Understanding air flow

[gdbud]

You do not want a FAN on the intake!!
This will cause one of two problems
1: If the intake fan is of less CFM than your exhaust fan CFM then your out put will be equal to the intake CFM.
2: If the intake fan is greater than your exhaust fan then your will have a postive pressure inside the cabinet and the smell will escape through every nook and cranny.

It is best to use passive intakes (no Fans)

 

[J23]

Thnx HosierDaddy. I'm gonna use bigger intake holes. Right now I'm reading some posts and it looks like I have finally found answer to my question. Looks like all axial fans (those small PC fans) are not capable of efficient air pulling through scrubber. I need a centrifugal unit. Since it's bigger than axial fan I might have to move it to another compartment of my box to save grow space. I have just found great article http://www.jasons-indoor-guide-to-organic-and-hydroponics-gardening.com/exhaust-fan-setup.html that might solve some fan/scrubber/vent questions. looks like there is some sale @ hydroponics and you can get fan-scrubber combo for $120. I think I'm gonna go for it.

 

[J23]

gdbud: I was thinking about using passive intakes for the box and fan on a cool tube intake... and you're right. fans on a intake may cause more problems with air circulation. You guys are really helpful. I'm planning my box for a month now but I'm total newbie so I learn everything from square one. I'd just rather do more planning/learning in the beginning than learn from mistakes (costly mistakes, time and $$$)

 

[hoosierdaddy]

If you can afford the lack of stealth, having the fan and scrubber on the outside of the box is the best thing you can do. They can be stealthy too...do a search and you will see some amazing stealth boxes with the works outside.

 

[J23]

stealth is the only way...

 

[gdbud]

J23,
Get a copy of Google sketch-up (it's free) then design your cabinet to scale to see how every thing is going to stealthy in side your cabinet. Here is my design I used.

 

[J23]

gdbud: what kind of fan are you using? I'm thinking about getting fantech 150cfm. don't know how loud/quiet it is... http://www.homeperfect.com/Ventilation-Fans/Fantech-FG-4-Inline-Round-4inch-Duct-Centrifugal-Fan-117-CFM-/invt/fanfg4.
as far as box, I have built my already. I still have to install all the equipment thou. Since you guys are so helpful I'm gonna ask your advice again... Please see my other post www.icmag.com/ic/showthread.php?t=158973 and let me know what do you think...

 

[yosun]

know i know how to set up my 2 fans,ty

 

[gdbud]

gdbud: what kind of fan are you using? ..

I'm using a 6" ecoplus inline fan out put is 440 cfm at 0" wc.
It's comparable to the fantech brand that you are looking at. You could probably get by with a 4" fan with an output of 170 cfm.

When look at fans the cfm rating is only half of the equation the other half is static pressure or inches of water column.

I know this is hard to read so I will put the information out that is easier to read.

Fantech fan 6" inline fan
Static pressure 0"= 483cfm, 0.1"= 466cfm, 0.2"= 450cfm, 0.4"= 409cfm, 0.6"= 369cfm, 0.8"= 329cfm, 1.0"= 289cfm, 1.25"= 243cfm, 1.50"= 201fcm, 2.0"= 103cfm.

As you can see the static pressure on your system can adversely affect you cfm.

On my cabinet I'm using a high performance furnace filter on the intake -> cool tube -> duct -> fan -> carbon scrubber.
The Scrubber is a flat bed has a screen -> prefilter -> 1 1/2" carbon -> filter.

I use the following formula to calculate my cfm.
cfm = (exhaust temp - interning air temp)/3.16 * total watts

When I turn the fan on high I'm running at about 210 cfm off of a 440 cfm fan, So by looking at the chart my cabinet has a total static pressure of between 1.25" and 1.50". I lost over 1/2 of my fans total cfm due to static pressure.

I'm able to keep my cabinet at 10 degrees above ambient tempture with only 85 cfm and running the fan at 1/2 speed.
I still have plenty of fan speed to work with.

