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Air Compressor Sizing Assistance Please

Air Compressor Sizing Assistance Please

Air Compressor Sizing Assistance Please

Hello There,

This is probably such an easy question I apologize for wasting anyones time on it. I really cannot figure out what size of compressor/pump i would require if my customer has come to ask me for a duplex lead/lag system that has sufficient capacity to supply 500scfm @ 60 psig for 5 seconds, with one minute intervals, then supply it again for another 5 seconds, etc.  
My compressor/pump will be supplying 100 psig air to the receiver.  It is our goal to mount both to a horizontal tank, and connect a desiccant dryer to get instrument air prior to supplying the equipment requiring the air.  If anyone can help me out, I would truely be so happy.  
best Regards,
Kris mcpherson
Central Air Equipment,
Calgary, Alberta

RE: Air Compressor Sizing Assistance Please

The average flow your system is 41 2/3 scfm.  That will give you a flow of 500 scfm for 5 seconds and then 55 seconds of no flow followed by another cycle.  So, the makeup compressor to your receiver will be at least this large, maybe round up and call it 50 scfm since it will be running all the time (or at least until the receiver is up to 100 psig or the maximum pressure)

During the 'use' period, the air receiver will supply the bulk of the air as its pressure drops from 100 psig to some value above 60 psig.  You can't go all the way to 60 psig since you will have some sort of pressure controller to drop from the tank pressure to your 60 psig IA supply pressure, allow for some dP through your drying system, a margin of safety, etc.  Let's say the minimum pressure in the tank at the end of a cycle is 70 psig.

During those 5 seconds of use, the demand is 500 scfm or 41 2/3 scf.  Your makeup compressor is still running at say 50 scfm so it supplies 4.16 scf to the tank leaving the balance to be supplied by the tank itself (37.5 scf).  37.5 scf is just a certain number of moles of air.  Calculate the size of the tank needed to supply this volume of air as it drops from 100 psig to 70 psig using the ideal gas law.

If your air drier uses some air in the drying process (a PSA type of system), you'll need to include that air consumption figure as well.  These numbers are just some that I came up with to walk you through the concept, you'll need to determine what makes sense in your case.

RE: Air Compressor Sizing Assistance Please

Hi TD2K,

You are a very smart person, and I hope someone is paing you big bucks for those brains!!!!
Thanks again, I have given this a purple star!!!
Have a great Weekend!

Best Regards,
Kris McPherson
Central Air Equipment, Calgary

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