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AirFlow Calculations/ Anemometer anyone?

  1. Feb 26, 2008 #1
    AirFlow Calculations/ Anemometer anyone??

    Hello all,
    I am trying to solve a problem with my anemometer. Im not sure if there is anyone here that has used one but here goes. My purpose is to calculate CFM of a particular AIR INTAKE of my truck. The problems lies in that CFM= Air Velocity Ft/Min X Area ft^2...with Area= PIE * (r^2)....however my intake is 8" diameter at the entrance, then the filter, then a 4" exit.....The air supply I am using is (shopvac) 2" diameter hose.....what do I use as the area?

    I have a chart that says @ 4500 feet per minute on a 4" diameter is 390cfm

    2nd question-- What am I doing wrong if I do 3.14 (2^2)= 12.56 Area

    CFM= 4500 * 12.56= crazy big number which is not 390cfm =0(
     
  2. jcsd
  3. Feb 26, 2008 #2

    FredGarvin

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    The last problem first...you need to convert your area value to square feet. You can not use square inches. You're a factor of 144 off.

    As far as the first problem, you can choose any section. The idea being that the mass flow is going to be the same through all of the parts. That being said, the velocities will change when there is a change in area. So, if you're interested in the volumetric flow rate, select the section of the shop vac. Take your measurements in the shop vac tube. Then you can calculate the volumetic flow rate in that section. From there you know you'll have the same flow rate at other locations. Just adjust the velocity based on the new area. The filter housing will probably not be a great spot for measurements.
     
  4. Feb 26, 2008 #3
    Yea wow Im retarded...as far as measuring at the filter housing goes. I am tryin to determine the difference in my intake vs the stock intake. I figure the only was is to hold the air supply a certain distance away (approx 9 inches away gives a 50mph wind) and then measure the flow rate threw the air filter and housing, etc to see how much the actual engine is getting. I know the calculations to determine the Volumetric Efficiency and how much CFM the engine needs at a specific RPM (Volumetric FlowRate)....how else would I determine which intake flows better other than this method?
     
    Last edited: Feb 26, 2008
  5. Feb 26, 2008 #4

    Danger

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    Amateur input here. (Sorry, Fred, but you know that I can't help chirping up when something bugs me.)
    Cst(etc.), just make sure that you take the flow rate of the shop vac when it's hooked up as opposed to free-flowing. With a filter in that housing, you'll get some resistance that will probably slow it down a bit. (Or not... Fred?)
     
  6. Feb 26, 2008 #5
    Danger....Yes the air filter gives resistance, but that is the whole point of creating a intake is to make one that supports more air and supports the CFM of a better filter (K&N) therefore, I would have to compare the air flow through the stock housing and filter to the new air filter and housing setup to see how much more CFM it will flow over stock or if it doesnt create one that will, etc.....thats why I was asking what to calculate the area of since in essence I am measuring the airflow through the bigger housing and not the air supply source......

    I have determined that the air supply (shop vac) has a velocity of approx 50mph @ 9inches from the point of measurement (where I hold the anemometer) therefore, I hold the air supply 9 inches from the entrance of the airfilter/housing (simulating 50mph wind speed) and hold the anemometer on the exit side of the airfilter/housing (after the actual filter) and then I would get the CFM that is capable of flowing threw that setup @ 50mph.....correct? (Fred) only problem is now is trying to calculate the actual CFM....anyone ever use a anemometer? Its a lil tricky
     
  7. Feb 26, 2008 #6

    Danger

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    At this point, I must turn everything over to Fred. The closest that I ever got to using an anemometer was reading the airspeed indicator on a Cessna, and that won't be of any help to you.
     
  8. Feb 26, 2008 #7
    Well thanks for the input at any rate ;0).....

    Also, One more question when a 6" pipe shrinks down to a 4" on a bend....how would I calculate the effect of this? Or would it even matter assuming I started using this device correctly =0)
     
  9. Feb 26, 2008 #8

    FredGarvin

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    Silly question, but are you using a handheld anemometer or is it an automotive part that is preinstalled in your intake assembly?
     
  10. Feb 26, 2008 #9
    Handheld
     
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