Understanding air flow

A dewd named Poiseuille figured out how air flows and published a formula that is used today to figure out the numbers for designing a proper air flow system.
The basis of "Poiseuille's Law" states that air will flow from an area of high pressure to an area of lower pressure.

We see examples of this all the time, although we probably aren't thinking much about the physics behind these events.
One of the most obvious examples of this law is the vacuum cleaner. When the vacuum is running, it has a fan that is sucking the air out of a canister which lowers the pressure inside the canister. The pressure on the outside of the vacuum hose is greater than that inside of the canister, so air flows from an area of high pressure (the atmosphere) to an area of lower pressure (inside the vacuum canister).

Now, put your hand slightly over the nozzle end and you can immediately feel that you have blocked some of the air flow into the nozzle. This does two things...first it lessens the amount of air that can come into the nozzle end. By having less air volume coming it the hose, the pressure inside the canister increases. You can easily hear the vacuum motor increase in noise, as it is trying to overcome the blockage of air flow that you have created with your hand. The only way to stop the vacuum motor from laboring is to take your hand away and allow the air volume to increase, which allows the flow to return to normal.
This is exactly what is happening inside our grow boxes. They are essentially vacuum canisters.

We know from Poiseuille's Law that air will not travel through an orifice if the pressure is the same on both sides of the hole.
Here we have a box with an incoming pipe of air. There is equal pressure in both the pipe and the box, so the air will just sit there and not move.


Once we turn on our fan, the pressure inside the box starts to drop. This immediately causes the air in the pipe to rush into the lower pressured box's first chamber, at the same time all the air in the first chamber is flowing out of the first chamber and into the second...
Let's assume the pressure in the pipe is 14 psi, and when the fan is turned on the pressure in the box is lowered to 10 psi. This means the air is entering into the box with 4 pounds per square inch (psi) of force, which makes it easy to see how air will flow into an area of lower pressure.


The air is now filling the first chamber and flowing through to the next chamber.
Now, there are some other things happening with this flow that we do not see. For one, every time the air mass passes through an orifice (or hole) it sees some turbulence and friction that causes little fluctuations in pressure.
What happens in the end is that the air flow that was traveling at 250 CFM sees a slight bit of resistance at the hole and now travels at 245 CFM.
It looses a little bit of CFM with every hole it passes through...


If there were no resistance caused by the holes, and if we had a fan that could pull 250 CFM through a 6" pipe, then our air would pass cleanly through all the holes until it reached the fan and exited at 250 CFM. But the fact is that we do see some loss at each hole.
With the exact same size entry holes as the exit hole, we lose some CFM.


There are two ways to get the full 250 CFM at the exit.
One way would be to increase the speed of the fan to a higher CFM rating. In my example we lost 15 CFM from start to finish, so if I started with a fan that could pull 265 CFM, then I could essentially end up with my 250 CFM using the holes as they are.

The problem is that most fans are running at peak to begin with. So you can usually not increase them.
The alternative to get the 250 CFM we want is to open up the intake holes. By making the holes bigger, you will be adding the needed air flow that the holes inside the box are going to rob you of.
HVAC men know that they need a minimum of 10% larger opening than exit, because they understand the physics that we are discussing here.
We often get the recommendation to open up our intakes twice the size of the opening, but that is normally overkill. All you need to have is enough air flowing in to satisfy the fan and it's CFM rating. The mass flowing out is equal to the mass flowing in.
Although you can never really have your intake too large, you can easily have it too small.

Orifice Area
To properly size our air flow holes, we need to know how to figure the area of each shape. Both a 10" x 10" square and a 11.28" diameter circle have a surface area of 100 square inches.


Here are some examples of duct and pipe sizes and the area figures for them:


Let's assume I have a 6" ducted fan.
We know that a 6" duct hole has a surface area of 28.27 sq. in.
I decide that I want to use 3" pvc pipe for my light blocking intake holes.
We know that a 3" pipe hole has a surface area of 7.07 sq. in.
28.27 divided by 7.07 = 3.999 So I need 4 3" pipes to equal the 6" exhaust opening. But, I want at least 10% more opening to account for my losses through the holes, so I will add another 3" pipe, which is more than 10% over sized, and should do the job.

Knowledge will forever govern ignorance; and a people who mean to be their own governors must arm themselves with the power which knowledge gives.
-James Madison

Very interesting...

very informative.

so is the 2x passive intake rule a myth? would this much intake help or impede greater airflow?

Bedlam said:

so is the 2x passive intake rule a myth? would this much intake help or impede greater airflow?

Click to expand...

If you have twice the opening as you have exhaust, you will always have enough air. The recommendation is not a myth, but rather a rule of thumb that is a good practice for folks who have few clues about their air flow should follow. Or...you can use my tutorial to size your intake to work properly with the system you have. (shrug)

hoosierdaddy said:

Although you can never really have your intake too large, you can easily have it too small.

