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Need a lil help on CFM

  1. Dec 13, 2007 #1
    Im trying to figure out the CFM on a certain project. To sum it up I am taking a 4" PVC Pipe and it is going into a 3" pvc pipe (had to shrink size due to bends and turns). My question is that if a 4" Pipe @ 4500 FPM can withstand 390CFM how will it be affected shrinking it down. Will it speed up the FPM for the 3" and make more CFM? Here is what I have

    3" @ 4500fpm= 220cfm
    4" @ 4500fpm= 390cfm

    Will it increase velocity? If so, how can I calculate how much it increases to calculate the new CFM. How would it affect the outcome if I added another smaller inlet pipe in addition to the 4" to increase the amount of air COMING INTO the 3".

    Thanks guys/gals!
  2. jcsd
  3. Dec 13, 2007 #2


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    Staff: Mentor

    The velocity will increase, so the airflow won't necessarily decrease quite so much, but that's an awful high velocity. What is the application? Generally, you can add up the total static pressure loss of a duct system and compare it to your fan curve to determine if you have the static pressure you need to get the airflow you want. Finding the static pressure of your system may be tough though because with such high velocities, every little fitting/elbow is going to create a huge loss. You're looking at needing a fan capable of generating 10" of static pressure, if not more.
  4. Dec 13, 2007 #3
    PM sent russ
  5. Dec 13, 2007 #4


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    Science Advisor
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    Gold Member

    Hi Cstm,
    Attached is a paper that discusses fluid flow through pipes. You can think of flow being driven by a pressure potential. It’s the pressure that is forcing the gas through the pipe. If the pressure is being generated by a fan or blower as Russ eludes to, then the pressure produced by that machine is dependant on flow as well. There is a pressure versus flow curve for the machine. But there are other situations such as a regulated source of air which provides a pressure which is independent of flow. In this case, the air regulator valve would open up more and increase flow in an attempt to maintain pressure.

    If you provide more detail of what you are doing, it would help.

    Attached Files:

  6. Dec 14, 2007 #5
    Basically I am designing an air intake that has to be a certain size (3 1/2") at the throttle body due to a coupe curves. My purpose is to get more air into the motor, however, it is limited at that point. I dont think there is any chance that the acutal pipe is filling up with air so I am tryin to get more air to it to make the velocity into the motor faster at a speed of 55mph (4840fpm) so I am make a 4" piping that pulls air from outside the engine bay as well as the existing hole on the intake itself to make more air available to be sucked threw the air filter/ intake. I have already done this to my truck and have noticed it helped a lot in fuel economy. Now Im curious as why and how, the amount of air, velocity, etc. =0)

    Thanks for your help!
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