someone please help me figure this out

for instance
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sears pro 25gal 3.5hp 2-stage pump 175psi w/ 140psi cut-in
SCFM Delivery:

SCFM Delivery At 40 psi

5.6 SCFM

SCFM Delivery At 90 psi

5.1 SCFM

Tank:

Compressor Tank Capacity

25 gal.

Maximum Compressor psi

175 psi

Compressor Tank Pump Style

2-Stage Oil Free

Compressor Tank Type

Vertical

Motor-Engine:

Amps

15.0

Horsepower (javascript: openWindow('http://www.sears.com/sr/advisor/glossary/popup_glossary_term.jsp?BV_UseBVCookie=Yes&vertical=TOOL&glossarykey=787','glossary','height=150,width=300, resizeable,scrollbars=1');)

3-1/2 hp

Type

Electric

Cooling Fan

Yes

Cooling Fan Type

Internal

Drive Type

Direct drive


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now heres another wth the same size tank. just different psi and hp. both a sears craftsman professional series
SCFM Delivery:

SCFM Delivery At 40 psi

8.6 SCFM

SCFM Delivery At 90 psi

6.8 SCFM

Tank:

Compressor Tank Capacity

25 gal.

Maximum Compressor psi

150 psi

Compressor Tank Pump Style

Oil Lube

Compressor Tank Type

Horizontal

Motor-Engine:

Amps

15.0

Horsepower, Maximum Developed

6.0 hp

Air Filter Type

Auto-type paper

Cylinder Liner

Cast iron

Overload Protection

Thermal

Voltage

120/240 volts




whats the deal with the SCFMs? Why would a tank with less psi have more SCFMs? My compressor i have now is a CH 20gal, 5hp and 125psi and it doesnt run my impact very well. I have an IR Thunder Gun 1/2" impact. supposed to be rated at 650ft/lbs in reverse and sometimes wont even take off some lug nuts. i was lookin at the 2 compressors above to buy just tryin to decide what i need.

Ok let me see if I can explain this without confusing myself. Basically SCFM = to Square cubic feet per minute. Its really what does the work especially in an impact. The pressure counts also but not near as much as the SCFM. Lets take your impact I don't know the specs on it but I know it uses some air. There should be a spec either on the box or instruction sheet that says what the recommended SCFM is. What this states is the amount of air volumn needed to run it. There is probably also a pressure recommendation. Now here is where it gets critical, keeping both of those recommendations at the same time for best performance of your airgun. Think of a tank of air thats 20 gallons in size sitting at 125PSI. As soon as you hook up your impact the pressure drops as the volumn is used up. The compressor has the job of keeping that tank at 125PSI. The SCFM rating is a measure of how fast it can put the air back into that tank. Also, the bigger the tank the more air volume that is available makeing the compressor job easier to "keep up" with discharge. This is why the bigger the tank the better off you are. Now back to that pressure. There are two types of compressors. Low pressure (single stage) and high pressure(double stage) the single stage is usually around 125 psi max. The dual stage is around 175 psi max. Obviously the more pressure the better some air tools run. Especially impact tools. (there are limits here so this is not a blanket rule). Hope I didn't lose you here and if I am not clear I am sure its my lack of ability to explain things. Just let me know and I will see if I can explain things better.

Now for my recommendation on air compressor. Take the specs on the tool you will use the most and match the compressure specs to that tool. When I bought my compressor last year I got the most SCFM I could afford. I think mine is around 27 at 90PSI and 21 at 175 psi. Obviously the higher the pressure the lower the SCFM will be. Good luck on your purchase and let me know if I can explain anything any better for you. Also think of your compressor as a tool thats going to be around for a while. There are a lot of different types of tools it can run if your compressor is big enough for the job. So if you might have any future plans for something like a sand blaster or something take that in consideration and spend a little more on your compressor now to get something to handle the job.

