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Author Topic:   What determines an air compressors CFM rating?
68 S-code GT
Gearhead

Posts: 605
From: Sayreville, NJ, US
Registered: Mar 2000

posted 10-25-2002 07:19 AM     Click Here to See the Profile for 68 S-code GT   Click Here to Email 68 S-code GT     Edit/Delete Message   Reply w/Quote
What determines an air compressors CFM rating? Is it the stroke and RPM? I have seen some lower horse power compressors put out more CFM than the higher ones!

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Just Strokin
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Posts: 471
From: Tuscaloosa, Alabama
Registered: Dec 2001

posted 10-25-2002 09:18 AM     Click Here to See the Profile for Just Strokin   Click Here to Email Just Strokin     Edit/Delete Message   Reply w/Quote
Generally the rpm is either 1850 or 3600 on an electric motor so it is usually not a factor.

I would think that volume of the tank and the displacement of the pump would be the determining factors along with the HP of the motor.


JMO

------------------
Larry

No fast Fords at this time but one fine cruising 96 F350 CC DRW Power Strokin diesel.
And one rusty 64 Fairlane nick-named the Rust Bucket....And sometimes called the Money Pit...

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[email protected]
Gearhead

Posts: 202
From: Lakewood, CO, USA
Registered: Jun 2001

posted 10-25-2002 01:28 PM     Click Here to See the Profile for JCQuinn@work   Click Here to Email JCQuinn@work     Edit/Delete Message   Reply w/Quote
What you have to look at is the CFM rating at what pressure. Often a high CFM rating is at low pressure.

You have to look at the CFM/psi rating of your tools to match the compressor to your tool set.

John

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68 S-code GT
Gearhead

Posts: 605
From: Sayreville, NJ, US
Registered: Mar 2000

posted 10-25-2002 10:22 PM     Click Here to See the Profile for 68 S-code GT   Click Here to Email 68 S-code GT     Edit/Delete Message   Reply w/Quote
What I have been comparing was either 40 psi or 90 psi. The only thing is that I noticed some 5hp for example have a higher cfm rating at 40 or 90 than some 6hp compressors with a larger holding tank.

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Just Strokin
Gearhead

Posts: 471
From: Tuscaloosa, Alabama
Registered: Dec 2001

posted 10-25-2002 10:59 PM     Click Here to See the Profile for Just Strokin   Click Here to Email Just Strokin     Edit/Delete Message   Reply w/Quote
If you are thinking about an HVLP paint gun, you best check the SCFM at 90psi....a lot of them require 12-16 [email protected]

I just recently bought a new compressor from Sams.....about $800 for a 2 stage, 6.5hp, [email protected] and my little [email protected] die grinder will drain it. DO NOT believe the cfm rating on some of the air tools. But boy does she run the DeVille-Blis Mellium paint gun.

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68 S-code GT
Gearhead

Posts: 605
From: Sayreville, NJ, US
Registered: Mar 2000

posted 10-25-2002 11:04 PM     Click Here to See the Profile for 68 S-code GT   Click Here to Email 68 S-code GT     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Just Strokin:
SCFM

Whats the "S" stand for?

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Just Strokin
Gearhead

Posts: 471
From: Tuscaloosa, Alabama
Registered: Dec 2001

posted 10-28-2002 07:16 AM     Click Here to See the Profile for Just Strokin   Click Here to Email Just Strokin     Edit/Delete Message   Reply w/Quote
I think it has to do with sustained cfm.

A lot of compressors will do 10, 12 15 cfm in bursts or short durations, but it takes a decent hp and a like a 2 stage pump to do sustained cfm.

Also, the more time the compressors spends off than running, the less heating of the air (moisture in the air supply verses dropping out of the compressed air into the bottom of the tank), the better off you are, especially when painting something.

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[email protected]
Gearhead

Posts: 202
From: Lakewood, CO, USA
Registered: Jun 2001

posted 10-28-2002 10:04 AM     Click Here to See the Profile for JCQuinn@work   Click Here to Email JCQuinn@work     Edit/Delete Message   Reply w/Quote
The S in SCFM means "Standard". That means the cubic foot of air is at "standard conditions" 14.7 psia and 60 degrees F.

John

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Ken&Shell
Gearhead

Posts: 468
From: Mocksville, NC
Registered: May 2000

posted 10-28-2002 04:51 PM     Click Here to See the Profile for Ken&Shell   Click Here to Email Ken&Shell     Edit/Delete Message   Reply w/Quote
first thing, Rpm on a motor can affect the cfm.. a 3600 rpm motor makes more cfm than 1850 rpm but you loose pressure making more cfm. If you go to Lowes or wherever and look at their small (cheap) ones that is what accounts for the differences in delivered cfm. A bigger tank does not change your cfm. It still only puts out the same cfm, your compressor just runs more to keep it filled. You should always know what psi you need your air delivered at before you look for your cfm rating. The two are interrelated. compressor will have a maximum cfm at a specific pressure. If you drop the pressure, you can get more cfm.

