did air compressors get weaker or did the CFM method change?

[Deuce Lee]

I'm noticing that newer 120v 1.8-2.0rhp oil lubricated air compressors are putting out about 5.8 CFM @ 90psi. (example is the two new craftsman professionals and the IR upright portable)

My dad has a mid 90's craftsman twin cyinder 120/220 (running 120v) 20 gal compessor that puts out 7.3 CFM @ 90psi.

For my garage, I just picked up a great deal on CL on a late 80's sandborn 20 gal, 3hp, 120v, twin cyinder that puts out 6.8CFM @ 90psi. $100 for the good condition AC and 50ft hose :D

Anyways, my question is, did the method to calculate CFM at 90psi change over the last two decades? or are today's compessors just getting weaker? You'd think as time goes by, that things get better and more powerful.

I do notice that the max psi has gone up, from 125/135psi to 150psi'ish. What's the deal?

[Dan Hintz]

Physics hasn't changed... what has changed is the motor's ability (i.e., design) to continue pumping enough air and the ratings they list (it looks better when they list the peak horsepower rather than the average/running horsepower).

[ian maybury]

Here's a definition of standard cubic feet per minute http://en.wikipedia.org/wiki/Standard_cubic_feet_per_minute, but I doubt that it's the method of measurement of cfm that's behind most of these situations.

Many makers of commodity equipment are basically lying left, right and centre in their literature. Or at least purposely setting out to mislead on key parameters and claims.

Some basic techniques include testing in non standard (and probably non existent in the real world) situations, and taking spot readings in situations that the equipment cannot sustain without e.g. overheating.

Even basics like HP seem to be getting smaller and wimpier. That's before we get to the patently ridiculous generalised claims made about everything. How can everything be the best, how can the cheapest still be 'professional quality'???

Somehow the more standards organisations we have eating up enormous amounts of public funds (here in Europe too) the more prevalent these practices seem to become.

It's in the end the result of lots of people with limited practical experience (with little knowledge, and not much to compare performance of what they buy to) - and of the factories becoming totally cavalier about this stuff.

It seems to be endemic, just about everything bought by the non expert buyer seems to be 'puffed' in some way or another.

My most recent experience was of buying a 'quality' European made panel saw to find it was plated as having 1kW less motor than the literature explicitly said it had. They didn't offer to take it back, despite not being able to offer a set up giving the advertised power - although they have since adjusted their literature.

Time for some honesty and integrity. Why is it that most marketing people see no problem in weasel words???? How can we hope for the world to become a better place if people more and more think overtly preying on others is a legitimate means of getting ahead?

[Will Overton]

Why is it that most marketing people see no problem in weasel words????

They are practicing to be politicians. :D

I think Dan answered the OP's question.

[Jack Clark]

My recent search for a decent air compressor revealed a couple of general guidelines.

Maximum running HP from a 120v motor cannot exceed 2. A true 3HP motor will require at least 220 volts with running amps rated at 15.

Figure compressor output at roughly 4 CFM per horsepower (again, the HP rating has to be somewhat honest).

In spite of a class action lawsuit against them, marketers are still fudging on the specifications for consumer grade compressors and shop vacs. Look for the key word "peak." Here's a pretty good definition of "peak horsepower:"

The max-claimed HP of a shop-vac, etc. is actually achievable. It is based on the max amperage the unit would draw if you jammed a screwdriver in the impeller. Just before the motor fried, you will have achieved maximum HP.

:rolleyes:

[Dan Hintz]

Maximum running HP from a 120v motor cannot exceed 2. A true 3HP motor will require at least 220 volts with running amps rated at 15.
I assume you meant a 120V motor should not exceed 2HP if run from a 15A breaker...

[Deuce Lee]

My recent search for a decent air compressor revealed a couple of general guidelines.

Maximum running HP from a 120v motor cannot exceed 2. A true 3HP motor will require at least 220 volts with running amps rated at 15.

Figure compressor output at roughly 4 CFM per horsepower (again, the HP rating has to be somewhat honest).

In spite of a class action lawsuit against them, marketers are still fudging on the specifications for consumer grade compressors and shop vacs. Look for the key word "peak." Here's a pretty good definition of "peak horsepower:"

The max-claimed HP of a shop-vac, etc. is actually achievable. It is based on the max amperage the unit would draw if you jammed a screwdriver in the impeller. Just before the motor fried, you will have achieved maximum HP.

