Aerodynamic Loading on a Cooling Tower.

[floridashaughn]

I am trying to find some information on dynamic loading (fan) on a cooling tower structure . Any help would be greatly appreciated.
Thank you,

 

[rb1957]

do you mean the delta p that the fan is creating ?

or the internal airflow ?

or the external wind loading ?

Quando Omni Flunkus Moritati

 

[floridashaughn]

I'm actually talking about the force being applied on the structure at the contact points, due to the fan rotation.

 

[floridashaughn]

So, I guess that would be delta p (fan thrust) on the structure. I am not sure what code book to reference. Thank you.

 

[IRstuff]

Wouldn't it be the generated thrust imposed on the mounting points; action -- reaction?

TTFN
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[floridashaughn]

Yes, I just did not know the required variables needed. I am a young structural engineer, not aerodynamic. I have calculated thrust being F=(mass flow)(air velocity). My calculations provided a value with the units lb-ft/s^2, so I'm assuming I divide this by gravity to get lbm. The answer is coming up with a very small weight. On, another note, what exactly is the propeller efficiency coefficient. Would this be given by the manufacturer? Thank you for your help

 

[IRstuff]

Not my specialty, but I would have expected more like this:

http://www.wired.com/2012/09/modeling-the-force-from-a-fan/

You should be looking at an overturning moment, rather than a weight, I think, but, you never stated whether the fan force was acting vertically or horizontally. the equation in the cite is already in force, assuming units usage is correct. Division by g to get force is not correct.

TTFN
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[floridashaughn]

Sorry about that. Yes the fan is mounted vertically, pulling air from below creating a thrust (Force) reacting at the contact points. The reason, I thought, to divide this thrust by gravity would be to get my value in weight, rather than force.

 

[floridashaughn]

Man I am tired. I guess that would give me what I need. Dividing by gravity is going to give me a mass value. So this would be right. Thank you. How would I get the propeller efficiency coefficient? Thanks

 

[IRstuff]

It shouldn't matter much. Your mass flow should pretty much account for efficiency. I'm a bit surprised that this is a concern at all, given that the mass of the tower must be at least an order of magnitude, if not two, larger than the lift capacity of the fan itself. I would have thought that rotational torque or nutation from the fan would be a bigger concern.

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[IRstuff]

BTW, this probably should have been posted in the Heat Transfer, HVAC, or even plain Structural forums; there probably would have been a bigger response pool

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[rb1957]

i don't think you need propeller efficiency ... if you know the flow rate you can calc the pressure ... pressure*area = force

flow rate ... ft3/min ? ton/min ??

if ft3/min, well that is area*v (yes?) ... what you want is pressure*A = 1/2*rho*v2*A ... yes?

torque should affect the reactions as well ... some of the motor torque goes into the airflow (as swirl) but some is reacted by the fan supports.

Quando Omni Flunkus Moritati

 

[chicopee]

I would think that the weight of the fan, motor and frame would far exceed the thrust created by the fan, so, I would be more inclined to calculate the support of said structure assembly.

 

[chicopee]

Oh! and of the vibration created by the fan and motor assembly.