Suppose I have nine machines in total which require a continuous supply of air at 2.5-3 kg/cm2. So, can I get maximum safe working pressure of the compressor by working on the formula of circumferential and longitudinal stress for pressure vessels which requires parameters such as internal pressure, wall thickness and shell diameter. Secondly, how do I calculate the capacity and HP of compressor which would suit my requirement. Also, at some websites, the CFM(cubic feet per minute) is explained as mass of 1 cubic feet of air which a compressor breathes per minute. But isn't it volumetric flow rate ?
You've got the 2.5-3 kg/cm^2 which is the pressure; approximately 2.5-3 bar. You still need to find the volumetric or mass flow too to fully determine your requirements.
Don't try to go with stress calculations if you're shopping for a compressor, and not building one from scratch. Get one rated for pressure you require with a healthy margin - not a hard thing with 3 bar required; most available on the market can easily provide more. Since you need continuous supply, tank volume is secondary - you really need the flow to exceed the requirements.
The CFM measurement is expressed that way because the flow will change with pressure - if the compressor takes 100 cubic feet of air per minute in, it will output 50 cubic feet of air per minute at 2 bar, or 25 cubic feet per minute at 4 bar. By using mass equivalent you can easily convert your pressure and volume requirement to CFM - just multiply the volumetric flow by pressure in bar, to get the multiplier required on the intake.