 

[hoosierdaddy]

Top shelf post, gdbud.

When look at fans the cfm rating is only half of the equation the other half is static pressure or inches of water column.

 

[simon]

I've read at least one report that they are more restrictive than the purchaser expected leading to ventilation and heat problems. I would definitely not consider it to flow as much as a 8"x8" open hole, possibly less than 50% as much.

As one who essentially introduced darkroom louvers to the growing community, perhaps I can shed some light on this. You're absolutely correct in saying that a 8x8 louvers offers substantial restriction. It's more than 50%, relative to a 64in^2 area. If I had to guess, I'd say ~75-80% with all 4 panels installed. Depending on the config of the room, however, all 4 panels don't need to be used. My C22s run 2 8x8 louvers on their sides, 2 panels per louver, and four 4" lovers on the backs of the cabs along the span of the floor. The boxes, individually, run at 1-2F above ambient, each with a 600HPS in ~6ft^3 of floor space.

The benefit to the louvers doesn't lie in their ability to flow air. Darkroom louvers offer a (physically) flat light trap, where PVC and the like take more space. If the spacial constraints aren't there, other options present a better flowing solution. For example, my main room uses 2x8" pipes configured as light traps/intakes. No louvers.

Simon

 

[yosun]

my exhaust is 4 by 9 = 36 inch area? 36/12.56=2.86? (using 4 inch hole saw)

 

[yosun]

thanks to this thread and the orifice calculation table hoosierdaddy made,i was able to drop the temps of my cab down to only a few degrees over ambient,with my passive intakes cover with filter and my out take..also his tips of placing my fans side by side ruled,any1 struggling with heat problems should read this thread and follow it,i will b sending people here

 

[hoosierdaddy]

Very glad it helped, yosun. Once you get a grow going be sure and give us a report!
Cool grows are cool grows.

 

[Flo Grow]

I have a Homebox Small tent (33" x 33" x 64") with an 6" CanFan (270cfm) and my temps run 7F - 8F above ambient. I also have a Stanley Blower. The blower is inside the tent hanging on speed 1, then into my Air-cooled hood, then to the Can Fan outside the tent and into the attic. I only have 1 out of 3 vents open for passive intake. I would like to drop the temp another 4F ?

 

[Strangely]

Top thread, appreciate you spreading the knowledge! I've got a 'is my big passive intake too big/small' querie but I'll wait til I'm back in front of pooter to upload a design pic and spare the head pain!

 

[hoosierdaddy]

I can answer it before you post it up.
Measure your exhaust. Your intake needs to be At LEAST 10% larger than the exhaust, and a pretty easy rule of thumb to follow is to just make the intake twice the size (100% larger) of the exhaust.

 

[Strangely]

I've worked to that rule as I'll be pulling through a layer of carbon (the red on the pic) and I'm using a home made light trap design which was more what I was quizzing you over. Does the bend create more turbulence, I was thinking a little but not enough to go above the x2 rule?

If you wouldn't mind having a little looksy to see if it passes muster! My general plan btw is to grow from seed initially then leave a bit of stem and leaf post harvest to reveg and clone from. Thanks in advance HD!

 

[hoosierdaddy]

The filter material will restrict the airflow a bit, but it will not be starved for air with that setup. It appears you are providing it with sufficient air volume.
If that light trap setup goes all the way across the back, you are probably offering it lots more air than it needs, which is fine to do.

 

[Strangely]

Good to know! Thanks HD. I figured the carbon will offer a fair bit of resistance but I've kept it as good/big a surface area as I could (TM ScrubNinja to help with that.

Whilst I have an experienced grower looking in the vicinity of my design, I can opt for either 2 x 40 watters (CFL) or 2 x 60 watters in there. Do you think I could get away with the 60s or would it be too Hot Hot Hot d'ya think?

Thanks in advance (last question promise! .