[TheForge] air compressors, CFM, and psig

terry l. ridder terrylr at blauedonau.com
Wed May 6 21:01:07 EDT 2009 

hello bruce;

that is what i originally thought but after graphing it and thinking
more about it that is not the case. the reason it is not the case is
that there is the 'time' factor that changes the relationship.

the 'time' factor has to be taking several other factors into account
other than the ones i can come up with.

assume for a amount that we are dealing with a single cylinder
reciprocating air compressor. the single cylinder hasa set volume that
is is able to compress per unit time. the reciprocating air compressor
has a fixed revolutions per minute when it is operating. most
reciprocating air compressors do not have the smarts to have a variable
rpm depending on the demand. there are some that have multiple cylinders
and are able to use either 2, 4, or 6 cylinders depending on the demand.
that type is beyond the scope of my original question.

the amount of air that can be compressed in one minute is the cylinder
displacement multipled by the revolutions per minute.

the cylinder has a diameter of 3 inches and a stroke of 2 inches.
this is a displacement of 14.13 cubic inches.
assume that the normal rpm is 1050.
14,13 cubic inches * 1050 rpm == 14,836 cubic inches per minute

1 cubic ft = 1,720 cubic inches

8.59 cubic feet per minute

the 'cfm' should be close to this even when the pressure varies.

140 PSI Twin Cylinder Air Compressor Pump for 3 HP Motor
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93785
<begin quote>
Construction Cast Aluminum Pump 
Cast Iron Flywheel 
Compressor Type Single Stage Twin Cylinder 
Maximum Pressure 145 PSI 
Maximum Speed 1200 RPM 
Lubrication Splash Oil type with Oil Level Window & Ball Bearing 
Check Release Valve 
Air Delivery Rate 
at 1050 RPM 
15.1 cfm at 40 psi 
13.5 cfm at 90 psi 
12.3 cfm at 115 psi 
Pulley 
Dimensions 
14-1/2" Dia. x 1-15/16"W 
with dual V-grooves for belts 
Piston 
Dimensions 80mm Dia. x 60mm stroke 
Overall 
Dimensions 15"L x 11-1/2"W x 17-1/4"H Net Weight 58.45 lbs.
<end   quote>

the equation has the limits . the low end is 0 psia and the high end is
the rated maximum psia for the reciprocating air compressor. in this
case 175 psig (189.7 psia). at 14.7 psia the equation should be 0
(zero).


On Wed, 6 May 2009, Bruce Freeman wrote:

> 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.
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