Static Pressure and Cooltube Mindfuck

So I've read a TON about ventilation, and learned a lot. It's an incredibly complex subject the deeper you dig! But I have this really silly question left I haven't seen answered anywhere.

What is the relationship between static pressure and the size of a duct? For example, say you have a cool tube, and the light takes up some of the area available for air to travel, correct? Obviously the fan is going to pull less CFM than without the obstruction there.

Now I'd imagine that half the area would equate to half the airflow. Now does that mean that if a duct is reduced from say 6 to 4 inches at any point, the airflow is basically cut in half, no matter how long the constriction is?

I've seen tables that show the static pressure per 100 feet of duct sizes, for example .65"wg for 4" duct, .2"wg for 6" duct, etc. So is that way to understand it, if you have a 6" duct with a light taking up the middle, is the static pressure simply increased in relationship to how long the light is?

What gets really confusing to me, is to imagine covering half of the intake side of a 6" duct. Makes me think there would be half the airflow coming out the exhaust side.

So say you want to hook a cool tube up to a 6" fan, and for the tube it's really only like the equivilant of a 4" duct the air can flow through (which is about half the space).

So now thinking about matching a fan to a filter, you see the issue. Say the fan max's out at 1"wg pressure, the filter is .7"wg already. Is reducing the size of a duct in half .3"wg?

Jeez I feel like I'm not making sense. But it seems like a really obvious question?

I guess to make it really simple. If I have a 6" fan, and hook up a 4" reducer to one side and run 4" duct, what is the static pressure increase?

Now I think it must be a function of how long the run is! So that would mean covering half the intake doesn't cut the flow at the exhaust in half? WTF....

Someone who knows about static pressure please help!

Airflow varies by the root of the change in static pressure. sq root(SP1 / SP2) = CFM1 / CFM2. In other words, a large change in static pressure results in a much smaller change in total air delivery. Static pressure have different impacts on different types and sizes of fans. Each fan will have its own performance curve that dictates the airflow and bhp at a given resistance. If you have a steep curve your fan can overcome the resistances more eficiently while a fan with a flat curve will have very poor performance as pressures increase.

While the free area of a 4" round (.09') is roughly half of that of a 6" round duct (0.196") that change will not reduce the airflow by one half. You are simply increasing the duct velocity and the static pressure, which will reduce airflow but not to such extreme levels. The longer the length of smaller ductwork the more impact you will have. In order to calculate duct pressure losses properly you need to know all of the components in your system including fan airflow, duct size, duct length, elbows, transitions, etc. but it really doesn't need to be that complicated. Typical velocities in these types of applications will be less than 2500 feet per minute (fpm). Just keep your airflows below these levels. Claculate fpm by dividing CFM by duct area (sq ft).

Also, the rating for filters is typically the "dirty rating" and will be based on a given airflow (probably much higher than you will be using). For instance, if the dirty rating of a filter is at 0.65" then the initial or "clean" resistance might be somewhere around 0.20". Also, if the filter ratings are based on 500 CFM and you're only using 300 CFM...then the final static pressure will only be 0.23".

Hope this helps.

Thank you, that was very helpful!

So I guess the only answer to the original question, how much static pressure does a reduction in duct size add, is that it depends on how long the duct segments are, and the only answer to the question of how that affects CFMs depends on your fan and how it handles static pressure.

I think I got it, thanks. Anyone have anything to add?

You are putting too much thought into it.

Just buy the most powerful fan you can afford and use that.

Airwave said:

You are putting too much thought into it.

Just buy the most powerful fan you can afford and use that.

Click to expand...

No doubt. It's not that hard.

Correct...it's not that hard...just keep it simple and buy what you can afford. Stay away from can style fans (inline plastic propeller style fan blades). The airflow on these goes in the toilet with just a little resistance. Inline, vortex style fans (backward incline, airfoil) are the what you want to use.

Airwave said:

You are putting too much thought into it.

Just buy the most powerful fan you can afford and use that.

Click to expand...

I'd actually rather understand the underlying principles of ventilation when designing a ventilation system. Love to learn about things. Not everyone does I guess. Some of the people who post here actually have very helpful information on such things, so thanks to those that post worthwhile info that addresses the topic! Good job getting your post count up to everyone else!

gardenhose said:

Correct...it's not that hard...just keep it simple and buy what you can afford. Stay away from can style fans (inline plastic propeller style fan blades). The airflow on these goes in the toilet with just a little resistance. Inline, vortex style fans (backward incline, airfoil) are the what you want to use.

Click to expand...

Max fans are considered inline fans right? They actually provide the best of both worlds. High volume and decent pressure. I am running a 12" max with 25' of insulated duct with several bends.
I am actually considering adding a motor controller to lessen the airflow

ill stick my finger in here and point out that you seem to think the amount of difference the globe creates in the duct is alot

the globe is say 2 inches wide right?
this does NOT turn a 6 inch duct into a 4 inch duct.

2d size of a duct is pi r squared

6 inch duct is 3 pi squared = 88
4 inch duct is 2 pi squared = 39

6 inch duct with a 2 inch light in the centre is 6 inch duct minus 2 inch duct
(6 inch duct is 3 pi squared = 88 ) - (2 inch duct is pi squared = 9) = 79

79 worth of air flow is MUCH more than 39 worth of air flow

so the globe in the 6 inch duct does NOT make it equivalent to a 4 inch duct.

dunno if this makes sense, but the point is that having a globe in the duct (cooltube) doesnt make all that much difference.

DaveTheNewbie said:

ill stick my finger in here and point out that you seem to think the amount of difference the globe creates in the duct is alot

the globe is say 2 inches wide right?
this does NOT turn a 6 inch duct into a 4 inch duct.

2d size of a duct is pi r squared

6 inch duct is 3 pi squared = 88
4 inch duct is 2 pi squared = 39

6 inch duct with a 2 inch light in the centre is 6 inch duct minus 2 inch duct
(6 inch duct is 3 pi squared = 88 ) - (2 inch duct is pi squared = 9) = 79

79 worth of air flow is MUCH more than 39 worth of air flow

so the globe in the 6 inch duct does NOT make it equivalent to a 4 inch duct.

dunno if this makes sense, but the point is that having a globe in the duct (cooltube) doesnt make all that much difference.

Click to expand...

Good info. This stuff is WAYYY over my head. I would also think that the fact that the bulb is somewhat aerodynamic it would have even less restriction.
But again as posted above. This is way over thought. Figure the SQ FT. of the room and size the fan appropriately.

When you're using an inline fan, aka vortex fan, its GOING to move air. PERIOD. Use the recommended cfm ratings and compare it to your grow room/closet/cabinet/attic and go from there. Beyond that, there's no need for any significant complex number crunching. Keeping your ducting tight, and not having it hanging is really the best approach to getting the most from your fan. You're pulling air thru the tube or reflector, you're not blowing air, minor things like bulb dimensions aren't going to equate into this...however bulb wattage will, as you emit more heat.

The ONLY reason you use fans on lights in the first place, is to keep temps from getting too hot. It can't get colder than the ambient temp. You're not making a proton collider here.

Airwave said:

You are putting too much thought into it.

Just buy the most powerful fan you can afford and use that.

Click to expand...

Yes I concur,
U r over analyzing things man!

personally i think its cool that he asks questions and wonders why.
its a pretty acedemic question as the reality is that it doesnt matter,
but cool that you ask anyway