I just bought an HVLP conversion gun (PC PSH1-love it!) but was concerned if my 20 year old Crapsman 2HP 20Gsl compressor was up to it. Its specs are 9.0cfm @ 40psi and 7.5 @ 90. I went shopping a few places to see what it would cost to better that and was surprised to find even 5HP compressors that couldn’t beat those numbers. I started looking at the motor dataplates and found on all these machines (Sears, PC, Husky, CH) the motor HP rating was “Spec.” which is meaningless. The current draw was about the same for any compressor I looked at from 1.5HP all the way to 5 which tells me all these motors are the same & explains the lack of performance. Are all these guys rating output the bogus way that Crapsman started years ago with their “Peak Horsepower”? I guess 20 years ago a 2HP compressor really put out 2HP of continuous work but today the specs tell a different story.
/endrant
Replies
The rule of thumb for compressed air is four (4) CFM per horsepower. A two (2) HP compressor should have an effective CFM output of 8 CFM.
It is my understanding that there is no standard (regulated body or otherwise) to ensure a 2 HP motor actually gives you 2 HP. A while back someone had written it is not possible to have 3 HP with only 110 power and was followed up with mathematical formulas to show it.
I wouldn't say 3 real HP is impossible on 120 V but it sure wouldn't be cord connected to a regular panel board circuit. 34 amps full load @125% would require #8 wire and a 60A dual element fuse. If a motor is integral to a tool I guess there is no standard. NEMA standards do apply to regular induction motors though and any NEMA motor has to make it's nameplate HP within +/- 10% of it's nominal voltage. If you rated a standard 3 HP motor like they rate HP on a 3 HP ShopVac it would be about 21!
John O'Connell - JKO Handcrafted Woodworking
Life is tough. It's tougher if you're stupid - John Wayne
Edited 3/14/2003 3:39:44 PM ET by ELCOHOLIC
If you would like a good article on compressor horsepower ratings and how companies rate their compressor's today, go to the following website.
http://www.kmstools.com
Click on the store entrance, go to products and scroll down to "compressor buyer guide".
good luck
Bill
Thanks, Bill--that is a great link! And the truth shall set you free...LOL
Well maybe the truth shall set me back a few hundred $$
Hers's a link that offers good understanding between scfm and acfm. Standard and actual.
http://www.sandcarver.org/air_compressor_selection.htm
Don
I have a 60 year old 1.5 HP rated (states 1.5 on the plate) motor on my planer (220V) that I bet would out do most 3 HP motors bought today. The beast is 10" diameter, 14" long and weighs more than I like to lift these days. Also has grease zerts!!
If you have 10 motors lined up, all with the same ratings, get the one that is the heaviest.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
Once read that only two companies give honest hp ratings when it comes to compressors, Ingersol Rand and I don't remember the other. The IR, L5 is listed at something like 14.5 cfm at 40 psi. That's a 5 hp motor. By other standards this compressor would rate about 35 cfm. Be aware that most quality guns rate the necessary cfm by honest ratings, not by what you see on most compressors. I've know more then one person that didn't realize just what perfomance they are missing out of their gun. At one time I sure didn't.
Don
CFM is the only real piece of information you need when shopping for a compressor. Horse power ratings are so exxagerated these days as to be meaningless.
CFM ratings are based upon free air displacement. In other words 8CFM@40psi DOES NOT mean the compressor will deliver 8 CFM of compressed air. The compressor will draw in 8 CFM of free air and compress it to 40psi. The numbers diminish as the psi grows because of increased resistance and decreased efficiency in the compressor head.
The good thing about all this is that tool manufacturers use the same displaced air figures in their tool ratings.
LeeLee Grindinger
Furniture Carver
Lee,
I sure didn't find that while doing my research before buying. Compressor ratings are as hacked as dust collector ratings.
Don
You're right, Don. I ran this issue to ground a few years back and at first blush I spewed the wrong information. I really should take better notes when I do this stuff. I'll try to remember what I learned and repost the right information.
