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Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

(OP)
The ASHRAE Fundamentals Handbook describes the conversion of equivalent duct sizes from circular to noncircular assuming that you start with a design of a circular duct and then convert.  I am starting with an existing rectangular duct arrangement and am trying to add a section of rectangular duct and need to calculate the pressure drop due to friction?  I realize that in order to use the Darcy equation, I need to convert to an equivalent circular duct size.  Here's my dilemma.  What velocity do I use?  Is it the velocity through the existing rectangular duct, or through its equivalent circular diameter?

RE: Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

If you are going to use the Darcy equation, it is the velocity in the circular duct.

But why not take the easy way?

Use table 2 "Equivalent Rectangular Duct Dimensions" in reverse to get the equivalent circular duct.

Enter Figure 9 Friction chart for round duct on the cfm axis and go up to the interstection of the equivalent circular duct size.  Read the friction loss per 100 feet on the Y axis.

RE: Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

(OP)
Thanks MintJulep for that quick response.  I guess to answer your question as to why I don't utilize the tables, it is because I've developed an Excel spreadsheet that will calculate the pressure drop/100ft.  I just wasn't sure which velocity should be used, but now I am.

Thanks again.

RE: Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

I think I didn't get it right. If you are using an equivalent circular duct for calculation, velocity should remain same.

RE: Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop


Graphs to convert the dimensions of rectangular cross-sections into equivalent round duct diameters, are presented in two ways:

a. with constant velocity (same mean hydraulic radius)
b. with constant capacity (equal friction per unit length)

Have a look at the Fan Engineering handbook published by the Buffalo Forge Company.

RE: Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

From SMACNA:

De = 1.30((ab)^0.625 / (a + b) ^ 0.25)

where:
a = length of one side of rectangular duct, inches
b = length of adjacent side of rectangular duct, inches
De = circular equivalent of rectangular duct for friction & capacity , inches.

For pressure drop in inches wg / 100 ft of duct I have been using:
PD = (CFM)^1.90 (0.0109132) / (De)^5.02

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