Click to expand...

If you are going to size the intake properly, then you need to also realize that the loss of flow happens each time the air passes through another opening.
Which means that had I just added a 1" pipe to get just about 10% over size, the next chambers resistance will cause the air flowing out of it to be less than 10% over. I hope that makes sense.
I'll try again if not...

Excellent information here. Thanks for that.

One thing I've been contemplating is that with a two chambered box, as you've pictured, how large should the opening connecting the two chambers be?

Is it simply enough to have the connecting opening as large as the intake? Presumably, any smaller will lead to increased resistance in the second chamber where you vent and drop airflow just as it would in a single chamber box. In other words, the connecting opening is just basically an intake dedicated to the second (exhaust) chamber.

Would it be wise to have the connecting opening a bit larger than the passive intake in order to compensate for turbulance caused by the passive intake?

Also, what about having a fan blowing from the inlet chamber and into the outlet chamber? Sort of like a halfway helper fan to keep air moving along a bit better between the two chambers. Basically, an exhaust on each chamber -- one venting into the second chamber and one venting out of the box completely. I would think that as long as the fan venting right out of the box is moving more air than the one venting out of the first chamber into the second chamber, then you would still have negative pressure throughout the box. I'm just not sure if it would actually move more air overall or if it would cause turbulence or other disruptions to the airflow.

great post I have a homeboxe xl with a 270Cfm can fan pulling threw my can carbon filter and cooling my cool tube I just put up the 3 1'6 by 4inch wide vent flap up so I don't think I'm chokeing it but what do you guys think

this is just what i needed to help plan my box/boxes. I truly appreciate it. i love threads like this that are full of information. thank you and keep up the great work.

-McChris-

Thanks for the kind words, folks.

HempHut, you are spot on my friend.
Because of the slight pressure differentials from chamber to chamber, you need to account for it by a slight increase of size for the next hole.

I needed to have four 3" pipes to equal the 6" exit hole, but I wanted an extra 10%. In my example I used a fifth 3" pipe. This was an increase of 25%.
With one or two more chambers, five 3" pipes should be golden in all dividers.

If you add a fan in between chambers, then you have essentially created separate systems. The flow in the first chamber would be controlled by the fan and the size of the intake opening.
The flow in the second chamber would be controlled by both the intake(fan you added in the middle) and exhaust fan.
Here is the problem with adding an additional fan in a middle chamber....

The extra fan changes the second chamber into a potential positive situation. Which means that the middle fan can actually force more air into the chamber than the final exhaust fan is taking out, which could cause air, that is full of funk, to escape from every nook and cranny in the last chamber, bypassing your scrubber.

Let me guess, you grew up watching Mr Wizard on Saturday mornings? Another fine post , sir.

LOL...nope, I was always too busy hustlin newspapers so I could score a lid of redbud!
However when I was a young punk, I used to take stuff apart just so I could figure out how it worked. Problem was, after I had it figured out, I would get bored or high, and never put it back together.

I was waiting for this to come.

Nice one Hoosier.

To add to this:
By having adequate intake you can dramatically reduce the heat buildup that occurs in your micro grow space.

I started with 4 1-inch holes in my veg chamber that allowed for it to be around 7-10 degrees within ambient.

I added 2 more 1-inch holes and that dropped it to 4-5 degrees within ambient.

I added 2 more 1-inch holes and that dropped it to 1-2 degrees within ambient.

I did the same to my flowering side. I was able to drop it almost 10 degrees. Now it actually sits at or under ambient with the light on.

By adding %100 more intake I was able to drop the increase in temperature my 8-10 degrees or so when the lights were on.

I'm not saying that adding intake will automatically decrease your temperature. I am saying that it is definitely something to consider if you are having troubles with it.

Thanks again for the great post Hoosier.

-Q

Glad things are working out for your, Quazi.

HempHut, I got to thinking about your post some more.
If we have a 2 chamber box with a 250 cfm fan, and the openings are the proper size, then the fan will indeed produce 250 cfm.


If we were to add another 250 cfm fan to the middle chamber, we will not increase the cfm at all at the final exit.
What a second fan does do is help with the static pressure. The static pressure rating of a fan is what should be looked at. Not all 250 cfm fans are created equal, and those with low static pressure ratings will NOT tolerate any shortages of air flow.
As a comparison, a Stanley blower creates about 250 cfm, as does a Suncourt inline duct booster. There is no comparison between the two as far as power goes, and the Stanley blows the Suncourt away, although both will create 250 cfm at 0 static resistance.


Now consider what happens if we put the extra fan at the exit, and add intake holes. Double the airflow.

