100 Percent Effective Duct Length Tool
The concept of 100% effective duct length is an important one in the HVAC industry. When air exits a centrifugal fan, the flow profile is distorted by the action of the fan wheel flinging the air towards its perimeter, causing more air to be concentrated at the outside of the fan scroll and thus, a higher velocity at that location. A similar deformation of the flow profile occurs at the discharge of an axial fan and a tubular centrifugal fan.
But at a point termed 100% Effective Duct Length, a uniform velocity profile will re-establish itself. Thus, it is helpful to be able to assess the 100% effective duct length point both during design and in the field. For the designer, establishing that distance will allow them to make better decisions about the discharge configuration of the fan and how to minimize system effect.
For someone out in the field, being able to get a sense of the 100% effective duct length point can be helpful in assessing how much of a fan performance penalty might be associated with a poor discharge condition on an existing fan. In other words, if the duct fitting or fittings in question are near the 100% effective duct length point, then the issue is likely not as significant as if they were at the 25% effective duct length point.
But at a point termed 100% Effective Duct Length, a uniform velocity profile will re-establish itself. Thus, it is helpful to be able to assess the 100% effective duct length point both during design and in the field. For the designer, establishing that distance will allow them to make better decisions about the discharge configuration of the fan and how to minimize system effect.
For someone out in the field, being able to get a sense of the 100% effective duct length point can be helpful in assessing how much of a fan performance penalty might be associated with a poor discharge condition on an existing fan. In other words, if the duct fitting or fittings in question are near the 100% effective duct length point, then the issue is likely not as significant as if they were at the 25% effective duct length point.
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The spreadsheet tool to the right has the equation for effective duct length built into it. It also calculates hydraulic diameter, the circular equivalent of rectangular and flat oval ducts, and the equivalent diameter of a rectangular duct. The Circular Equivalent Duct page of the website also discusses why we use different duct shapes.
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