Tutorial Ventilation 101

[JBlaze]

Alright guys I'm trying to build a new flowering cab right now for a SOG run. The cab is 4'x2'x6'. I want to stick a 600watt in a cool tube in my cab. What kind of cfm and ventilation setup am I looking at to pull this off? Or should I just go with a 400?

 

[Amstel Light]

HEY RED WE NEEd your epertize over here
Thanks
http://www.icmag.com/ic/showthread.php?t=133297&page=3

 

[hoosierdaddy]

Running the two 6" fans Y'd into a 6" duct would nearly double the airflow in the 6" ducts. The velocity would be nearly doubled as well so would the duct noise.

Red, the airflow is the velocity.
I think you will find that the airflow, or velocity at which the air is flowing, does not double by adding an additional fan. What it does is increase the ducts ability to overcome pressure, but the CFM will remain constant.

You must either open up the ducting, or increase the speed (cfm) of the fans to increase velocity. You could Y ten 250 cfm fans up to a 6" duct, and it will still flow air out the exhaust at the same velocity as if one were hooked up.

 

[Mr. Hugeblunts]

This thread should be stickied. I've spent like 4 hrs trying to find it.

 

[VerdantGreen]

awesome thread, thanks!

V.

 

[jump117]

... You could Y ten 250 cfm fans up to a 6" duct, and it will still flow air out the exhaust at the same velocity as if one were hooked up.

Really? Are you sure the energy applied to that extra 9 will go to noise production solely? This isn't true imo.

 

[hoosierdaddy]

If you run two 6" fans into one 6" duct, which is what a Y does, you can only increase the pressure resistance of the set-up, and not the CFM. You will also cause the fans to labor (and yes gets noisier). The single 6" duct size is the key to all this.
Run the same two fans into the same area with their own separate ducting, and double the CFM.
I did a thread with graphics to help explain the physics behind all this.

http://www.icmag.com/ic/showthread.php?t=100823

A vacuum cleaner is something that makes it easier to understand airflow...
Turn on a vacuum cleaner and it runs at a certain speed and pressure. Choke the nozzle down to half the size it is now, and immediately the motor of the unit raises in RPM and it has harder suction at the nozzle. This was due to the fact that you decreased the duct area the air can travel through, which increased the pressure resistance (it sucks harder), and as a result the motors labor and increase in speed and noise. but the volume that the air flows remains constant, and is dictated by the size of the nozzle. The speed of the air over the nozzle end may well increase, but only because of the nozzle being restricted. It is not getting additional air, it is just having to pass through a smaller area with the same volume of air.

 

[jump117]

The increased pressure gradient will increase airflow through the same duct.
There is a little misunderstanding here.
Imo airflow is a volume of air moved by time (cfm or cmh) but velocity (here) is a linear speed of air stream (foot,yard,mile per second,minute,hour).
I hope it was lost in translation, otherwise we meet disappearing of energy that breakes the main physics law.
or where does added power will go in your opinion? Only into noise and heat producing? What about 1000 vs 10 into the same nozzle?
Bookmarked your link to read.

 

[hoosierdaddy]

 

[BudBag]

Ouch this is far to technical for me to follow. I already own 3 Axial fans that move 31.65 cfm each. My cab is 3.6 cf. I have a 6"x10" opening and a 3"x10" exit. Should I have my 3 Axial fans pushing air through my filter or pulling air through it?

 

[hoosierdaddy]

Pulling

 

[BudBag]

Ok I have read through this thread again and was able to absorb enough to rethink how im cooling and cleaning the air. I already own 3 Axial fans that move 31.65 cfm each. My cab is 3.6 cf. I have a 6"x10" opening and a 3"x10" exit. So I thought that ment I had 94.95 cfm.. Is that right or wrong? I have a "diy" scrubber that I did myself... It is an air tight tray that I fill with carbon and slide into the light trap. Then I have my 3 fans on a rack that I slide right under the tray. the scrubbers tray underside is coverd with weather strip so it actually sits right on top of the fans. I dont know if this will be enough to combat my grow. From what I am reading it is going to get real bad. So I am looking to be pro active. So do I need a new solution? I can redesign my cab if can get some solid advise.

 

[guest 77721]

Hey Hoosierdaddy,

I like your graphs for explaining parallel and stacked fans. Good work!

 

[guest 77721]

Hey Budbag,

If you can measure the ambient, cabinet and exhaust temps, we can extract your effective air flow from the CFM/Watts chart.

Your cabinet with 168 Watts needs 58 CFM to keep it within 10*F of ambient.

There are 3 restrictions blocking your airflow, the intake, exhaust and obviously the scrubber. I think your intake and exhaust are fine for now. It's your scrubber that is blocking up the airflow.

To get more air through a scrubber, it has to be larger in area or the media depth has to be reduced. Or you can swap out axial fans for a model that produces more pressure.

Check out my cabinet, it has 4 x 42W lights with a single 65 CFM computer fan for the light section and a small scrubber, 14" x 6" with a second 65CFM fan. I cool my lights with an unrestricted fan which allows a small scrubber to work effectively.

