How to calculate compressed air requirement for a plant?

If you do the calculations, DON'T FORGET TO ADD THE VOLUME OF THE FLOW LINES BETWEEN THE VALVE and THE ACTUATOR. That is required volume each cycle and can be significant when actuators and valves are some distance apart.

Good point, Bud!

The other "gotcha" to watch out for, especially when using the manufacturers specifications is whether the flow is average or peak. For example, a machine may have several pneumatic devices that work independently as product flows through the machine. A single piece may not trigger all the devices at once, but a constant flow of product (up to the maximum throughput rate of the machine) will keep all pneumatics busy and consuming air pressure. A manufacturer may decide that the duty cycle of the machine normally will be about 50% on and 50% off, and rate the flow based on the average consumption, but in practice the machine may be pushed hard and approach 100% duty cycle at times. The same can be said for multiple machines in a production system. Normally they may consume air pressure at different times, but occasionally their "on" times might coincide, causing a temporary peak in consumption.

Should your compressor capacity be sized for this peak? Well, that depends on how you want to spend your money and what you believe will be the true duty cycle of your overall system. Larger compressors cost more money, require larger main lines, take up more floor space, etc. An alternative, and why you see them mounted on most small compressors, is to use a large reservoir tank. This works the same way a hybrid vehicle with a small gasoline engine, electric drive motor, and a storage battery works. A smaller power source device (engine or compressor) costs less and works more efficiently if it works continuously, rather that starting and stopping all the time. The storage unit (tank or battery) allows the consuming entity (electric motor in the hybrid or pneumatic cylinders in the machinery) to be fed and not starved, during a peak demand period (acceleration/peak production), then be replenished by the source during off-peak periods.

The other nice thing is that storage reservoir tanks can be distributed locally to feed individual machines or groups of machines. This allows the feed lines from the compressor to be kept relatively small, which can make a big difference if the equipment is on the other side of a large plant from the compressor.

When I was a manufacturing engineer installing a new production system, the system builder did not include a large enough reservoir, and underestimated the peak demand. We had product failures because pneumatic screwdrivers would stall out as air pressure dropped during peak usage, and they did not complete the installation of screws to the proper torque. I solved the problem by finding a very large surplus pressure tank (that had once held propane fuel) in our plant's "bone-yard" and had it refurbished. This included totally airing out to remove any residual propane, removing rust (it had been sitting outside), pressure checking and welding shut any leaks, installing new fittings and re-painting it to match the other equipment on the line (from green to dark grey). No more low torque screws!