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