Closed Circuit Air Line

On a simple rule-of-thumb basis of 30 ft/s allowable velocity, this sounds okay. Depending on where the usage branches are located, your main header could be reduced toward the far end of the loop from the compressors. Just add up the usage flows in each segment of the main header, and make each segment large enough to keep the velocity ≤ 30 ft/s. Note that at 6 bar, your intake air of 230 cfm will reduce to about 40 cfm.

If your compressors operate for a considerable time before using the air in intermittent large blasts, these quickie calculations will need adjustment.

Copied and pasted pipe sizing information for you. Tornado's rule of thumb will work. I would carry the header full size and cap at the end with 2 x 2 x 1 black malleable Tees at the user points. I would also recommend installation of a receiver - 80-120 gallon capacity depending on the frequency of air usage to reduce compressor run time and avoid frequent compressor start/stops.

Suggested Pipe Size for Compressed Air Flow at 100 PSI Length of Run, Feet

* On a compressed air distribution system, pressure losses greater than 3% are considered excessive, and a well-designed system having a steady rate of air flow is usually designed for not more than a 1% loss or 1 PSI for a 100 PSI system. The pipe size depends not only on the volume of air flow but how far it must be carried. To hold the distribution loss to 1 PSI, pipes of larger diameter must be used on longer runs to carry the same flow that can be handled by smaller pipes on shorter runs.

Figures in the body of the chart above are pipe sizes recommended on a 100 PSI system to carry air with less than 1 PSI loss. When measuring lengths of runs, add 5 feet of length for each pipe fitting. If carrying 120 PSI pressure these sizes will carry slightly more air than shown, or pressure loss will be slightly less than 1 PSI. If carrying 80 PSI pressure these pipes will carry slightly less air at 1 PSI pressure loss than shown in the chart.

The left column of the chart shows the volume of air to be carried. It is difficult to estimate the air flow volume to be carried in each leg of the distribution system. This varies with the application. On some applications, like in a large plant with many legs in the distribution system serving dozens of air-operated machines, the air usage may be at a fairly steady rate. Other applications, usually on small systems, may have to carry a high surge of air if several machines happen to be operated at the same time. Then there may be a period with almost no flow.

To make a realistic estimate of air flow volume, the far right column of the chart showing compressor HP may be used. On steady pumping, a compressor will produce a minimum of 4 SCFM air flow for each 1 HP of capacity. This is a conservative figure, as most compressors will produce 5 or 6 SCFM. For example, a 25 HP compressor will produce at least 100 SCFM of air. This is shown in the far left column on the same line as 25 HP.

With that type of volumes, pressures and especially the distances it is mandatory for you to use the proper configuration for user lines. This entails not simply piping in a "Tee" and dropping from there. In order to keep the user line water to a minimum the branch of the tee must go up to an elbow, across to another elbow and down towards the user line. At the bottom of the user line pipe install a tee with a dead leg, a auto drain valve and a valve tee'd to collect moisture. Next is a compressed air filter on the user line, a regulator if pressure drops are required and finally the user equipment,

If this is not followed you will get water in the compressed air which will be havoc to the user equipment. By going up from the main line the moisture remains in the main line where it can be drained out with drain valves along the main line.

Good Luck, Old Salt