement of SCFM at atmospheric pressure. IE the
amount of air that the compressor
will pass at 14.7 psi.
To get a CFM rating at a specified pressure you have to regulate the
output of the compressor so that it actually develops that
pressure. Then measure the airflow.

The problem is that it all depends on the manufacturer and which way
they actually measure.

SCFM - The amount of air that the compressor will suck in at a set rpm
with open ports at sea level pressure (14.7 psi) at 68 degrees
temperature and 36% RH.
Expressed as only a number IE- 25 SCFM


CFM@ XX PSI- The amount of air the compressor will pump at that pressure
with 14.7 psi at the inlet. This is due to the volumetric efficiency
losses from the increased pressures. Measured at the output of the
compressor to eliminate the calculations of the VE losses inside the
unit. These are the ones you see on most consumer rated units -
3.5 CFM @ 90 psi, 2.8 cfm @ 120 psi It is not a linear table though as
the VE losses vary with temperatures and pressures


ACFM - This is measured in actual use. Take a known volume tank, pump
air into it while measuring the time it takes to get to a set pressure.
Use a nice equation to calculate out what the compressor is actually doing.



"Air gets denser as you compress it, less dense as you heat it. In
order to provide accurate flow measurement to an end user or to
provide an "apples to apples" comparison of flow, that variability
has to be removed.

So a "standard" cubic foot was created based on a
standard pressure and temperature.

Typically being 1 atmosphere and temperature of either 68F or
15C. In other words, 1 scf is the space that 1 cubic foot of air
occupies at atmospheric pressure and a standard temperature. At 90
psig that same cubic foot takes up a lot less space than it did at
atmospheric pressure so 1 cfm @ 90 psi contains 7 scf.(standard cubic
feet)

In real life, cfm is the useful output rate and it is a measure of
exactly how many cubic feet leave the compressor at the rated
pressure in a minute. There is no regard for the density of that
cubic foot or whether or not my compressors cubic foot contains as
much air as your compressor. That's where scfm comes in - it
equalizes the playing field, providing standardized correction to
capacity claims at 110 psig versus those at 90 or 150 psig for
instance.

In reality no one using compressed air to drive a tool really cares
about scfm - we're all concerned about how many cfm at my required
pressure can I get so I know I can drive this air wrench etc. As long
as I get 30 cfm of 90 psi air and my wrench works I don't care how
many scf that air contains."





Centrifugal water pump capacity varies with the pressure difference between
the inlet and outlet so a centrifugal pump is specified by a set of graphs
( pump curves) that show the flow rate along the X axis and pressure
differance along the Y or vertical axis. They vay with the speed of the
impeller. Positive didplacement pumps such as lobe pumps or "Mono"pumps will
deliver a flow rate porportional to impeller speed into any pressure
difference provided the drive has sufficient horsepower. Of course the
maximum pressure is limited by the pressure rating of the system.

And to test for rated flow you connect up the inlet to a known water
source (for fire apparatus you are supposed to draft with no inlet
pressure) Run the pump up to the correct rpm and measure the output
flow. If all is well then you switch to the next rpm/pressure level and
measure again.
It is one of my least favorite things to do. Set up the test tank.
Connect up the suction, put on hearing protection, start pumping water.
Do your initial test. Then run the pump for an hour while testing every
15 minutes. Then move up to the next test. I will say that running some
of the older rigs when your testing at the 250psi level can be
interesting. With the engine doing all the work it does place a lot of
strain on them. The old Detroits REALLY scream then.

We used to measure the old piston rigs the same way.

As far as air compressor ratings go, the only difference between CFM
and SCFM is that SCFM attempts to standardize the inlet temp, pressure
and humidity. CFM is still a measure of volume at more or less
atmospheric pressure and temperature, not the volume of the
pressurized air at the outlet. I suppose there may be exceptions for
specialized pumps, but not for typical shop air compressors.

Tossing pumps for water, an incompressible fluid, into the discussion
only confuse the matter.

--
Ned Simmons

Rule of thumb:

Whatever you THINK you need, multiply by 3 and you'll probably be close.
But not even guaranteed then.

Steve

"Steve W." wrote

With the engine doing all the work it does place a lot of
strain on them. The old Detroits REALLY scream then.

We used to measure the old piston rigs the same way.
--
Steve W.

We had six on the barge that we used to jet pipe underwater with a
mechanized roller device. They sure would make noise. We worked sometimes
48 hours at a stretch, and could fall asleep on a coil of hose next to them.
When they shut off, we automatically woke up. Time to jump an anode or take
a measurement.

Steve

Steve B wrote:
Rule of thumb:

Whatever you THINK you need, multiply by 3 and you'll probably be close.
But not even guaranteed then.

Steve



Sure give him the easy way out...
I usually multiply by 4. Plus never discount a larger/pair of tanks for
the system. That can really help out if you buy a slightly small
compressor. You really don't want to run out of air halfway through a
coat of paint!!!

--
Steve W.