Nice tutorial and diagrams, hoosierdaddy! Heat control is the biggest challenge facing cabinet and micro growers. We need more tutorials such as this to help us set up the ventilation and air flow in a small cab.

Very nice!

Hey HoosierDaddy:

I hope this is OK to add to your thread. I'll pull it if you believe it doesn't tie in well enough. Poiseuille's Law caught my eye. That law and it's application were a BIG part of my former career

Airway Resistance

Airway resistance is the opposition to flow caused by the forces of friction. It is defined as the ratio of driving pressure to the rate of air flow. Resistance to flow in the airways depends on whether the flow is laminar or turbulent, on the dimensions of the airway, and on the viscosity of the gas.

Poiseuille's Law: R= 8ln/pi r4 where l=length of tube, n=gas viscosity, and r=radius of tube

The most important variable here is the radius, which, by virtue of its elevation to the fourth power, has a tremendous impact on the resistance.

Thus, if the diameter of a tube is doubled, resistance will drop by a factor of sixteen.

For turbulent flow, resistance is relatively large. That is, compared with laminar flow, a much larger driving pressure would be required to produce the same flow rate. Because the pressure-flow relationship ceases to be linear during turbulent flow, no neat equation exists to compute its resistance.

The tube in our case is the ducting in our ventilation system. The flow in our applications is NOT laminar. So using this formula for resistance is really a "best case" scenario.

Several things should interest growers.

Making holes bigger greatly reduces the resistance to airflow. This in turn reduces the work load on the blower.

Use the largest diameter ducting you can. Again, this reduces the resistance of the ducting and work load of the blower.


Minimize turns in your ducting. Make the turn radius as large as practicable. This reduces turbulance and work load.

Use as few reducing adapters as possible. These restrictions in radius(bottlenecks) dramatically increase resistance and work load.

I've done some pressure/vacuum measurements on my grow HydroHut tent, blower assy. and ducting. The resistance of the scrubber, ducting, reducer, etc drops my Dayton 465 blowers 'rated' output from 465 ft3/min (unloaded) to about 227ft3/min (~.5 inches Static Pressure from blower specs) Actually .5" of vacuum since I pull through my scrubber, etc. I measured the vacuum with a Dwyer Magnehelic differential pressure gauge.

From what I read, many people don't take into account the "loading" effect that resistances like their scrubber have on the blower/fan they use. Loading is why these cheap lil computer fans just don't cut it when it comes to pulling or pushing air through some "airways."

The more resistance the blower has to work against, the more powerful it must be. Period. A fact of physics. It doesn't matter whether the resistance is from a small hole or bends and kinks in the ducting, or whatever. The bottom line is the blower just knows it has to work harder the more resistance it sees.

And this in turn is why some blowers are better than others in terms of performance and quietness. As the work load increases for a blower it draws more current...up to a point...in an attempt to maintain its rated RPM. More current, more heat, and efficiency drops, noise increases.

Well designed blowers have poweful efficient motors designed to work against a "load range". My 465 is working at about peak at .5" and down to about 200ft3/minute.

Toodles


Toodles

hoosierdaddy said:

Click to expand...

Are we sure on this?

I'm not a vent doctor nor have I played one on TV. I'm going on memory so ... Seems to me we had a tutorial posted on how two 250s side by side did not a 500 make and, that that stacked was more efficient than side by side.

toodles, that is exactly the additional info this post needs. I was hoping we could eventually get into the meat of these matters.

Freeze, I am as certain as I can be on that issue. As long as you have the proper intake openings to allow for the least amount of resistance, then you will definitely have 500 cfm using two 250's in that manner. Stacking them is only going to make them both work less to accomplish a ceiling of 250 cfm.
It helps to look at it like this...a single fan or a stack of 10 fans for that matter, are bottlenecked by the orifice size of it's port. A 6" fan has only 27 sq in of area to get the air through. Two of the same fan doubles the area, and the amount of air that can be put through with the same apparatus'.
But again, intake orifice and path must allow enough air for the fans to function as rated.

OK, here we go. I may have mis-remembered the first part but, here's my source on the stacking of fans. Airflow engineering

There's all kinds of formulas for pressure, noise, power consumption. Converting cfm to meters per minute or hour.

Give me a sec, I am going to post up those graphs. They are perfect for explaining what I am trying to convey on this issue....


OK, Look at the graph at the bottom showing the two fans stacked. Notice how the lines both come to the very same point when it comes to the air flow. That is because no matter how many fans you stack in series, you cannot increase the air flow, only the power they have to overcome static pressure, which the lines also show.

Note how the top graph showing the parallel setup the lines do indeed show in increase in air flow with the dual fans. However, the two in parallel did not increase the power they have to overcome static pressure.

I hope this made things more clear.