Keep us informed on how you make out

First, your problem with the air gun is probably the regulator not flowing enough. Buy a good quality regulator and make sure it is tapped directly to the tank. A 20 gallon tank should have enough reserve to run an air gun just fine.

SCFM Standard Cubic Feet per Minute. Should me taken at 0 PSI. This is more or less the max flow of the compressor with no load. All the other specs should be CFM @ some pressure and show how well the pump performs as the pressure rises. The only one you are interested in would be the CFM@90.

With compressors you have two things to consider. The reserve capacity (tank size) and the pumping capacity (pump output). The bigger the tank, the longer the time between cycles, the bigger the pump, the faster the tank recovers.

For short burst tools like the air gun, they run off the tank. Long run tools like spray guns and grinders need enough pump to keep them fed.

SCFM is measured at the tank...In other words, SCFM is the useable CFM you'll get out of that particular compressor..
In general Ryan, a 1/2" impact requires about 3.5 CFM...
The small tanked air compressors, as fingers said, will run burst tools just fine provided you have a good regulator....If you're going to run something like an orbital sander, air chain saw, grinder, etc..you need more air...


this is just what you need...see post #16....http://www.dieselplace.com/forum/showthread.php?p=655590#post655590 ):h

I agree that your current setup is plenty. I have a little 2 cfm compressor that works okay for me-but I have a total of at least 25 gallons of capacity (everytime I see a little air tank on an auction go dirt cheap I buy it and add it to the system). Of course a bigger compressor is always better as I can only use my cut-off saw or grinder a little at a time before having to wait for the compressor to catch up, but the my old CP impact works fine.

If your problem does not lie in your regulator, you might also check your lines and fittings for rust and scale build up. If you don't have a drier on the line, the fittings can start building up corrosion like a water pipe, and slowly shrink passage until you are trying to get air through a tiny little hole.

If your running a regulator and impact first thing first take that regulator out of there! Number one with that small of an compressor you certainly won't exceed the pressure limits of that impact. I only use regulators for precision tools like nail guns etc. Everything else is plumbed straight to the tank. If your trying to run something that takes a lot of air like sander, impact or grinder that regulator will only slow the air down. As your running the tool I guarantee your not keeping the pressure up anywhere near where an regulator is necessary. Most compressors that I have seen, if they even come with an regulator, has a port straight on the tank and another port for the regulator.

shew, some of this is comfusing, well what im gonna do is go take my reg off. actually i might take a pic and let you guys see what im workin with so maybe you can better assist me. if i take the reg out of the equation then how does my comp know when to kick on? should i maybe "T" it off?

Ryan, are you going to use 220? If you get a 15 amp 110, you will probably hate yourself, especially if you have any 14 awg house wiring (allowed here).

If you have 10 awg and a 20 amp circuit, go for it. Otherwise, 220.

my compressor has a SCFM of 5.7 @ 90psi

one of my CP Impact requires 4 CFM @ 90

a comparable IR impact requires 5 CFM @ 90

so my current compressor should be doin a little better job. must be all the reg crap

Ryan - if you have the type of compressor I think you do, don't remove the regulator...Doesn't it have the switch and wiring incorperated into it?

Ryan - if you have the type of compressor I think you do, don't remove the regulator...Doesn't it have the switch and wiring incorperated into it?

yea thats what i was just lookin at why cant i stick in a T-fitting and just let it sit on one side to control the compresssor and use the other side of the T for an unregulated source

Didn't realize your regulator was part of your pressure switch when I said to remove it. Sorry about that... Definatly T it off like you suggested.

Try using a 3/8 inch air line and fittings hooked to your impact. Better flow should make it work better. I know with my dad's old Craftsmen compressor the 1/4 inch line won't run the impact at full tourque, but it works fine with a 3/8 inch line. Same with a framing nailer.

Take a pic of where you plug in the hose Ryan. Then we can be sure the reg is seperate from the controls. I have yet to see them combined on any compressor.

