cfm ratings- tools vs compressors

[byrdman]

There's gobs of info out there to help you decide what you need in a air compressor. I've read a good bit of it. I can't figure out why this problem is not addressed more often...

I'm trying to go about deciding between two Ingersoll-Rand compressors - the TS51N5 and the TS7N7.5. Everybody will tell you to use the air requirements of your hungriest tools to dictate how much CFM you should seek in your compressor. Well, here's the problem I'm running into with that:

given:CFM varies with PSI. That being said, compressor specs tend to list CFM at one PSI only, usually the compressor's maximum (175PSI). Air tool specs tend to list their CFM requirements at a lower PSI(~90PSI). What I really need is a breakdown of compressor CFM output at different PSI's, but I've yet to see that.

So, ignoring other factors- if a blast cabinet requires 22CFM@100PSI, how do I determine if the 5HP model producing 17CFM@175PSI is adequate?
(Yeah, yeah I know, buy the "biggest one I can afford". I can respect that line of thought. I just don't want to spend more than I have to!)

Any thoughts/advice/tips/financial donations?

 

[bmwpower]

Nice choices. Just looked at these models this week.

The only thing I can think of is to call the manufacturer and speak to someone who knows the specs. With out several other points of reference, you really can't "figure out" how many cfm at a particular psi you will be able to produce. Even then the curve is probably not going to be linear, so extrapolation using points of referene may be impossible.

Without knowing for sure, both of those units might NOT work for you.

 

[Glen]

In general, compressor output in CFM goes up as pressure comes down. The proportion depends and you will have to call the manufacturer for exact numbers, however, you can safely assume that if the compressor delivers 17 CFM at 175 PSI, it will deliver MORE CFM at 90 PSI.

 

[Major Ramifications]

Well, with enough data points, you could interpolate what the volume would be at 100 PSI, but you only have one data point.
So all we can do with this is to find out is how much work the compressor is capable of doing. Converting it to units of electricity; 17 CFM (amps) X 175 PSI (volts) is equal to 2975 units (watts). Given that the blasting cabinet requires 2200 units when run constantly, you should be OK. Add to this that you will not be blasting continously, and that the compressor has a large reserve tank, and you are even more comfortably OK.
If you don't envision yourself using more than one pneumatic tool at a time (kind of hard for one person to do), and you don't plan on adding any equipment that uses more air than the blasing cabinet, (not much uses more air than that) you should be happy with your choice.

 

[iiibdsiil]

I remember a bunch of people saying that it SEEMS like the air tool people always under rate the CFM, and the air compressor people always over rate the CFM. I believe this on the lower priced stuff, but not so much on the big $$$ stuff.

 

[bdaz442]

One more thing that you should look at, is the motor running the compressor.
Make sure that you are getting a 5 HP motor.
A true 5 horsepower motor will draw 24.5 amps at 230 volts.
AND Never buy a compressor that uses a 3450 RPM motor. It should turn 1750 rpm. if you want your compressor to last.

 

[rhandwor]

You need two things volume and capacity. If you have a large storage tank it will handle surges air grinders and ratchets have a large demand. But the compressor can catch up after the job.
Good ASME rated tanks will probably cost more than the compressor. Look for a garage going out of business. Most garages use two compressors and tanks so only one is normally running to save on power.
Average for small compressors is appx. 6cfm per horsepower. Most home units are undersized. Try a 60 gallon tank and 5hp comp for good home use.
With a smaller unit you have to stop and wait for the compressor to catch up.