Determining the size exhaust fan I should use

So I want to do an indoor plant grow tent 27"x27"x63". The thing is I need to vent the heat(it gets hot AF during the summer)/smell to my chimney vent in the wall about 12 foot away. There is going to be about 9-10 90° bends in the duct leading to the chimney vent which will make the total length of the duct to be about 25'.

I know I'm planning on growing in an area that is approximately 30 cubic feet. I have to pull the hot/stinky air from a carbon filter (inside the tent), into the exhaust inline fan, through all the duct/9-10 90° bends (approximately 25' long), and into the chimney vent which probably extends another 12' into the open sky.

I am trying to figure out how to calculate different stuff. I don't need to have someone give me the correct answer but maybe just a formula. Although straight up answers are taken as I don't turn down any given advice!
Sources are fine too!

What I'd like to know:
1. What should be the rated CFM ( I know it should be rated at least 30 cfm, but all the bends/traveling should account for some air velocity to be lost, right?)
2. What size duct/fan should I use (4" would be ideal for the size for the space my duct location could fit)?
3.How do I calculate the loss of velocity through the length/bends of the duct?

Thanks so much in advance! Hope y'all are staying cool

Edit* ps: I'd like to use 4" flexible ducting if at all possible. I was planning on using an ac infinity 4" exhaust fan with a rating of 205 CPM, aswell as the carbon filter (about 14" long)

TBH with that many bends you will definitely have static pressure loss. Really don't know how much heat load you are cooling. It's going to be very hard to estimate the absolute minimum requirement. Best to use a larger fan and if it's too much you can use a speed controller to slow it down.

Probably won't work nearly as well as it looks without some things to keep in mind here:
10 bends will have elipticals and turbulence to overcome...you will need to support all bends with supports.
That is a serious lot of pressure from resistance, turbulence, temp/ air pressure, height to overcome.


The AC infinity fans can't suck through a carbon filter very well, so you need to get the largest carbon filter that can fit (lesser resistance to overcome) and mount it as close as possible to the filter. See if a stubby 6 inch filter with larger diameter will fit, and flange it down to 4 inch.
A tjerland M-4 fan is cheaper and better choice for this setup, more static pressure and it will work on your next larger tent
Take a quick look at hvac standards for flexible ducts, very informing.

Not sure the ac infinity would cut the mustard and handle that resistance.
You might be better off with a squirrel cage fan (centrifugal) as although they have a bigger initial drop off they handle bends and long lengths better.

The math to work it out is super complicated , I would probably opt to go for a fan that will take care of the heat and then double up on cfm and use a speed controller.

This may be the one time I say a duct booster could be useful.

Cant imagine why 9-10 bends would be needed?

coreywebster said:

Cant imagine why 9-10 bends would be needed?

Click to expand...

The exhaust goes out the fireplace, so its in a living area was my thought about the bend #s.
Those AC infinity fans are best used to increase airflow due to environmental triggers, not be the main source.

I've had it in my cart ten times for 2 different designs before reason took over. As nice and shiny and gadgety as they are, they are for supplemental cooling, or feeding supply air (which can't overwhelm the negative pressure a tent needs and audio racks don't need).

Some rough info just to show how much 9 bends is going to effect the resistance

You want to look at fans with high Pa ratings