Stop me if you’ve heard this one before: Fans are constant volume machines. While this is not a joke, it often requires an explanation. Imagine the fan wheel being a series of scoops. As it rotates, each scoop gets filled with material. The volume of material is always the same, whether it is feathers or rocks. Of course the rocks weigh more, so it takes more effort (or brake horsepower [BHP]) to move them. Remember, a scoop is a scoop!

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Now expand this concept to a system: throw a scoop of rocks against a wall (resistance to flow). The energy imparted to the wall is greater than the energy you would get from the scoop of feathers hitting the wall. The same is true of duct systems. It takes more energy (static pressure) to push a denser material through the duct. In the same manner, a fan is unable to generate as much static pressure when handling a less dense material.

This illustration should help explain why we make corrections to the static pressure -- and not to the airflow rate, or cfm -- when looking at calculations when choosing the proper fan for an application.

Standard air is defined as sea level, 70°F (21°C), 29.92" Hg, and 50 percent relative humidity. Therefore, standard density is, by definition. 0.075 lb/ft3. Fans are rated at standard air, so corrections must be made to the static pressure only before selections from the rating tables can be made.

Most applications require operating conditions other than standard air: temperature, elevation, molecular weight, moisture content, barometric pressure, etc. Some of these corrections are quite complex, so often, they are handled by a computer program.

Here is an example. Assume the following conditions exist: 5,000 cfm, 1" static pressure at 2,500' elevation, 170°F (77°C), saturated air. The density corrections are as follows:

Elevation x Temperature x Saturated Air = Subtotal
0.913 x 0.842 x 0.831 = 0.639
Subtotal x Standard Density = Actual Density
0.639 x 0.075 = 0.048 lb/ft3

The selection would be made at 1.56" static pressure (1" divided by 0.639) from the standard density tables. That will select the fan at a higher rpm, which will compensate for the lower density air. The resulting BHP needs to be multiplied by 0.639 to account for the lower density.

ACFM vs. SCFM

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ACFM is the actual airflow cubic feet per minute (cfm) required at the operating conditions. Sometimes, you need to know the equivalent airflow to move the same mass of air at standard density. That value is called scfm. In our previous example, the actual cfm needed was 5,000 acfm. When you convert that to scfm, you get 3,194 scfm. Make sure you select the fan using the acfm.

Here is the best way to avoid problems: First, specify the actual cfm at the point in the system that the fan operates. Also, obtain the actual static pressure rise needed at that point, and the nonstandard conditions. Take this information, calculate the density, correct the actual static pressure to the equivalent standard static pressure, and select the fan. Don’t forget to adjust the brake horsepower as well. You may need to select your motor size for some other density during startup.

Don’t be afraid to ask your fan supplier for help.

Editor's Note: this article was originally published as "Flow and Density" in the March 2010 issue of Process Heating.