We all know about the 'peak horsepower' lie... but what about CFM?

My $100, 8-gallon HF compressor is rated at 4.5 SCFM @115 psi.
There are 7.48 gallons per cubic foot meaning that my 8 gallon
compressor is just a little over 1 cubic foot.

Assuming I am basically at standard pressure & temperature conditions,
this would seem to imply that my compressor should fill in under 15
seconds, i.e., (8/7.48)*(1/4.5)*60

But it takes more like 2 minutes to fill...

What am I missing here?

On Oct 27, 1:04*pm, blueman wrote:
We all know about the 'peak horsepower' lie... but what about CFM?

My $100, 8-gallon HF compressor is rated at 4.5 SCFM @115 psi.
There are 7.48 gallons per cubic foot meaning that my 8 gallon
compressor is just a little over 1 cubic foot.

Assuming I am basically at standard pressure & temperature conditions,
this would seem to imply that my compressor should fill in under 15
seconds, i.e., (8/7.48)*(1/4.5)*60

But it takes more like 2 minutes to fill...

What am I missing here?

The charging flow's resistance isn't linear as the pressure increases
in the tank.
The answer to the original question is no.

On 10/27/2011 1:04 PM, blueman wrote:
We all know about the 'peak horsepower' lie... but what about CFM?

My $100, 8-gallon HF compressor is rated at 4.5 SCFM @115 psi.
There are 7.48 gallons per cubic foot meaning that my 8 gallon
compressor is just a little over 1 cubic foot.

Assuming I am basically at standard pressure& temperature conditions,
this would seem to imply that my compressor should fill in under 15
seconds, i.e., (8/7.48)*(1/4.5)*60

But it takes more like 2 minutes to fill...

What am I missing here?

You appear to be expecting approximately 1 CF of air to fill the tank.
And it will, but not to 115PSI.

On 10/27/2011 3:36 PM, Mike wrote:
On 10/27/2011 1:04 PM, blueman wrote:
We all know about the 'peak horsepower' lie... but what about CFM?

My $100, 8-gallon HF compressor is rated at 4.5 SCFM @115 psi.
There are 7.48 gallons per cubic foot meaning that my 8 gallon
compressor is just a little over 1 cubic foot.

Assuming I am basically at standard pressure& temperature conditions,
this would seem to imply that my compressor should fill in under 15
seconds, i.e., (8/7.48)*(1/4.5)*60

But it takes more like 2 minutes to fill...

What am I missing here?

You appear to be expecting approximately 1 CF of air to fill the tank.
And it will, but not to 115PSI.

And, when account for that, (115 + 15)/15 = 8-something so the SCF input
required is 8X times the final volume. Thus 8X the 15 seconds is for 1
SCF yields about 2 minutes for 8. Amazing!!!

--

dpb writes:

On 10/27/2011 3:36 PM, Mike wrote:
On 10/27/2011 1:04 PM, blueman wrote:
We all know about the 'peak horsepower' lie... but what about CFM?

My $100, 8-gallon HF compressor is rated at 4.5 SCFM @115 psi.
There are 7.48 gallons per cubic foot meaning that my 8 gallon
compressor is just a little over 1 cubic foot.

Assuming I am basically at standard pressure& temperature conditions,
this would seem to imply that my compressor should fill in under 15
seconds, i.e., (8/7.48)*(1/4.5)*60

But it takes more like 2 minutes to fill...

What am I missing here?

You appear to be expecting approximately 1 CF of air to fill the tank.
And it will, but not to 115PSI.

And, when account for that, (115 + 15)/15 = 8-something so the SCF
input required is 8X times the final volume. Thus 8X the 15 seconds
is for 1 SCF yields about 2 minutes for 8. Amazing!!!

DOH me!!!! of course I forgot the old pv=nrt
and therefore it's not 1 SCF in there but as you say (115+15)/15...

That makes a lot of sense of course...