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Calculating Friction Rate – It’s Not a Constant

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Friction rate is a pretty simple topic. It’s just a straightforward calculation that gives us the design friction rate from the two quantities: available static pressure and total effective length. The available static pressure (ASP) is what’s left when you take the total external static pressure and subtract the pressure drops for all the things that aren’t ducts and fittings. That’s how much of a push you have left to move the air through the ducts. The total effective length (TEL) is the sum of the duct lengths and the fitting equivalent lengths. Here’s how to take those numbers and calculate the friction rate (FR).

Calculating friction rate

Let’s just jump right into it. Here it is:

Let’s say the ASP is 0.31 inches of water column (iwc), a reasonable number for an air handler rated at 0.5 iwc. And let’s say the TEL is 424 feet. Plugging in those numbers, the friction rate would be:

That number tells how much pressure drop to expect in each foot of that ductrun. Notice it’s a very small number because we’re dividing a small number by a big number. No one likes a lot of zeroes to the right of a decimal point before you get to something significant. Scientists use scientific notation to get around that. Engineers use engineering notation, which is a knockoff of scientific notation. And poets use poetic notation, which really doesn’t make any sense but it’ll make you cry tears of joy. Unless you’re a scientist or engineer. Then you just won’t understand what they’re talking about.

So, friction rate is usually given not as the pressure drop per foot but instead as the pressure drop per 100 feet. Let’s redo our work now with that new convention.

See what happened there? Multiplying by 100 gets rid of two ofthose offending zeroes. Now are result for friction rate looks like this:

In this case, our friction rate is 0.073 iwc/100′. That’s the number we use to size the ducts.

How to think about friction rate

The big problem a lot of people have with friction rate is they don’t actually calculate it. They assume it. And the number they usually assume is 0.1 iwc/100′. That’s close to what I calculated above, but if you use 0.1 instead of 0.073, your ducts will be undersized. Do you know why they’ll be undersized?

When you calculate the friction rate for a particular design, you’re finding out how much pressure drop you can get away with for each 100 feet of effective length of your duct system. If the number is higher, you can use smaller, more restrictive ducts. If the number is lower, you have to use larger ducts.

Make sense? Maybe not. We’re used to thinking of friction as something that we want to decrease, right? Now I’m telling you that a high friction rate allows us to use smaller ducts. It seems counterintuitive. But it’s not. I think the mental hangup here is because of the word “friction.” Think of it in terms of pressure drop allowed and it makes more sense.

Another way to think of it is in terms of the two numbers that went into the calculation. We want available static pressure to be as high as possible and total effective length to be as low as possible. Higher ASP (good) and lower TEL (good) both make the FR go higher (good).

If it doesn’t make sense yet, you may have to chew on it for a while and it will. Once you have the friction rate, you can use a duct sizing calculator to find what size ducts you need for each run.

Allison Bailes, III, PhD
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Posted In: Building Performance

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