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Fan/Blower's functioning at different density of fluid

  1. Jul 15, 2017 #1
    Suppose we have a fan/blower that is working at sea level. It has a certain RPM against a specific voltage and current. We all know that. Now, that fan/blower has been brought to the top of a mountain where air is sufficiently less dense and the same voltage and current has been applied to the fan/blower. What will happen? Whether the RPM will increase or it will remain the same?
    Common sense tells that RPM will increase as the air that it has to blow is less dense. And higher RPM means the speed of air is faster in comparison to the sea level. Whatsoever, it's my thought and I want to check it.
  2. jcsd
  3. Jul 15, 2017 #2


    Staff: Mentor

    I was going to say that question can't be answered without specifying the type of motor. But then I see that you said both voltage and current are unchanged. In other words, constant power.

    So I say yes, to hold constant power with decreasing air pressure, we must increase fan RPM. That is a mechanical argument that has nothing to do with motors.
  4. Jul 15, 2017 #3
    Thanks for the reply. Kindly tell me that what will happen to the air that will be blown by the motor. As it's less dense, does that mean it will be blown with higher velocity?
    And, what is most important, is it necessary to change the configuration of a motor if the density of fluid blown is changed sufficiently? As for example, we have a fan/blower made to work at atmospheric pressure. I want to know whether whether we can use the same fan/blower (without any kind of change in configuration) to blow air at 1/10th atmospheric pressure? Is it necessary to change configuration of that motor?
    Suppose that power supplied will be same in both cases.
  5. Jul 15, 2017 #4

    jim hardy

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    Have you looked up "Fan Laws" ?
  6. Jul 15, 2017 #5
    I have asked the question from practical viewpoint. It's mere theory and can be easily understood. What I want to know is that do we face a problem if we want to use a fan/blower that is made for air/gas at atmospheric pressure to air/gas at lower pressure.
  7. Jul 15, 2017 #6

    jim hardy

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    You're being impractical.

    Work done by a pump(fan) at constant volume is roughly in proportion to density of whatever it's pumping.
    density is under the square root sign with Δp in flow calculation so you need to define what this fan is pumping against
    If you're going to force constant power into it it's got to move more cubic feet of the less dense fluid
    and it's not clear how a propeller or centrifugal blower will do that.

    An induction motor's speed won't change appreciably
    but a DC motor might well speed up

    To remove same heat from the motor its cooling system will require a lot more cfm of cooling air at 1/10th atm than at 1 atm

    A question well stated is half answered and yours is neither.

    The answer it deserves is "The motor will probably overheat because the rarified air doesn't cool its windings adequately."

    old jim
  8. Jul 15, 2017 #7
    That simply means if special means for cooling the motor is installed, it can perform. Right?
  9. Jul 16, 2017 #8
    Depends on what you mean by perform.
    Will it go round in circles? Yes.
    Will it move air? Not so much.

    As Jim stated, a blower is a constant volume device, but it has variable mass flow. Air density is 0.075 lb/ft3 (at 70°F) at sea level which decreases as altitude increases; that is, less mass is moved the higher you go. Moving mass around is the purpose behind many fan applications, and there comes a point where you've gone too high, and are just spinning your wheel.
  10. Jul 16, 2017 #9
    I can understand that. Just tell me whether the mass moved will be at higher speed or not. If the power consumption remains the same, then that can only be concluded (consider the loss due to friction very little).
  11. Jul 16, 2017 #10
    for instance, where Q=flow volume (CFM), A=cross sectional area (ft2), and V=velocity (FPM).

    As air density decreases, and there is less mass to move, in order to maintain constant power (which is determined in part by how much mass is moved) blower speed must increase, and so too will CFM, and hence velocity. However, how do you propose to do that when both motor current and voltage are held constant?

    For example, the rotating field for a 4 pole, 460V, 3 phase AC induction motor at 60 Hz revolves at 1800 RPM. Fully loaded rotor speed is somewhat less (perhaps 1750 RPM), and approaches 1800 RPM as physical shaft loading decreases. It won't go faster than that unless a VFD is used, and frequency is increased, but maximum rated voltage remains 460V, and the only way to maintain constant power is to allow current to increase.

    Another way to maintain constant power is to somehow change the physical nature of the blower or fan - add blades, change their pitch, increase overall size, or tweak another such factor "on the fly" - so it must do the same amount of work as air density decreases. Variable pitch is difficult but do-able (helicopters do it all the time), but nothing else jumps out at me.
  12. Jul 16, 2017 #11
    What I can understand with common sense is that if density decreases, then velocity will increase. If density will decrease to 1/4th of the previous value then velocity will be twice as before.
  13. Jul 16, 2017 #12

    jim hardy

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    Is that all you saw there ?

    Any practical fan with fixed geometry and speed is essentially a constant volume device.
    Mechanical engineers express head in feet of fluid flowing
    so when you reduce density you reduce delta-pressure available to force air through the ductwork
    and power goes down
    if you counter that by increasing power (speed ) you can bring dp back up

    fan laws tell us power is proportion to cube of shaft speed.
    and head is in proportion to square of shaft speed

    and i'll leave it to your common sense to ratio out those cube and square functions
    and apply the speed/torque characteristic of whatever motor you envision..
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