Efficiecy of Cooling Tower Fans

[corrosionman]

Gentlemen, Cooling tower fans have 4 blades, typically 60 inches long 15 inches wide and of a fixed section for the full lenght without any change to the "angle" of the blade. Such a blade must be extreemly inefficient as a rotating "wing". Please can anyone hazard a guess as to the possible efficiency of this fan compared with an aerodynamic designed one. My thoughts are that for something running constantly the wasted power (cost) must be very sizeable, and a correctly designed profile blade would soon recover the initial outlay.
Any comments much appreciated.
Corrosionman

 

[FieldTeam]

I don't believe these fans run constantly, at least on the more modern units. Variable speed motors and cascade systems control temps and airflow.
There is a pretty interesting post in the HVAC/R engineering forum on cooling towers.

 

[Tmoose]

Are you working with a particular cooling tower?

I've been around cooling towers whose blades' "angle" did vary along their length. They had blocks cast in to provide a flat reference surface to set intallation angle.
Might have been longer than 5 foot.

Here's some spx/Marley info about Cooling tower fans as small as 54 inch Dia with "true twisted airfoil" design.
It's from this literature -

http://spxcooling.com/pdf/PRG-08.pdf

On page 36 there is a fan for low noise applications that appears to have uniform shape, but also mentions low tip speed, which would be a powerful tool to make a low noise fan.

 

[berkshire]

I did some work on a cooling tower last year, replacing corroded splash grids. that tower had composite, full aerofoil blades, with a tapered section and aerodynamic twist in the blade, also there were 5 blades.
B.E.