Question about air compessors and what drives CFM

[larryq]

I have a Craftsman Pro compressor, 25 gallon tank and a 1.9 HP oiled, belt driven motor that does 150 psi max. It puts out about 6 cfm @ 90psi. Pretty good for what it is; it's no Quincy 2 stage 60 gallon but beats a pancake unit.

I see other compressors such as this one-- (Campbell Hausfeld 26 gal, model VT6271) that claim 6hp peak (ha ha) power and go to a max 135 psi, but somehow manage 10 cfm @ 90 psi? Could someone explain why a unit that maxes out at 15 psi less than another can manage so much more cfm?

Campbell Hausfeld is a respected company and while I know HP numbers are often fudged, cfm seldom is, so there must be a reason, I just don't know what it is.

 

[warweapon762]

I have a Craftsman Pro compressor, 25 gallon tank and a 1.9 HP oiled, belt driven motor that does 150 psi max. It puts out about 6 cfm @ 90psi. Pretty good for what it is; it's no Quincy 2 stage 60 gallon but beats a pancake unit.

I see other compressors such as this one-- (Campbell Hausfeld 26 gal, model VT6271) that claim 6hp peak (ha ha) power and go to a max 135 psi, but somehow manage 10 cfm @ 90 psi? Could someone explain why a unit that maxes out at 15 psi less than another can manage so much more cfm?

Campbell Hausfeld is a respected company and while I know HP numbers are often fudged, cfm seldom is, so there must be a reason, I just don't know what it is.

Pump design, likely very different dimensions in bore and stroke between the two.

 

[On-Wheel]

Don't let HP fool you .Putting a 5 hp instead of your 1.9 hp on your AC turning pump at same speed,doesn't change the out put of your pump.

Pump RPM will change out put.I have a pump that's ok running 400 rpm min. and 900 rpm max.

There's a math equation to get cfm.

 

[theoldwizard1]

What drives CFM ? Displaced volume over time. Either a bigger bore and the same RPM or the same bore and higher (pump) RPM.

I don't think there has been any "breakthroughs" in piston sealing or other things that cause losses in a long time. Synthetic oil helps.

 

[jar944]

How many amps does the sears draw vs the CH. And not the bs "peak draw"

 

[DekeT]

that claim 6hp peak (ha ha) power and go to a max 135 psi, but somehow manage 10 cfm @ 90 psi? Could someone explain why a unit that maxes out at 15 psi less than another can manage so much more cfm?

There is no functional relationship between max psi and cfm. I have seen little 12 volt cigarette lighter driven compressors have a max pushing over 150psi. Delivery rate is probably a cubic foot/hour. Max psi is mostly a cost driven point that is limited by tank specifications, piston rings, component quality etc. It is more expensive to produce and safely contain higher psi/high cfm than simply the basic piston, stroke, and horsepower concerns of higher cfm/lower max psi. High psi means nothing without some useful cfm driving it. It is cfm that does the work. Tradeoffs.

More food for thought. That higher cfm compressor may be able to reach as high of psi as the other compressor. It may be rated lower max because the manufacturer chose to use a cheaper air tank to reach a price point or pump life by keeping it within lower stress limits.

 

[guy48065]

I have a Craftsman Pro compressor, 25 gallon tank and a 1.9 HP oiled, belt driven motor that does 150 psi max. It puts out about 6 cfm @ 90psi. Pretty good for what it is; it's no Quincy 2 stage 60 gallon but beats a pancake unit.

Hmmm...compared to my HF pancake putting out 4.1CFM@90psi I don't feel too impressed by that Craftsman Pro!

Yeah--I think the numbers are often fudged.

 

[larryq]

Thanks everyone, very educational. I'm not sure how many amps my unit draws compared to the CH unit, though it's a 120v vs 240v thing so certainly mine draws fewer amps.

Hmmm...compared to my HF pancake putting out 4.1CFM@90psi I don't feel too impressed by that Craftsman Pro!

Lol, my unit is pretty solid but yes, wish those CFM numbers were a bit higher. I got it used in very nice shape so I can't complain too much. It does put out better cfm than the 20-30 gal Husky uprights at Home Depot, so that makes me feel better..

 

[Lans Company]

Air Compressors sold in Big Box stores have to go by guidelines, so they do in fact fudge the numbers..

