Intake size double the exhaust size?

ScrubNinja · May 9, 2008

Sup fellas. I keep seeing the general rule to make the passive intake size twice the exhaust size. I definitely agree and think it's a good idea to have as much flow needed, but I got to thinking.

Seeing as the bottleneck in pretty much every micro exhaust system would be the carbon filter, wouldn't the exhaust/intake rule then apply to the "size" of the carbon filter as the exhaust? All generally speaking of course.

minigreens · May 9, 2008

yea i thought about that too... i have a 120mm fan, that probably 4x4inches but with the filter on it looses airflow

Guest · May 9, 2008

I think the idea is to have your exhaust twice the size as your intake

FreezerBoy · May 9, 2008

Intake is twice the size of exhaust. You can't suck out more than you pull in.

Guest · May 10, 2008

Yeah, thanks I was too skink last night, Now that I think about it, thats how mine is.

FreezerBoy · May 10, 2008

Georgia Green said:
Yeah, thanks I was too skink last night, Now that I think about it, thats how mine is.

Nonsense, my good man. You simply weren't stinky enough. Remember: Smoke two joints before you smoke two joints and then you smoke two more. :wave:

ripOG · May 10, 2008

FreezerBoy said:
Nonsense, my good man. You simply weren't stinky enough. Remember: Smoke two joints before you smoke two joints and then you smoke two more.

FullMetalJacket · May 26, 2008

I mean this is a very general rule that intake should be twice the size of exhaust and doesnt hold much value. In fact i think it is useless and misleading. Always makes me laugh as i hear it everywhere.

If it were that simple, scrubninja in correct you would go by the surface area of the carbon scrubber...lol.

Other factors like how many cfm you are exhausting are more imporant than size. What the rule is trying to help people with is-
Displacement vs dispersion

You want to displace the air in the box so that the air is exchanged more effeciently. If the air comes in w/ a high velocity it mixes the air causing more dispersion and less displacement. All that being said if your circulating the air internally well you are dispersing it anway.

Because of this I feel you can have a smaller intake or same size intake as your exhaust. I think it is better, because it saves room, more discrete, and easier to light proof. It is just dependent on your exhaust fan being able to overcome the additional pressure (static pressure) a smaller intake requires it to overcome. Also consider any noise the velocity of the air rushing into the intake will create.

My point being just think about things scientifically instead of using "genearal rules" taht people just repeat, when it can negatively impact your design.

bud_me · May 26, 2008

FullMetalJacket said:
Because of this I feel you can have a smaller intake or same size intake as your exhaust. I think it is better, because it saves room, more discrete, and easier to light proof. It is just dependent on your exhaust fan being able to overcome the additional pressure (static pressure) a smaller intake requires it to overcome. Also consider any noise the velocity of the air rushing into the intake will create.

My point being just think about things scientifically instead of using "genearal rules" taht people just repeat, when it can negatively impact your design.

Gotta agree with FMJ there, i keep to the one to one ratio, if the intake is too small, your exhaust fan faces more work than it needs to.

minigreens · May 26, 2008

what would be the disadvantage if the intake is too large?
wouldnt air only be sucked in at the rate of the exhaust if its passive? would there be pressure build if the intake is too large?

bud_me · May 26, 2008

a build up of stagnant air would result.

also, people working with micro grows are often limited on space and light proofing is easier the smaller the hole is.

riddler · May 26, 2008

also wouldnt it matter on the speed of your fans?

FullMetalJacket · May 26, 2008

minigreens said:
what would be the disadvantage if the intake is too large?
wouldnt air only be sucked in at the rate of the exhaust if its passive? would there be pressure build if the intake is too large?

disadvantages of larger intake

1. You want negative pressure in the cab to keep the smell in. Well depending on how many cfm you are exhausting you can have an intake very large. I overkilled mine, i could probably not have any doors and still have negative static pressure. So as long as you are running the exhaust you should have negative pressure. If you stop the fan their is more chance for the air to seep out of a larger intake.

2. Room
3. Harder to light proof
4. Less stealth

advantages

1. Displacement (which you eliminate if you have good internal circulation anyway)

FullMetalJacket · May 26, 2008

riddler said:
also wouldnt it matter on the speed of your fans?

Of course it would, not only cfm but static pressure (SP) it can handle. Did you read the posts before yours.

Congrats on your fist post by the way... welcome..

hoosierdaddy · May 26, 2008

I have found that the larger intake becomes necessary to get efficient air flow and cooling. Sure, there may be some very slight advantages to using a small intake, but I can't see them overriding the benefits of a larger one. For one thing, the intake needs to be sized appropriately to the exhaust and cfm of the fan, plus taking in account any baffles or scrubbers.
If you close your doors, and the fan increases in sound, it means there is insufficient intake and starts putting work on the fan blades. This can be corrected by increasing the fan cfm...but much easier to do, and for far less coin, the problem can easily be corrected by increasing the intake size.

I'd much rather cut a few more holes, than to shell out good money for another gadget.

legalizekindtok · May 26, 2008

all this applies to negative pressure for controlling pest... but if pest arent a prob or smell, you want whats going in to come out the same... lets say you have no pest then 300 cfm in and 300 cfm out, but you have probs with pest then you want like 150cfm going in, and 300 going out to create negative pressure. :joint:

FreezerBoy · May 26, 2008

A smaller intake requires bigger, louder fans at greater expense and cost of stealth. With matching intake, I could cool my 150 HPS with a $10 76 CFM axial but, my 250 CMH turned my cab into an easy bake oven. In a flu induced panic I flushed $300 down the drain on a 150 CFM inline fan and the unsuccessful means to tame it.

Turns out all I needed was another intake hole. The inline's gone, the axial's back, The cab's cool and, with larger intakes, the axial is less stressed and runs quieter than before.

Anyone want to buy an inline? And a Speedster? And a fan isolation box? And some insulated ducting? And a muffler? ... Not enticed? It's gauranteed to blow your stealth to hell. Now what would you pay?

bud_me · May 26, 2008

Yeah when stealth is to be taken into consideration stress on the fan of an inadequately small intake translates to noise. Nobody likes noise. The basic rule of listening to the fan when a side is open and when the box is completely sealed will usually reveal whether or not your fan is under additional stress.

FullMetalJacket · May 27, 2008

Once again, relative. I use inline fans..big ass fans for a cabinet. But i use a speed controller and turn them down a bit. I cannot hear my fans just some wind noise. I can turn them down further and not hear the wind noise but if i wanted that then i should have gotten an axial. The reason why i got rid of the axial and muffin fans were because they didnt move air fast enough and my muffin fans were loud.

I would never recommend axial fans for anything larger than a small cabinet and definitely not for a carbon filter. I would even go muffin minimum for the light cooling. Just my opinions.

You can keep stealth with inline fans if you get the riight ones and use them properly...

MDsmoke · May 27, 2008

Exhaust the light, then exhaust the grow (separately).... Then this all gets simple.

Intake size double the exhaust size?

Grow like nobody is watching

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