The Stovebolt Site Forums The Shop Area The Tool Chest CFM vs Tank size

What's more important, CFM or the size of the tank?

For instance, Harbor Freight has a 30 gallon unit that puts out 14.5 CFM @ 90 psi and 16 CFM @ 40 psi. Seems to me that the tank size is of little consequence if the pump can't produce enough air to replace what's being drawn from the tank.

Isn't it true that a larger tank just extends the amount of time the compressor is idle when you start draining the tank by using a tool. And if you're using a 9 CFM tool and you have a 7 CFM compressor the compressor will never catch up until you stop working.

But conversely, a 9 CFM will maintain operating pressure regardless of how long a 7 CFM tool is running.

Or am I missing something?

The longer the compressor is at rest the cooler it will run. Hot air = water extraction from the ambient air. With a larger tank the less time the compressor will run = a longer lasting compressor.
Tom

Czechman,

I don't think you are missing anything. CFM is the holy grail when it comes to pumping air. While it's important to state at what pressure you can maintain that flow, the volume is the key.

I think lots of marketers of air compressors these days purposefully attempt to confuse this fact with overrated HP's and ambiguous CFM/PSI ratings.

Stuart

Stu is right on Woody, and you have it. Just picture a bucket of water vs a 55 gal barrel with the same size hole in the bottom the bigger the hole (cfm) the more fluid is flowing out. If the hole that goes in is the same size as the hole that goes out the container volume doesnt change. Now alter either the hole that goes in or the hole that goes out and you can see the relationship. The container is simply a buffer for short bursts of output. if you had a 10,000 gal air tank all that would mean is that you could probably run your air grinder for couple of hours before the pump kicked on again. Of course the pump would probably burn up refilling the tank unless it had a 100% duty cycle.
The cheaper compressors that most of us can afford are simply not designed to run continuous ly.
And from my point of view, if you could afford a compressor that put out 25cfm you probably wouldn't need much of a tank at all because it would keep up with just about any thing that you might add to your tool box.
Denny Graham
Sandwich, IL

I use the most air when I am sanding, when the drain on the system is continuous. (My 5 hp single phase, which is more like 2hp, 40 gal) system can't keep up with the high volume tools that are in continuous use, but for 99 percent of my needs it works very well. The only reasonable way to increase the performance of my unit would be to go to a 5 hp 3 phase motor, or to a gasoline powered unit ( which would be inconvenient for me )
The larger tank helps to ensure that I will have the minimum required pressure for a longer period of time.
Bob

I'm with Denny & Stuart. CFM should be your main concern. Beware of the CFM ratings listed. Few tools work at 40 PSI. The 90 PSI rating is more important to me. The purpose of the large tank is to reduce the amount of total run time & allow a little cool down time between runs. The IR I bought has an 80 gal tank & is rated at 18.1 CFM @ 90 PSI continuous with IR oil in it. The max pressure is 135 which is of no concern since nearly all air tools work at 90 PSI +/-. I've never come close to overworking it. They are sold at Tractor Supply and are only slightly more expensive than the Harbor Freight stuff.

Thanks for pointing me in that direction Tiny. TS has THIS sweet IR compressor for under $600. A bit more than the HF unit but I'm getting an excellent unit from a reputable company and a first class dealer... AND they are building a TS store about 5 miles from my house.

That seems like the way to go for me.

Nice choice my man. One bit of advise, before you fasten it to the floor, lay it on it's side and replace the draincock on the bottom with an elbow & enough pipe to clear the edge of the tank. Put your draincock there....unless you like laying on the floor to drain the water out of the compressor.

Great suggestion! Helps make maintenence easy!

Look at this web site for a clear picture about compressors. Look at the compressor descriptions. I bought one from the factory and would buy another.

www.eatoncompressor.com