 

[larry_g]

On a properly matched motor and pump the rule of thumb is ~4cfm per horsepower. So amps x volts = watts and ~750 watts per HP. So a 15 amp 120 v circuit will deliver at most ~ 2hp or 8 cfm. Those numbers are approximates and many factors can make them lower and not much can increase them any appreciable amount. So do the math on what the power draw is and you can make a fair guess of what the compressor can deliver. If the delivery numbers are wildly above what the math supports then call BS.

lg
no neat sig line

 

[sberry]

It works out to about 6 cfm max on a 20A circuit. 750 watts to hp is mathematical and doesn't figure for any conversion losses.

 

[jrcampbe]

To understand CFM vs pressure, it may help to think of a car engine. The displacement of the engine is the volume of air moved through the engine per revolution. The air compressor is the same. There's a piston in there that compresses the air exactly the same way. (Roughly) the volume of air at BDC is compressed into the volume at TDC, then this is released into the compressed air chamber (tank). The higher the compression ratio, the higher the pressure difference. You can have a very tiny piston with a very big compression ratio and achieve high pressure, but the size of the piston means there is very little air actually being compressed each stroke. This would be a high pressure / low flow design. Similarly, we could have a huge cylinder bore and stroke, but a very low compression ratio and have a lot of air compressed to a lower pressure.

Pressure and volume are related proportionally and this relationship is called the "ideal gas law" (PV = nRT). So a given amount of air can produce *either* more pressure or more volume (flow), but not both (unless you heat it). Different designs simply exchange pressure for volume by varying the compression ratio inside the pump. To get more pressure at the same CFM, or more CFM at the same pressure, you either need a bigger displacement or higher RPM

 

[jrcampbe]

Let me also add: if I remove your 1.9 hp motor and put a 5 hp motor, or even a 500 hp motor on your pump and run it at the SAME SPEED, the pump will put out exactly the same performance. Displacement, compression ratio, and speed of the pump determine the performance. If I double the RPM, I will roughly double the CFM. Making the motor bigger does not help unless it changes how the pump displaces air.

I will also say that rules of thumb like 6 CFM per 20A or whatever MUST assume a certain pressure, usually 90 psi.

 

[rsanter]

what drives the CFM is what is called the 'swept volume' which is a function of the bore and the stroke.

and then the RPM that the crank is spinning

bob

 

[CompressorPros.com]

There was a ruling a few years back that made compressor manufacturers advertise actual or "running" HP instead of the fake "Peak" HP. Your 6HP is most likely a 3 or 3.5HP unit. Always look at CFM instead of HP when choosing a compressor. For the max psi, most single stage (pistons the same size) will give you 140 psi or less and a two stage (two different sized pistons) compress the air twice, which will give higher pressure, typically 175 psi max. Manufacturers, especially in the consumer market definitely fudge their numbers.

 

[sberry]

I will also say that rules of thumb like 6 CFM per 20A or whatever MUST assume a certain pressure, usually 90 psi.

Yes, that is absolutely a rule of thumb aimed at small single stage 120V units. What kind of makes it a good rule is that its fairly accurate. I think the very best units in this class are about 6.5. 2 stage units are about 4 per hp and while its not perfect for these conversations most of it doesn't need to be.
The real math is great for understanding but can be a little fuzzy, the estimations are solid and good enough to make most sizing decisions by.

 

[AmishFury]

i'd say the only relation HP has to the CFM is as simple as bigger motor can run bigger pump at higher rpm... so the higher the HP the more likely it is to be paired with a pump that moves more air

it would be a waste to put the 10HP motor from the compressor at work on the pump of my little pancake compressor and i wouldn't dream of trying to operate the pump on the compressor at work on my pancake's 1.5HP motor

 

[PCMusicGuy]

Reading this thread makes me wonder why there doesn't seem to be any sort of variable air compressors on the market targeted at for the pro-sumer population. I assume its all cost/benefit driven. I've seen electric motors that have a switch between a lower speed and higher speed RPM. You could have that on a tank with an air pump capable of operating at both RPMs, and still be inside the efficiency curve for the pump at both. At the slower speed it could be making like 5 cfm @ 90 psi and at the higher speed it could do 8 cfm @ 90 psi. That way you have typical home owner air compressor uses covered, and then you can temporarily have some higher output for dabbling in things such as soda blasting and grinding.

 

[nh_yota]

Where pressure (PSI) comes into play is how much air is stored in the tank, so to speak. A greater air pressure in the tank means a greater buffer between the CFM input and CFM output.