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Old 01-27-2005, 12:33 PM
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choosing a compressor

Lately there has been a lot of questions about what is the best compressor to buy. To be sure a compressor can be a major purchase and probably the most important air tool in your garage since all of your other air tools are dependent on it. Myths and misconceptions abound about compressors and unfortunately a lot of the manufactures only add to this misunderstanding with misleading if not downright fraudulent specifications and other sales gimmicks. Since 1968 my livelihood has depended on welding machines and air compressors and I have built and installed many air systems for service trucks and shops over the years and have learned from experience what works and what don't. Your most important concern when choosing a compressor is air delivery which is given in cubic feet per minute(CFM), this is what determines if a compressor will keep up or not. Motor HP ratings mean nothing if the CFM is not there. Manufactures usually base CFM ratings on calculated CFM which is pump displacement x rpm and don't take into account pump efficiency which means that actual CFM is somewhat less. When checking CFM ratings always go by the rating given at the highest PSI(usually 90 psi) as CFM ratings will look much bigger at 40 psi,but most tools usually run higher than that. As a rule of thumb a compressor should have at least a 30% higher CFM rating than the tool it is operating. CFM RUNS YOUR TOOLS, NOT HP AND TANK SIZE! All to often someone will choose a compressor based on HP and tank size and overlook the most important factors and the manufactures play on this by mounting small air pumps with over rated "peak" HP motors on a large tank. While that big tank may LOOK impressive I cannot stress strongly enough that a tank only STORES air it DON'T make air and a bigger tank will NOT make a small compressor keep up with an air hungry tool such as a DA sander or sandblaster. The tanks main purpose is to control the on/off cycle rate of the motor and pump and it cannot provide more air than the pump is rated for. Any advantage the extra storage capacity offers(usually less than a minute) at initial start up of a tool is quickly lost by having to wait longer for the bigger tank to recharge so you do not gain anything. Also bear in mind that the salesperson at Lowes,Home Depot or where ever is usually just that and not an expert on compressors.

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Last edited by oldred; 01-27-2005 at 12:39 PM.
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Old 01-29-2005, 08:11 AM
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Thanx for posting this Oldred. This info is repeated over and over in various threads but in view of the obvious confusion out there, it can't be repeated enough. How many times have we seen a question like "My compressor has a 60gallon tank. Is this big enough to run such-and-such tool"?

I would add to your post these [color=orange][size=5]MINIMUM[/SIZE][/COLOR] requirements for a serious auto hobbyist;
  • 5hp => 10cfm@ 90psig
  • 2-stage
  • belt driven
  • 60 gallon tank

Again those are minimum specs that will get you buy. Anything bigger in cfm, HP, and tank size are icing on the cake.
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Old 02-04-2005, 12:20 AM
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It is impossible to have too big of a compressor. You will never wake up in the middle of the night screaming, "I bought too big a unit." The opposite is a possibility.
If you are going to paint, set your minimum capacity above the requirements of your gun.
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Old 02-04-2005, 09:18 AM
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Julmer--This is true, scrimp now and be sorry later! But my point here is that a big tank does NOT make a big compressor! I keep seeing the same things asked, "my compressor wont keep up with my DA do I need a bigger tank?" or "my 30 gallon compressor is to small should I get a 60" when the fact is that tank size tells almost nothing about a compressor's performance. While some will simply not accept this it is a fact, a bigger tank WILL NOT-REPEAT, WILL NOT increase a compressor's ability to keep up with a tool. It will help the compressor last longer and to a lesser extent consume less power, both due to fewer high load start up cycles. It is a fact that an 18 CFM compressor with even a tiny 10 gallon tank will keep up with a 12 CFM tool just fine while a 12 CFM compressor will not keep up with an 18 CFM tool even if it had a 100 gallon tank. One would use up all the reserve in that 100 gallon tank in short order and then he could go have lunch while waiting for it to rebuild pressure. Both examples are too extreme to be practical in a shop but the 10 gallon example IS used on some service trucks. The bottom line is a tanks main function is to control pump and motor on/off cycles and not to make up for insufficient CFM from the compressor! A large compressor needs a big tank to keep from cycling on/off too rapidly if a lot of volume is being used while a huge tank with a really small (like some that are popular) compressor can be a PITA due to the long recharge times.

Check out this handy guide from an HONEST manufacturer, especially the parts about HP and tank size.