:rolleyes:

I could see that...and that's why most compressors you see today running on 120v has under 2.0rhp. It's kind of like my buddy who though his Berhinger EP4000 has 4,000 watts rms compared to my EP2500, it's the same amp, the EP4000 is just rated at peak power, because there's no way physically a 4,000rms amp can run on 120v.

i guess my question is: Is my cheapie sandborn overrating it's 6.8 CFM at 90psi rating? J/w cus I don't have any physical way to test. And are you guys saying the 6.8CFM is probably measured in a different way then today's 2.0rhp compressors?

My original thought with my dad's compressor is that he'd need to run his at 220v to achieve the 7.3 CFM at 90psi, does that sound correct?

[Jack Clark]

Deuce,

My best guess is that the 6.8 CFM rating of your 120v Sanborn is much more accurate and honest than its 3HP rating.

It's also my understanding that converting a 120v compressor motor to run on 220v (assuming it is a motor that can be rewired for 220), will not result in higher CFM output. It will just run more efficiently (i.e., draw less amps).

[Jeff Bratt]

A guide to evaluating real air compressor performance - http://www.truetex.com/aircompressors.htm.

General rule - about 4 CFM per real HP.

My page about electric motor horsepower - http://home.roadrunner.com/~jeffnann/WoodWorking/Shop/HP/Horsepower.html.

You can run a 1 1/2 HP - or newer, high efficiency 1 3/4 HP motor - on a US standard 120V 15 amp circuit. Higher HP requires more volts, more amps, or both. There's no such thing as a 3 HP motor that runs on 120V 15A.

[Dan Hintz]

It will run at half the amps for twice the voltage... same power draw, no change in efficiency. There is no direct tie to HP versus CFM... I could use a 100HP motor with a 1" diameter piston on a 0.5" stroke and never get close to 6.8CFM. HP is a general measure of how much force/power is available... a higher HP rating means (if designed correctly), I can compress a larger quantity of air on each stroke, which can lead to a higher CFM. If not designed properly, that extra HP can mean bupkus.

[Jack Clark]

I assume you meant a 120V motor should not exceed 2HP if run from a 15A breaker...

Dan,

My assumption was that most of us don't have 30 amp breakers on our 110v circuits. Perhaps I'm wrong.

Deuce might find the information on this webpage useful:

http://sawdustmaking.com/ELECTRIC%20MOTORS/electricmotors.html

[Dan Hintz]

20A breakers are common code these days, which gives a bit more room than the older 15A code days.

[Jack Clark]

My page about electric motor horsepower - http://home.roadrunner.com/~jeffnann/WoodWorking/Shop/HP/Horsepower.html (http://home.roadrunner.com/%7Ejeffnann/WoodWorking/Shop/HP/Horsepower.html).

Let's all give Jeff a standing ovation! :D

[Jack Clark]

20A breakers are common code these days, which gives a bit more room than the older 15A code days.

No argument there. I have several myself. :)

[Deuce Lee]

ok cool...thanks for the info guys...i'll check out those links...i had a good feeling the 3hp was not running hp...i was just confused as to why newer compressors are not making the same (less) CFM as these older compressors...so from what Dan's saying, it's probably the design of the twin stroke pistons...but then i'm confused why they don't make newer mid size (20-30 gal) compressors have 7'ish CFM like they use to...different type of users now a days? no more need for that much CFM?

thanks for the reads...

[Jack Clark]

...but then i'm confused why they don't make newer mid size (20-30 gal) compressors have 7'ish CFM like they use to...different type of users now a days? no more need for that much CFM?

I would say that marketing types sometimes take engineering specs and mess with them (otherwise known as "the craftsman school of horsepower rating").

