[TheForge] air compressors, CFM, and psig
marc3rd
marc3rd at marc3rd.com
Wed May 6 21:39:06 EDT 2009
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 ><>
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>
>
>
> --
> Bruce
> NJ
>
> The total lack of evidence is the surest sign that the conspiracy is
> working.
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> --
> terry l. ridder ><>
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