Ok I have read through this thread again and was able to absorb enough to rethink how im cooling and cleaning the air. I already own 3 Axial fans that move 31.65 cfm each. My cab is 3.6 cf. I have a 6"x10" opening and a 3"x10" exit. So I thought that ment I had 94.95 cfm.. Is that right or wrong? I have a "diy" scrubber that I did myself... It is an air tight tray that I fill with carbon and slide into the light trap. Then I have my 3 fans on a rack that I slide right under the tray. the scrubbers tray underside is coverd with weather strip so it actually sits right on top of the fans. I dont know if this will be enough to combat my grow. From what I am reading it is going to get real bad. So I am looking to be pro active. So do I need a new solution? I can redesign my cab if can get some solid advise.

 

[2fast4u2]

ventilation is easy.

go overkill with a thermostat, no more problems

 

[hoosierdaddy]

Thanks, Red...it's one of the best graphics I have seen on the topic. I believe it was Freezerboy who may have found them...not sure.

Going big isn't always an option, 2fast.

 

[BudBag]

Hey Budbag,

If you can measure the ambient, cabinet and exhaust temps, we can extract your effective air flow from the CFM/Watts chart.

Your cabinet with 168 Watts needs 58 CFM to keep it within 10*F of ambient.

There are 3 restrictions blocking your airflow, the intake, exhaust and obviously the scrubber. I think your intake and exhaust are fine for now. It's your scrubber that is blocking up the airflow.

To get more air through a scrubber, it has to be larger in area or the media depth has to be reduced. Or you can swap out axial fans for a model that produces more pressure.

Check out my cabinet, it has 4 x 42W lights with a single 65 CFM computer fan for the light section and a small scrubber, 14" x 6" with a second 65CFM fan. I cool my lights with an unrestricted fan which allows a small scrubber to work effectively.

I will get those temps for you! Not sure how to measure my cab temp. Under the lights about 90 half way down about 78-82... this is where I usually have my thermo to measure. My exhaust temp is 82-88 but have only a carbon filter laying in the tray, no carbon in the tray atm. My intake comes from under the house where the temps fluctuate, also its cooler than the inside of the house. I also have a pc fan 120mm 52 cfm I could use for something. I was thinking about pointing it across my lights. I am growing a very potent smelling plant so if you think I should scrap what I got let me know. I could build something else to replace my 3.25" light trap section. I am felxeble there. The overall size of the cab cant be adjusted... because of stealth reasons.

 

[lysol]

Good info on the fans, I'm a little confused, redgreenry's explanation applies as long as I increase square inches of ducting the proportionately to the cfm increase I need?

so 2 fans in parallel will double CFM as long as I cut the resistance by more than 50% by at least doubling my duct size, yea? ( or maybe a little more than 50% to account for things I overlook with my simplistic calculation )

A = Π * r^2

6" duct = 28.27 square inches of opening
8" duct = 50.26 square inches

so theoretically two 6" 400cfm blowers can move close to ~750cfm thru an 8" duct? As long as I don't introduce pressure bottlenecks...

Also lets say you had two 6" fans on a 6" duct, would the % increase over a single fan be dependent on the type of fan, for example two inline fans might only add 10% cfm but if there were centrifugal or other industrial fans couldn't the fans build up more pressure inside the 6" duct ( and sleighly more cfm ) before the fans reach their critical point or whatever ( and start sounding like a blocked up vacum )

So if you have X cfm in two 6" ducts, piped into one 6" duct you get X * 1.1 ( small increase only and inefficient fans )
So if you have X cfm in two 6" ducts, piped up to two 8", piped into 1 8" duct you get X * 1.9 ( large increase and efficient fans )

is my understanding correct?

 

[Me2]

So if you have X cfm in two 6" ducts, piped up to two 8", piped into 1 8" duct you get X * 1.9 ( large increase and efficient fans)

It also depends on the duct lengths and the airspeeds in each section. With straight duct runs and no Y fittings:
400cfm into 10ft of 6" duct = 34ft/sec (loss: 19.9 Pa)
750cfm into 10ft of 6" duct = 63.7ft/sec (loss: 69.2 Pa)
750cfm into 10ft of 8" duct = 35.8ft/sec (loss: 15Pa)

I agree with Hoosierdaddy

Run the same two fans into the same area with their own separate ducting, and double the CFM.

 

[guest 77721]

It sounds like your getting about 45-55 CFM effective airflow with no scrubber. You're just ok right now but installing the scrubber will drop your airflow down enough that the temps will be unusable.

This is an interesting problem. Three parallel fans using the intake and exhaust space of one fan yeilds an effective airflow of 130%. The reason is each fan only produces X amount of pressure. CFM = Sqrt(pressure)*Area. Parrallel fans don't increase the pressure so the airflow remains relatively the same.

If you want to get more airflow you'll have to increase the intake and exhaust size or replace your fans with higher pressure models especially if you want to plug it up with a scrubber.

It might be easier to section off the light section with a piece of glass and make a small scrubber for the grow section.

I will get those temps for you! Not sure how to measure my cab temp. Under the lights about 90 half way down about 78-82... this is where I usually have my thermo to measure. My exhaust temp is 82-88 but have only a carbon filter laying in the tray, no carbon in the tray atm. My intake comes from under the house where the temps fluctuate, also its cooler than the inside of the house. I also have a pc fan 120mm 52 cfm I could use for something. I was thinking about pointing it across my lights. I am growing a very potent smelling plant so if you think I should scrap what I got let me know. I could build something else to replace my 3.25" light trap section. I am felxeble there. The overall size of the cab cant be adjusted... because of stealth reasons.