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68 S-code GT
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Posts: 605
From: Sayreville, NJ, US
Registered: Mar 2000

posted 10-28-2002 09:38 PM     Click Here to See the Profile for 68 S-code GT   Click Here to Email 68 S-code GT     Edit/Delete Message   Reply w/Quote
I?m looking for a 120V that I can use for a small blasting cabinet. The 2HP one I have is ok for tires or airbrushing. I noticed that there was a difference in the 5HP compressors with the CFM and wondered why.

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Just Strokin
Gearhead

Posts: 471
From: Tuscaloosa, Alabama
Registered: Dec 2001

posted 10-28-2002 09:51 PM     Click Here to See the Profile for Just Strokin   Click Here to Email Just Strokin     Edit/Delete Message   Reply w/Quote
The compressor I have is rates @

[email protected] and [email protected]

It keeps up with most any air tool I have, but the cutoff and diegrinder will tend to put th emost drain on it and they are only rated at [email protected]

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n2oMike
Gearhead

Posts: 1073
From: Spencer, WV
Registered: Jan 2001

posted 10-29-2002 11:03 AM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
Most electric motors nowadays are mis-rated.

First, they are 'rated' on the amount of current they consume, not put out. PLUS, this is usually rated by the large 'startup' current, not the sustained operational consumption.

746 Watts = 1 horsepower

Watts = volts x amps so, amps = Watts/volts

5hp = 5 x 746
5hp = 3730 Watts

3730 Watts/120 Volts = 31 amps!

My 120V '5hp' compressor easily runs off a 15 amp circuit. It's not even consuming 5hp worth of electricity, let alone producing 5hp worth of power!

You ever notice how HUGE an old 2hp electric motor is???? and they were used to power huge, high volume compressors! THESE were rated at their true output.

If you want a POWERFUL compressor, go with one that runs off 240V. Forget about the 120 Volt units. Plus, spring for the 2-stage compressor. These produce WAY more air than single stage units, produce less heat, and produce less moisture as well.

Lastly, when looking at compressor rpm, you not only pay attention to the motor speed, but the pully sizes on the motor and compressor as well. A higher torque motor can be geared a lot taller, and spin the compressor a lot faster (or drive a larger pump) than a lower powered unit.

Remember, TWO STAGE PUMP and 240 VOLT MOTOR for any serious work. These start around $800, but you get what you pay for.

Single stage 120V models (regardless of tank size or HP) are fine for blowing up tires and air ratchets, but if you're going to use any 'air hog' tools, such as sand blasters, drills, grinders, sanders, etc.... go for the bigger units. Else, you'll spend more time waiting than grinding. Blasting is a MAJOR pain with a single stage unit. When they start working hard, they produce a LOT of water... which freezes up the nozzle of the blaster. (but I guess it will defrost as you wait for the air to build back up.)

One more note... Larger pumps run at lower speeds produce less heat (and moisture) than smaller pumps that are spun faster. They also last longer. You can even get compressors that are equipped with intercoolers now.

Good Luck!

------------------
Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

[This message has been edited by n2oMike (edited 10-29-2002).]

[This message has been edited by n2oMike (edited 10-29-2002).]

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DutchD58
Journeyman

Posts: 97
From: West Milton, OH
Registered: Jul 2000

posted 11-03-2002 07:56 PM     Click Here to See the Profile for DutchD58   Click Here to Email DutchD58     Edit/Delete Message   Reply w/Quote
I bought an Ingersoll-Rand single-stage compressor back in May. It has a true 5hp, 220V motor (23 amps) with a 100% duty cycle and a 5000hr rating, 80 gallon tank and is rated at 18.1 cfm @ 90 psi(and over 20cfm @ 40 psi) and 135psi max. I've run an HVLP gun, die grinder and other tools and it's never even broken into a sweat. Not bad for $750 from Tractor Supply. The only larger compressor they carried was a 2 stagr I-R, but it was nearly double the price. I figured I could buy a few tools with $750 saved............
As far as air consumption of die grinders and some other tools (air files, DA's, etc), many times if you read the fine print in the ratings, they'll tell you that a tool (die grinder for instance) will be rated at an average of 4cfm...........for a 15 second burst!! That equals 16cfm @ 100% duty.

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68 S-code GT
Gearhead

Posts: 605
From: Sayreville, NJ, US
Registered: Mar 2000

posted 11-06-2002 07:23 AM     Click Here to See the Profile for 68 S-code GT   Click Here to Email 68 S-code GT     Edit/Delete Message   Reply w/Quote
My Dad has a nice 2-stage Ingersoll-Rand if I really need to paint of some thing, plus he set up for painting. I figure the cost for one of these compressors along with running a 220 line out to the garage would leave me little for my cars. I was just trying to figure out why there is such a variety of CFM outputs with comparable HP and size tanks?

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