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Possible airflow formulas

  1. Feb 17, 2008 #1
    Is there any kind of formula that exists or I can put together that will allow me to determine airflow from a fan being funneled into a tube that decreases in size over the length of the tube? Something specifically that would allow me to determine the air pressure (psi ?) coming out the end of the tube?

    An example: a 5" diameter fan that consumes 30 watts of power and spins at 4,000 RPMs. Directly attached to the fan is a funnel that channels the air flow into a 2" tube that gradually tapers down to a 1/4" tube.

    At what rate would the air come out of the 1/4" opening? How could I measure the PSI? Does length of tube have an impact? Whats the best method to channel as much of the air as possible into the 2" opening reducing push back, or drag (?), that might reflect back into the fan because the air can't all fit into the 2" opening at once (so it must go somewhere right?)?

    Essentially, I am trying to create a low tech air compressor that regular room temperautre air forced at high rates of speed to create a high (or higher) pressure through a series of channeled tubes.

    Any thoughts or insights into the project, beyond the answers to the above questions, would greatly be appreciated.

    Thanks.
     
  2. jcsd
  3. Feb 18, 2008 #2

    russ_watters

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    You'll need to actually take the measurements unless you get a commercial fan that has that data given in the catalog. Also, bladed fans don't produce much pressure, so this air compressor idea isn't going to work with one. You'll need a centrifugal fan.

    Look into Bernoulli's equation, for starters...
     
  4. Feb 18, 2008 #3
    I looked into centrifigul fans. I am not certain I understand how it would be better than an axial fan.

    The axial fans consume way less power, have a smaller foot print, weigh less but have way more SCFM airflows. I am assuming that a higher airflow CFM means that the fan is going to have a bigger, more powerful blast of air. Is this incorrect?

    How are these two different :
    http://www.blowerwheel.com/blower-fasco-clamshell.htm (105CFM)
    and
    http://www.qwikflow.com/AC_Fans/1538 HighAir.pdf (166CFM)

    This might be a futile project, but the research into the subject material is helping me understand a lot of things.

    Thanks.
     
    Last edited: Feb 18, 2008
  5. Feb 20, 2008 #4
    Neat idea. Have you tried Rolling your tubes ie, snail shell sort of design, or funnle I guess? I believe that acceleration is a big part of this idea of yours.

    Your main issue here is the losses your going to have with the use of the axial fan. The axial fan's don't compress air. Your using the tubes to compress it so at the point of where the fan blow's into the tube, if theres any gap or spaces, the air will attempt to go to. Even back wards through the fan will occur. Air moves path of least resistance. If too much air can't get into and fit in the tube it will attempt to go back wards down the sides of your tube. It in turn will cause resistance to incomming air and your psi will be almost nothing.

    Russ watters is trying to show you the easiest path to get your end result. The centrifugal fans are (I believe) designed almost like a turbo or compressor, so naturally it will have a high psi output over a smaller area.

    CFMs or Cubic Feet per Minute is pretty much a volume over time. PSI is Pounds per Square inch, which is kindasorta lol is Weight per volume (maybe density is a good word).

    You however are attempting to use a High CFM (not psi, similar, but different), channeling it (focus) to essentially compress the air. Theres heavy math in this, I can point you out to websites I've found for similar equations/calculations to understand CFM/PSI, and how they relate to your "problem". Just ask in your next reply as it will take a few minutes for me to log onto my "work" computer.

    The axial fans comsume less power because they have less LOAD on the fans, then essentially the motor. With less load on the motor, they consume waaaaaay less ampherage and therefor consume a MULTIPLIER less of wattage.
    Power(watts)=Volts x Amps. Less amps, way less wattage, but still at same voltage system.
     
  6. Feb 20, 2008 #5
    Thanks for everyones responses.

    I have found some pretty high outputted computer type axial cooling fans that claim 200+ CFM. The power consumption is as low as 30 watts for these fans too. The centrifugal fans can do about the same but also claim a higher PSI. Understanding this CFM vs PSI relationship is a bit taxing.

    Assuming a 2"x2" fan face, with a 2"x2" funnel over the fan face (so as little air escapes), my idea was to use a custom made tube that tapers from 2" to 1" to 0.5" to 1/4" over an over length of about 12"...so each segment of the tube would be 3" long. The design of the tube will be a coil (snail) shape to keep the design compact with least amount of flow resistance. Is 12" to small, should it be longer at 24" allowing 6" per tapered section of the coil? Does coil length matter?

    At what PSI would the air from a fan (axial or centrifugal) with 200 CFM exit the coil at?
    and/or
    At what PSI would the air from a fan (axial or centrifugal) with 25PSI exit the coil at?

    Are there other factors I need to consider in determining this?

    If I know this about the fan:
    * 200 CFM
    * 15 PSI
    * 3800 RPMs
    * 32 Watts
    * 120mmx120mmx38mm (dimensions)

    I am interested in this idea b/c if it can work I have essentially created a reliable (computer fans run forever), low wattage (25watts), electric and quiet air compressor that hopefully can exceed 150 PSI. Change out the fan, and for very little extra $$$ you can potentially create an air compressor with twice the PSI.

    Thanks again for the help everyone.
     
  7. Feb 20, 2008 #6

    russ_watters

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    Yes, it is. The clue should be the power. What you are missing is that the pressure generated is the force in the work/power equations (force times mass flow rate). Axial fans generate less pressure.
    The axial fan has a fan curve shown, the centrifugal does not, so it isn't possible to compare their actual performance.

