CFM Guidelines

[TimWilborne 108]

Does anyone have any guidelines for estimating CFM requirements of a plant? I know that air motors and other constantly running air equipment would have to be taken into full account but what about machines with air cylinders that occasionally operate? Chances are slim that every air cylinder in the entire plant will actuate at the same time.

[Steve Bailey 118]

Multiply the volume of the cylinder by its cycling rate. For example, a 2" bore x 6" stroke cylinder that operates every 30 seconds. Since we're just estimating, we're going to ignore the difference in volume between extending and retracting. Cylinder volume is PI * 1^2 * 6 = 18.85 In^3 = .01 Ft^3 / stroke Operating every 30 seconds = 0.01 Ft^3 / stroke * 4 strokes / minute (extend and retract twice) = .04 CFM.

[TimWilborne 108]

I can figure out the CFM of devices, and an entire machine based on the steps it goes through to figure out the maximum number of pneumatic devices operating at the same time. Where I am running into trouble is estimating the total load of a plant because you never know what each machine is doing at a particular time. Kind of like your house. You may have 200 Amps going to it but if you add up all of your loads, it is greater than 200 Amps.

[Steve Bailey 118]

It really depends on the size of your tank. Let's say you have a 5 CFM pump and you're using air at 5 CFM. If your usage is constant, you're (nominally) OK. But let's say your usage is biased so that you use 5 cubic feet in 10 seconds followed by 50 seconds of zero usage. If you have a 10 cubic foot tank, your system pressure is going to drop roughly in half during the 10 seconds when you use those 10 cubic feet and then recover over the next 50 seconds. If you have a 100 cubic foot tank, then your pressure will drop by roughly 5 percent during the period of peak usage. The higher the ratio of peak to average usage, the larger the tank needed. Obviously, you don't want to buy a 5 CFM compressor for 5 CFM average usage, but beyond that, there is a lot of leeway. You need to strike a balance between capital cost and reserve capacity.

[Chris Elston 79]

Have you tried downloading and looking at this? http://forums.mrplc.com/index.php?autocom=downloads&showfile=133 I use this to size valves and FRLs. I updated this sheet, it's at work. I will updated it tommorrow and let you know to download it again after I update it.

[Beuwolf_1 0]

We have five compressors, ranging from 150 hp down to a couple of 50's usually on stand by. Normally the 150 takes care of the entire plant but if we step up production, and those cylinders do start cycling at the same time one of the stand by's goes in. Usually tho, the only time the stand by's go online is if the reserve tanks run low like was mentioned earlier. We have roughly four times the "storage" of air then our computed usage, with the tanks placed evenly around the plant so that the reserve doesn't have to get to the long range spots thru the headers. Our usage right now is somewhere above 300 CFM at 95 PSI and the pressure is monitored for low air faults. Since adding the additional storage in the plants, the only time we experince a loss of air is when we have a mechanical failure. Extra storage makes up for a lot of unused and expensive compressor operating times.

[TimWilborne 108]

The air tank. Why didn't I think of that. Knew I was forgetting something Thanks everyone