# Duct Flow and Friction Loss Problems



## JoeysVee (Oct 17, 2009)

When do we use Dequivalent and when do we use Dhydraulic???

De = (1.3*(short side * long side)^(5/8))/(short side + long side)^(1/4)

Dh = 4*(area of flow/perimeter of flow)

They are both used when working duct problems and I' not sure when to use which type of diameter. Please help...thanks!


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## Yingli (Oct 18, 2009)

I am also not clear on this concept, I did google , here is the information and link.

http://www.engineeringtoolbox.com/hydrauli...eter-d_458.html

*Note!* The hydraulic diameter is not the same as the equivalent diameter. The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as a rectangular duct or pipe.



The hydraulic diameter - dh - is used to calculate the dimensionless Reynolds Number to determine if a flow is turbulent or laminar. A flow is

•laminar if Re &lt; 2300

•transient for 2300 &lt; Re &lt; 4000

•turbulent if Re &gt; 4000

The hydraulic diameter is also used to calculate the pressure loss in a ducts or pipe.

The hydraulic diameter is not the same as the geometrical diameter in a non-circular duct or pipe and can be calculated with the generic equation

dh = 4 A / p (1)

where

dh = hydraulic diameter (m, ft)

A = area section of the duct (m2, ft2)

p = wetted perimeter of the duct (m, ft)

*The equivalent diameter *is the diameter of a circular duct or pipe that gives the same pressure loss as an equivalent rectangular duct or pipe.

The equivalent diameter of a rectangular tube or duct can be calculated as (Huebscher)

de = 1.30 x ((a x B)0.625) / (a + B)0.25) (1)

where

de = equivalent diameter (mm, inches)

a = length of major or minor side (mm, inches)

b = length of minor or major side (mm, inches)


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## buick455 (Oct 18, 2009)

This is from Lindeburg in regards to this subject in the Errata for the 12th edition 1st printing which is what I have:

p. 16-6, Section 9: I've perpetuated sloppy engineering terminology. Section 9, "Equivalent Diameter" should be renamed "Hydraulic Diameter". What is being calculated here is known as both "hydraulic diameter" and "equivalent hydraulic diameter." Engineers get sloppy and call it "equivalent diameter" all the time, which confuses the issue. Hydraulic diameter, equivalent diameter, and hydraulic depth are three different things. Hydraulic diameter is a characteristic length used in the calculation of the Reynolds number and friction loss. Hydraulic depth is a characteristic length used in the calculation of the Froude number and other open channel flow parameters. Equivalent diameter is the diameter of a duct or pipe that would have the same friction loss per unit length as a noncircular duct. Basically, every place on pp. 16-6 and 16-7 that "equivalent diameter" and "De" appear, they should be "hydraulic diameter" and "Dh". 2/23/2009


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## JoeysVee (Oct 18, 2009)

Yeah I saw that errata too. I don't think that helps us much though.

Most of the time books use De when using the duct friction loss chart but sometimes they use Dh. I think the difference is...

Use De for the duct loss chart (figure 20.4 MERM) when the cfm is known...and this is the case most of the time. So if you have a 10 x8 duct and you know the cfm you would calculate De and get the friction loss from the chart using De and cfm.

Use Dh for the duct loss chart (figure 20.4 MERM) when the velocity is known...and this is a rare case. So if you have a 10 x8 duct and you know the velocity you would calculate Dh and get the friction loss from the chart using Dh and velocity.

Can someone confirm this? I'm not 100% sure this is correct. I just know at times people will use Dh and I think it's when velocity is known.


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