How to calculate pipe sizes for air flow

Written by timothy banas
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How to calculate pipe sizes for air flow
The pipe sizes for your ductwork will require knowing the necessary air flow volume for you building. (air conditioner vent image by Tammy Mobley from Fotolia.com)

Installing new ductwork in a building requires pipes of the correct size based on the building's air flow needs. To begin the determination of the necessary pipe sizes, consider the volume of air flow that the building requires to regulate temperature and air freshness. Also consider the velocity of the air coming out of the ductwork; you probably do not want the air flow from your pipes blowing the furniture away. Once you have taken these considerations into account, figuring out the necessary pipe sizes for your air flow can be accomplished by applying some relatively simple equations.

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Instructions

  1. 1

    Determine the air flow velocity and volume that your building requires from its ductwork. Doing this may involve consulting a building engineer or the manufacturing company that made your furnace.

  2. 2

    Write this equation that relates the diameter of cylindrical pipes to the air flow velocity and volume.

    Diameter = Square root of ((air volume x 576) / (air velocity x pi))

    pi = 3.14 Air volume is given in units of cubic feet per minute (cfm). Air velocity is given in units of feet per minute (ft/min). The same methodology can be applied using metric measurements.

  3. 3

    Replace the variables in the equation with the values for your building to calculate the necessary pipe diameter for your ductwork. For instance, if your building requires an air flow volume of 1100 cfm moving at 1200ft/min, the diameter calculation would look like this:

    Diameter = Square root of ((576 x 1100 cfm) / (3.14 x 1200ft/min)) = 13 inches

    Your duct pipes would need a diameter of 13 inches.

  1. 1

    Determine the air flow velocity and volume that your building requires from its ductwork. Doing this may involve consulting a building engineer or the manufacturing company that made your furnace.

  2. 2

    Write down this equation that relates the cross-sectional area of a pipe to the air flow velocity and volume.

    Area (square inches) = (144 x Air Volume)/Air Velocity

    Air volume is given in units of cubic feet per minute (cfm). Air velocity is given in units of feet per minute (ft/min).

  3. 3

    Replace the variables in the equation with the values for your building to calculate the necessary cross-sectional area for your ductwork. For instance, if your building requires an air flow volume of 1100 cfm moving at 1200ft/min, the area calculation would look like this:

    Area = (144 x 1100)/1200 = 132 square inches

    Your duct pipes would need a cross-sectional area of 132 inches.

  4. 4

    Determine individual dimensions by using the equation for area.

    Area = height x width

    For example, if you want to use pipes with a width of 11 inches, then the height would have to be:

    Height = Area / width = 132/11 = 12 inches

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