passive intake, math of many small holes?

blanonamous · Sep 8, 2008

setup:
it's a tall cabinet (lowe's stor-it-all storage cab) with 2 equal size flower chamber rooms (1 on top of the other). the very top of the cabinet is a small room that the modded stanley blower sits in. this is a drbud micro sog setup (cfl's).

i want to put passive holes on the bottom floor of the cab. then the exact same passive holes in the shelf going to the second chamber above. finally, air'll go through the diy carbon filter (durbanpoison dyi style) hooked up to the modded stanley blower and exhaust out the top back.

active exhaust size: 5in duct (stanley blower). it's pulled through 6in duct carbon filter. so the active exhaust size should be a 5in hole.

problem:
i'm not sure how to do the math to do many small passive holes all over the floor and shelf and have negative pressure. this should equate to 10in hole.

would 10 1in holes be the same as 1 10in hole?

also i'm worried many holes would weaken the structural integrity of the floor and shelf i.e. plant cave-in collapse, especially if there is water damage in the melamine (paper wood cabinet material) shelf/floor later.

my solution to this would be to use tubberware to set the soda bottle pots in after watering. but this would also block the passive intake holes. the soda bottle pots will take up the whole area of the flower chambers.

any ideas? :wallbash:

cocageo · Sep 8, 2008

id go with like 2 4" passive intakes both bottoms

petemoss · Sep 8, 2008

Usually people recommend passive intakes at least twice as large as your exhaust. By twice, I mean twice the cross sectional Area. For example, your 5" exhaust has an area of 19.6 sq inches or (Pi) times (radius squared). An intake hole 2" in diameter has an area of 3.14 square inches (again area=pi times r times r). So you would need 12.5 little 2" passive intake holes.

Drilling 12 holes and then making and painting PVC elbows for those holes would take me all afternoon! Better to buy darkroom louvers and drill one or two big square holes IMO. The black louvers below are 8" X 8".

blanonamous · Sep 9, 2008

i wouldn't do elbows as air would come in from the back-bottom and up the bottom holes. i don't think there would be light leeks. also both rooms would be 12/12 at the same time.

it seems more effective and time effiecent to put many small 1-2" holes for air flow around the plants. 12.5 2" holes seems like my best bet, unless something else is better.

gramsci.antonio · Sep 9, 2008

many small holes has less airflow than a bigger one...

btw i have senn an intake that is the half of the outtake... the only effect is that it doesn't have to buy a fun, since a nice airflow form in the cabinet...

NanoScroger · Sep 9, 2008

pvc intakes work really well. with a 350cfm fan i used 5 or 6 2.5 or 3" holes.. cant really remember right now. worked great.

i think 3x the exhaust port is generally accepted for passive intake...
passive intake calculator

hoosierdaddy · Sep 9, 2008

grams...dewd...you are stating things that go against physics.
If the openings are the same size in area, then the air flow through both a single orifice and multiple orifices will be the same. A fan running at a given speed will pull the same amount of air through both configurations.

And having your intake smaller than the exhaust is only choking down the full potential of your cabs airflow.
You need a fan no matter what, no fan no flow.
And it is a rule of thumb to have at least 10% more intake vent than exhaust.
Having less does nothing but make the fan work harder to accomplish the task. Problem is the fan reaches its potential before that task is completed, when a choked down intake is used.

gramsci.antonio · Sep 9, 2008

hoosierdaddy said:
grams...dewd...you are stating things that go against physics.

Imagine the airflow as many infinite sheet of paper (planes) that slides against each other... so each sheet is frictional with the closes ones... and the boundary ones are frictional with the conduct...

imagine now a giant square-shaped fan in a huge room without any airflow inside. So now the layer of air are still, and no friction is there... Now you lit the fan and the sheet start moving parallel to each other... the fan just give energy to the sheets, and they spend it for moving or to the frictional force...

Now put a wall in front of the fan (at a given distance D), with a circular hole in the middle...

Now the sheet are forced not to be parallel anymore but they are contracted in such a shape:

http://home.cc.umanitoba.ca/~gunderso/model_photos/topology/hyperboloid_of_one_sheet.jpg

http://en.wikipedia.org/wiki/Image:HyperboloidOfOneSheet.png

with the knot in the middle smaller of the hole in the wall.

Since the sheet are compressed to each other the frictional force increase as more as you get closer to the center... being none in the center (or infinte, no air flow thus no friction) and at the border. But on the border is zero because the flow vector caused by the wall force the sheet of air parallel to the wall before entering the hole (a hole of potential) and changing direction. Just like when in car you make a turn to the right, but you don't go on the very right side of the street, you go on the center due to the inertial force pushing you on your old direction.

So the optimal area in a hole for airflow is a torus with a very small hole at the centrum. Thus i've demostrated that in order to maximize the airflow over a given surface, the surface has to be as big as possibile, in order the minimize the border area/total area.

