Make-up air for air compressor

[highlander11]

Hi all,

I have an air compressor (7.5 HP, 16.8 acfm @ 175 psi) in a confined space. I was thinking of putting a small louver in the exterior wall for make-up air. How big should my louver be? Or what is the required free area? Thanks.

 

[NickE]

use P1/V1=P2/V2 to find out how much volume you need per min. at STP.
you may find that it really isnt too much. then it should not be too hard to convert your needed Absolute cfm to an areal flow rate. Then just size your louver 200% and no troubles, even when cottonwood and other fluff fills the screens.

 

[highlander11]

Nick, are you using P1V1/nRT1 = P2V2/nRT2 ? If T1 = T2, then P1V1=P2V2. But for the air compressor, isn't T1<>T2 ?

 

[NickE]

I dont really know for sure, just suggesting a good method. I suppose that the outlet air of the compressor is hot and thus takes more volume, however I was figuring this was a compressor/tank etc. setup. Usually those seem to give delivery pressure and flowrate for use in tools and such and then it would be at 70F on the outlet.

 

[zdas04]

highlander,
I think you are confusing temperature rise across the compressor with the conversion from ACF to SCF. At sea level on a 60F day, ACF = SCF. In Garfield County Colorado(one well in that county sits at 10,000 ft and has an atmospheric pressure of 10.1 psia) on a -30F day then the conversion is:

SCF = ACF * (10.1*(460+60))/(14.73*(460-30) = ACF * 0.829

The reason for the conversion that flow equations are stated in SCF. Your inlet plennum should be sized for either a specific pressure drop or a specific velocity. I like to use velocity less than 5 ft/sec on the suction side of an air compressor (this is really arbitrary, I picked it up from a grey-haired old fart early in my career) so at sea level I get 3.2 ft/s in 4-inch for your 16.8 SCF/m. If you want to go with NickE's advice of doubling the size, a 6-inch would give you 1.4 ft/sec.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[crjones]

As the little recip being asked about the 16.8 acfm is the inlet. Take the acfm times 1.5 for make up air and remember the louvers are only 60% flow area and will have a pressure drop if you are not careful in your selection. Another concern in your confined space is temperature. The elevated temperature will effect performance and bearing life.