Here's my experience. I bought a Craftsman 2HP oil lubed compressor about 5 years ago. Nice machine. Built by DeVilbiss. Had to rewire my garage with a dedicated 110 volt 20 amp circuit because the motor on the compressor would trip the 15 amp breaker when the garage light was on at the same time. The pump design has been around for years, and if you look on Craigs List you'll find plenty of them for sale. I suspect that many of the other brands that sell this class of home compressors pretty much use the same pump. When I bought mine, the specification plate on the compressor states that it puts out 8.6 CFM at 40 psi–pretty much what I needed for my amateur spray finishing habit. For some strange reason I bookmarked the listing for my compressor model on the Sears website. The specifications were listed there and matched the plate on my machine. About that same time the the ruling on the class action lawsuit came down. A while later I took another look at that bookmarked webpage and low and behold, the CFM rating had dropped to 7.4 at 40 psi. Same compressor model, same motor, I'm assuming same pump, etc.

I guess my point is the same that has been made by others on this topic. That is, at the consumer level, specifications can be a minefield of confusion and misrepresentation. Pump designs probably haven't changed that much. Marketing lies can change overnight.

Now Sears doesn't even sell that compressor. Profit margin must have been too small.

If you want a good compressor, that is the last one you'll every have to buy, look at Jenny or Quincy. But be prepared for sticker shock.

[Jeff Bratt]

I would say that marketing types sometimes take engineering specs and mess with them (otherwise known as "the craftsman school of horsepower rating").

Even your pics point out small specification "enhancements". Note that the HP rating on the motor nameplate is left blank - the 2 HP is listed only on the compressor tank (along with the 6 HP blatant lie). Even this 2 HP rating is fudged a little, just not as much as many other products. As I said previously, a rating of 1.5 to 1.75 HP is expected for a motor which is speced for 120V 15A.

You stated that you are happy with your compressor - so this is not a criticism of how it works. I'm just pointing out how it's almost impossible to make reasonable comparisons when motors with "enhanced" HP ratings are so prevalent.

[Jack Clark]

You stated that you are happy with your compressor - so this is not a criticism of how it works. I'm just pointing out how it's almost impossible to make reasonable comparisons when motors with "enhanced" HP ratings are so prevalent.

Jeff,

After researching and searching for a bigger compressor to replace the Craftsman, I really understand what you're saying above. One of the problems is we amateurs get a distorted view of cost by the prices on compressors at big box stores, Harbor Fright, etc. When you really start looking at performance factors such as duty cycle, suddenly a lot of that imported stuff can start appearing pretty junky.

Well, now I have to admit something else. Over time I've become unhappy with the Craftsman. Not because it runs poorly–it has been trouble free. But over time I've developed a desire for more CFM's. I'm about to take delivery on a new 3 HP Jenny that will fit right where the Craftsman used to sit, but kick out about 3 times the volumn (14 CFM at 100 psi). Paid an arm-and-a-leg (more like an arm and two legs), but it will be the last compressor I ever buy.

Thanks again for that link on your website that really explains the horsepower deceptions. I hope folks here on Sawmill Creek have bookmarked it for future reference.

[Duane Bledsoe]

Here's a couple of formulas you guys may find useful...

Volts x Amps = Watts

Watts / 746 = Horse Power (because 746 watts is 1 HP) [edit] -- had to correct this formula, I had the W = HP number wrong, previously listed at 784

So, assuming that the amperage draw of any motor on a tool is listed correctly, all you have to do is multiply it's voltage input (120v or 240v) times the current draw to get watts and then divide by 784 to see what horse power rating it would be putting out.

Many times I do this math listed above and come up with less HP than what is advertised. For instance, tonight I bought a router that states 2 HP on the box, but doing the math for an 11 amp motor, I get considerably less horse power. Comes out to be 1.68 HP only, so not even 1 3/4 HP but still rated at 2 anyway.

Trouble is, did they rate the HP under a load or just free spinning?? I think most times ratings are considered "no load". If it is rated at no load, then wouldn't putting it under a load (cutting wood) cause it to increase current draw and thereby increase HP as well??