LeeLee Grindinger
Furniture Carver
Lee,
I'm assuming sarcasim?? As the info on the KMS site discusses, cfm is rated at zero pressure which isn't helpful. Do you agree?
Don
Allright, I've had a night's sleep and I've dredged up the info I'd stored in the cess pool I call a mind.
All ratings are free air. By free air I mean ambient pressure. Ambient pressure at sea level is around 14.69 PSI.
10cfm@90psi means that 10 cfm of ambient air is delivered at 90 psi. It does not mean there is 10 cubic feet of air at 90psi.
I really can't point you in a direction other than to spend a day on the phone talking to people that do not understand this. Not one salesman I've ever talked to understands this, they spew the ratings they are taught.
Think for a minute, this is the way I finally got the truth about this matter and that came from an engineer that actually understood the issue...Let's assume a rating of 10cfm@90psi. To deliver ten cubic feet of air that is compressed to 90psi means that the compressor head has to compress 61.226 cubic feet of sea level air PER MINUTE! (90 divided by 14.69 equals 6.1226 10 times 6.1226 equlas 61.226) Not even trying to account for inefficiencies in the compressor head that comes out to a cubic foot of air entering the head PER SECOND! Approximately 30% is lost to leaks in the compressor head.
In real numbers the actual amount or air a compressor head would have to suck in to produce a 10 cubic feet of air compressed to 90 psi is 79.59 cubic feet of ambient air. That's 1.326 cubic feet of air PER SECOND being drawn into a compressor head.
Do you have any kind of idea how great a suction that is? Heck, I doubt my shop vac does that! How about your dust collectors? I'm being facetious now but do compare these numbers to a dust collector and try to equate the two.
At any rate, I spent a day running this to ground a few years back. My most trusted tool salesman gave me phone numbers and watched as I talked myself blue in the face. I cannot substantiate my information, I don't care to. Do the math.
I'm not being defensive or sarcastic, I'm just remembering the resistance I met running this to ground.
As I said earlier, it's all just a mental exercise anyway, tool ratings are based on the same measure.
Lee
Lee Grindinger
Furniture Carver
Lee,
Seems we might agree more. My reasearch led me to this. CFM is is meaningless. It is nothing more then a number referring to speed and displacement of the compressor. With this logic, all automobile engines of the same displacemnt would deliver the same hp regardless. SCFM is a better number, but still misleading for a couple of reasons. First, this number does not take into account effeciencies of the compressor. It's a no load and no back pressure number. A lot of examples of how this number can skew reality can be given. I'd say the best would be, using this number there would be no difference between one and two stage compressors. We know that's not true. Then there is ACFM. This is by far the best number because it is based on the actual output of the compressor, but still, like SCFM, it is based on sea level, temp. and relative humidity numbers that may not be representative for some. In my area I need to multiply ACFM by .9 to get a better representation of what I would really get. The link I posted put Dever's factor at .68, or something like that. That's over a 30% loss from the best possible numbers posted by a manufacturer.
What I was referring to when you stated that CFM was a good number because it's standard for all is that it's not really representative of what a compressor will do, nor where it's going to do it. There is a reason why an Ingersol Rand compressor costing $1200 to $2k costs what it does, while a Porter Cable with numbers way ahead of the Ingersol Rand and costing half as much, still won't compete with the IR.
Don
Wow, there's much more specs BS than I could have imagined. I was just noticing that the range of compressors at my local Quality Farm & Fleet (and elsewhere when I started looking into it), rated from 1.5 to 5HP all had 120V cords & 15A. maximum current draw. The CFM specs were pretty similar for all as well. Something obviously wasn't adding up. After visiting the links you guys posted I have a much better (and more disgusted) understanding of what the manufacturers of consumer grade compressors are up to. Thanks.
Of all the numbers given as specs CFM still seems to me to be the most relevant one. I know it's innaccurate in some instances but CFM is given as a spec universally.
