Right Size for an Air Compressor

[Jimmy240Z]

Need some help. As you know I have been working on my floorpans, etc. I went out and bought an air saw and a cut off tool. I have a 2 HP 12 Gallon Air Compressor. Neither tool is cutting the metal. Not sure if it is a problem with the Air Compressor or what. I have tried to adjust the tools and that doesn't help the cutting. Someone said I need a larger air compressor. Can anyone give me their lessons learned on the right size for an air compressor?

Thanks,

Jimmy

[moonpup]

The tools you bought should list somewhere, either in the instructions or on the box, what the minimum amount of CFM's are required from your compressor in order to operate them. Compare that to your compressors CFM rating. Sounds like your compressor isn't generating enough.

[CoastGuardZ]

Jimmy,

I see you're in Charleston. What part? Charleston is home for me... I grew up in the Summerville/Goose Creek area and I am doing everything it takes to get back... hopefully next year.

I feel pretty confident in saying that your Compressor is too small. I have a Craftsman 1.5hp (that is the sustained hp the max developed is 3hp but that seems to be an arbitrary number) 15gal rated at 3.5 SCFM @ 90 psi... which is way too small for what I ask of it. I am currently in the market for a new compressor too, (Yes E, you can say I told you so). From what I can tell the minimum you should look for is a 6.0 SCFM @ 90psi. Also, it seems that all the larger "oil free" ones are quite LOUD. Mine is really annoying and so is my brother-in-laws... also a Craftsman, the 33 gal 2 hp (6hp?) which makes 6.4 SCFM @ 90. We have a BIG and I mean BIG compressor at work which is not as loud as my little "home-owner" one. I've decided that I'd like to get somewhere around 9 SCFM @ 90 psi / 30 + gallons and an oil lubed pump, which I guess means I'll be into one for around $400-$500 bucks. Good luck, and look for a PM from me.

Nate

[webdawg1]

I guess there's some truth in the old saying "Bigger is Better" Get the biggest honking compressor you can get your hands on....You won't be sorry you got the bigger one...Well...maybe you will...when it comes time to move to another house you might have some second thoughts...

webdawg1

[kurtwwalters]

I just got a 6hp 30gal Craftsman. I got a hold of a 120gal air tank from an industrial compressor that I want to use for sandblasting or anything else that will use a lot of air. I just need to get a few fittings for it. Anybody ever tried it? I got it for next to nothing so I thought I'd put it to use....

[CoastGuardZ]

I've heard of people doing but never myself. I would be worried about overworking the compressor motor to fill that big beast though. I don't know. I also have a feeling that, like mine, the hp number you have there is the "max developed" number and not the rating at wich the motor will be running at to fill that big 'ol tank, which is probably closer to 2 hp. So it may take a while and on top of that the motor may meet it's maker quite a bit sooner. Just a thought.

OTOH, it might be worth rigging up just to see how it goes. I would watch it the first few times to see how long it runs and how hot the motor gets.

Good Luck!

[jmortensen]

Adding the big tank should help because the motor won't be on as much if you're not running an air sander or something that you can run for an hour at a time. So if you're using say, an impact gun to break loose your lug nuts, with the small 15 gal or whatever size tank, the pressure in the tank might go down enough just while you're breaking them loose to cycle the compressor on. With the 120 gallon tank hooked up, no way.

I have basically been trying to break my 20 gallon, 5hp compressor. It is an oilless and it is so freakin loud I can't stand it. In an attempt to justify a nice 80 gallon oiled 220V compressor I've been not waiting for the compressor to catch up. I swear I've had that motor running for 2 hours with about 5 minutes of rest quite a few times now. It's gotten noticably looser sounding, especially when the air tank is empty, but it keeps going.

But yeah, the bigger is better idea is really the way to go when it comes to compressors.

[CoastGuardZ]

I have basically been trying to break my 20 gallon, 5hp compressor. It is an oilless and it is so freakin loud I can't stand it. In an attempt to justify a nice 80 gallon oiled 220V compressor I've been not waiting for the compressor to catch up. I swear I've had that motor running for 2 hours with about 5 minutes of rest quite a few times now. It's gotten noticably looser sounding, especially when the air tank is empty, but it keeps going.

That's awesome! Good luck with that.

Yup, the noise is really begining to annoy the crap outta me too. At first I thought about trying to break it, but decided it would be better to make $50-$100 off of it to apply to the new one.

I too am looking at the 220v models... it really sounds like the way to go.

[DeesZ]

I am by no means an expert on compressors, but have learned a lot about them over the last several years. I decided to get a "larger" unit than the little guy I got several years before, and did some research before getting one. I'm glad that I did, because my normal impulse purchasing habits would not have served me well for this purchase. Here's a few things that I've learned... anyone with corrections or additions, please post them (again, I'm not an expert).

Proper compressor sizing will provide you with suitable performance and longer compressor life (because you will not 'overwork' the unit). Air compressors are rated by horsepower, pressure, and cubic feet per minute (CFM) of output at a given tank pressure. A good way to choose a compressor is to first decide which tools you are going to use with it (you have done that, but will you get bigger ones as time goes on?); the compressor should exceed the CFM requirement of the largest tool you will use by probably at least 1.5 times. If you plan to run more than one tool at a time off the same compressor, add the CFM's of the tools together before multiplying by 1.5.

