Filter, fan, vents and CFM loss.

What's up guys,

I'm setting up a new 4x4x8 grow room with a 600 watt air cooled light, a can-50, and a 590 cfm inline fan. I have about ten feet of 8" duct off the fan and connecting to the light, with another vent connecting to the can filter.

I'm wondering how to do the math to account for cfm loss in the bends in the ducting since the can-50 is rated for 410 cfm with a minimum of 210. The ducting goes straight down from the ceiling from the fan, then a 90* bend to connect to the light, then another 90* bend to connect to the can-50.


(if that makes any sense)

What I have read is that a 90* bend will restrict air flow (up to) 60% and the filter will restrict by 25%.

590*.6= 354cfm (for bend 1)
354*.6= 212cfm (for bend 2)
212*.25=53cfm (for can)

So that would only give me an airflow of 53 cfm. Waaay lower than the needed rating on the filter, not to mention cooling the light. Please tell me my math is wrong here.

Also these bends I am going to try to make as long as possible for the space I have, instead of making tight 90* bends.

Im setting everything up tomorrow, so just doing a double check before I do. I appreciate any input.

-M.GT

Start with an 8" round flexible duct being good for maybe 200 ish CFM

search google for duct calculators...

So what we are talking about is called Velocity reduction (friction) its hard to understand ,but I will try to put it in a way you can ,

lets sat you are driving your car down a 3 lane highway, and then you see the sign construction 1,500 feet ahead merge One lane head .

well what happens as all three lanes of cars try to move in to that one lane , so you see this is something like what happens to air as it travels down its path
if you do not size your duct properly and if you put up road blocks (bends) .

So no matter what size fan you have (cfm) it will not perform to its full potential unless you size your duct correctly.

you must also understand that the fan needs to be fed a constant amount of air to perform to its full potential (cfm)

in your situation the total cubic feet of you area does not have enough square inches of air for the fan to perform at its full potential so I suggest you need to what is called makeup air (passive air)

If this was my room and was using 590 cfm fan I would increase the round duct from the can 50 to 10'' round all the way to the fan.

then on the exhaust side of the fan I would increase to 12'' round from the exhaust flange to where ever you are venting.

on the make up air side (passive air ) a 14'' x 12'' opening to obtain enough fresh air so your fan can operate to its full potential.

Now on your question about how much loss occurs on adding 1) 90 degree elbow well again this is in the friction loss method of equating. ,but for a good idea it is equivalent to adding 40 feet of straight duct to your system .

As Snaps pointed out 8 inch duct can handle 200 cfm , so you see you are already in that construction zone down to one lane.

all so another note of sizing the duct to and from your fan correctly is that it will offer a much more quite fan operation and as air travels through proper size duct it also will be less noisy .

my personal opinion is that 590 cfm is way over kill for your cubic feet of space.

^^ agreed, now that I got it set up, way overkill (but better to get an over sized fan and a speed controller than a fan too small!) ^^

Ya it didn't restrict air flow half as much as I thought it would. Actually need to get a fan speed controller to lower the speed.

Major negative pressure going on in there, but light an incense stick and could smell it coming out of the exhaust. I figure I can cut the fan speed in half and be gtg.

Even with 3 90* bends its really not dropping the airflow of the 590cfm fan that much. Filter is rated for 210-410 so fan speed controller aught to sort me out perfect.

Also I have a 150 cfm fan running as an active intake. Should be good once I get the fan speed controller.