Altitude effect in fans

[xj25]

Hi,

We have been doing some airflow measurements in a ventilation installation on site, at 1500m above sea level (0.84 density factor).

We have been a little surprised by the fact that the flow was even higher than at our place (300m above sea level). We expected to get similar results and about 84% of pressure drop.

Searching about this I found the following in an well-known german fan manufacturer FAQ:

47. How does fan and blower performance change at 1,000 and 2,000 meters altitude?
The pressure developed by an air moving device will change proportionally with the air density if the RPM is kept constant. An air moving device powered by an electric motor will change RPM with a change in altitude due to the change in density (less load at higher altitude). This RPM change generally will compensate for altitude changes. The exception is for AC motors that are designed to run very close to synchronous speed at sea level. These will not change the speed with altitude change and will therefore produce less pressure at high altitudes.

My question goes like this, if the FAQ is true and the RPM increases it may explain the increment in flow, makes it sense for you?
The fans are low power AC axial type.

Regards and thanks for reading!

 

[miningman]

Well if this is true, it may be the explanation I was looking for a few years ago when running an axial fan at about 1200 metres at temperatures around 30-33 degrees C. Despite all the corrections I could make for air density, that thing put out way more air than the curves suggested, with considerably less power draw than the curves suggested.

 

[xj25]

Thanks minigman,

anyone more can confirm in its experience this issue of higher flow at higher altitude?

 

[willard3]

Where are you getting anything but AC power?

 

[Compositepro]

I believe most low power AC fan motors generally operate with a great deal of slip, i.e., the speed changes significantly with load. That is consistent with your observation that flow was higher at altitude. It is because the motor sped-up.

 

[msquared48]

If you are dealing with 84% in the pressure due to gravity at this level, you are also dealing with 84% in the friction force that the fan bearings see, hence more rpm for the same energy input.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[317069]

- is it a constant or variable flow fan?
if it is constant flow fan, then change in desnsity will not change the motor RPm but will change the work point on fan curve.
- fans and pumps don't produce pressure, they give pressure difference.

 

[Drazen]

Lol, the biggest problem is that no standard that I know it requires compensation for altitude.

Your fan transfers volume, but what you actually need is mass, for instance all internal pollutions will be the same, but there will be lower mass of air to dilute it.

So your fan draws less energy, but you receive less mass of air in space.

So, your ventilation requirements will remain intact, but if you use air for heating, than it is mass of air what counts and you will need more air volume to transfer the same amount of heat energy compared to lower altitude.

And guess, need for increase volume will also increase power consumption, and what determines whether it will be lower or higher demand compared to low altitude is where your efficiency curve will peak.

 

[Artisi]

xj25
Quote - "My question goes like this, if the FAQ is true and the RPM increases it may explain the increment in flow, makes it sense for you?" end quote.

If running with AC motor re-read the the FAQ - Quote "The exception is for AC motors that are designed to run very close to synchronous speed at sea level. These will not change the speed with altitude change and will therefore produce less pressure at high altitudes." end quote.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[xj25]

Hi Artisi, thanks for the reply

the motor is AC low power (<300W) and I think that it is not of the type described.

Unlike high efficiency AC motors, i.e. it is designed to allow speed regulation by variable voltage, thus I think that very probably indicates that it hasn´t a very "hard" torque characteristic vs speed.