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Max and Min RPM for a Fan

11/17/2016 1:24 PM

Hi all,
I need to use a fan. I have calculated the internal curve of my system. I saw fan suppliers websites and got the characteristic curve for a fan which works in my range of interest. All the fan curves which I saw are calculated for a certain RPM. So I calculated the fan curves for different RPMs with the fan laws, in such a way I can calculate different working points.
My problem is that I did not find any indication about max and min RPM for a certain fan. So my question is: is there any rule of thumb which would allow me to estimate the max and min rpm at which a fan can rotate?
Thanks so much

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#1

Re: Max and Min RPM for a fan

11/17/2016 2:18 PM

It depends more on the motor driving the fan. AC induction motors run at a narrow range of speeds, slightly slower than synchronous speed. DC motors can run over a wide range of speeds, depending on the voltage supplied to the motor.

The fan will have a airflow versus rpm curve. From this you can determine the fan's torque versus rpm curve. Likewise your motor will have a torque versus rpm curve. These two plots will cross at some speed where torque supplied by the motor equals torque required by the fan. (You can, of course, modify the torque versus speed curve of the motor by incorporating a belt and pulleys drive.)

http://www.tcf.com/docs/fan-engineering-letters/application-guide-for-selecting-ac-motors-capable-of-overcoming-fan-inertia---fe-1800.pdf?Status=Master

Bottom line: The speed at which the fan will run is the speed that the torque supplied by the motor equals the torque required by the fan.

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#2

Re: Max and Min RPM for a fan

11/17/2016 2:31 PM

The minimum speed a fan can spin should be obvious, it is 0 RPM. This speed will often happen when no power is applied to the motor or when the motor shaft is locked from a seized bearing or other obstruction. For the maximum speed the limiting factor use usually the type of electric motor and its power source. A shaded pole motor is a common type of motor used on many fans and the maximum speed is set by the frequency of the AC power source. If one is using a variable frequency drive or a DC motor then the mechanical load by the impeller onto the motor can be the maximum speed. As the number of gas molecules being propelled reduces the maximum speed can increase to astonishing speeds. It is not unheard of for a UHV turbomolecular vacuum pump to spin at thousands of RPM at low pressure levels. To reach these speeds the rotor must be stabilized in all six degrees of freedom.

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#3
In reply to #2

Re: Max and Min RPM for a fan

11/17/2016 4:28 PM

Redfred stole the only part of the answer I am qualified to give: 0 rpm is the minimum.

The only exception is if for some strange reason someone might have a convention where cw fan rotation is positive and ccw fan rotation is negative rpms, in such case, the maximum should be nearly -1.0 * the minimum. This is sometimes done on cooling tower fans in winter to prevent system freeze up.

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#4

Re: Max and Min RPM for a Fan

11/17/2016 5:23 PM

This approach you are taking is like trying to build a clock to know what time it is and not knowing how to tell time.

It has been established that, under normal conditions, the fan speed can go from 0 RPM to infinity, or catastrophic failure of the fan blades.

You are most likely a new "engineer" and you are vastly over thinking the problem. Pick a fan that runs in the most efficient part of its performance curve that moves the amount of air per unit of time that you need, buy it, have it installed and move on to something important.

When all else fails you CALL A FAN SUPPLIER on the phone or contact one on the web. They will tell you what fan you need to use and you will look smart to your seniors!

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#5

Re: Max and Min RPM for a Fan

11/17/2016 5:52 PM

Fan blade and motor characteristics are unique to each setup many factors come into play....some motor and blade setups may exhibit undesirable characteristics at certain speeds....

https://eurovent.eu/sites/default/files/field/file/PP%20-%202015-05-23%20-%20Attachment%20-%20Hudson%20tip%20clearance%20example%20-%20page%2015%20-%20figure%206.pdf

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#6

Re: Max and Min RPM for a Fan

11/17/2016 6:22 PM

What sort of fan?

What are your maximum rpm's..?

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#7
In reply to #6

Re: Max and Min RPM for a Fan

11/18/2016 6:45 AM

At least I can guess which curves you're thinking about.

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#10
In reply to #6

Re: Max and Min RPM for a Fan

11/18/2016 10:10 AM

I'd like to spin that fan to the max!

