Subject: SCFM, how to determine?

From: [email-masked] (Jay McMunn)
Date: 4 Jul 1996 06:50:50 GMT

I am involved in a project where we supply the customer with an
air compressor/dryer system as part of the package he is buying.
Our company is selling him a HUGE compressor, in my opinion, for
the real requirements of the machine that uses compressed air.
Can someone tell me how to calculate the SCFM that the machine is
using if I run it from a known tank size and known off/on pressures, 
and using the cycle time of the compressor?

Any help appreciated,

Jay

---

From: [email-masked]
Date: Thu, 04 Jul 1996 23:26:39 GMT

>and using the cycle time of the compressor?

There are several ways to accomplish this task, but I will only
describe one method.  

Step 1.  Calculate the volume of the tank and piping system.  You said
that you know the volume of the tank, and the piping system may or may
not be significant.  

Step 2.  Calculate the volume of air contained in the system.  This is
easily done by converting the pressure to atmospheres of pressure
first.  An atmosphere is roughly equal to 15 psia at sea level.  The
volume of air in the system =  P(atm) * V(cuft.)

Step 3.  There are two states that are important.  The high pressure
point where the compressor is off, and the low pressure point when the
compressor comes on.  You must calculate the air contained in the
system at both points.  The difference between those two volumes is
the amount that is used by the pneumatic device.  

Step 4.  The flowrate is calculated most easily when the compressor is
off.  Allow the compressor to reach maximum cutoff pressure.  When the
compressor cuts off, start a stopwatch.  Time the interval until the
compressor restarts.  Convert the time to minutes.  

Step 5.  Calculate the flowrate by dividing the volume of air used by
the pneumatic device from step 3 by the time in minutes from step 4.
This value will be the average usage rate of the pneumatic device in
SCFM.  

There are other ways of getting more precise measurements, but this
method may be adequate for your usage.  

J. M. Stuart