backSCFM, ACFM, CFM, ICFM & ESP
By definition, standard air is 70°F at Sea Level with 50% relative humidity. A cubic foot of standard air at sea level weighs .075 lb./cu.ft. Unless you are lucky enough to be in Seattle in the summer, you almost never have Sea Level and 70°F air for your fan. Your loss!
Since fan tables and catalogs are generated for these conditions, we have to adjust a fan’s performance to match the conditions at the specific temperature, altitude and air density where it is to be used. What we are really saying is that we almost always have ACFM (Actual Cubic Feet Per Minute), something other than SCFM (Standard Cubic Feet Per Minute). Normally, our customers call us and ask for just so much CFM (Cubic Feet Per Minute) because they feel their altitude, temperature or density is not sufficiently different than standard air (.075 lb./ cu. ft.).
Having 40°F air (.0795 lb./ cu. ft.) or 100°F (.0710 lb./ cu. ft.) air doesn’t normally make a big difference (5.6%) in fan performance, whereas 3,000 ft. elevation reduces air density to .067 lb./cu. ft or 11% less than standard air, as would 129°F.
When a fan handles air that is 11% lighter than standard air it produces 11% less pressure than cataloged. Think of the fan as being designed to centrifugally sling marbles out the discharge and the air is now lighter and you are slinging feathers. A fan at Sea Level in Seattle will develop 11.2" SP when delivering a given CFM throughput but will only deliver 10.0" SP at 3000 ft. elevation at the same flow. The horsepower at the higher elevation will be 11% less as well because you developed 11% less pressure.
Another instance we rarely adjust for, but should, is fan inlet rarefication. If a system fan is selected for 50" SP and all 50" is on the fan intake, we are essentially stretching out the air as we draw it to the fan. This rarefication reduces the inlet air density by 12.3% [(408"- 50")/408"]. This means the air the fan wheel is taking in is .0659 lb./cu.ft. not .075 lb./cu.ft., so it’s back to slinging feathers. To properly develop 50" at the fan inlet, we must allow for the rarefied fan inlet density. By selecting the fan at an ESP (Equivalent Static Pressure) of (.075/.0659) x 50 = 56.9". We then use the standard density catalogue charts to select a fan speed to develop the ESP of 56.9" SP. Then when the fan is operated with an inlet density of only 0.0659 lb./cu.ft., the fan will actually develop 50" SP as required.
Quite often with positive displacement blowers like M-D Pneumatic’s and high speed centrifugal blowers like Invincible’s Turbo Blower, we use the term ICFM (Inlet Cubic Feet Per Minute). Most often this just means SCFM if the intake air is .075 lb./ cu. ft. It is easier to define the working air volume at the inlet of these blowers because after the blower does it’s job, the air is heated and compressed.
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