# math behind reducing main trunk size over lenght

14046 Views 3 Replies 4 Participants Last post by  beenthere
I'm just doing some of this in my head and was wondering about how you calculate the reduction of side in a main trunk duct as it gets longer..

does it have to do with the number of takeoffs over the length, or is it just length based?

It seemed like as you take air away you'd want it to get smaller to keep the velocity the same as the amount of CFM would reduce as you add more take offs... is this how it works?

never looked into this, tried to look it up on google, but haven't found much good info
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You are correct, it has to do with static pressure drop in the duct which is a function of (among other things) velocity. You are dealing with airflow volume (cubic feet per minute, CFM) and duct area (sq ft) and this gives you velocity (CFM/SF= velocity in feet per minute, FPM). So it's not just a matter of number of take-offs, but how much air (CFM) goes into each, or, more specifically, how much is left in your main.

I'm sure someone here will give you some contractor "rules of thumb", but to do it properly you need to know how many CFM you have in your main at (or immediately after) each take-off so you know how/when to reduce the size of the main.
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