Needed: Compressor Operation for Dummies

gator · 08-22-2007, 07:20 PM

Sometimes this old brain just doesn't understand things. Let me explain what I think I know and then chime in with what is real.

The air you use to run air tools comes directly from the holding tank via pressure valves/gauges. If a compressor is rated at say, 7CFM@40PSI/4.9CFM@90PSI/4.3CFM@115PSI, that is the rating of the compressor pump and how fast and at what pressure it fills the tank. Thus if I have an air compressor that came with the first rating and find a pump/motor that is rated at say 7.76CFM@115PSI and put this pump/motor on my tank, would my tank output still be at what it is now or would it go to the higher spec. Remember, I have not changed the valves/gauges on the output side of the tank. In other words, is the rating of an air compressor related to the pump/motor and how fast it fills the tank or to the effeciency of how well you get the air out of the tank to the tool. To me the two are not directly related. (I do realize that if you take more from the tank than can be replaced in the same amount of time, then the compressor will run behind and run continous). All this being the case why can one not run high CFM tools from a holding tank, just in shorter spurts as the pump tries to catch up?

Confused!!!!!

George

FredP · 08-22-2007, 07:46 PM

the motor and pump determin CFM@??? PSI. the size of the tank doesnt matter much. a larger tank will take longer to fill than a small tank if the CFM@PSI is the same because it holds more air. however for high consumption tools a larger tank allows you to use the tool for longer periods of time before the pump kicks in and if the CFM@PSI is large enough you could run the tool as long as you like because the pump will replace the tank air faster than the tool uses it. small tanks can be used for most air tool but in moderation. any motor and pump needs to cool or they will stop working in short order. using a high volume tool off a small tank even with a high CFM motor and pump will eventually burn up the compressor because it will run hot unless you let it cool down between those short bursts. my shop compressor puts out around 17 CFM@ around 100 psi. but it only has a 40 gallon tank so things like sandblasters would require a break every now and then to allow the motor and pump to cool. I can use a blow nozel or impact wrench continuousely and the compressor will cycle fine however i do let it cool down once in a while just because.

[ because i realy like my compressor! ]

fred p

Travis Porter · 08-22-2007, 10:28 PM

Totally agree with Fred. One side note, on most compressors I have seen there are no valves or such on the output unless you install one yourself. The hook up is straight off of the tank. The pressure switch is in between the compressor and the tank and kicks on and off when the tank reaches certain pressures just like a well pump pressure switch does.

gator · 08-22-2007, 11:09 PM

Originally Posted by Travis Porter

One side note, on most compressors I have seen there are no valves or such on the output unless you install one yourself. The hook up is straight off of the tank.

I think that all compressors have a pressure adjust valve on the output line. I can adjust my output pressure from about 20PSI to 115PSI. It has two gauges, one showing tank pressure and one showing output line pressure. Output line pressure will never be higher than tank pressure.

But, I guess the basic question from all my ramblings was if I want to run a spray gun needing 6.5 CFM but my compressor is rated at 4.3, I can do it but only in spurts, and trying to upgrade only allows the tank to fill faster. Or, is there something I am missing and I really need a compressor rated at 6.5 or higher before I can run that spray gun. You know how people say the pancake types won't run spray gun but if I only run it for 10-15 seconds, wouldn't it work?

George

Gofor · 08-22-2007, 11:31 PM

It sounds like the compressor/motor are fine, but you have volume limiting restrictions between them and the tool. Your problem may be the fittings between your air compressor tank and your tool. 1/4" NPT fittings/hoses do not allow enough volume of air to supply an HVLP gun for example regardless of the tank pressure. If your regulator, oil/water separator, etc are 1/4" NPT fittings, you probably are not getting the volume needed to supply the tool. You may get by with one 1/4" fitting (ie, the 20 gal tank exit), but by the time you add the regulator, several quick disconnects, and a few feet of 1/4" id hose, you cannot get the volume of air you need without upping the pressure to 300 psi. Try upgrading the regulator to a 3/8 ID min inlet/outlet fitting, quick disconnects (air hose fittings) to "high volume" (normally 5/16" id), and go with a minimum 3/8" air hose until you get to within 5" of the tool. If you can go with a larger ID fitting coming out of the tank, do so. The regulator is most likely your main restriction.

Go

DavidF · 08-23-2007, 12:13 PM

Originally Posted by gator

But, I guess the basic question from all my ramblings was if I want to run a spray gun needing 6.5 CFM but my compressor is rated at 4.3, I can do it but only in spurts, and trying to upgrade only allows the tank to fill faster. Or, is there something I am missing and I really need a compressor rated at 6.5 or higher before I can run that spray gun. You know how people say the pancake types won't run spray gun but if I only run it for 10-15 seconds, wouldn't it work?

George

I would say yes, you can run the gun for short bursts, unless the volume of air you are consuming robs the tank so quickly that the available pressure goes below that required. I run my tank outlet at 90psi and re regulate to 30psi at the gun when spraying with HVLP. If you drop below 25 - 30psi at the gun then you will not get good atomization. With other high volume tools it is not quite so important as they just stop working!

FredP · 08-23-2007, 01:40 PM

the problem with spraying finish from too small a tank and less than required CFM is you may run out of air before you get to a stopping point that is condusive to a good finish. IE... if you need to make four passes and run out of air after only 2 passes your finish will suffer.

fred p

jeff... · 08-23-2007, 02:03 PM

Don't know about you guys but I love my air tools

NCPete · 08-24-2007, 07:48 AM

gator, I have an air tank you could use to increase your available capacity, it was my plan to put it in line with my pancake compressor to increase available air, but then found to good a deal on a 2hp (almost real 2hp- one of the re-rated machines formerly labled 6 hp at sears) compressor with a 33 gallon tank. lemme know, we can make the North Carolina Woodworker Express go to work!

gator · 08-24-2007, 08:19 AM

Pete,

Thanks for the offer but I have a 21 gal compressor now. The first specs I listed, the lower ones, are the specs of the one I have now. I saw a reasonably good deal on a 5HP pump and motor and was considering upgrading but after thinking I came up with the supposition I made in my original post. That is, that a bigger pump and motor wouldn't do anything except fill my tank faster and that I really would not have any more capacity (CFMvsPSI) in real life than I do now.

George

Travis Porter · 08-24-2007, 11:41 AM

IMO, with a 21 gallon compressor, you can do some spraying. When the compressor starts up, it is time to finish that pass and let it catch up a bit. Adding an extra tank to the mix will keep it from having to kick in as frequently.

Back on the discussion of the regulators, Gator, you are right. I have a 20 gallon compressor and it does have regulators for the output. On my bigger compressor (60 gallon stationary) it does not have regulators for the output. Just a connection right off of the tank.

Residency requirements