This is a demostration as intuitive as i was able to... sorry for my poor english, it would have been better if i was to made it in german

And sorry for the imprecisions... but a strict proof would require a reader knowing hight topics in PDE and infinitesimal calculus....

hoosierdaddy · Sep 9, 2008

grams, I understand fully about the back pressure that is created with the multiple orifices...but for all intents and purposes, and for this application, the calculation for back pressure is not needed, as the excess of back pressure that exists with the multiple orifice opening is only a very, very slight difference from that of the larger single opening. For our purposes, we can disregard the back pressure differential.

petemoss · Sep 10, 2008

NanoScroger said:
pvc intakes work really well. with a 350cfm fan i used 5 or 6 2.5 or 3" holes.. cant really remember right now. worked great.

i think 3x the exhaust port is generally accepted for passive intake...
passive intake calculator

Thanks for that very nice link, NanoScroger. I've seen that heat transfer equation applied before, but that calculator puts everything in a nice tabular form so you can see how the required CFM changes when you vary the desired temp differential or the size of your light. Only problem is that most higher wattage setups use air cooled lights. This model assumes you are running the light bareback - without an air cooled hood or tube. If you use air cooled lights you would need a much smaller fan than indicated by the table. Interesting to know, but not with a lot of practical implications.

NanoScroger · Sep 10, 2008

well really i was just using it for the info under the table.. 1 sq ft of intake for every 300cfm of fan flow. using that equation you know how many holes to drill and of what size so you are not choking your fan. the cab in that pic is cool tube cooled, and took a 300cfm fan running full tilt to keep her cool with a 400 in it. cab was 2x2x5

blanonamous · Sep 11, 2008

the problem with using a big hole is that i'm going to have soda bottles taking up who whole inside of the floor space. i figure if i place many small ones careful i can fit all those in and have air pushing on the leaves and branches of the plants.

is 12.5-2" holes the best way to go?

FreezerBoy · Sep 11, 2008

NanoScroger said:
passive intake calculator

Dude, great link! Rep points for you!

NanoScroger · Sep 12, 2008

FreezerBoy said:
Dude, great link! Rep points for you!

thank you seniore. just trying to do my part

blano - you can use several small holes just figure out the surface area of whatever circle you have a hole saw for and you will be golden.

if you got big holes you can of course take that wire shelving and cut it down and line the floor with it to give your bottles something to sit on and distribute the weight.

this is however why i ported the side of my cab, not the bottom

blanonamous · Sep 17, 2008

math is still wrong

math is still wrong

i decided to go with 2-3 big holes (with wire over them), but i still don't know the math to make the correct sized holes. i looked at the size guide but i'm not sure of my cabinets wattage, cfm, or temp. I havn't hooked up the cfl's or the fan, also the carbon filter changes the cfm's.

anyways, i have the modded stanley blower (let's guess 300cfm) + carbon filter. there is a small piece of 5" duct connecting the filter to the fan.

What size should i cut 2-3 holes to get well-balanced negative pressure?
Thanks everyone so far for the help!

zolar · Sep 18, 2008

that pic of the darkroom louveres is the way to go put it on the bottom of the back side i'd drill 3 or 4 inch holes and put pvc elbows on em again on the back wall and point em down and glue a 2x2 accross the inner back along the top edge of the holes
and sides then staple black screen to it should cut light leaks and be enough in size plus putting them on back means you could add computer fans to intakers later if needed

FreezerBoy · Sep 18, 2008

blanonamous said:
i'm not sure of my cabinets wattage, cfm, or temp. I havn't hooked up the cfl's or the fan,

If you're not willing to look up your own numbers and do the math, then simply double the exhaust. One 4" exhaust = Two 4" intakes. One 8" exhaust = Two 8" intakes. If that don't do ya, cut a third hole.

blanonamous · Sep 19, 2008

that kinda puts me back at square one, and the sooner i put these holes in the sooner the clones can start. am hoping to get pointed in a good direction.

if i were to wing it i'd go with 2-5" holes and see how it runs. maybe put some cpu fan intakes in the middle shelf and make bigger holes if needed. which is fine, except it might be better to get the intake and exhaust right from the start. maybe 2-5" holes isn't gonna work the best.

the math feels more complicated than i am up to try. i'd mess up some variable and defeat the purpose of attempting intake math. :violin:

for instance if i went with the calculator provided, would it change having two rooms and using cfl's? trying to keep it simple. also is the 5" connector between the filter and stanley blower and hole size i should double?

FreezerBoy · Sep 19, 2008

blanonamous said:
the math feels more complicated than i am up to try

That's why I gave you a 100% math free option.

Let's try it this way. Go to Home Depot and buy the hole saw of your choice. Drill 3 holes for every fan. One at the top and two at the bottom. Fan goes on the top hole.

blanonamous · Sep 19, 2008

lol, will end it on that note unless anyone has anything else to contribute.

passive intake, math of many small holes?

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