# How do I determine air flow in a duct?

1. Jan 6, 2016

### TSN79

Picture a Ø160mm duct 10m in length, open at each end. Then I install an in-line fan at one end so that it blows into the duct and just let it run at full power. How can I determine what the air flow will be? I startet thinking about it and was just surprised that I didn't know how to approach it as I'm doing duct sizing and such pretty much every day. The fan has a capacity diagram, and the operating point will be somewhere along the outline close to maximum flow, but where exactly? Do I need to assume some pressure drop in order to get anywhere?

2. Jan 6, 2016

### SteamKing

Staff Emeritus
You have to construct a system curve for your duct for various assumed air flow rates, and then find out where the duct system curve intersects the fan output curve.
As long as the air velocity in the duct is kept below about 330 ft/s, air can be treated as an incompressible fluid for the purposes of making pressure drop calculations.

If you have an HVAC handbook, this should be explained somewhere.

http://nptel.ac.in/courses/112105129/pdf/R&AC Lecture 38.pdf

3. Jan 6, 2016

### Staff: Mentor

And if you do this with a spreadsheet, you can check a bunch of different airflows in a short period of time, until you get to the one that falls on the fan curve.

4. Jan 8, 2016