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#11
In reply to #10

Re: Max and Min RPM for a Fan

11/18/2016 10:27 AM

Down boy!

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#8

Re: Max and Min RPM for a Fan

11/18/2016 7:41 AM

Most of the replies so far seem to assume speed is limited by the motor power available (power varies as speed3). My guess is you know what air flow you want, and the pressure, as you've prepared the system curve. So you can already estimate the motor power, which apart from small variations in efficiency won't vary with fan speed (to give the required duty). And the motor must be selected to suit.

Why not just pick a fan based on the rpm in the published data? You haven't said anything about the duty, if it's a big fan it might be worth going for a smaller (cheaper) fan at higher rpm. Depending on the published rpm figure, you might be able to go higher, but I would ask the fan supplier for a maximum.

Also the drive arrangements affect it. If it's direct, you need an inverter to vary the speed (which can save running costs if the duty can vary). If it's belt drive, easier to choose a speed.

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#9

Re: Max and Min RPM for a Fan

11/18/2016 8:42 AM

The minimum speed is actually zero, as has been pointed out above; invariably, they are operated above that point.

The maximum speed of a fan is that where the tip speed and shape is such that the air speed is just under Mach 1.0; invariably, they are operated below that point.

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#12

Re: Max and Min RPM for a Fan

11/18/2016 11:39 AM

Thanks all for your replies.

Well, the fact is that my airflow will have to change quite often. So I want to choose a fan which works well for the most likely airflow and then I want a way to calculate what is the max airflow at which my fan can work.

Is it correct to assume that the limit in terms of airflow comes from the limit in terms of rpm? Is it the right way to evaluate the max airflow which a fan can handle?

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#13
In reply to #12

Re: Max and Min RPM for a Fan

11/18/2016 12:13 PM

If the system losses are all dynamic, for a given fan, flow varies as speed, pressure rise as speed2, and power as speed3. Maximum flow is at maximum rpm, as you might expect. So if you select a speed for your most likely airflow, and then find maximum speed from the supplier, or by trying it, or otherwise, you can find maximum airflow.

Whether or not that is satisfactory depends on the ratio of maximum required airflow to most likely airflow. If the maximum is still too low, you probably need to try a bigger dia impeller at lower speed for your most likely airflow, or accept a fan for which the most likely airflow is at a less than ideal point on the performance curve.

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#14
In reply to #12

Re: Max and Min RPM for a Fan

11/18/2016 12:28 PM

I think codemaster covered this, but I have not read his post as yet. Really, if the fan tip speed becomes supersonic, it will stop moving air and "stall", just as in a jet engine.

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#15
In reply to #14

Re: Max and Min RPM for a Fan

11/18/2016 12:50 PM

I don't think tip velocity is likely to be the limiting condition for speed in a fairly ordinary fan, as this appears to be. Just did a quick check on high-speed centrifugal air blower (used eg in aeration in activated sludge plants). Typical impeller dia 400mm, speed 15000rpm, gives tip speed 314m/s, vs sonic velocity ~ 330m/s. Fan would be nowhere near those figures.

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#16
In reply to #15

Re: Max and Min RPM for a Fan

11/18/2016 1:36 PM

I agree, Codemaster, however, do you not agree that subsonic is a strict physical limitation in any fan situation?

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#17
In reply to #16

Re: Max and Min RPM for a Fan

11/18/2016 2:13 PM

Subsonic fan speed is not really a limit on the fan speed. I believe it is a limit for effective "air" movement because faster fan tip speeds will not significantly move more "air" due to cavitation effects. However with proper mechanical supports and balance I see no reason why the fan cannot move faster. My turbomolecular vacuum pump example spins far faster than the speed of sound.

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#19
In reply to #17

Re: Max and Min RPM for a Fan

11/18/2016 2:41 PM

Thanks for the very fine point of clarification, sir.

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#20
In reply to #17

Re: Max and Min RPM for a Fan

11/18/2016 3:04 PM

This thread is the first time I've heard of turbomolecular vacuum pumps. I don't know how big these things are, but if say 100mm dia it would have to do > 60000rpm to reach speed of sound. Do they go much faster than that?

