Volumetric Air Flow Rate 1

[APengineer]

I am trying to determine the difference in the volumetric air flow rate in two different scenarios:
1. Two 3" ID pipes (2 feet long) each flowing 2 cfm of air join together into one 3" pipe (10 feet long). What is the volumetric air flow rate coming out the 3" pipe.

2. Two 3" ID pipes (2 feetlong), each flowing 2 cfm of air.
Each pipe reduces to 2.25 ID pipe (10 feet long). What is the volumetric air flow rate coming out of each of the 2.25 ID pipe?
thanks

 

[DesignerMike]

I believe that we would need more information.

Primarily: air pressure and source

A more realistic number would be expected pressure drop over the two different scenerios. 2 cfm is not very much air through a 3" line, so I would expect your pressure must be quite low....Also identify how much pressure loss you can accept. The other factor will be linear velocity.

 

[BobPE]

Even though we are talking about air, flow is still conserved. You provided in your problem defination the air flow rate in the pipes. Changing pipes or pipe diameteres does not affect flow (as you have stated the problem). So unless you have an outlet/inlet condition between the 2 pipes and the single pipe, the flow will simply be the summation of the 2 pipes. Pressure has nothing to do with the problem (as you have stated it).

BobPE

 

[APengineer]

I am trying to determine which automotive exhaust system would be the best.
Actually, it is two 1 7/8" (3 feet) to two 3" (5 feet) to two 2.25" (~ 10 feet).
Or
Two 1 7/8" (3 feet) to two 3" (5 feet) to one
3" (~10 feet).
Thanks for your responses.

 

[ko99]

All other things being equal, the system ending with two 2.25" pipes will be slightly better. The reason is simple: cross-sectional area of two 2.25" pipes is more than one 3" pipe.

The difference is small and the actual performance will depend much more on how you handle turns and transitions.




ko (

http://www.ecooling.biz)

 

[APengineer]

Thanks so much for the reply. The velocity would change due to different diameter
piping which was throwing me trying to calculate the actual volumetric air flow rate in each scenario.
I do not know how to determine the back pressure in each scenario as well.
Thanks again.

 

[quark]

BobPE is right that flow is conserved. You will get 4cfm in the initial case and 2cfm each in the second case. You have to note that automotive exhaust is done by a positive displacement device. So flowrate will not vary drastically.

 

[monkeydog]

APengineer,
It has been quite a while since I have played with an automotive exhaust system, but as I recall what was critical in the design of the system was the back pressure in the system. Is that what you are trying to do? Back into, a specific back pressure (play on words not intended)?

 

[JStephen]

Ditto to the comment above about bends and transitions being more the critical factor. Not obvious is that the only way to really tell the difference in the proposed systems may be to test it.

The back pressure mentioned should be just the pressure drop through the system (including abrupt exit at the end). So you are wanting to find pressure drop in a piping configuration.

Auto exhause is pulsed- don't know how that affects anything, but maybe some auto engineers could let us know if that makes any difference. If you have a muffler or catalytic converter, they'll probably be more important than the exhaust pipe.

I have heard that reducing exhaust backpressure by putting headers on a car can cause burnt valves if it isn't retuned. No personal experience there, though.

I remember also reading that there have been headers sold that actually had higher exhaust restriction than stock exhaust manifolds.

I remember on the headers, they would always make them equal length so that backpressure on each cylinder was supposedly the same. Never figured out why that mattered, though. The path isn't the same on stock manifolds.