Don’t Install your Industrial Fan in a Vacuum: Know your Inlet Air Density

If the conditions surrounding your application are standard, then you might not need to worry about calculating the inlet air density before selecting your fan. That being said, you need to do some extra calculations with non-standard conditions, as they can have a big impact on your fan sizing and design.

Standard conditions are considered to be 70 degrees Fahrenheit, with an elevation of zero feet (sea level) and an inlet pressure of zero inches of water column, giving a density of 0.075 pounds per feet cubed. Fan engineers often assume these standard conditions/density when asked to size a fan for a given air volume and static pressure when a client does not specify the inlet air density or conditions affecting the density.

Factoring in Surrounding Elements for Inlet Air Density

Industrial fan engineers calculate inlet air density using three key factors:

  • Temperature
  • Elevation of the installation
  • Fan inlet pressure

A fourth factor, moisture content, has minimal impact on density for most applications and we won’t really address it at this point. Just know that saturated air will affect the density and should be mentioned to your fan engineer.

Temperature at the Inlet of the Fan

Temperature, especially on really hot fans, will greatly affect the density. For example, once you reach around 150 degrees F, your inlet air density factor is already down to about.065 pounds per feet cubed. This can change the specification quite a bit as you relate it back up to the volume and static pressure.

Elevation of the Application

Elevation doesn’t have as great an impact on the overall calculation of inlet air density as temperature until you get into the 1000-plus feet elevations. But for mountainous regions with elevations higher than that, it becomes a critical component of air density, and thus an easy important factor to check.

Negative Pressure, or Vacuum at the Inlet

The negative inlet pressure, or vacuum required to pull the air in at the fan inlet, is the third important factor. It is most important when you have a great vacuum. For example, if you need a pressure blower to pull 60 inches of water through your system, that pressure is all negative at the fan inlet. This would impact your inlet air density and affect your fan selection.

Hear it from the Application Engineer

Senior Application Engineer Chet White outlines what is standard, what’s not, and how to make your calculation to determine fan inlet air density in this 3-minute video.

When you’re ready to start your project, reach out and connect with one of our application engineers to discuss the details of your specification.

Related Content: Other Inlet and Discharge Considerations

There are many factors to consider in every industrial fan application. If you’re thinking about inlet air density, you’re probably also thinking about other elements. Learn more about two of them:

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