Non-Circular Duct to Circular Pipe Conversions

Convert rectangular or annular cross-section to hydraulic diameter



Make selections:    Rectangular cross-section 
Dimension, b [L]: 
  Dimension, c [L]: 
Ratio, b/c [L/L]: 
Hydraulic Diameter, D [L]: 
Flow Area, A [L2]: 
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Velocity, V [L/T]: Flow Rate, Q [L3/T]: 

Units in hydraulic diameter calculator: Numbers may be entered in any units, so long as they are consistent. [L] means length units. [L/T] means length per unit time. [L3/T] means flow rate in cubic length per unit time. For instance, if b and c are cm, then area will be cm2. If velocity in cm/s and area in cm2, then flow rate will be cm3/s.

Cross-Sections and Equations for Rectangular and Annular Hydraulic Diameter Calculator

Cross-Section Diagrams
For both geometries: Q=VA, Q=Flow Rate, V=Velocity, A=Flow Area
Rectangular:  A=Area=bc.  P=Perimeter=2(b+c).  D=Hydraulic Diameter=4A/P
Annular:  A=Area=π(c2-b2)/4.  D=Hydraulic Diameter=c-b      References

Using our hydraulic diameter page:
If you have a non-circular duct, the cross-sectional geometry can be converted to hydraulic diameter using this page. Then, velocity (not flow rate) can be computed using a) Design of Circular Liquid or Gas Pipes or b) Design of Circular Water Pipes based on the hydraulic diameter. However, (a) and (b) will not compute the correct flow rate since they compute area based on a circular cross-section. Therefore, you must multiply the velocity by the actual area of the duct. The actual area is computed on this page.

If you wish to determine the cross-sectional geometry of a non-circular duct, you can use a) Design of Circular Liquid or Gas Pipes or b) Design of Circular Water Pipes if you know the velocity, not the flow rate (flow rate can only be converted to velocity if the actual duct area is used, not the area based on hydraulic diameter). From the hydraulic diameter output from either (a) or (b), this page can be used to determine the geometry of the duct.


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