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Evaporative cooling
Evaporative cooling  
Dear Stephen,

We want to use the cooling effect of evaporation to lower the temperature of water flowing through a coil inside a vessel. This will be for a real HVAC experiment not related to education (not homework problem)

A steel finned-tube helical coil about 40cm high and 25cm diameter is inside a steel vessel of similar size. Hot water flows into the coil at the bottom, and cooled water will flow out at the top. A cold water nozzle will apply water to the coil, and evaporated water exits the vessel via a vent at the top. Like shown in the attached sketch.

The question is: we can choose any method of applying the water (nozzles can penetrate the vessel at any point, any number of nozzles, any type of nozzles). The three main ways would be:

(a) "drizzle-stream" - liquid water streams onto the top loop of the finned-tube helix, then cascades down to the next-level loop below it, and so on. Like "thin-film" cooling effect  

(b) "spray" - any number of nozzles spray liquid water anywhere onto the coils

(c) "mist" - a high-pressure, low volume water nozzle with an impinger atomizes all of the cold water before it hits the coils. Suffuses the whole chamber of the vessel with a cool water mist.

Which of these is basically the best approach for maximal cooling of the water flowing inside the helix ?

I don't know the physics of evaporation well enough to decide upon it, so I am certainly ready to be re-educated !

Thank you!

from Andrew

Sorry, I've been on travel and forgot to set vacation settings.  There are a few considerations here that I'm familiar with from nuclear reactor design, but the correct answer will depend on the amount of heat you're hoping to dissipate, and at what temperature.  This is a very small chiller.  You're going to get vapor out the top, and not liquid water, just as your diagram proposes.  If you're looking to dissipate large amounts of heat at high temperature, the "mist" approach would just blow out the top and not cool anything.  In that case you'd want the drizzle-stream or the spray approach.  If you're looking for lower flow rates, then the smaller the droplets the higher the surface-to-volume ratio will be and the more efficient your chiller will be.  So think about small droplets for efficiency, and larger flow rates for high temperature and power (like if the water is going to be basically boiling on the surface of the fins).

Again, this might be a question better suited to an engineer and not a physicist.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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