Air compressor - a bit more technical

Old 05-16-06, 08:38 AM
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Air compressor - a bit more technical

Hey all,

While looking for a decent air compressor to buy, I have started drilling more into it and confused myself with some technical questions:

1. Assuming I have a 50lt compressor capable of 250lt/Min @ 90psi. Simple math says that I can use a 9cfm airtool for 12 sec at 90psi before it consumes all the air in the tank (1/5 of a minute). Is that correct? Using the same math it means I can use the same tool (9cfm) for 36 sec at 30psi. Did I get it right? So little? It makes no sense!

2. What makes the litre/Min in a compressor? I came across so many compressors lately and surprisingly some have the same basic specs but different free air delivery. For example, two 2.5hp/40lt air compressors, one can deliver 6cfm and another 12cfm. Is this just a big lie? If not, what makes the difference in the free air delivery capabilities?

3. Is there any other benefit to belt-driven air compressors aside for reduced noise? I came across a belt-driven 2.5hp/50lt/7cfm air compressor. Why would this one be better than a direct-drive compressor?

Any answer will be very appreciated ..

Old 05-16-06, 10:21 AM
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Here's what's missing: The supply pressure does not remain constant with continual use unless the compressor can keep up with the demand.

Let's say that you have a 6 cfm @ 90 psi compressor attached to a 12 cf tank that has been filled to 120 psi and nothing is demanding air supply. Assume that a pressure regulator on the output limits the pressure to 90 psi. Assume the compressor comes on below 100 psi and shuts off at 120 psi.

Connect a 9 cfm tool and fully open the valve on the tool. The tool will see 9 cfm @ 90 psi UNTIL the pressure in the tank drops below 100 psi. The compressor cycles on but the pressure continues to fall since demand exceeds what the compressor can supply. Continued use of the tool will stabilize the supply at a pressure below 90 psi while the tool and compressor continue to run. The tool will lose power or volume since it will now be running at 9 cfm but only 30-40 psi.

If the tank is larger, the initial run of the tool will be longer, but it will take the compressor longer to refill the larger tank.
Old 05-16-06, 10:43 AM
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advantage of a belt drive

Less noise, yes. But more importantly you can easily replace/upgrade either the pump or the motor or both. Not so easy with a direct drive. You can also play with the flywheel sizes to slow down or speed up the pump, where as with a direct drive you are destined to run the compressor at the same speed for the life of it. The disadvantage is that you have to tighten the belt periodically.

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