Generally, tools like sanders, grinders, sandblasters, etc. need a continuous flow of air and will need larger, more powerful compressors to work properly.

Another issue to get a grip on is what is known as the "duty cycle". The duty cycle is usually expressed as a ratio - how much time the compressor can safely run within a given period of time. As an example, a common "duty cycle" for compressors is 50%, meaning that the compressor should not run more than 50% of one hour... During the "on" time, the motor is pressurizing the tank. During the "off" time, your tools are running on the air stored under pressure in the tank. If your tools are draining the pressure off too fast, the compressor must run at more than 50% of the time that you have it in use. (With the 50% duty cycle the maximum pumping time per hour would be 30 minutes.)

Horsepower ratings of electric motors can be measured several ways and the figures have been abused by manufacturers over the years, so they are not a reliable way to measure performance. The CFM and pressure ratings of an air compressor the absolute best measure of how a compressor will meet your needs.

You'll probably end up looking at a piston type compressor. There are several types of those to consider..... single-stage and two-stage. Single-stage compressors have one piston usually (sometimes two) that compresses air and delivers it to your storage tank. The single-stage system is normally found on light or medium-duty compressors with a maximum rating somewhere below 150 pounds psi. Single-stage compressors are adequate for many of our needs. Two-stage compressors have two pistons that compress air. The first piston compresses the air and pushes it through a check valve to the second piston. The second piston further compresses the air and delivers it to your storage tank. The two-stage system is usually found on better heavy-duty compressors with maximum ratings above 150 psi. Two-stage compressors are good choices for heavy use environments.

Then there is the decision to get an "oil-free" or "oil-lubricated" unit. Depending on what your needs are you may not have a choice. Oil free units require less maintenance, no oil changes, etc., but some people claim that they do make more noise and will not last as long. They are good for a lot of applications, and I know many happy users of that variety. I have very limited experience with oil-free units, and I have no complaints with them. "Oil-lubricated" units are most often found in more demanding environments. Some people claim that they are a bit quieter and that they last longer. Of course there is maintenance involved with oil changes. Personally I prefer the latter variety despite the maintenance (I have this 'thing' about parts rubbing each other without lubrication.... what can I say?) Any variety that you will get will be loud. I have never been around a quiet one, so consider 'placement' in your garage, and maybe some noise control.

Sticker price can be an eye popper for a nice unit, but don't skimp if you can possibly swing it. Consider what you may wish to use it for in the future, beyond your immediate needs. A good heavy-duty unit new will likely be between $900. ~ $1500. This is where patience can come in. I decided to look for a used well cared for unit. Found one (DeVilbiss brand) on eBay for $300.(plus a 150 mile drive) with about 50 hours of use - looks and performs like new, 80 gallon tank, oil-lubricated two-stage, and delivers 17.4 SCFM @ 100 psi & 16.9 SCFM @ 175 psi. I am very happy with the unit and it will meet any demands that I put upon it now and in the future. When I got home with it I asked my wife to help me get it out of the back of the truck... It was a bit big for me to do alone safely.... her only comment was "holy $^!#!"

[=Enigma=]

Thanks for the great info John. I've been meaning to do some research on this myself to replace the 6 gal 1.5hp unit I have. About all that's good for is filling tires, blowing out clogged lines, drying cleaned parts, and freaking out the neighbor's cat when she's taking a dump in my garden.

[Jimmy240Z]

I found two compressors at Lowes today, both are by Kobalt. Both are 220 volt and they are oil lubricated. The larger is 7HP 60Gallon 13.3 SCFM@90 PSI and the price is $499.00. The smaller is 6HP(?) 45Gallon 11.? SCFM@90PSI and the price is $399.00. I'm thinking that I am goping to go for the bigger. The extra $100.00 is only a question at the time of purchase and by the time I start using it for all the body work etc. I am going to be glad for the extra capacity.

Jimmy

[geezer]

Great info. John. I would say Jimmy, you get what you pay for when buying a new unit. There are many different ways to decribe a compressor and when they are marketed they tend to get creative sometimes, clouding the decision. Ultimately the SCFM rating should give the best indication of the units capability. HP claims don't always hold true either. A few things that come to mind are, the intended use, space constraints and noise levels. I have two. One is a portable and one is a larger industrial (took 4 guys to get off the truck). I don't know the specs. offhand but the smaller is rated at 2HP and runs on both 220V or 110V. It will run an air cutoff wheel for about a minute, but because of the small 20 gal. tank and small pump is not practical. The larger unit I bought back in 1984. It is a true 5HP and has an 80 gal. tank. It will run the same cutoff wheel all day and never leave me waiting for enough air. It will power my sandblaster at about a 50% duty cycle. It still isn't big enough for me though and I borrow a diesel unit for larger sand/bead blasting projects. The one you are considering sounds to be a good choice and the price seems reasonable too.

I built an enclosure for mine, for several reasons and it has served me well. It is located out of the garage and thus doesn't take up valuable space. The enclosure is constructed using a staggered stud arrangement and insulated with sound proofing materials. The compressor can barely be heard from in the garage. It is supplied with filtered air from a stack. It stays clean also. The only thing in the garage is the automatic oiler running from one line, the regulator, the moisture bottles, a recoil spool for the hose (all wall mounted) and an overhead swivel airline. A compressor this size will need a starter box also. They can be on the pricey side too. Just a few things to consider. Good Luck