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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
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        • EBCx Workshop Series 15 >
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        • EBCx Workshop Series 16
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Hydraulic Diameter

One of the common equations used to calculate the pressure drop for a pipe or duct is called the Darcy Weisbach equation, which is illustrated to the right.  The square law that we use to project a system curve for a given duct or pipe network in a fixed configuration has its roots in this relationship as do most duct friction charts.

One of the terms that shows up in the Darcy Weisbach equation is hydraulic diameter, which is illustrated to the right, below the Darcy Weisbach equation.   If you do the math, for a circle, the hydraulic diameter and what we would simply call the diameter of the circle are the same.   In other words, a 12 inch round duct or pipe has a hydraulic diameter of 12 inches.

Interestingly enough, a 12 inch square duct (one square foot) also has a hydraulic diameter of 12 inches.  But, if you hold the cross-sectional area constant at one square foot and vary the aspect ratio, the hydraulic diameter starts to decrease as the aspect ratio the duct increases above ​1.  ​ 
Picture
The Darcy-Weisbach Equation
Picture
Hydraulic Diameter
The table below illustrates this and also some of the other duct dimensional characteristics for different duct sizes and aspect ratios.
Picture
 Hydraulic diameter is not the same thing as the circular equivalent of a rectangular duct or equivalent diameter.  And, just to make things interesting, each of those terms has a different definition .   For more on that, visit the Circular Equivalent Duct page of the website.  Meanwhile, the downloads below will give you the .wmf files for the Darcy-Weisbach equation and the Hydraulic Diameter equation so you can use them to document your work.  You will find more information on the specifics of doing that and why it matters on the Useful Formulas page of the web site.
darcy_equation_for_friction_loss_in_duct_with_renolds_number.wmf
File Size: 24 kb
File Type: wmf
Download File

hydraulic_diameter.wmf
File Size: 6 kb
File Type: wmf
Download File

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