Fan cfm output: reliant on speed or torque?

In summary, the conversation discusses the factors that determine the output of a fan, including power, speed, and torque. The relationship between these factors is explained using equations and the concept of affinity laws. The conversation also touches on the efficiency of the fan and its size, and how these factors affect the power required to move a fluid. The link provided offers more detailed information on fans.
  • #1
basically i want to know for a fan whether the air flow created is more reliant on speed or torque, if there's some sort of equation out there that'd be helpful too!
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  • #2
Its funny, I would say that the output of the fan would depend most directly on Power (watts).

Power = (Rotation Speed)* (Torque)

This equation is a good first step for thinking about motors in general.
  • #3
The flowrate across a fan depends upon the fan speed and the primemover should have enough power to move the air. You can use affinity laws of fan.
Q1/Q2 = N1/N2, H1/H2 = (N1/N2)^2 and P1/P2 = (N1/N2)^3, where Q is flowrate, H is head developed and P is power. When flowrate is high, the primemover develops power till it gets overloaded and trips off. [Broken]
The above link provides you basic and detailed knowledge on fans.
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  • #4
If your in the same medium, isn't the flow rating a function of the speed which is a function of torque?
  • #5
Pengwuino said:
If your in the same medium, isn't the flow rating a function of the speed which is a function of torque?

The volumetric output will be a function of speed. Saying the speed is a function of torque is a bit misleading since the torque is going to be very dependent on the fan design. Like Crosson and Quark said, one should really think in terms of power for the prime mover.
  • #6
ok thanks guys...but one slight question. What're N1 and N2? lol, because this is from air to air.
  • #7
N refers to the rotational velocity of the motor/fan.
  • #8
quark said:
The above link provides you basic and detailed knowledge on fans.
Thank you so much for providing that; it's exactly the sort of thing I've been trying to find to aid in my hovercraft design. :smile:
  • #9
if N refers to the rotational velocity (im assuming that's rad/s) why do i need N1 and N2 if i only want to find out some information for 1 speed? And shouldn't size of the fan be an issue?

also another sort of random question. Since (according to part of Bernoulli's equation) the energy density of a moving fluid is 1/2pv^2 (where v is velocity in m/s and p is density in kg/m^3), then would this make sense:

Say the fluid we have is water (p = 1000 kg/m^3), moving at about 1 m/s (just to make it simple. thus the kinetic energy density is 500 j/m^3. If the water is flowing at a rate of 2 m^3/s then would this mean the energy (or power rather) of the water is 1000 j/s (which is equal to 1000 W) because 500 j/m^3 * 2 m^3/s (the m^3 cancels and you get j/s) = 1000 j/s

does that make sense?
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  • #10
I didn't get what you exactly intended to ask. Flowrate through any fluid moving device, fundamentally, depends upon the speed of the moving device(or the linear velocity of the fluid) and its cross sectional area. For a fixed flowrate, if you increase cross sectional area, you can work with lower speeds. This is a trade off between initial investment and operational cost.

For a fixed flowrate and head, power varies only with the gross efficiency of fluid moving device+prime mover assembly. Fluid kW(or Fluid HP), a term referred to indicate the theoretical power required to move a fluid at a certain flowrate and against a certain resistance, is always constant(if flowrate and head are constant) irrespective of the speed of the fluid moving device.

Hope this clarifies your doubt.
  • #11
ok so the flowrate of a big slow moving fan can be the same as a small (very) quickly moving fan?

and what about the question in my "edit" post?

1. How is fan CFM output affected by speed?

CFM (Cubic Feet per Minute) is a measure of the volume of air that a fan can move in one minute. As the speed of a fan increases, the blades are able to move more air, resulting in a higher CFM output. Therefore, fan CFM output is directly proportional to the speed of the fan.

2. Is fan CFM output reliant on the speed setting?

Yes, the speed setting of a fan directly affects its CFM output. A higher speed setting will result in a higher CFM output and vice versa. This is because the speed setting controls the rate at which the fan blades rotate, which in turn determines the amount of air that is moved.

3. How does torque impact fan CFM output?

Torque is the amount of rotational force applied to the fan blades. It is a measure of how much work the motor is doing to turn the blades. The higher the torque, the more power the motor has to rotate the blades, resulting in a higher CFM output. Therefore, fan CFM output is reliant on torque as well as speed.

4. Do different types of fans have different CFM outputs at the same speed?

Yes, different types of fans, such as centrifugal fans and axial fans, have different designs and mechanisms for moving air. As a result, they may have different CFM outputs at the same speed. For example, a centrifugal fan may have a higher CFM output compared to an axial fan at the same speed due to its design.

5. Can a fan's CFM output be increased by increasing both speed and torque?

Yes, increasing both speed and torque can increase a fan's CFM output. However, it is important to note that there are limits to how much the speed and torque can be increased before the fan reaches its maximum CFM output. Additionally, increasing both speed and torque may also result in higher energy consumption and noise levels.

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