Time to Fill a Tank

Making a few assumptions: the compressor is probably rated at pumping 2.41 m³/minute at 1 bar (this is FAD, or Free Air Delivery), and you're filling a 1 m³ tank to about 7 bar, (which means you're adding about 7 m³ of air at 1 bar).

This suggests your measured time is about right.

<...the compressor should take the time in terms of second and not a minute....>

On a process plant, if it happens in a second, it isn't real.

Flow rate = Volume / Time

Time = Volume / Flow Rate = 1 m³ / (2.41 m³ / min.) = (1/ 2.41) min. = (60/2.41) sec. = 24.896 seconds

But the flow rate quoted is (almost certainly) relating to the volume/time at 1 bar.

Agree with John DG.

7.5Kg/cm2=121.37 PSIA.When the tank is full you a 35.0 CU Ft of 121.37 PSIA air.

p1*v1=p2*v2(neglecting change in temperature)

So 121.37*35=V(scf)*14.7

So scf=289.0

If compressor delivers 80SCF/m then minutes=289/80=3.6 minutes.

SCFM ordinarily means Standard Cubic Feet per Minute, and usually (but not always, in reference to compressors) refers to air at 1 bar. Your tank will be at about 7 bar, so will contain 7 x 35 SCF = 245 SCF. Your fill time seems about correct (although a 35 cf tank is very large). The air in the receiver tank will be at elevated temperature, so will not actually contain 245 SCF when the compressor shuts off.

Compressors displace different masses of air at different pressures, so, for example, the compressor in the example (in the link) delivers 11.1 cfm at 7.5 bar and 8.2 cfm at 10 bar.

I hope this will make sense to you, you will need to know the rated cfm of your compressors and then you will be able to see if your units are operating efficiently.

I had sent this to a poster much earlier who was contemplating adding another compressor to his plant to improve his current situation.

#1 Concern is certainly leaks in your system.

#2 Current machines efficiency, you really need to know that you are not adding two other compressors into the mix with units that are in need of repair. For the price of a new unit you may be able to repair what you already have.

DO THIS SIMPLE TEST FIRST. I copied this a while back for checking compressors.

The compressor must feed a receiver or tank of a known volume. (stamped tag)
To conduct the test, close the valve that isolates the receiver from the plant's distribution system and turn off the compressor. Vent the receiver down to approximately 30 psi and turn the compressor back on. Study the pressure gauge on the receiver and use the stopwatch to time how long it takes for the pressure to build from 50 to 100 psi. Use the following formula to calculate the flow (scfm) of your compressor:
0.45 V_R t = O_C
where:
V_R is receiver volume, gal
t is time, minutes
O_C is Compressor output, scfm
Dividing the results of the test by the manufacturer's flow specification will indicate what percentage of the original capacity the compressor is operating at.
As an example, lets say a 712-hp compressor has a factory rating of 28 scfm and an 80-gal receiver. It takes 2 min and 6 sec to build the pressure from 50 to 100 psi. Then:
0.45 80 2.1 = 17.1 scfm, and
17.1 28 = 0.61
So the compressor is operating at only 61% of its original capacity. This indicates that the compressor is operating at a severely diminished capacity. This information should be carefully considered before replacing the unit.