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=A0pm, blueman <[email protected]> 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 <[email protected]> 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...