The maximum length of exhaust ducts

[Felix the Dog]

I'm an outdoor dude, but am thinking of a very small indoor gig. A tent I have my eye on includes a Hurricane 4" 165 CFM Inline Fan. I'm wondering how far such a fan can blow exhaust effectively. 25 ft.? 50 ft.? I may need to do 50 feet because of the layout of the building I am dealing with.
Thanks for any suggestions

 

[Weaselcracker]

I have a fairly weak 6” fan (not sure the cfm) in my veg room pushing air ahout 16’. It can barely push the air down that length- there is so much backdraft coming out where the fan is mounted that I’m surprised it’s doing anything at all, but it does seem to help cool the room. However, in veg, my intake air temps are low and I’m not producing much heat in that room either - so cooling is much less critical there.

In flowering- with two 600w HPS lights, I have a 12” hurricane fan pushing air only about 8’. A little overkill but it allows me to set the fan speed control low to keep the noise down.

50’ for a 4” fan sounds very unrealistic. Maybe there’s a way to shorten that run? For 50’ I’d be looking at a 12” fan.

 

[TorturedSoul]

To do more than guess, someone who was knowledgeable in this kind of thing would probably need to know information such as what kind of fan it is (e.g. axial, radial), whether you're going to be moving air though a restriction such as a carbon filter (if so, data on that), and how well the fan performs against pressure (some manufacturers publish graphs with flow on one axis and pressure on the other).

In other words, there are numerous variables to consider. Some of the cheaper fans advertised as being for the purpose of grow room exhaust fans are barely adequate to move air a few feet if that run includes a good-sized carbon filter, lol.

On the other hand, if the fan in question happened to be a Dayton blower salvaged out of a decades-old furnace, lol - and given minimal maintenance (IOW, its first cleaning and oiling in years) - it would, figuratively speaking, of course, "think it was enjoying a nice vacation."

I suggest you upsize your selection; that is to say, if your calculations indicate that a 4" fan of a certain type and rating are adequate for the job of exhausting or exhausting and sanitizing (whichever is applicable) your grow space with a minimal duct run, choose the 6" model with the next-higher rating instead.

If that ends up being a little more airflow than you need, well, that's what fan controllers are for.

 

[TurboBucket]

As noted a good manufacturer can provide you with a fan pressure curve, this will help you to understand how much air volume a fan can actually move with "x" static pressure. I've contacted AC infinity for theirs, they refuse to disclose that unless you agree to be a vendor. Hyperfan publishes theirs for what it's worth. I think I have seen fan curves for Max fans as well.

To move 300cfm of air through 4" duct @50' you're looking for a fan capable of 5" w.c. static pressure, it's also going to be around 3000fpm screaming through that duct.

For reference a normal ducted furnace system operates at 0.1"w.c. and around 500fpm.

Buy an 8" hyperfan and temperature speed controller and don't look back.

Don't forget the intake openings will probably be the bottle neck in the system. 8in round duct is approximately 50sq in which means your looking for at least 100 sq in of passive inlet grills.