Negative Pressure for Dummies

FreezerBoy · Jul 4, 2009

by King of the Dummies. :wave:

Just a quick primer on negative pressure and why we want it.

I) Two air particles are moving along. We'll call them Fred and Ethel. Suddenly, they're separated by a wing. Fred takes the long route over the wing and Ethel takes the express route underneath. Somehow, they meet up on the other side. I can't tell you why this happens (I'm a dummy) but, I can say Fred traveled much faster to do so. This difference in air speed creates air pressure zones. Low, negative pressure above. High, positive pressure below. Negative pressure sucks; Positive pressure blows. The wing is pulled up from above and pushed up from below and, voilá, you're flying.

II & III) In a standard passive intake system, the increased airflow through the cab creates negative pressure. Negative pressure sucks. All air in the vicinity tries to get IN to the cab. The only escape is through the scrubber. Like negative results from the doctor, negative pressure is a good thing. It's our goal.

IV) You can't exhaust more than you intake. You CAN intake more than you exhaust. Thats positive pressure. Positive pressure blows. Interior air tries to force its way out while the relatively negative pressure outside the cab tries to suck it out. Combined, air can bypass the scrubber and compromise stealth.

The fastest way to positive pressure is intake fans. This doesn't mean you can't use them but, you can't be a dummy. You need to know your stuff. Ventilation 101

uncle louie · Jul 4, 2009

good info-- thanks

Kizzattack · Jul 4, 2009

Good thread. This will certainly come in use to many people.

FreezerBoy · Jul 5, 2009

You're outside the scope of the thread and my knowledge. This is a pre-school primer on why negative pressure is good and why positive pressure is bad. The Dummies answer to your question is: NEVER, EVER, EVER use intake fans! NEVER!!

Venting for Dummies says: Buy a hole saw the size of your fan. Punch 3 holes. Two at bottom, one at top for exhaust fan (guarantees negative pressure every time). The End.

If this doesn't cover your needs (and it wont for many) then your answers are elsewhere.

FullMetal has been gone for a year now so, you can't ask questions of him. Still, FullMetalJacket's StealthCab may be worth a read

Anti's still around and is doing the multi-chamber thing here Anti's MicroStealth Cab

And, as always Ventilation 101

Sleepy · Jul 5, 2009

tagged...

good info, i'm a dummy!

DiscoBiscuit · Jul 5, 2009

should be a sticky

hoosierdaddy · Jul 5, 2009

Good post, Freeze.

smokinshogun,
The velocity and the pressure that the air creates is controlled by any restrictions such as the intake, exhaust, and the power source (fan/s).
Nothing on the inside of the cab can change the above factors to any degree.
Now, a fan on the inside can effect the turbulence of the air within the chamber. And turbulence is a directional thing which has nothing to do with the force or speed of that air.
If there is no fan within the space, then the turbulence will be controlled by the configuration of the cab itself, how it's contents are arranged, and by where the intake and exhaust are located.
If your intake vents are on the same wall as the exhaust vent, then try to imagine smoke being added to your air so you can see how it flows. Imagine it coming in the vents, and being sucked right up that wall and out the exhaust.
Now, if we place that intakes on the opposite wall, in the opposite corner, then our smoke would have to travel from those holes, diagonally across the space up to the exhaust vent. By just changing where the vents are located you can control how the air flows withing the cab, or the "turbulence" of the air that the fans and ducting creates.
So in essence, yes you can control the air.
Hope I made sense.

tmu · Jul 5, 2009

So, negative pressure helps with transpiration and positive pressure in the root zone will also be beneficial, right?

hoosierdaddy · Jul 5, 2009

I don't think the air pressures effect the plant to any extent at all. None measurable anyway.
Although air turbulence does seem to effect stem growth.

The manipulation of the air does a few important things like;
keep temps in check
keeps humidity levels in check
helps to prevent disease
provide the plant with Co2
control and contain odors

hoosierdaddy · Jul 5, 2009

So, odor is not a priority?

hoosierdaddy · Jul 5, 2009

One of the points Freeze is trying to make concerns just what you are trying to do.
Positive pressure is not a good idea in most every situation. The only time you should use a positive pressured cab is if you have your shit together, as Freeze explained.
And the ones who actually understand all this stuff would not use a positive system unless it was absolutely necessary.

Have your exhaust fan at the end of the cabs air flow, and make sure you have proper intake openings to satisfy the flow, and the rest will take care of itself fairly well.

