fan perfomances at high elevation 1

[mielke]

Is there a rule of thumb or equation used for correcting a fan cfm for different elevation if you know the the cfm at sea level but do not have any fan curves.

 

[dcasto]

Yeah, you adjust for atmospheric pressure. The ACFM is constant. The standard CFM is a function of inlet pressure.

WHY, because you need mass of air to be equal (a slight assumption here) , lower inlet pressure at constant volume, you have a lower density and less mass.

 

[mielke]

I understand that atmospheric pressure will change, thus change the density.

The actual ft3/min (ACFM) is the same? so for a fan operating at the same rpm but different temp/pressures will produce the same actual air flow?

when you say you need mass of air to be equal are you saying that for fans operating at different elevations (ie different operating pressures) the mass flow rate (lb/hr) is constant? doesn't this contrast with ACFM being constant?

 

[reidh]

Fans (or propellers) are constant volume devices. Given a few assumptions, a fan will move a given volume of air per revolution regardless of air density or temperature. To answer your original question, the volume flow rate for a fan should be independent of altitude.

Fans are typically used to aid in heat transfer. Since heat transfer is proportional to mass flow rate, a higher pressure altitude will have a negative effect on a system's heat rejection capability due to the lower air density.

Counteracting the negative effect of lower air density with increased altitude is the fact that temperatures decrease at a rate of 2 C/1000 ft up to 36,000 ft (assuming a standard lapse rate). This equates to about .7% temperature decrease per 1000 ft near SL, compared to an approximate 3.5% decrease in air density per 1000 ft. It is clear that the decrease in air density easily offsets the decreased temperature, resulting in reduced heat rejection as altitude increases.

We could open up another can of worms here if this cooling question was related to a naturally aspirated engine (less power produced at altitude), but we will leave that for another day.

-Reidh

 

[imok2]

Let's assume a fan will operate at 25,000 feet and will need to cool 340 watts and maintain a 15 °C temperature differential. Using the equation we get:

CFM = 2074 * Q (kW) / (? * ?T (°C))
CFM = 2074 (.340 kW)/ (.549 kg/m3 * 15°C)
CFM = 86 ft3/min

Where at sea level we only needed 40 CFM.
For more info go here:

http://www.comairrotron.com/high_altitude_cooling.shtml

 

[mielke]

if we have two fans with all things being equal except that they operate at two different air densities then they produce the same ACFMs from what i understand here.

but wouldnt they also be operating with two different static pressures (due to two different air densities) and thus produce two different air flows based on their static pressure???

 

[rmw]

Same volume flow, different mass flow due to density difference.

rmw

 

[mielke]

FYI i am asking in reference to axial (propeller) fan performance. if that makes a difference

 

[rmw]

The air doesn't care what kind of fan you use to move it. The performance and efficiency curves might however.

rmw

 

[ione]

Both changes in pressure (for example due to installation elevation) and in temperatures affect the air density.

Volume flow rate of the fan remains unchanged as the density of the fluid operated changes.
Static and dynamic pressure vary with fluid density.

P1/rho1 = P2/rho2

P1 = fan total pressure at altitude 1
P2 = fan total pressure at altitude 2
rho1 = density at altitude 1
rho2 = density at altitude 2

You can get air density vs altitude at the link below.

http://www.engineeringtoolbox.com/air-altitude-density-volume-d_195.html

Note that the power varies as well with the density of the fluid.

 

[speco]

Mielke,

I understand that you are designing some type of air-cooled exchanger, right?

In that case, to answer your last question, yes, the static pressure across the cooler bundle changes with altitude. Generally, pressure drop calculations are on the order of mass velocity squared divided by density. So as as the altitude becomes higher, the acfm stays constant, the mass flow decreases, and the static decreases.

Here's a quick altitude density correction factor from a well-known fan manufacturer:

correction factor = e^(.000037*alt)

altitude is in feet above sea level

However, I think you are going about this in a somewhat backwards fashion, unless you are moving an existing unit to a higher altitude and want to see if it will still work. Normally, you would select your fan based on the actual ACFM and static for the given conditions. If you are working with some type of fixed, off-the-shelf designs, then you should have some really significant safety factors built into the selection.

Regards,

Speco (

http://www.stoneprocess.com)

 

[mielke]

Im sorry to beat a dead horse but i understand centrifugal fans are constant cfms, but this is true for propeller fans also?

 

[ione]

mielke,
Be sure that both for propeller (axial blowers) and centrifugal fans the volume flow rate is constant. As density varies, pressure and power vary directly (and the same way) both for centrifugal fan and propeller (axial blower).

 

[speco]

Mielke,

Here's a correction to the correction factor calculation. It should read:

correction factor = e^(-.000037*alt)

Speco