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Old 02-11-2013, 09:16 AM
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Originally Posted by oldred View Post
Fellas you are being concerned about the wrong thing, an 80 gallon tank will make no difference vs a 60 gallon, how could it? Think about it. The tank only stores air the pump puts out it does not nor can not add air, I know what the usual argument is and that's "reserve air" but honestly it don't work that way. Let's take that 60 vs 80 gallon example since that's what usually comes up, if your compressor is running out of air at an annoying rate with a 60 gallon tank just how long would it run if it only had a 20 gallon tank? Mere seconds before cycling on right? Well that's all you would add to the run time but then even that little bit is lost to the proportionally longer recharge time so what was gained? Obviously nothing is gained, all that happens is that with the larger tank you will get a slightly longer run time (usually only seconds under high demand) that is immediately off-set by the proportionally longer recharge time so exactly NOTHING is gained during any given work period, that's NOT the purpose of the larger tank in the first place. No matter how it is cut, sliced or diced over any given work period you will have the same amount of air available regardless of whether the tank is 60 gallons or 80 gallons, the larger tank will have fewer but longer cycles vs the smaller tank's more frequent but shorter cycles but the time spent running vs recharging, all else being equal, is going to be EXACTLY the same! You need to be concerned about the CFM and the compressor's duty cycle and quality because whether you believe it or not a 16 CFM 60 gallon tank compressor would outperform a 15 CFM 80 gallon tank compressor or for that matter the 16 CFM with only a 40 gallon tank would outperform a 14 or 15 CFM unit with an 80 gallon tank! A bigger tank DOES NOT make a bigger compressor!!!

So why are bigger tanks used? A tank is chosen by the designer to balance the run time vs recharge times (duty cycle and cooling) based on CFM and expected demand (on a quality compressor anyway, cheap Chinese units are for another reason) and not to help run time performance. A larger tank can help with wear and power usage over a long period of time due to fewer high load start cycles but over the life of the compressor this is going to be almost negligible between a 60 vs 80 gallon tank with all else being equal. The point is DON'T WORRY ABOUT THE SIZE OF THE TANK! If you choose the right compressor for the job based on the important factors such as CFM, duty cycle and quality then the proper tank size will be chosen by the designer, if it's 60 or 80 gallon then either is just fine and it would be a huge mistake to pass up any of the other factors (especially CFM because if it's low then nothing else matters!) just to get a bigger tank but unfortunately it happens all the time. That's what I meant when I said the cheaper outfits choose a tank for another reason, they are aware that most people simply do not understand the physics behind an air compressor and will go straight to the biggest tank in the store, hey that big tank just looks like a big compressor right? For this reason there are too many compressors out there with woefully under-sized pumps and under-powered motors mounted on uselessly oversized tanks that LOOK big and bad but in operation would be lucky to make enough air to properly blow the hat off your head, that big tank is nothing more than a sales gimmick. When choosing that compressor ask yourself, do I want a compressor that LOOKS impressive or do I want something that will supply the air I need? To get the air you need will mean CFM available at the tool not a big empty tank with a little pump straining to fill it!
Listen to what Old Red is saying as he turns blue in the face repeating himself --- Big tanks mean NOTHING for your usage, they are just a marketing ploy for your misdirection.

Like OR said - in an engineered system the sizing of the tank vs. air consumption vs. run time is considered.

IF you were to commission a study, you would quickly find out you need a compressor about 4 times as large as the maximum anticipated air usage.

That means the air tool that says it uses 12 cfm@90 - you will need an air compressor that makes around 45-50 cfm@90 to be properly sized.

Does this happen, yes in industrial settings.

But it doesn't happen in your garage or mine or most body shops for that matter.

Like OR said, AMPS @ volts and cfm along with the duty cycle are the true measurements of what you are buying.

To step up a bit, large air compressors do not even have storage tanks. The pipe directly into a system that has air receiver tanks located around the facility. The tanks supply a large surge usage supply, but the do NOT increase the output of the compressor(s)

One more time - BIG TANKS MEAN NOTHING to the amount of air the compressor can produce.

Hp X 746 Watts
---------------- = AMPS

5 hp x 746w / 220v = 17 amps IF you had 100% efficiency but you wont so multiply the 17 by 1.2 = 20 amps

I would search for a compressor that has a 20+ amp 220 volt motor and not focus to much on the rest of the details.
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