Try using a 3/8 inch air line and fittings hooked to your impact. Better flow should make it work better. I know with my dad's old Craftsmen compressor the 1/4 inch line won't run the impact at full tourque, but it works fine with a 3/8 inch line. Same with a framing nailer.

Ideally only the last 5 ft should be regulated air. But that isn't real practical. I have an inline pressure guage I use right at the tool. That allows me to predict what each tool needs back at the regulator to give the tool what it wants. It is a must for spray guns to get a quality job.

Just what I was going to say. Ryan, the regulator doesn't shut the compressor off, a pressure switch does. But, some compressors have the regulator built into the same module as the switch. I don't see any harm in leaving the regulator on, we run Dad's 1" impact through a regulator (of course this isn't a tiny little regulator we are talking about).

This is a dumb question, but where is your regulator set at? Maybe it is just turned down to far.

turns out my reg was not integrated to the thing that kick the compressor on. i unscrewed the reg and then run the air line straight in. its like a whole new impack. i took a breaker barw/ a cheater bar on it and cranked a lug nut down REALLY tight. to the point i know that in the pas the impact wouldnt take it off. Put the impact on it and a sec later it was off. i actually started goin over my lug nuts retightening them and was gettin anoth 1/4-1/2 turn on what the impact had previously done. now i wanna plumb an extra 5-10 gallon tank in the system for a little extra capacity.

RyanU, use a torque wrench on those lug nuts. Overtight is not better, nor stronger. You can stress the lug or even break it off. First you'd loose your nuts, then you could loose your life...Yikes!

As for using impact wrenche without regulated air pressure, my IR 1/2 pro model says 90 psi right on the gun. So should I say IR doesn't know what they are talking about it and run it at the 135 that my IR 60 gallon compressor (just got it from TSC for $400, nice!) puts out? It makes sense it'll crank even harder at more air pressure but I was thinking I'd be rat-tat-tat-tatting at too intense a pressure or frequency and exceed the design of the gun. Since it does 600 ft lbs at 90, it out to work no matter what I need.

My 2 cents on compressors: buy all you can afford, consider it a lifetime investment (or at least long time, with proper maintenance) and you'll be glad you did.Too much air volume and pressure is a nice problem easily regulated down, too little is a royal PITA. I just retired my original purchase Sanborn from Sam's (20 years old), and wonder why I waited this long!

Stay away from anything Sears makes except hand tools. Buy a two stage Ingersol Rand.

Personally, I am an Emglo fan. I went through a series of compressors till I got my Emglo's. My shop compressor is a 4cyl 2 stage Emglo that is now 25 yrs old, and I still have both portable construction compressors I bought 10 and 13 yrs ago for my construction business.

you guys are hurting my craftsman compressors feelings.........

Compressors are rated by their max discharge pressure and intake flowrate. Hence the name, they compress air, so the intake volume flow is different from the output volume flow. HP is a bit misleading since the efficiency of the compressor varies, and it's compressed air you're after. HP is just how it gets that way.

They don't have infinite compression ratio. When the piston comes up, the air is forced out. When it hits TDC, the discharge check valve closes, and there is high pressure air caught in the little headspace. This trapped air reexpands as the piston heads down and the intake check doesn't open until the cylinder pressure drops to 0 or slightly below. So, a bit of the intake stroke is wasted and this reduces how much air can get drawn in.

The higher the discharge pressure, the more trapped air, and the more flowrate is reduced. They all do this, even the good ones.

Whatever you buy, make sure to get one with a decent quality water drain on the tank. Draining that water is important to the life of the tank.

A regulator is a needle valve attached to a pressure-sensing diaphragm. As the outlet pressure drops the needle valve is opened. How much flow depends on the valve size, and how much the valve opens depends on how far below setpoint the pressure drops. A bigger regulator can flow more air with less drop. I retrofitted mine with a 3/8 regulator, and that's plenty for an impact wrench without overpressuring the wrench.