Equivalent Duct Diameter (Circ-NonCirc) and Pressure Drop

[749KDV]

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?

 

[MintJulep]

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.

 

[749KDV]

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.

 

[imok2]

749KDV you can go here and download free software for ducts and piping :>)

http://www.mcquay.com/eprise/main/mcquay/Home/HomePage

 

[quark]

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

 

[25362]

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.

 

[lilliput1]

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