I have a motor in my shop that claims 7.5 HP. It's a little larger than a loaf of bread. I know it's closer to two horse than three and I mention this to illustrate just how inflated HP ratings are.
I'd like to believe that CFM ratings are a bit closer to the truth. SCFM is more accurate but it's not given for comparison when you go shopping. Based on the info universally given I'd say CFM is the most relevant spec but an educated shooper will know to compare motor size and such as well.
LeeLee Grindinger
Furniture Carver
Lee,
I agree. What can happen though is CFM for the tools used with compressors are also based on input. Obviously what happens is then the cfm of a compressor does not translate to output which is in turn input for the gun, tool or whatever. As I said, I need a multiplyer of about .9 to get actual for my area. .9 at the compressor and then 1.1 at the tool, and collectively I start with about a 20 percent additionally needed cfm from specs. 20 percent is a significant loss. As the chart shows from the one link, in Denver, this loss can total 60 percent collectively. Ouch. What cfm does do is give a reference of how to adjust needed requirements whereas hp is meaningless. With this each hp should deliver so much cfm, my compressor should be deliver 20+ cfm, yet manufacturer specs are less then 15. Add this with the bogus universal motor special compressor hp ratings, and my little PC jobsite compressor delivers more cfm then the L5. I don't think so!
Don
How do like the HLVP conversion and compared to what were you using before? I'm a member of a woodworking guild and we just had a factory rep give a demonstration on this product, He said as long as your compressor puts out 120 psi it would work with the conversion sprayer, witch I was thinking about purchasing one myself,but debating to buy their whole unit. and my twin head sears compressor threw the bearings and can't get the seal for that compressor till June , I'm a pro furniture builder and can't be with out that long , priced a Inger Sal/ Rand, 3hp 60 gal $560. and the price for a whole unit for a HLVP is close to the same/ what to do?
Good luck / with the problem. Furniture builder/ Grand Rapids,MI
Odd advice from that factory rep. Pressure isn't the issue with HVLP--continuous airflow is. Per the PC manual; I'm running my compressor at 90 psi and with the gun regulator set at about 20 psi I'm getting pretty good atomization.
There's no problem with my compressor, it turns out. While waiting for the gun to arrive I shopped around a little looking at compressors "just in case". My old Sears is an honest 2HP and seems to be putting out adequate air--maybe 20% duty cycle or so if I spray continuously. The PC gun itself is a treat. Very well made & finished. The job I'm using it on is spraying latex which is difficult for ANY spray gun to do well but I'm much happier with the results than I was with either my Wagner airless or my standard-pressure detail gun. I'm sure I'd get even finer results if I had the proper nozzle size for latex. The PC comes with a 1.5mm (only), which is pretty small. Nobody but Lowes around here carries this gun yet and Lowes has no accessories. Online sources also didn't list nozzles. PC has a factory repair center nearby and they want $53.00 per set for each size. The gun only cost $80.00 plus shipping so 53 bucks for each needle/nozzle set seems outrageous. That is my only quibble with the gun. That price probably isn't bad at all for a quality Binks or DeVilbiss but is out of line with the selling price of my gun.
I bought one of those cheapie Harbor Freight HVLP gun/turbine systems at the same time but I haven't tried it yet. I've heard some pretty good things about it but I'm sure not impressed with the plastic gun & cup. For $70.00 though....(and it came with 3 needle/nozzle sets!)
I hope I've answered your questions?
--Mark
The horsepower rating of motors used in applications where the motor starts under full load have become "peak horsepower" ratings rather than "continuous horsepower" ratings. It's a different way of rating that takes into accound that these motors must put out large amounts of initial power to overcome the compression resistance. Same thing for dust collectors where the inertia of the large fan rotor requires large initial power. Like other things, marketing claims are what sells compressors so manufacturers look for ratings that maximize these numbers.
Standard power tool motors are rated for continuous power which is the power at a limited amperage draw and heat build up.
That said, for compressors, the only figures that make any real difference is CFM and capacity.
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