[Callan Campbell]

It's worse than you think. For years this whole trickery was going on, and the people who sold air tools UNDERestimated the air consumption of their tools. While the lawsuits about motors and power ratings might level some things out from a horsepower rating standpoint, you and others are still in the dark when it comes time to fully understanding how MUCH compressor you need to run that new sander or grinder you bought. You can read about the levels of air needed, but the key is that they're stated in 1 minute intervals usually, and not the nominal time that a tool might be used at a constant rate of consumption.
How many of us run the tool for 1 minute or less? Size of the tool is no indicator, as Dan H. stated about motors and pumps, the desired work output needs to be reached. If there is a tiny amount of binding inside the air motor, or a restriction to the tool that limits the pressure and volume of air to run it at the correct spec, then your compressor may be worked harder as your air usage goes up. There was an old adage about pneumatics, "everyone brags about how much air their compressor makes, and everyone lies about how much air their tools consume":p:p:p

[Chris Parks]

The ultimate measurement is the Free Air Delivery. In Australia I think this is the standard now used IIRC or is going to be. This link explains....

http://www.ferret.com.au/n/Improved-measurements-for-air-compressors-n716167

[Jack Clark]

Watts / 784 = Horse Power (because 784 watts is 1 HP)

Hey Duane,

Minor point, but I always thought it was 748 watts per HP (or was it 746?).

[Jeff Bratt]

Watts / 784 = Horse Power (because 784 watts is 1 HP)

That equation only works for a theoretical, 100% efficient conversion of electrical to mechanical power - such a motor does not exist. Real-world efficiencies are in the 50-80% range for small motors. Real horsepower is a measure of mechanical work, and one HP is 33,000 lb/ft per minute. The physics is explained on my second HP page - http://home.roadrunner.com/~jeffnann/WoodWorking/Shop/HP/Horsepower2.html (http://home.roadrunner.com/%7Ejeffnann/WoodWorking/Shop/HP/Horsepower2.html).

The conversion factor is 746 W / HP.
Your 11 amp router is really 1 HP or lower.

[Duane Bledsoe]

Ouch!! Sorry guys, got my numbers messed up. You are right with the 746. Don't know why I had 784 on the brain. It's been a long time for me since that electronics/math class and I thought I remembered it correctly when I wrote that. I went back and edited my comments to show the correct number in an effort to prevent bad information from circulating. Thanks.

[Gary Herrmann]

I'd like to pick up a 60 or 80 gallon compressor to run sanders and grinders primarily at some point, but I only want to buy once.

I saw the statement about Jenny and Quincy compressors but am unfamiliar with them.

Is Ingersoll Rand still making good compressors? Sorry if this is a bit of a hijack.

[Jack Clark]

Sorry if this is a bit of a hijack.

Gary,

I'm often guilty of "hijacking," so I don't want to drone on here.

But if you copy the text from your post and start a new thread, I'd be happy to share my experiences purchasing a Jenny. I'm sure others with Ingersol-Rand and Quincy experience would contribute too. There's also Eaton, Castair, Curtis and probably several others in the "professional" category I'm unaware of.

[Callan Campbell]

I'd like to pick up a 60 or 80 gallon compressor to run sanders and grinders primarily at some point, but I only want to buy once.

I saw the statement about Jenny and Quincy compressors but am unfamiliar with them.

Is Ingersoll Rand still making good compressors? Sorry if this is a bit of a hijack. Old I-R compressors seem to have a better rep than the newer stuff. I've heard good and VERY bad stories about the last few years with people buying commmerical grade, not DIY, 2 stage I-R units. It seems I-R is so globally spread out with their production, that all the plants have to be running quality at the same time for the units to meet our expectations.
I've heard of out-of-balance pumps that shake the unit around the floor, weak or flat out bad electrical controls that short out after the first run session. But, I've also heard good things, so it seems you want the "middle-of-the-week" made units, and not the "Monday or Friday" production.:p
Quincy makes some nice stuff, they have pressure lubed units too with a spin-on oil filter if you have the money[most units are splash lubricated to save on pump costs]. Champion, Saylor-Beall and the others mentioned are also good. For a sander, you want a strong 5 to 7.5HP, 2 stage unit, that puts out 15CFM or better at 90 psi. Depending on the sanders thirst for air, a good 5HP unit may keep up with it. Ask around once you decide on which sander you want to use, as getting "real" experience for compressor run time with the sander is nice info to have while you're deciding on what unit will fit your needs best. 7.5 HP might seem like overkill, but it depends on your usage and needs. Also, a 7.5 HP unit could run slower than a 5HP with regards to pump RPM, so you may end up with a much quieter unit if it's running a few hundred RPM slower than a fast moving 5HP compressor.
My 3/5HP Saylor is like this, the pump is made for either motor, it's just pulley sizes that determine the pump RPM, AND output. I have the smaller 3HP motor on it with the correct factory pulley,so it's a slow, quiet, system that puts out as much air as a cheap single stage 5HP unit in many cases.

