Rain falling speed

[russmech]

Does anyone out there know how fast I would have to run air out a vertical open ended duct in order to overcome an average rain fall (and not rain into the duct). What are your sources or where should I look? Thanks.

 

[plawrence]

If you're looking to keep water out, I would recommend a zero-loss stack. I don't have my ventilation handbook in front of me so I'll need some backup on this. On whatever the size of the exit stack, mount a piece of duct 1" bigger in diameter to the stack and extend it at least 4 diameters up. For example, a 6" stack leaves the process and exits the building. Mount a 7" x 28" long piece to the 6" piece (with angle, channel, etc). In theory, rain doesn't fall straight down so the rain that hits the inside of the 7" duct will run down the sides and out the annular area between the 6" and 7" piece. We have this design all over our plant with no complaints of water in the ductwork.

Or you can put a hat on it.

 

[witchdoc]

Typically, a stack velocity greater than 20 fps will push water vertically up the inside of a stack. You must have a contingency plan for times when the exhaust fan is not operating.