air cpm. I am not an engineer and I had to determine a way to evaluate whether
the Used Dusty Dirty air compressors I looked at had a working pump that output
my required free air cpm. Upon searching the internet and comparing physics sites
and engineering sites I came up with the following technique.
1) First you need to a have stop watch or a watch with a seconds hand.
We will measure Time in minutes so 1 min 30 seconds will become 1.5 .
Time in decimal minutes will become our TIME variable.
2) Determine the tank size of the air compressor in cubic feet (cuft).
This will often require converting from gallons to cubic feet.
1 gallon = 231 cubic inches
1 cubic foot = 12in x 12in x 12in = 1728 cubic inches
hence a 30 gallon tank = 4.01 cubic feet (cuft)
( 30 gallons x 231 )/ 1728 = 4.01 cubic feet
60 gallons = 8.02 cuft
80 gallons = 10.69 cuft
We will use this cubic foot number as our Volume.
3) Determine the max cut off pressure of the air compressor. This is the
pressure reading when the pump cuts out.
The compressor I bought had a max cutoff pressure of 130 psi.
We will use this number as our Pressure.
4) We are trying to determine free air cfm. We will call this
free air cfm air flow C.
5) The standard atmosphere PSI (lbs/cuft) of AIR is 14.7 . The standard is
based upon an air temperature of 69F. Close enough for what we need.
6) The Formula:
C = PV/(14.7T)
C = free air cfm (cuft per minute) to meet tool air demand
P = Compressor discharge Pressure at cutoff (see 3 above).
V = Compressor Tank volume in cuft (see 2 above) .
14.7 = constant (see 5 above).
T = time in decimal minutes are read from our stop watch.
7) The process:
With stop watch ready bleed off the air compressor until it reaches
0 psi. Start the air compressor and stop watch simultaneously. Stop
the stop watch when the air compressor reaches its cut off psi.
Record the Time in decimal minutes.
An example:I bought a used air compressor with a 60 gallon tank, 130 psi cutoff
pressure.
It has 220v single phase 15 amp 5hp (5hp bull not 23-25amps) electric motor.
What is its free air cpm?
P = 130 psi V= 8.02 cuft T = 6.00 minutes
C = PV/(14.7T)
C = (130 x 8.02) / (14.7 x 6.00)
C = 11.82 of free air cpm ready to supply my yet to be purchased
air tools.
How accurate was my estimate. I contacted the manufacture C-aire custom made
in Minnesota and they said the scfm for the unit was 11.9 . Pretty close.
Proves the pump is still fairly tight.
How much free air cpm does your compressor have?
I paid $300 USD for this commercial body shop compressor that once
supported two paint guns, a tire changer, impact wrenches and 1/2 air drill.
Not all at the same time.
It was low RPMs and fairly quiet as compared to his 7.5 hp 25+ cpm 80 gallon
Ingersoll-Rand. An ok (not great) price for a 10 year old compressor that did
not see constant use.
Rob Sexmith
Rebel 722R awaiting delivery
rsexmith@fuse.net
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