Need Air Compressor HELP!!!

I need some help with selecting a new Air-Compressor.

This will be a very seldom used, back-up/emergency compressor to run some machinery and conveyor lines in our warehouse.

The PRIMARY compressor we run is a BINKS(Quincy) Model 340-32. We had a small electrical problem and switched over to our "back up" compressor and found that it was not large enough to operate our machines. The current back up compressor was purchased long ago when we used some different machinery that required less air. So the new machines simply over-tax our back-up unit.

Under normal circumstances, our "back up" compressor only operates a few days each year.

I need a total of 100psi at 12cfm to operate the 2 primary machines. The air required ot operate the convyor is minimal, so lets say my maximum draw is going to be 100psi at 13-to-14cfm to EASILY cover my needs. Anything beyond that is excess. I think I should look at something of MODEST quality that will deliver 100psi at 16cfm, that gives me extra capacity without buying a giant size compressor. Again, this may only run for a few days a year.

My main question is for you guys who know air compressors --because I understand that compressor ratings are about as misleading as tractor specifications-- do you think this will do the job?

I found this at Sears for $819.00, it is roughly 1/2 the price of a comparable industrial grade compressor like I use for the primary unit.
Craftsman Professional 80 gal. Air Compressor, 5.5 hp, Vertical, 2-Stage Pump w/Splash Lubrication

SCFM delivery @ 100p.s.i. = 17.4 SCFM
SCFM delivery @ 175p.s.i. = 16.9 SCFM
Sears item #00916780000 Mfr. model #16780

In my opinion it will do the job you describe. The key is the 2-stage pump. Few compressors with a single state pump will meet your requirements, but it's fairly easy for a 2-stage to reach those numbers, even when powered by only 5.5 hp. The 2-stage compressor I used to own had a 10 hp, 3ph motor and would achieve 25 CFM all day long. I used it to run a fiberglass chopper gun with air knives, resin pump and MEK injector which used most of the capacity.

I'm not familar with the specific compressor you cited but I relaxed as soon as I saw it had a 2-stage pump.

I'll always opt for the two stage as well, but really it doesn't accomplish that much for your required pressures.

Your needs actually leave me in a quandry, you're looking at less than 50hours a year; yet that will be over a couple continous sesions.

The motor looks a bit wimpy [though their rated HP is at least somewhat in their rated CFM ballpark, which somewhat impresses me - either they are semi-honest or very very dishonest; I don't think Craftsman wants to risk the latter]. It probably has reed valves, which aren't the #1 choice for continous operation; but shouldn't be problematic either.

So, if may answer a quesiton with a question: If your losses during a failure [say the motor burning out or valve snapping] are less then the savings over a true industrial unit, I'd certainly pick the unit you have pictured.

Very curious too, what it's pump-up times are; this is indicative of CFM, but I don't have a chart handy or remember the equation right now (about 3AM probably!).

Not nearly 3AM, actually not even in bed, but a few #'s come to mind [don't bank on them though]:

About 1 amp of 220 per CFM (100psi). About 4cfm per motor HP. A little math with the first expression indicates electrical draw at perhaps 75% of the rated-HP-equivalent; makes sense.

Take (final gage pressure - initial pressure [0 for pump up time])/14.7
Take that times tank size/7.5 [ft^3/gal] = compressed air
Divide by time [in minutes] and you have CFM(average over test range)

I neither recommend for or against these guys for your needs, but I do love how they explain compressors (I built a 19CFM 2stage with a pump and motor from them; their 5HP motor is truely heavy duty! the pump's "fit and finish" wasn't great, but works nice and the head unloaders are awesome!!) :

http://www.eatoncompressor.com/catalog/item/504747/172993.htm

I know.....not 2 stage, but for maintaining 100psi continously you need the continous run [head unloaders] worse than 2 stage.

Bob, That compressor should do fine for your needs. I have a Devilbiss 80 gallon vertical two stage with a 5HP Baldor motor on it. It looks identical to that unit. The original 6.5 HP motor burned up on me. I replaced it with a Baldor motor. Baldor's 5HP motor had more power (amps) than the original 6.5HP motor. This was one of those develops, or should I say, can develop up to 6.5HP. My compressor was around that same price range when I bought it back in 1993. I use it almost every day at least a few cycles. Some days it runs for quite a few hours. The only other problem I had with it was when the head bolts loosened up and blew the head gasket. All in all I'm quite happy with it. My other compressor over in the barn is an industrial 5HP unit that puts at least 25 CFM. That unit was made by American kellog. I bought it used back in 1978 for $900.00 and haven't done a thing to it except to change the oil.

Stumbled across this the other day on the internet; didn't check the accuracy, but seems at least in line with what I was thinking.

SCFM x HPF x 1.1 = HP
HPF (Horse Power Factor)

at tool rated pressure:
-----------------------

psi......HPF.........HPF
1-stage....2-stage
-------------------------

40......0.107...... -----
60......0.136......0.128
70......0.148......0.138
90......0.170......0.156
100.....0.179......0.164
120.....0.196......0.178

What does that mean -if anything- here? First, what it means is that 2stage holds efficiency better as pressure increases. It's about 10% better at 120psi.
But for this application, it's not running enough hours/year to care about the power savings. However, efficiency also means that much less torque against the same motor, and futher, two stage pumps not only have less total stress, but with 2 cyliners for each cycle, less stress on rods, etc. All topped off with a lower pump temperature; the intercooler improving cooling litteraly, and of course, the augmented the efficiency means less temp to heat the pump.

Why ever buy anything but 2-stage? The extra components cost extra $$. In the low pressure/low cfm occasional use applications; it's obvious. Where I get worried, is that nothing is sacred to marketing people anymore; so if 2 compressors are the same size and $$$, but one is 2stage; either the latter is a better buy, or they cheapened it somewhere else to make the $$$ mark.

Doesn't change the fact that [barring substantial failure costs] the Craftsman unit will likely be fine for Bob's use; but thought I'd toss the info here in case anyone else was following it.

Bob, would you mind posting pump up times [X seconds between Y and Zpsi] for whatever unit you get? I'm always curious how honest these compressor ratings are.

I have that compressor in my garage. I have had it about two years. Use it probably at least a few cycles per week. All I have had to do is change the oil.