How air velocity effects heat transfer of evaporative cooler?

[aeroUB]

Example

There is a closed loop evaporative cooler. The cooler consist of coils/tube bundles made of 1" OD x 0.065 wall tubing that are bent into serpentines to form 6-passes per tube/circuit and 42 tubes/serps wide. There are several coils/tube bundles set side by side and two large fans with back-draft dampers. The coils are 'flooded' with spray water on the outside of the tubes and the process that's being cooled is flowing through the inside of the tubes. The air is drawn over/thru the coils by the fans.

Say the air velocity is at about 750 ft/sec over the coils with both fans on. Is it safe to say that if both fans are identical and one goes out and assuming the dampers are working to block off the failed fan...that the air flow and therefor the air velocity over the coils/tube bundles would be cut in half? Resulting in an approx. air velocity over the coils being around 375 ft/sec?

Also...if that is the case, how would you go about figuring the effects on the heat transfer from the coils with the air velocity cut in half? This wouldn’t be a linear response right?

Thanks for any input
aero

 

[IRstuff]

Neither is quite that straightforward. Removal of one fan probably does not drop the velocity by 50%.

Even if it did, the transfer coefficient drops by less than 50%.



TTFN

 

[Yorkman]

One way to estimate the loss is to plot it out, the air side, on a psychrometric chart. You would need to know the CFM of the cooler entering DB and WB temperatures and leaving DB and Wb temperatures. Find the BTU/HR with two fans and then plot it again with a single running. I'm just throwing a guess that you drop to about 65 or 70% of full load.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[KiwiMace]

Refer my answer on your Mechanical Engineering forum posting. Well... half the answer. The airflow is likely to reduce much less than half. Not exactly sure of airflow/heat transfer relationship.

 

[aeroUB]

yorkman,

That makes sense and the entering DB, WB and exiting DB, WB is info that I would have or could calc. if I knew what happened to the air. I guess my real issue in this problem is determining what happens to the airflow. Which, I believe I have figured out through another post that was specificly about the fans.

Thanks for the input!
aero

 

[willard3]

Fans in parallel have a specific operating curve, just like pumps in parallel.

Flows add for parallel operation but the graph is strongly quadratic and not linear.

On fan will likely do lots more nthan 1/2 the air due to the pressure drop reduction.