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Building Science

Using Total Effective Length in Duct Design

For air flow purposes, a duct system is often much longer than it appears

One method of duct design relies on calculating the total effective length, which is the sum of all the lengths of the straight sections of duct and the equivalent lengths of the fittings.
Image Credit: Energy Vanguard

Today I’m going to explain an important concept in one of the most popular ways of doing duct design. I’ve been writing a series on duct design over at my blog and began with a look at the basic physics of air moving through ducts. The short version is that friction and turbulence in ducts results in pressure drops. Then in part 2 I covered available static pressure. The blower gives us a pressure rise. The duct system is a series of pressure drops.

We can divide the pressure drops into two categories: those resulting from the ducts and fittings and those resulting from all of the components that aren’t ducts and fittings (e.g., registers, grilles, filters…). When we subtract the non-duct/fitting pressure drops from the rated pressure rise (total external static pressure) of the blower, we get the available static pressure. That’s the total pressure drop we have available for the ducts and fittings and is what sets our duct pressure budget.

What we want to get out of this in the end is the proper duct and fitting sizes. We have a certain amount of available static pressure to use up. If our ducts are too small, we can end up with either too little air flow in the case of a fixed-speed blower (PSC, which stands for permanent split capacitor), or we get the air flow but use too much energy with a variable-speed blower (ECM, which stands for electronically-commutated motor). The first step in finding the proper duct and fitting sizes is to find the total effective length (often called equivalent length), the topic of today’s article.

What is effective length?

Length is length, right? Why do we need something else called effective length? The answer lies in…

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