Grant Erwin

2003-12-22 15:07:55 UTC

I have been thinking about the issue of model equivalency between an air

regulator and a transformer again. Recall that I had postulated that the

equations of air flow and pressure with respect to an air regulator might

be similar to the equations of electricity flow and pressure (amps/volts)

with respect to a transformer. For an ideal transformer, of course, the

product of amps and volts on either side of the transformer is constant.

Thus I had postulated that if air were flowing e.g. at 10 cfm at 180 psi

and it were regulated down to 90 psi through an (unachievable) ideal

regulator, the output would be 20 cfm at 90 psi.

That discussion generated much heat but little light some months ago (GTA).

Many people felt that if you have an air compressor which can generate

e.g. 10 cfm into 90 psi that you cannot ever get more than 10 cfm out of it

no matter what. (It is possible that they didn't feel this way, but that is

what I perceived, but as usual I may have been wrong.)

As an interesting corollary to this discussion I just found an interesting

equation which I had not known, which is the mathematical relationship between

SCFM and CFM when the air pressure is expressed in pounds per square inch (psi):

SCFM = CFM * SQRT[(Pg + 14.7)/14.7] ;; Pg expressed in psi

For example if a compressor is rated to deliver 10 cfm into 90 psi then it

could equivalently be rated to deliver approximately 26 SCFM. So beware of

SCFM ratings unless you have the above equation handy!

(In case anyone is curious, I got the above equation from a Sylvania Web page:

http://www.sylvania.com/pmc/heaters/air/using.htm)

I propose an experiment: an air regulator with an airflow meter on either

side of it, plumbed in series with it. If I attached my compressor to the

input of this and a big air grinder (running unloaded) or some other fairly

constant load, let the tank charge up fully and then open the tank valve

to energize the system, let the system stabilize and then pull the trigger

on the air grinder and take readings off the airflow meters for several

different regulator settings, that should produce data which would either

support or refute my postulated equations of flow/pressure on either side

of the regulator.

So. Does my proposed experiment make sense? Does anyone have a couple of

suitable airflow meters they would like to loan me or sell me real cheap?

Grant Erwin

Kirkland, Washington

regulator and a transformer again. Recall that I had postulated that the

equations of air flow and pressure with respect to an air regulator might

be similar to the equations of electricity flow and pressure (amps/volts)

with respect to a transformer. For an ideal transformer, of course, the

product of amps and volts on either side of the transformer is constant.

Thus I had postulated that if air were flowing e.g. at 10 cfm at 180 psi

and it were regulated down to 90 psi through an (unachievable) ideal

regulator, the output would be 20 cfm at 90 psi.

That discussion generated much heat but little light some months ago (GTA).

Many people felt that if you have an air compressor which can generate

e.g. 10 cfm into 90 psi that you cannot ever get more than 10 cfm out of it

no matter what. (It is possible that they didn't feel this way, but that is

what I perceived, but as usual I may have been wrong.)

As an interesting corollary to this discussion I just found an interesting

equation which I had not known, which is the mathematical relationship between

SCFM and CFM when the air pressure is expressed in pounds per square inch (psi):

SCFM = CFM * SQRT[(Pg + 14.7)/14.7] ;; Pg expressed in psi

For example if a compressor is rated to deliver 10 cfm into 90 psi then it

could equivalently be rated to deliver approximately 26 SCFM. So beware of

SCFM ratings unless you have the above equation handy!

(In case anyone is curious, I got the above equation from a Sylvania Web page:

http://www.sylvania.com/pmc/heaters/air/using.htm)

I propose an experiment: an air regulator with an airflow meter on either

side of it, plumbed in series with it. If I attached my compressor to the

input of this and a big air grinder (running unloaded) or some other fairly

constant load, let the tank charge up fully and then open the tank valve

to energize the system, let the system stabilize and then pull the trigger

on the air grinder and take readings off the airflow meters for several

different regulator settings, that should produce data which would either

support or refute my postulated equations of flow/pressure on either side

of the regulator.

So. Does my proposed experiment make sense? Does anyone have a couple of

suitable airflow meters they would like to loan me or sell me real cheap?

Grant Erwin

Kirkland, Washington