# Airflow question regarding pipes of different diameters

Hello all! I think I have come to the right place!

Can someone explain to a mathematically challenged person such as me which of the following systems would create the most airflow:

System #1 - System number one begins with (Point "A") a 1 foot long piece of 2" (I.D.) PVC pipe, connected to a 90* elbow, connected to a 3 foot long piece of 2" PVC, connected to another 90* elbow. (Point "B")

System #2 - System number two begins again with (Poiint "A") a 1 foot long piece of 2" PVC pipe, connected to a 90* elbow, connected to a 2" to 5x1" distributor, connected to five three foot long 1" (I.D.) PVC pipes, connected to five 90* elbows.

In other words, the air comes in at point "A" in both systems. It runs through the first foot of 2" pipe in both systems. It runs through the first 90* elbow in both systems. It then either runs through five 1" pipes and elbows OR through a single 2" pipe and elbow.

I am fully allergic to mathematics, and would be ETERNALLY grateful to anyone who can help me figure this one out.

Dan

Gold Member
It depends On how you define airflow. The total mass flow will be the same in either system assuming you have the same inlet mass flow at point A.

If you are just looking for air velocity, system A will likely have a higher velocity since it has a smaller outlet area.

minger
as boneh3ad said, in the absence of friction the flow rates will be the same through both systems assuming the same difference in pressure between inlet and outlet

However, in the real world, there will be different frictional and minor losses. If flow rates are small, then these could be nearly negligible, but they will be "slightly" different.

AlephZero
Homework Helper
The 5 1" pipes have 5/4 times the cross section area of one 2" pipe.
They also have 5/2 times the internal surface area of one 2" pipe

The larger cross section area will tend to make the flow rate higher

The larger surface area will tend to make the flow rate lower, because that is where the "frictional resistance" to the flow comes from.
Five smaller elbows and the distributor will also restrict the flow more than one large elbow.

Which of these effects will "win" and make the flow larger or smaller depends on more information than you gave us.

Honestly, a conductor of the same size throughout its length will give you excellent flow in any case. I you gradually decrease the size toward the outlet, you'll end up with faster moving air.

Increasing the flow rate depends purely on the amount of air being pushed or pulled through the system. But you can also destroy a system with too much pressure or vacum.

russ_watters
Mentor
According to my ductulator the friction on a 2" is lower than on five 1" ducts.

OK... this is helping greatly!!!

So, if I am understanding everyone correctly -

The volume of air drawn in to each system will be the same, due to the fact that the 2" pipes at Point "A" are present in BOTH systems.

The amount of friction MAY make up for any gains found with the increased volume of five 1" pipes

Faster moving air will be the result of having five (actually ten... I'm doubling the recipe) one inch inlets.

So... if I can eliminate all or some of the elbows at Point "B", I'd be better off? Honestly, I am so excited by the answers that you all have provided me I can hardly control myself!!!

Gold Member
So, if I am understanding everyone correctly

The volume of air drawn in to each system will be the same, due to the fact that the 2" pipes at Point "A" are present in BOTH systems.

The volume of air being drawn in was an assumption on our part since you didn't tell us anything about what was drawing air into these pipes. The amount of air going in is essentially independent of the piping system.

The amount of friction MAY make up for any gains found with the increased volume of five 1" pipes

There are no gains for increased volume. The amount of air that goes into the system has to be the same as the amount of air coming out. For that reason, the total mass flow rate of air coming out of both designs will be the same. However, since there is more exit surface area between the 5 pipes of that design, that mass will be distributed over a greater area and so your exit velocity will be lower than the single pipe design. Again, this is assuming equal inlet air flow.

The friction in the pipes will just exacerbate the problem. The 5 pipe design will have greater friction losses, making it even slower than it otherwise would have been compared to the single pipe design.

Faster moving air will be the result of having five (actually ten... I'm doubling the recipe) one inch inlets.

No. And if you do 10 outlets, you will cut your velocity in half. If you want the air to move faster than it is when it goes in the pipe, the exit needs to have less area than the inlet.

russ_watters
Mentor
The volume of air being drawn in was an assumption on our part....
For me, that assumption is that each system is connected to a centrifugal fan running at the same constant rpm. In that scenario, more air flows through the 2" pipe.

OK... you math folks can sure get a guy frazzled...hahaha.

Let me clarify a couple of things. These systems are two different designs that I have been scheming up to try and build the most airflow friendly snorkel setup for a high performance 660cc carbed ATV motor. I would AUTOMATICALLY go with two 2" pipes leading in to the airbox... however, with the tight spaces I am forced to work with, I need to "spread out the airflow" a little, hence the need to run smaller lines, while attempting to keep as much air as possible moving towards the airbox.
When I mentioned doubling the recipe, I should have been clearer (sorry about that). What I meant was that I will be doing TWO 2" lines OR TWO 2" lines split down to five one inch lines each (ten lines total into the airbox)
The amount of air being sucked into the systems will, of course, vary with throttle, so I cannot give anyone a pinpoint answer on how much air I am trying to move. Basically, it will run the gamut from very little to a great deal.

Again, let me say how much I appreciate all of your input on this. Bear with me. As I said, I have never claimed to be a math whiz. And the math on this particular problem is so far over my head I needed expert advice!

Just a bump-up of the thread... I really need an answer on this. Sorry if I am breaking a forum rule.