Seems like the whole thing with your situation is that you don't want to put a fan in the box. Why not? Not that you need it, if your ventilation system is working well with negative pressure, the air will move in the box just fine...depending on how you have your vents situated. A fan is just a helper...and a much needed one during many times.

BurmeseDragon · Jul 5, 2009

Ya I learned the lesson of not using intake fans, you can have negative pressure at the top of a chamber with a positive pressure at the bottom. That doesn't may physical sense, but I think it has to do with timing.

A good fresh air intake is a good exhaust, just not worth the hassle any other way.

exploziv · Jul 5, 2009

You are right. Good thread!

Btw.. I used negative pressure on my grow boxes every time and most of them had lower temps inside (with the lights on) than temp. of the room they were in. (summer time)
All that just because of good air movement & negative pressure.

ureapwhatusow · Jul 5, 2009

DiscoBiscuit said:
should be a sticky

SmokeyGriff · Jul 5, 2009

I was wondering if excessive stalk development caused by an over abundance of turbulence in the grow chamber could actually be detrimental?

I did a ton of research on the subject when I was getting into modding my Antec 1200 computer case. I'll try to find some of the threads regarding negative and positive air flow but google does the trick just as well if you want to get to it. Without a doubt case modders have some of the most in depth information regarding the subject and if you want to read more, it'd be a good place to look.

However if I remember the general theme of it correctly negative pressure is a lot more effective at maintaining a consistent temperature and humidity. Double the efficiency for half the watts. Along with a reduction in equipment and the noise it creates. It allows you to control airflow easier. also because you can put you highest producer of heat next to an exhaust greatly aiding in cooling.

Dead zones are created anytime an obstruction diverts the free flow of air. Say the opposite side of the negative intake. Fan use would just be to make sure the fresh air is distributed evenly through any obstruction. Otherwise you get mold ect. This is also where positive pressure creates problems because it has to pump so much more air in to be just as effective so you run into excessive swirling turbulence anytime there is the slightest obstruction that leads to pockets of dead air.

To me the added cooling along with the vastly superior odor control are the reasons I'm designing my cab around negative pressure.

All this talk reminded me of a very cheap but effective air filter for intakes, pantyhose.

FreezerBoy · Jul 5, 2009

SmokeyGriff said:
All this talk reminded me of a very cheap but effective air filter for intakes, pantyhose.

You sir are an honorary dummy (you're obviously not a real one) Filtering intake keeps bugs and dust out of the grow but, more importantly, off your lamp. Those with enclosed hoods are especially vulnerable to dirt accumulation inside the glass which robs your plants of light.

Other materials include air filters, coffee filters, aquarium filter pads ...

Filter Your Intakes

SmokeyGriff · Jul 6, 2009

Coming from The King, I'm honored

J-Blunt · Jul 6, 2009

FreezerBoy said:
by King of the Dummies.

IV) You can't exhaust more than you intake. You CAN intake more than you exhaust. Thats positive pressure. Positive pressure blows. Interior air tries to force its way out while the relatively negative pressure outside the cab tries to suck it out. Combined, air can bypass the scrubber and compromise stealth.

im sorry im a noob, im not trying to contradict u at all - just trying to understand a little more thoroughly

y cant u exhaust more than u intake? if i have 2 intake fans bringing in ~55 cfms each and an exhaust of 150 cfms thats attached to a scrubber, then what will happen? i dont see y this wouldnt work

if u were in my shoes would u just take the 2 intake fans and put them in the room to cause some airflow? leaving the two ~4in holes and having one ~4in hole for my exhaust would then allow negative pressure and everything rushing through the scrubber...if im getting this right finally

FreezerBoy · Jul 6, 2009

J-Blunt said:
y cant u exhaust more than u intake?

For the same reason a thief can't take more money than you have.

Nature abhors a vacuum. You can't subtract from nothing. Every bit of air you pump out had to be pumped in first. If it weren't there, you couldn't move it.

You CAN pump in more than you exhaust. That's positive pressure. Positive pressure bad. Positive pressure comes from positive pressure generators, aka intake fans. What would I do with your intake fans? Build two cabs and use them for exhausts. Intake fans bad for dummies.

J-Blunt · Jul 6, 2009

yes but there are spaces inbetween the fan blades, could it not be drawn in the cab thru there and u still have neg pressure WITH intake fans?

thanks for the quick reply

Negative Pressure for Dummies

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