    I need to leave, but I'll see if there is anything else I can help with (perhaps help find you a fan).
     
  8. Feb 20, 2008 #7

    russ_watters

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    Here's some data on a centrifugal fan: http://www.continentalfan.com/pdfs/axcind.pdf

    It's in table form, not graph form, but to compare a few points:

    Your fan's static pressure is listed in mm of water. Usual is inches, so I'll convert.

    s.p.----airflow
    -----axial---centrifugal
    0 -----195----106
    .125"--150-----99
    .25"---100-----93
    .375"--85------87
    .5"----60------80

    Your axial is 36W, the centrifugal is 55.

    The big thing to notice about these numbers is the strenghts and weakensses of the two types of fans: The axial is good for producing high airflow at low static pressure, but as soon as you try to add a restriction (like your funnel), the flow goes down fast. A centrifugal has a lower maximum flow, but will generate much more pressure in a flat profile and as such can push air through a restriction better. This is the reason attic fans are axial (no ductwork) and air conditioner fans are centrifugal (ducted). 1/2" of static pressure it typical for a fully ducted residential air conditioning system.
     
  9. Feb 20, 2008 #8

    russ_watters

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    Just about zero.
    Not sure what you mean by "a fan with 25PSI" there is no fan that I've ever seen that can generate that much pressure. But if there were a fan that could generate 25PSI, it could generate 25PSI.

    Read up on Bernoulli's principle. The gist of it is this, though: the total pressure along a streamline is constant. A streamline is the path a particle of air will take through your apparatus (along the flow, not perpendicular to it). There are two kinds of pressure, static and velocity (dynamic). Static pressure is what is in a balloon. Velocity pressure is what you feel when you let the balloon empty into your hand. Now since the total pressure is consant, that means that:

    S.P. + V.P. = T.P. = Constant

    Inside the balloon, S.P. is something and V.P. is zero. In the throat of the balloon, V.P. is something (the same something) and the S.P. is zero. There is nothing you can do to generate more pressure than you start with.

    Now I can tell without doing the math that the velocity pressure we're dealing with here is small fractions of an inch at these fans' best operating points. For the centrifugal, the max S.P. is 1.25", and with a relatively insignificant V.P., that means that that's about the max you'll get. And in psi, 1.25" is only about 0.05psi.
    Yeah, I gathered you were trying to cheat the concept of a compressor. You want to be able to generate pressure with lower power input even if it takes a while. It can be done -- but only with a compressor. There is lots of energy in the compression itself, so the only way to lower the wattage input is to do the compressing very, very, very slowly with a standard reciprocating compressor.
     
    Last edited: Feb 20, 2008
  10. Feb 20, 2008 #9

    russ_watters

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    Oh, one other thing: you may not be familiar with the concept of "inches" of pressure. It's actually inches of water column, just like a mercury barometer, but with water. Atmospheric pressure is 14.7 psi or about 30 inches of mercury, or 35 feet of water. And we're dealing with 1 or 2 inches.
     
  11. Feb 20, 2008 #10
    Nice, I couldn't have summed it up as readable as that. Hey russ, do you know alot about compressors? I was wondering if you could explain the relation of CFM and PSI. Cubic feet/minute, and LBS/square inch. One is volume over time, and one is weight over area. Is this a linear relationship? or even quick calculation?
     
    Last edited: Feb 20, 2008
  12. Feb 21, 2008 #11

    russ_watters

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    There is no relationship between cfm and psi (*Caveat). Typical air compressors of any size can give you 150 psi or so at a wide variety of cfm. To get more cfm, you buy a bigger compressor.

    Compressors are basically cylinders like a bicycle pump. The compression ratio is determined by the length, the cfm by the surface area (width).

    *Caveat: By that I mean in the contxt of the OP's goal - the cfm of a fan or pump tells you nothing about its pressure. But similar to with a fan, a pump does have a range of cfm and pressure, they are just fixed as a function of geometry and rpm with the pump. They vary according to the ideal gas law - you put a regulator on an air tank to knock the pressure down and the flow goes up as the air expands or just set the pump to shut off at a different pressure. Fan curves are more difficult because they are a function of aerodynamics.

    Yet another caveat: there are also centrifugal pumps that are essentially just high pressure centrifugal fans.
     
    Last edited: Feb 21, 2008
  13. Mar 19, 2008 #12
    How about putting an array of fans in line ahead of the restriction like in a jet engine?
    It seems like you could build up some modest pressure with good air flow that way.
     
  14. Mar 19, 2008 #13
    I had to put some of this on hold as business travel got in the way. But I am back on it now. I have been researching different centrifugal fans and air compressors. I will look into centrifugal pumps now too.

    Ultimately, I need to generate a minimum of 70 PSI....but hopefully something that can push over 100 PSI. So CFM is not a requirement even though I have been asking a lot about it. I also need this solution to be electrical, low wattage power, quiet and, hopefully, inexpensive.

    Not too much to ask is it? :-)
     
  15. Mar 19, 2008 #14

    russ_watters

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    Sure, pressures are additive when you put fans in line with each other. But to generate 70 PSI, even with good-sized commercial fans, you'd need about 500 of them in line with each other.
     
  16. Mar 19, 2008 #15

    russ_watters

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    Quiet might be difficult, but low wattage and inexpensive isn't too much to ask:
    http://www.sears.com/shc/s/p_10153_12605_02875117000P?keyword=air+compressor

    400 watts
    120 psi
    1 cfm
    $50

    Do you have an actual application in mind here? It would help a lot to know how much air you need.
     
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