[TheForge] air compressors, CFM, and psig

[Bruce Freeman]

Terry,

Volume is inversely proportional to pressure.  This can be written,

P1 x V1 = P2 x V2

If solving for volume, rearrange:

V1 = V2 x P2/P1

To make the numbers come out right, you have to use absolute pressure
units, like psia (psi absolute) instead of relative pressure units
like psig (psi gauge).  The conversion is:

P(in psia) = P(in psig) + 14.7psia

Where 14.7 psia is atmospheric pressure.  (If you were in Denver, you
might want to use a smaller value here.)

You have the I-R at 50 CFM @ 175 psig = 189.7 psia.  (Round off if you
like to 190.)

CFM is a volume per unit time measurement and need not be converted to
anything else, but can be used in place of V in the above formula.

This gives us:

V at 90psig = 50 CFM x (175+14.7)/(90+14.7) = 90.6 CFM

V at 40psig = 50 CFM x (175+14.7)/(40+14.7) = 173.4 CFM





On Wed, May 6, 2009 at 5:38 PM, terry l. ridder <terrylr at blauedonau.com> wrote:
> hello;
>
>
> i am still researching air compressors and have not found a good
> explanation or chart for determining the CFM for a given psig for an air
> compressor.
>
> for any given air compressor take the last one i last below, the
> ingersoll-rand, is 50 CFM at 175 psig. what does that equate to at 40
> psig? what does that equate to at 90 psig?
>
> the 40 psig is what my paint sprayer works at.
> the 90 psig is what my 3/8inch air ratchet works at.
>
> air compressor specifications generally list
>
> IndustrialAir 80 Gallon Hi-Flo Single Stage Air Compressor
> http://www.farmandfleet.com/catalog/product.aspx?i=605037&h=050104001
> <begin quote>
> IndustrialAir
> 80 Gallon Hi-Flo Single Stage Air Comp
> Mfr #: ILA4708065
> Blain #: 605037     Price: $778.99
>
> Capacity: 80 gallon tank
> Maximum pressure: 155 PSI
> CFM @ 40 PSI: 16.1
> CFM @ 90 PSI: 14.0
> Volts: 208-240
> Power: 4.7 HP
> Dimensions: 31.5" x 34.75" x 77"
> Weight: 540 lb
> UL & CSA certified
> <end   quote>
>
> IndustrialAir 80 Gallon 2 - Stage Industrial Compressor
> http://www.farmandfleet.com/catalog/product.aspx?i=605035&h=050104001
> <begin quote>
> IndustrialAir
> 80 Gallon 2 - Stage Industrial Compressor
> Mfr #: IV5248069
> Blain #: 605035     Price: $1,269.99
>                Sale Price: $1,199.99 Through 5/10/2009
>
> Capacity: 80 gallon tank
> Maximum pressure: 175 PSI
> CFM @ 100 PSI: 17.5
> CFM @ 175 PSI: 15.8
> Volts: 208-240
> Phase: single
> Power: 5.2 HP
> Dimensions: 31.5" x 34.75" x 77"
> Weight: 610 lb
> <end   quote>
>
> Ingersoll Rand Electric Stationary Air Compressor
> http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200319399_200319399
> <begin quote>
> Ingersoll Rand Electric Stationary Air Compressor (Fully Packaged) -
> 15 HP, 50 CFM At 175 PSI, 230 Volts
> Item# 1592042C
>  Was $4991.99
> Sale $4989.99 <--big sale a whole $2.00 usd off the regular price.
>
> Max. PSI    175
> Volts   230
> Amps    39
> HP  15
> Motor Phase(s)  3
> Stages  2
> Air Tank Size (gal.)    120
> Tank Description    Horizontal
> Pump    Two stage, splash lubricated, belt-driven
> Air Outlet Size (in.)   1
> Drain System    Automatic
> Portable or Stationary  Stationary
> Dimensions L x W x H (in.)  78 x 30 x 56
> Ship Wt. 1117.0 lbs
> <end   quote>
>
> sidenote: nearly every air compressor manufactuer makes the same mistake
> and just use 'psi' when it should either be 'psig' for pounds per square
> inch gauge or 'psia' for pounds per square inch absolute. just using psi
> is meaningless.
>
> what is a type-30 air compressor? northern tools and other web sites
> throw that term around but never define it.
>
>
> --
> terry l. ridder ><>
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-- 
Bruce
NJ

The total lack of evidence is the surest sign that the conspiracy is working.