CFM's in compressors

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Anyone know how this rating works? Why are ratings of cfm's higher at lower pressures? If a compressor is rated for 8 cfm's at 40 lbs, is there enough stored air in it's 30 gallon tank to run a 10 cfm consumption tool?

 

[davesl708]

The rating is stating you can use 8cfm @ 40 psi continuously. You can run a 10cfm tool for a short time. You will have to stop and let the compressor catch up or loose cfm. The larger the tank the longer the high demand can be maintained before the compressor has to restore air at a given pressure.

 

[BHD]

the simple of it air compresses, fits in a smaller space at higher pressures, so there is X micro grams of air in a cubic foot of at 40 pounds of compression, at 100 psi it may take twice as much or more air (in micro grams) to fit in to a cubic foot of space,

so it take more air at a higher pressure to get a cubic foot. thus the compressor has to run longer to make a cubic foot of air at a higher pressure,

 

[BHD]

the storage tank works some what like a water tower, say your pump can pump 8 gallons a min of water, as it pumps in to the water tower, but your tanking out 12 gallons a min, you can continue to pull 12 Gallons out until the tank nears empty and all you have is the pump pumping it 8 gallons a min,

now if your compressor is running all the time and only using the tool for short bursts, it will do fine, it is when the PSI drops down (tank getting empty) will it adversely effect the tools out put,

(air is a little more complicated, but I think you get the picture),

many air tools are more bust or short term runs, air impacts, and most air chiseling, nailers,
sanding and paint spraying are more continuous, and usually need a pump near the rated out of the tool being ran,

adding more air storage can help, when I was working and the compressor had to be a long distance away from the tool I would run a air hose with a check valve in the line to a "air bubble" (portable tank for compressed air) and then the final hose out of it to the tool, yes the tool would work of the final tank and in between using (nailer for example) the tank would recharge, and would act like I was close to the compressor,

Another time when I was having problem I put a 100 gallon tank on a small 6 gallon compressor and could work a good time with the small compressor, (down side, it took a long time to pump up and recover). and hard on the small compressor running continuously.

 

[SPYDERLK]

Anyone know how this rating works? Why are ratings of cfm's higher at lower pressures? If a compressor is rated for 8 cfm's at 40 lbs, is there enough stored air in it's 30 gallon tank to run a 10 cfm consumption tool?

CFMs are higher at low pressures because a compressor has a compression ratio. Most air goes to the tank, but some air is left in the cyl after each stroke. That residual air is at whatever tank pressure is at the moment. The compressor must now take a new lung full of air to compress, but its intake stroke is starting out with a little pressurized air in the cyl. The 1st part of its intake stroke must expand this residual to below atmospheric before it can start to suck in new air. The higher the tank pressure gets the more of the intake stroke goes to waste this way.
,,,Compared to a single stage compressor, a 2 stage compressor has a much higher effective compression ratio and retains almost constant CFM as pressure rises.

 

[4570Man]

You could run it for a short period of time. Your tool probably needs 90 psi, and your output is much less at 90 psi.