When designing a compressed air pipeline, remember that size matters. The internal size of the pipe, along with the pressure and air volume, determine the velocity of the compressed air in the pipeline. The higher the velocity, the higher the pressure drop in the pipe work from the compressor room to the point of use. High-pressure drops increase your energy costs and affect production rates.
What generally happens is that shop floor staff complain about low compressed air pressure. This leads to operators increasing the air compressor’s discharge pressure, which in turn means that for every 1 bar increase in pressure, you are paying an extra 7% on your power bill. The increase in pressure will also increase the discharge temperature and the wear and tear on the compressor, and may also affect the air/oil reclaimer life span.
This is when bigger is better. Do not use 15 m/s as a guide for main line sizing. Use the 6 m/s selection guide. These guides are for bar applications, which will change if the pressure is lower or higher. Slower air velocities equate to lower pressure drops. This means better compressor performance and less power wasted. Use the higher speed chart for very short air take-off points only, nothing else.
The charts do not show the pressure drops for valves, bends, tee pieces or flex hoses. These charts are merely a guide – treat them cautiously. The manufacturers of compressors often set a bad example for pipe sizing. We frequently see very small undersized outlets on air compressor packages. This suits the manufacturer as the piping is cheaper and easier to fit into tightly engineered compressor packages.
Check the charts above and oversize the piping accordingly, especially in the compressor room. Low speed equates to less back pressure. High velocities and bad pipe design lead to compressors bouncing from load to unload. This is not good for the controls, the filtration system, the air dryer or the compressor system itself.
Use a correctly sized air receiver to act as a buffer between the compressor and its ancillaries, such as dryer and filters. This will reduce your load and offload cycles and improve compressed air quality. High velocities and pressure drops in the compressor room frequently lead to the air compressor fighting for lead position or control. If possible, install a compressor controller where you determine the lead and lag air compressor as it will generally save you a small fortune on energy costs, compressor repairs and maintenance. Lower air speeds also improve compressed air filtration and reduce pressure drops across the filter bank.
Call us for a copy of ‘Compressed Air Piping Design Hints’.
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