[TheForge] air compressors, CFM, and psig

Wed May 6 22:01:44 EDT 2009

This all reminds me of going to work at my actual job.   All of this is a 
remarkable treatise on the theory of work and how to accomplish it, but 
there just isn't a damn thing actually getting done.

CraigS
Gresham, OR
----- Original Message ----- 
From: "marc3rd" <marc3rd at marc3rd.com>
To: "terry l. ridder" <terrylr at blauedonau.com>; "Blacksmithing List 
Sponsored by ABANA" <theforge at mailman.qth.net>
Sent: Wednesday, May 06, 2009 6:39 PM
Subject: Re: [TheForge] air compressors, CFM, and psig

> This sounds good on paper, but in reality I aren't there other factors?
> If I have a cylinder that is 10 cubic inches with the cyl retracted and 1
> cubic inch when extended. at the max pressure that can be achieved is 10
> times atmospheric.  so lets say 100 psi
> That 100 psi is going to move out of that cylinder and into the tank much
> faster and more completely when that tank is at atmospheric than when it 
> has
> 95 psi. In fact when it finishes its stroke and starts to return it should
> have 95 psi in it. So wouldn't it only lose 5% of its volume and there for
> only be refilled at retracted position with 5%. So wouldn't that make the
> efficiency 5% at that point?
> So I guess what I am saying is the efficiency of the system decreases as 
> the
> tank pressure increases. and there are a lot of factors that effect it
> including the orifice size and tube size between the cyl and tank, the
> resistance of the check valve,
>
> --------------------------------------------------
> From: "terry l. ridder" <terrylr at blauedonau.com>
> Sent: Wednesday, May 06, 2009 9:01 PM
> To: "Blacksmithing List Sponsored by ABANA" <theforge at mailman.qth.net>
> Subject: Re: [TheForge] air compressors, CFM, and psig
>
>> 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 ><>
>>> ______________________________________________________________
>>> TheForge mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/theforge
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:TheForge at mailman.qth.net
>>>
>>> TheForge mail list group photo site is
>>> http://www.photoaccess.com
>>> Login: blacksmithblacksmith at hotmail.com
>>> Password: anvil
>>>
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>>>
>>
>>
>>
>> -- 
>> Bruce
>> NJ
>>
>> The total lack of evidence is the surest sign that the conspiracy is
>> working.
>> ______________________________________________________________
>> TheForge mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/theforge
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:TheForge at mailman.qth.net
>>
>> TheForge mail list group photo site is
>> http://www.photoaccess.com
>> Login: blacksmithblacksmith at hotmail.com
>> Password: anvil
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>> -- 
>> terry l. ridder ><>
>
>
>
>> ______________________________________________________________
>> TheForge mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/theforge
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:TheForge at mailman.qth.net
>>
>> TheForge mail list group photo site is
>> http://www.photoaccess.com
>> Login: blacksmithblacksmith at hotmail.com
>> Password: anvil
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>
> ______________________________________________________________
> TheForge mailing list
> Home: http://mailman.qth.net/mailman/listinfo/theforge
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:TheForge at mailman.qth.net
>
> TheForge mail list group photo site is
> http://www.photoaccess.com
> Login: blacksmithblacksmith at hotmail.com
> Password: anvil
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> 