Many pumps are now run with synthetic oil to extend the life and deal with heat better than plain straight weight oil[non-detergent 30 wt is the usual older spec'd oil for air compressors] You might also look in rotary screw units, as they seem to be getting cheaper with time.
Now might a good time to get a real commerical used unit from a closing business. If it hasn't been run to death, and the oil and air filter were changed regularly, they can be outstanding units. Watch out for 3 phase motors though, much more common among industrial users who have access to 3 phase power, unlike we woodworkers who rig up conversion systems to run a 3 phase motor.:D

[Lorne Horvath]

While not an expert on this I do have a fair bit of experience with compressors being in the autobody repair buisiness. I Agree that a rule of thumb is 4 cfm per hp but that is with a 2 stage compressor. A 2 stage compressor generaly runs about half the speed and will produce more air than an equivalent single stage even using real hp numbers. I have an eagle 3 cyl single stage in my home workshop that seems to be between a good 2 stage and the cheap single stage models. While it runs at the higher rpm it does at least provide close to the same cfm as a 2 stage all be it at a lower pressure. Pressure is the other difference between the two as a 2 stage will provide 175 plus psi and single stage rarely will get above 125 -135. If you run long air lines or even a 50 ft 1/4" hose the pressure drop can be enough that you don't have enough pressure to run some tools properly. I generaly consider the cheap single stage compressors regardless of hp to be for filling tires or very light intermittent use. They will not keep up to a grinder or hvlp spray gun and run steady with ro sander.
I have three dvilbiss 2 stage compressors in the body shop, one of which has been running for the better part of 8 hrs a day since 1977 with only regular oil changes.
Just my 2 pennys worth.
Lorne

[Jeff Bratt]

Thanks - glad I could help.

[Deuce Lee]

no problem with the hi-jacking guys...

I brought this up because I was trying to find something could give out 7.X CFM @ 90psi and still run on 120v. From what you guys are saying and what I'm seeing, doesn't look like there's any...If anyone know of one, feel free to link it. My sandborn is decent but is only 120psi max, would like at least 135 or maybe 150psi. And it was cheap, if I sell it, I'd get all my money back. :P

I'm just a home owner hobby addict so I don't need a nor want a big 60 gallon that puts out 15CFM. Just going to be spraying a fender or hood one or so a year. For the trimmings, I bought a little 34lb PC pancake compressor that's a cinch to carry.

[Jack Clark]

Deuce,

If you ever decide that you need a really good compressor, take a good look at the Jenny lineup. Although pricey, they have a huge selection of size and option configurations. Plus their heavy duty quality dovetails nicely with the old adage of "buy once cry once."

[Callan Campbell]

While not an expert on this I do have a fair bit of experience with compressors being in the autobody repair buisiness. I Agree that a rule of thumb is 4 cfm per hp but that is with a 2 stage compressor. A 2 stage compressor generaly runs about half the speed and will produce more air than an equivalent single stage even using real hp numbers. I have an eagle 3 cyl single stage in my home workshop that seems to be between a good 2 stage and the cheap single stage models. While it runs at the higher rpm it does at least provide close to the same cfm as a 2 stage all be it at a lower pressure. Pressure is the other difference between the two as a 2 stage will provide 175 plus psi and single stage rarely will get above 125 -135. If you run long air lines or even a 50 ft 1/4" hose the pressure drop can be enough that you don't have enough pressure to run some tools properly. I generaly consider the cheap single stage compressors regardless of hp to be for filling tires or very light intermittent use. They will not keep up to a grinder or hvlp spray gun and run steady with ro sander.
I have three dvilbiss 2 stage compressors in the body shop, one of which has been running for the better part of 8 hrs a day since 1977 with only regular oil changes.
Just my 2 pennys worth.
Lorne Perfect timing Lorne!, can you add your work experience on using air sanders so that the poster-Gary H., might learn how much air they actually use, and what size or brand compressor he'd need to look into for powering one up at a consistent rate while working in his shop.
Totally agree with your post, that's been my experience too between single stage and 2 stage compressors.