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    • Bureaucratic Affairs Building HHW System Logic Exercise
    • Bureaucratic Affairs Building Scavenger Hunt
    • Chilled Water Plant System Diagram Exercise
    • The HIjend Hotel >
      • Chilled Water Plant Scoping Exercise
      • Ballroom AHU Scoping Exercise
      • Cooling Tower Scoping Exercise
  • Tools
    • Altitude Correction Factor
    • Economizer Evaluation Checklist
    • Eikon for Educators and WindLGC
    • Effective Duct Length Tool
    • Excel Third Axis Tool
    • Excel Time Value Conversion
    • Insulation Savings Tools
    • Logic Diagram Tool
    • Mixed Air Calculations
    • Monitoring Plan Spreadsheet
    • Pipe Friction Chart
    • Plot Digitizer >
      • Plot Digitizer Pump Curve Example
    • Square Law Spreadsheet
    • System Diagram Symbols
    • Thermodynamic Diagrams Spreadsheet
    • Universal Translator Data Analysis Tool
    • y = (m * x) + b Spreadsheet
  • Useful Formulas
    • Affinity Laws
    • Circular Equivalent Duct
    • Effective Duct Length
    • FT Guide Appendix C
    • HVAC Equations and Concepts
    • Hydraulic Diameter
    • Pump Power and Energy
    • "Square Law"
  • What's That Thing?
    • Cooling Towers
    • Pumps
    • Valves
  • Resources
    • Bill Coad's Writings
    • Data Logging Resources
    • EBCx Skills Guidebook
    • Energy Design Resources
    • Fisher Controls Valve Cavitation Bulletin
    • Functional Testing Guide
    • Honeywell Gray Manual
    • MCC Powers Bulletins
    • NBCIP Reports
    • PEC Tool Lending Library
    • PID Resources
    • Pneumatic Control Resources
    • Resource List
    • Scoping Resources
    • TAB Resources
    • Vintage Carrier Design Manual
    • VRF Systems
    • Williams' Wisdom
  • Videos
    • Bureaucratic Affairs HHW Logic Answers
    • Cooling Tower Flow Variation
    • Economizer Stratification
    • Functional Testing
    • Induction Principles
    • Monitoring Plans
    • Ripple Effects
    • Testing a Pump
    • The Garden of Low Entropy
    • Variable Flow Systems
  • Training
    • Materials from Classes and Presentations >
      • ACEEE
      • ASHRAE
      • ASHRAE - Engineers Notebook
      • BCxA and NCBC
      • BEST Institute
      • Case Studies
      • ERDC 2020 RCx Academy
      • ELPNW
      • ICEBO
      • Marriott AEP
      • Magazine Articles
      • NAVFAC
      • Pacific Energy Center Design, Performance and Commissioning Issues Classes >
        • PEC Class Materials Archive
      • Pacific Energy Center EBCx Project Review
      • Pacific Energy Center EBCx Workshop Series >
        • EBCx Workshop Series 12
        • EBCx Workshop Series 13
        • EBCx Workshop Series 14
        • EBCx Workshop Series 15 >
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        • EBCx Workshop Series 16
        • EBCx Workshop Series 17
      • Portland General Electric
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      • Fans, Ducts and AHUs
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100 Percent Effective Duct Length Tool

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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.  
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.
effective_duct_length_and_circular_equivalent_vweb.xlsx
File Size: 2608 kb
File Type: xlsx
Download File

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