Air Compressors and Air Tools

[mrX]

I'm looking at buying an air compressor and want to be able to use it for a variety of things. The one I'm looking at getting is this:

 

http://www.dewalt.com/us/products/tool_det...?productID=4774

 

What I'm confused by is the 3.2 CFM @100 PSI. I'm looking at tools like the following:

 

http://www.sears.com/sr/javasr/product.do?...pid=00919685000

 

http://www.sears.com/sr/javasr/product.do?...pid=00918820000

 

Both have CFM numbers higher that 3.2. Does this mean the tool won't work with that compressor? or is something else figured into this?

 

Just not sure what I should be getting here.

[All Kill3r]

cfm is flow and psi is pressure. . unfortunately your compressor links not working for me so i cant help much more. is its max rating 3.2cfm @100psi ? whats the HP ?

 

I think Facehole knows a bit about automotive spraying if you wanna shot him a pm about that part of it. the ratchet will work will less pressure then its rated for but with less torque, it wont matter though unless your doing some serious torqing, most nut/bolts on vehicles dont need 100psi.

 

if your asking how to convert psi to cfm thats a pretty complicated question that math involved there is beyond my knowledge.

 

i would shoot for an air compressor with something like 6cfm@80/90psi. Thats what the Lowe's near me carries for around $200.

 

-edit- was going to search for the product # on dewalt site but i cant get www.dewalt.com to open either

Edited May 26, 2004 by All Kill3r

[mrX]

Maybe this link will work for it from Lowe's:

 

http://www.lowes.com/lkn?action=productDet...01991-70-D55151

 

It's 2 1/2 HP. Costs $259.

[All Kill3r]

[Bubblegum Bandit]

MrX....The simple answer is yes, you need to match the cfm required by the tool with the compressor's rating. Especially with the paint sprayer. Basically, if you hooked that paint sprayer up to that compressor, it would spray for a couple of seconds, maybe a minute and your tank would be out of pressure and you would have to stop and wait for the pressure to build back up before you could continue. Not ideal if you are looking for a smooth finish. Killer was dead on about the wratchet though. Yes it will work, however there will be less torque than if you ran it at the higher cfm.

 

That compressor you have linked would be great with air nailers and simple tools that require a short burst of air to do their work. If you want to do paint spraying, orbital sanding, or much wratcheting, you will at the very least need to go with one with a substantially bigger tank. In order to keep up with the sanders and paint sprayers you will definately need a bigger tank and motor.

 

As far as converting pressure into flow, I'm pretty sure its a dynamic value, meaning it depends on several different variables that are changing the whole time you have the tool running. I would think the flow would have a direct correlation with the pressure as far as, as pressure goes down, flow goes down. This would be why the compressors usually list a flow at 90psi and 40psi. ex. Using the sprayer, as the pressure in your tank lowers, the less air flow and paint that is going to be coming out of the sprayer nozzle. (this is where the larger tank would come in handy, it would take longer to lower the pressure if there is more volume in the tank at the higher pressure. )

 

Another thing to look at when buying one of these compressors is the lubrication. Whether it is an "oil-less" or "oiled" compressor. Basically from what I have been told and have experienced, the "oil-less" are less maintenance but run considerably louder and tend to have a shorter life span than their "oiled" counter-parts.

[All Kill3r]

you could just make the tank hot and increase the psi its only air, no kaboom, just some shrapnel

 

glad you knew something about the paint gun... i didnt want to mislead about that one, not my area

 

For painting you would need to spray continuously until the job is done, so the compressor must have at least enough output to run the paint gun in continuous mode. But don't get scared by a paint gun that claims to need 8 to 12 CFM, because most guns will spray automotive paints (relatively low viscosity) at much lower pressure and volume that the stated numbers. I never run my paint gun above 40 psi, and I nearly always use 1/4" hose for the last 10 feet. The compressor does run continuously, but the pressure never falls below 40 at the regulator. In other words, that 7.5 CFM at 40 PSI is plenty enough to run most paint guns.

 

If you ever use a tool that sucks more air than the compressor can put out, then you start calculating duty cycle and trigger time. Duty cycle is supply/consumption, so if you have a 9 CFM tool and a 6 CFM compressor, it can only run 67% of the time at full power, and has to rest the other 33% of the time. The rest of this is a tough lesson.

 

Trigger time (how long the tool can run continuously) is determined by the size of the air tank and the magnitude of the swing between maximum and minimum tank pressure, and the relationship between cunsumption and supply. A 20 gallon tank (2.66 cubic feet) bleeding down from 100 PSI to 40 PSI will give up about 4.7 Standard Cubic Feet of air (see formula below). This can be supplimented by flow from the compressor while the tool is running. As such, the tool draws all of the flow from the compressor, and the excess flow required is drawn from the tank. In this example the 9 CFM tool would be drawing 6 CFM from the compressor and the other 3 CFM from the tank, so it could run with full output for 4.7/3 = 1.57 minutes (while the tank pressure was falling from 100 PSI to 40 PSI). That's the allowable trigger time. Divide this by the duty cycle (.67) and you get the total cycle time (2.34 minutes). As such this tool running with this compressor and tank can run full bore for 1.57 minutes out of any 2.34 minute period (assuming it is shut completely off for the rest of the time).

Edited May 26, 2004 by All Kill3r

[mrX]

Thanks guys!!

 

With what's here and what I've found on some different sites, I think I got the answer. Gonna need a bigger tank and motor. While I don't plan on using it much as a painting machine, I do plan on using grinders, cut off tools, ratchets, drills, and sanders with it the most, which seem like pretty air-intensive tools.

[Fatty]

Will these things be available to play with at FF04?

 

<runs to catch plane>