Wiki says "Because the compression of each stage is ~10, each stage closer to the outlet is considerably smaller than the preceding inlet stages." but it doesn't look like that on the cutaway or the schematic, perhaps I'm missing something.

Does speed of sound still have a meaning at pressures 0.01-0.1mbar? I think that puts it in the Knudsen flow range.

The OP of course wasn't asking about vacuum pumping and I'm pretty sure the fan speed will be limited by other matters well below sonic tip speed.

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#21
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Re: Max and Min RPM for a Fan

11/18/2016 11:17 PM

I believe the little tm pumps at work are about 250 mm in diameter and spin at a frequency of 4000+ Hertz.

As for an example of a "fan tip" exceeding the speed of sound at STP think of that helicopter blade making that speed of sound breaking "thump".

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#23
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Re: Max and Min RPM for a Fan

11/21/2016 10:18 AM

I suppose helicopter blades count as fan blade, since they rotate and move air. A wind turbine blade counts also, although wind is moving it. If wind turbine were to go supersonic, it presents a local problem with noise as I understand it, and maybe that has a deleterious effect on efficiency as well? This is what leads to changing the rpm setup for larger and larger machines.

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#24
In reply to #17

Re: Max and Min RPM for a Fan

12/10/2016 1:12 PM

Sorry, but it is the 1st time I read about "air cavitation". Could you PLEASE give more input.

Thanks

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#25
In reply to #24

Re: Max and Min RPM for a Fan

12/10/2016 4:50 PM

It isn't a strict cavitation effect because there is no phase state change. However, a loss in thrust does happen. Yahoo answers does a reasonable explanation.

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#26
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Re: Max and Min RPM for a Fan

12/11/2016 3:47 AM

It is well known that under circumstances the pressure distribution on a wing profile can change with respect to the "normal". The turbulence can generate a zone where the air flow does not have any contact with the profile. But this is NOT cavitation. Boundary layers are again under circumstances unstable far enough from the profile edge and the flow separate from the profile. But again it is NOT cavitation.

Cavitation is a complex phenomenon since the gas present in the fluid as solution or vapor generated due to local higher temperature and low pressure generates bubbles which implode where the pressure rises again and lead to shock waves. As well experimental as simulations have shown that in the boundary layer due to high shear stresses a local high temperature can occur leading to the separation of gas or vapor generation. To give an example in water cavitation is due to the vapor since water has a high vaporization pressure and in oil at much lower pressures and it is due to the air present in oil.

I cannot imagine air bubbles imploding is air, may be I have a limited imagination.

For me words have a meaning especially in technical discussions because wrongly used they lead to errors.

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#27
In reply to #26

Re: Max and Min RPM for a Fan

12/11/2016 10:00 AM

I agree that the meaning of words matter. CR4 is a forum not a formal publication site. I certainly agree that the correct word must be selected in a technical publication like a white paper, research paper or text book.

Correct word choice is one of the reasons these publications go through at least one technical editor before publication. An editor will identify typographic errors "air bubbles imploding is [in] air" and when possible offer a correct word for a sentence. It would have less of a technical error to use a simile (a cavitation like effect) in my explanation why a faster than sound fan tip speed would produce no additional air movement. However, similes are also frowned upon in the above formal technical publications due to their inherent need for interpretation. AFAIK there is no single word to identify this well known phenomena. To keep my analysis brief I chose the shorter of two inaccurate methods to get my point across.

If you prefer for complete accuracy on the subject to perform a complete analysis on how the Reynolds Number effects the performance of small scale propellers then be my guest.

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#28
In reply to #27

Re: Max and Min RPM for a Fan

12/11/2016 12:18 PM

Thank you for the important correction. I did not reread my comment it was an error I shall not repeat.

The paper is interesting I shall read it with care but not to day.

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#18
In reply to #16

Re: Max and Min RPM for a Fan

11/18/2016 2:33 PM

I would have, were it not for redfred's #17

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#22

Re: Max and Min RPM for a Fan

11/21/2016 9:51 AM

What is the back pressure and are fan blade tips shrouded? High speed helps with pressure but not efficiency. Low speed and larger diameter give more efficiency like BAF. A squirrel cages gives flow and efficiency, and are generally low speed.

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