www.toolnewz.com/102000/compressor_howtobuy.html
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Old 02-07-2005, 03:04 AM
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I think you said it a couple of times above. The rule is still CFM is the only way to compare compressors and air tools.
Big tanks solve a different problem. Because I got it for a song, I have a 300 gallon ASME tank in series with the 80 off the compressor. What I discovered is that I no longer have any moisture problem in my lines. The second tank is just so big, the air cools and the moisture fall out before it reaches the outlet. I used to get some water out of the sander when I ran the 8" DA hard for a long time. I added my surge tank and no more water under the worst conditions. The edge I have is a 20+cfm
2-stage pump that will keep up with 2 sanders.
Remember always, gallons don't count, horsepower doesn't count, amps almost mean something but Cubic Feet per Minute is the only true test.
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Old 02-07-2005, 09:15 AM
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julmer-EXACTLY! I guess I may have given the impression that I think a large tank is not a good idea but this is not at all the case. The point I have been trying to make(just as you did also) is that if the CFM is not there then a huge tank and inflated HP numbers are not going to help. I keep seeing these P.O.S. things everywhere, Sears,Tractor Supply,Lowes,Home Depot and most auto parts, big impressive LOOKING tanks with ridiculously inflated HP numbers and tiny compressor pumps that are suited for little more than airing up tires. I am not trying to be a smart *****know-it-all here it is just that I see posts from guys who say they are limited on how much they can spend so they can not afford not to get it right the first time. But then all too often the decision boils down to "Do I get the 5(peak) HP 60 gallon or save up for the 7(peak) HP 80 gallon" when in reality either one or both may just barely make enough air to blow your hat off let alone run air tools properly. MY advice to them is to Make SURE the darn thing has enough CFM to run the tools FIRST, THEN, as Willys said earlier, the rest is icing on the cake! Your set-up is a good example of what DOES work, that is, you first have the CFM to run your tools and then the capacity(WOW! do you ever have capacity) to make it all work smoothly and last a long time.
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Old 02-09-2005, 08:44 AM
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Compressor Advise

Does anyone have any recommendations. I am looking at a 60 gal 2 stage compressor from Eaton Compressor

http://www.eatoncompressor.com/catal...747/206949.htm

From all of my looking around they seem to be the best around. The motor runs at 800rpm which is half of the store bought units and it comes with a lot of industrial options.
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Old 02-09-2005, 11:02 AM
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daveh-Now we are talking about REAL compressors! They tell it like it really is and if you have chosen that set up you will have everything you will need and you will not be disappointed. Unless you are going to set up a business shop with multiple employees I would think that is probably all the air you will need and it should last for a lot of years. Compare that outfit to the Craftsman(and several other similar brands) 7 peak HP 80 gallon 2 stage for $849.99 and it will prove the point I have been trying to make. The craftsman's(and some others) 7 HP is phony, the pump only puts out a little over 16 CFM and whole thing is of questionable Quality. Anyone who thinks that bigger tank and phony HP rating on the Craftsman is better than all that extra CFM from the Eaton can save themselves $200 plus they will have plenty of time to stand and admire that big tank every time they have to stop and wait for it to recharge. You however, if you buy that Eaton, wont have to do that and your compressor will still be running when the "bargain" units are in the scrap yard.

BTW--People are starting to catch on to the "peak HP" nonsense so some manufacturers are starting to use the term "MAXIMUM DEVELOPED HP" instead but that is just a diffrent name for the same bull#@&^!

Last edited by oldred; 02-09-2005 at 11:08 AM.
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Old 02-14-2005, 01:43 PM
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Has anyone used the US General brand from Harbor Freight? My local store has on simular to this one http://www.harborfreight.com/cpi/cta...temnumber=3848
Except I think it had a 6.5 HP motor. The CFM was the same. The local store had it for 367.00. Just wondering about the brand.
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Old 02-14-2005, 06:49 PM
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Mavrick-If you look closely at the description at the bottom of that ad you will see 6.5 HP which is "peak" or "maximum developed" HP and means exactly nothing as to power output. The 3.5 HP listed as rated is the real power output and the 6.5 you mentioned on the other compressor is almost certainly the same thing at that price, check the amp rating I bet it is only 15 amps and 6.5 HP needs about twice that. Also 12.8 CFM from a single stage compressor driven by only about 3.5 (real running) HP seems kind of suspicious to me and since the CFM numbers can be juggled like the HP numbers one needs to check the SCFM to be sure. It may be starting to get a little confusing but that is how these outfits get away with selling this junk! After having said that I have looked at the US General compressors at Harbor Freight and I guess for the money one could get by if that is all that is affordable at the time but don't expect the kind of performance you would get from a similar size compressor from one of the more reputable companies. This machine could be used to run most body shop tools BUT be prepared to spend some time waiting on the compressor to catch up! If you can afford the extra cash for an Eaton or Ingersoll(or similar quality) now it would be cheaper than having to up-grade later. Remember CFM is what counts not the physical size of the outfit and if the manufacturer has to inflate the CFM and HP numbers so that they look better than they really are , what else are they hiding?

www.toolnewz.com/102000/compressor_howtobuy.html
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Old 03-13-2012, 01:29 AM
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Hey! I am looking for buying a compressor. I am confused in selecting the right size air compressor to run my air tools. Please guide me to choose right size air compressor. Thanks!
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Old 03-14-2012, 06:06 PM
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It's nice to see people searching for the info they need, but did ya notice the thread is 7 years old
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Old 03-14-2012, 09:40 PM
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Yep it's 7 years old but I would like to up-date something here. My comment about the U.S. General compressor was more than a little off the mark, at that time they had just started selling the rebadged Belaire compressors and most folks including my self assumed they were the usual HF quality "Central Pneumatic" compressors from China, not so! Some time later while looking at one of the U.S. Generals in a HF store I noticed the Baldor motor (some models use A.O. Smith motors) and Square D electrics then it occurred to me that thing looks familiar, after some discussion here and a bit of research it turns out they are indeed re-named American built Belaires at a really good price and not a Chinese clone as was originally thought. The U.S. General compressor (the black compressors) have got to be one of HF's best bargains, in fact at that price it's a great bargain compared to any other store. These compressors are American built except for the pump but even that's a quality built pump made in Italy not China like most other compressors in this price range including some that cost a heck of a lot more!
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