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# Compressed air system

## Compressed air system

(OP)
I need help to design/size compressed air system that includes compressor, air dryer and receiver tank. Given data are the air demand, air pressure and running time to start an engine. It would be great, if you direct me an example on how to size air compressor, air dryer, and receiver tank.

### RE: Compressed air system

The first, second, and third thing you need to do is to understand flow numbers into the compressor, demand, and how they relate. Terminology alone will kill you if you don't have rigor on your side. My approach is to convert the STUPID numbers you get from compressor manufacturers and pneumatic tool manufacturers into mass flow rate before anything else.

Air compressor manufactures want to rate their machines in ACF, which (if you filter through the cultural BS) means "actual cubic feet per minute at suction conditions". If you are at sea level and air temperature is always 60°F then that number is SCF/min. If you are at 7200 ft elevation and normal suction temperature is somewhere between -40°F and +100°F, it is not so much SCF/min. Determine the mass flow rate at the least favorable temperature.

Now figure out what your Maximum humidity will be. Convert the water vapor flow rate to a mass flow rate and subtract that number from the compressor mass flow rate.

Pneumatic tool demand is often stated as "CFM", but the definition of that number is quite murky. The main confusion is that they rarely say what inlet pressure they are using to calculate a cubic foot at actual conditions is. You have to dig it out.

Sum your loads, add a 50% safety factor for leaks (many people prefer 100% for leaks). Your air receiver should be big enough to carry you through 3 shifts without a compressor. You can do that with receiver size or with receiver pressure. You choose, you're the engineer.

If your compressor runs more often than once per shift it will be obnoxious. Size the compressor to give you 8 hours of air in less than 2 hours.

I don't have a clue if this is what you were looking for or not, but the canned "size a compressed air system" lead to bad systems (in my experience) since they never ensure that you are comparing apples to apples, not apples to lug nuts.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

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