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How to Calculate SCFM in an Air System

Standard cubic feet of air per minute (SCFM) is used to measure air flow in a ventilation system, and requires rigid standards to meet the technical definition. These standards include an air temperature of 68 degrees Fahrenheit with relative humidity of 36 percent, and it must be measured at sea level. There is some variation in the relative humidity requirement, as some sources say it must be zero and others say 50 percent. SCFM is generally treated more as a theoretical measure of flow; its impact is minimal in determining the compressed air flow needed to do work. For calculating air flow in systems, CFM is the preferred unit.

Things You'll Need

  • Pencil
  • Paper
  • Calculator
  • Differential zone pressure sensor
  • Pitot tube assembly, which includes static and total pressure probe assemblies
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Instructions

  1. Determine the Flow Velocity

    • 1

      Connect the total pressure probe to the high port on the differential pressure sensor.

    • 2

      Connect the static pressure probe to the low port on the differential pressure sensor.

    • 3

      Note the sensor's output, which is the velocity pressure ("P).

    • 4

      Calculate the flow velocity (V) from the equation V = 4005 * ("P)^(1/2), where V equals flow velocity in feet per minute and "P equals the velocity pressure.

    Determine The Duct Cross-Sectional Area

    • 5

      Determine the profile of the duct: round, square or rectangular.

    • 6

      Note the formulas for each profile. For circular profiles, cross-sectional area A = π * r^2, where π equals 3.14159 and r equals the radius of the duct in feet. For square or rectangular profiles, A = x * y where x equals the duct height in feet and y equals the duct width in feet.

    • 7

      Calculate the duct cross-sectional area using the appropriate formula.

    Determine the Air Flow in Cubic Feet Per Minute (CFM)

    • 8

      Note the flow velocity (V).

    • 9

      Note the duct cross-sectional area (A).

    • 10

      Calculate the air flow rate (Q) using the formula Q = V * A.

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