Ventilation duct size cfms, 6" cooltube to 8" ducting.

B

bishs

Active Member
Hey guys,

I need some help with duct sizing..

1). I have 6" cool tubes.. how many cfms can be pushed through? I've read that 6" metal ducts will allow 85-100 cfm.. but it does not state the distance of the duct. If the distance is short.. can more cfms be pushed through?

2). Should i be using 8" ducts to the 6" cool tubes? Or should i use 6" ducts to the cool tubes? Which is the more correct way?

3). I will be running 2 400 w hps lighting a U shaped scrog. Should each light get its separate ducting? Or can they be run on the same duct? The 6" cool tubes are really causing an issue for me.. as it seems that I will not be able to push enough cfms through them.

4). I have a Max Can Fan 8" which is capable of pushing 667 cfm. Would a 10" metal duct be sufficient to push 667 cfm or should i go larger? Maximum of 3 ft distance, which will be placed after the fan. If i were to use a rectangular type vent in take.. should i use 26x6" or a 24x6"?

Here is the chart i am using.. it seems like the figures are overkill..

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDIQFjAA&url=http%3A%2F%2Fhvac-talk.com%2Fvbb%2Fattachment.php%3Fattachmentid%3D488%26d%3D...&ei=UHCgTs3WFoGviAKVt4Vc&usg=AFQjCNGLfkOgHYqxbEjokFVAzDUDD2lsoA

If anyone has some experience with ventilation please help :D.
 
trichromalicious

trichromalicious

Active Member
1. CFMs are usually measured in ideal conditions.
2. Use the 6" ducts unless you plan to get 8" fans for 8" ducts bigger is better and then just get reducers.
3. The lights can be run on the same duct. DO NOT put a inline fan right after one of the lights it will torch the fan with heat and burn it out. Instead put the intake fan right next to the light blowing in.
4. 10" duct is huge I think you will be great with 8" duct the whole way with reducers at the cool tubes.
 
F

fred flintstoned

Well-Known Member
Your fan is plenty big enough to cool those lights. Probably overkill. 6" ducting will work fine. Don't know where you got the info on duct cfm capacity, but it doesn't sound right. I run about 1500 cfm in summer and 750 in winter through 8" with no trouble.
Run the lights in series with the fan pulling air through the tubes. It's more efficient.
A quality fan like Vortex, Canfan, etc. will have no trouble with the heat. In fact, check the manufacturer specs, they usually say the fan will work either pulling or pushing the air. I've been running Vortex fans pulling through 3600 watts of light for years with no failures, ever.
Hope this helps,
Fred
 
canniboss

canniboss

Well-Known Member
I'm not even sure if the size of the ducting matters all that much for CFM, I know that the smaller the ducting the louder it will be.
Your tent is probably something like 6'X4' or 7'X4' by about 7' high which would give you about 170' square. For scrubbing with a filter you would want a fan that would push about 1.5X the total amount of your CFM (about 250CFM) (The idea is to cycle all of the air in the tent every 3 minutes). However if your just trying to cool your tubes with a straight line you should be able to get away with a cheap 6" inline.
 
firsttimeARE

firsttimeARE

Well-Known Member
bishs said:
Hey guys,

I need some help with duct sizing..

1). I have 6" cool tubes.. how many cfms can be pushed through? I've read that 6" metal ducts will allow 85-100 cfm.. but it does not state the distance of the duct. If the distance is short.. can more cfms be pushed through?

You can push as many cfm's as you want through them. When you increase CFM's through a fixed diameter duct the only thing that happens is you increase the Static Pressure(friction loss) and you increase the FPM(velocity) of the air. Yes, since friction loss is determined also by the length of the duct, a short distance would be better if you have a fan that cannot overcome a huge amount of friction loss, the FPM will remain the same however(noise).

2). Should i be using 8" ducts to the 6" cool tubes? Or should i use 6" ducts to the cool tubes? Which is the more correct way?

The correct way is the 6" duct. If you used reducers that also increases friction loss(bad, you want this number as low as possible which would negate the fact you're using a bigger duct, IMO. 1 reducer on both ends is another 10 dollars also

3). I will be running 2 400 w hps lighting a U shaped scrog. Should each light get its separate ducting? Or can they be run on the same duct? The 6" cool tubes are really causing an issue for me.. as it seems that I will not be able to push enough cfms through them.

The only thing determining how many CFM's you push through that 6" cool tube is the fan you have attached to it.

4). I have a Max Can Fan 8" which is capable of pushing 667 cfm. Would a 10" metal duct be sufficient to push 667 cfm or should i go larger? Maximum of 3 ft distance, which will be placed after the fan. If i were to use a rectangular type vent in take.. should i use 26x6" or a 24x6"?

Circular ducts are ideal, or square, rectangular just increases resistance. A 10" spiral duct at 667 CFM will push the air out at around 1280 FPM at a friction loss of .24 over 100' of duct. Since you say 3' divide .24 by 33 roughly and you get .007" of static pressure to overcome, most mixed inline fans(your can fan) can handle 1" of static pressure. You're fine. I got 550CFM going through an 8" with carbon filter(these increase SP ALOT) over 15ft without problems. Unless you need or already bought the 667CFM I think thats overkill.

Here is the chart i am using.. it seems like the figures are overkill..

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDIQFjAA&url=http%3A%2F%2Fhvac-talk.com%2Fvbb%2Fattachment.php%3Fattachmentid%3D488%26d%3D...&ei=UHCgTs3WFoGviAKVt4Vc&usg=AFQjCNGLfkOgHYqxbEjokFVAzDUDD2lsoA

If anyone has some experience with ventilation please help :D.

I didn't check that link out but i'm using a Ductolator I have from when I did HVAC designing
Click to expand...
See above in red.

EDIT: I saw above you said this was for the lights? Thats wicked overkill mate. I use a 240CFM fan for my 600W and even I think thats overkill.
 
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