Precipitation Cooling Tower

[troubleMakr]

-----I read with great interest the pages on precipitation-based cooling towers here, and had a few ideas. All the articles mentioned that water escaping was a big problem - they all had open air cooling towers.
-----Now if the cooling is caused by the water expanding, then why not seal the tower, and atomize the water by forcing it through a set of very small holes. The atomized water will cool, and precipitate onto the sides of the tower further down, and eventually drip down to the reservoir at the bottom, to be pulled back into tubing going to a water block.
All that you would need is a "shower head" type device with very, very small holes, and a pump pulling water out of the reservoir, into the cooling block, back out, and up to the top of the tower, to be forced through a large number of very small holes.
-----Some problems I foresee - the rate at which the water precipitates back onto the tube, then into the reservoir will depend on the ambient temperature in the room - with a fluctuating ambient temp, more or less water would be required, so it would be good to go with MORE, just in case. This, however, will reduce the effectiveness of the entire cooling tower - if the water has to sit in the reservoir at the bottom of the tower for too long, the cooling effect of the atomization will be negated - this will require a delicate balance.
-----Also, perhaps the pressure from the pump will not be enough to force the water to atomize, and reduce cooling effectiveness - so move the pump to the top, forcing water directly into the dispersal device, and pulling water through the system, rather than pushing it.
-----I do plan to test out this idea, and any comments from anyone who has tried anything similar are welcome, either here, or emailed to me.

 

[William]

the principle of it though it blowing cool air in the bottom to cool down the water as it falls. That means you have to have an opening at the bottom and at the top. Just put something like an S at the top and you should be good to go.

 

[troubleMakr]

I understood that, but thought that by sealing the tower off and forcing the water to atomize via a shower block with very small holes and a high pressure behind it, you could acheive similar results without having to have the system open to the air, and all the humidity and water loss problems that entails.

 

[Colin]

Having played with a few cooling towers and a whole bunch of heat, it will interesting to read about your results.

For others reading this, my cooling tower now has a six gallon "Igloo" insulated reservoir to deal with the heat from my soon to resume dual pelt experiment.

 

[LimeyGreg]

The whole principle is based on evaporation, the loss of water molecules is generating the cooling effect. You can achieve this in multiple ways, but the idea is to accelerate the process. One way is to use a water deck and force air through it to provide the evaporation and hence the cooling, in this case the water is static and the air is moving.

In your case you propose to atomize the water, which provides a greater surface area for evaporation, in the first example the water deck accomplishes that. The second thing is that you are propelling the water droplets through the air rather than air past the water. Either way the mechanisms are the same.

Unfortunately you are forgetting the one important physics law that makes it work - EVAPORATION. If you seal the system it won't be long before the air inside is saturated and no more evaporation can take place, that means no more cooling.

You can not have a closed evaporative cooler unless you have some way to precipitate out the water and return both the air (now dry) and the water (now condensed) back to the system. The only way I know to do that is to cool the humid air below it's dew point and to do that you need energy - say using a refrigeration device - in which case why not just use the refrigeration device in the first place.

You can however try to minimize the loss of coolant. The air exiting the tower will consist of water saturated air and water droplets or aerosols. The droplets can be recaptured to a certain extent if you pass the exhaust through a collection of 'S' tubes. The droplets slow at the bends and impinge on the surface sticking to the walls until enough of them collect and gravity pulls them back into the system. Maybe if these 'S' pipes were copper and insulated and you applied an electrostatic charge they would attract more droplets - that would make it more efficient.

 

[troubleMakr]

That is my point, this is based upon a different principle, not simply evaporation to carry away heat, but atomization to force evaporation even in already saturated air. Simply atomizing the water provides cooling as well though. Putting pressure behind it (via the pump) to force it to atomize even in the very saturated air will cause cooling of the water - the water will be cooled as it exits the atomizing block, the hottest parts vaporizing, evaporating even in the already supersaturated air, letting the cooler water then fall down and recondense, dripping to the bottom to be put throught the heating process again. I do however realize that this cannot go on forever in a closed environment.

 

[troubleMakr]

one more thing, you force the cooler water to precipitate out and the air to partially dry out by exposing the saturated air to the newly heated water being forced out of the atomization block. It will be much hotter than even the water that is already in vapor form in the air.

 

[LimeyGreg]

The amount of heat removed from the water is equal to the amount of heat transported by evaporation. In your closed system evaporation will quickly reach a plateau and only increase slightly as the temperature increases - the hotter the air the more water it can hold. Dry air at a fixed temperature and pressure can only hold a finite amount of water. The water in your tower will indeed become saturated, at which point let us say the water vapor will precipitate out on the cooler walls of your tower.

The heat escaping through/from the tower walls to the outside is now the limiting factor. The further evaporation and heat transfer will be equal to the heat lost to the outside through the towers walls. This will be considerably less than what can be achieved by evaporation. Just because you form a fine mist spray doesn't mean you are cooling the water, it is not expanding as a compressed gas does when it is vented to atmosphere and so gets cooler, you are merely turning the water into droplets.

I suggest you try it as an experiment. Make a cooling tower with atomizers, a fan etc and close it up and see how long it is before the temps get too high. Then open up the top and unseal the fan and turn it on and see how fast it cools. Incidentally, you will need more than just your "run-of-the-mill" mag-drive pump to accomplish this, they can not develop enough pressure.

 

[Spewn]

The energy that comes out of the water MUST go somewhere. If it goes into evaporation, then when you condense it, that energy has to go somewhere else. The only way to re-condense the water out of the air and have a sealed tube would be if you had a unit powerful enough to take all that energy away. But if you could do that, what would be the point in using the cooling tower?

The water has x units of energy above ambient room temp, upon a certain amount of that water evaporating, x units of energy are used in that process. The energy is, however, still there. It was used to cause the molecules to separate apart. When that evaporated water condenses, x units of energy are given off. Unless you get rid of this energy(with a cooling device of power equal to that of the cooling tower), you've just released it back into the system.

 

[Sharp]

Actually as the temperature increased the pressure would also thus the air's ability to hold moisture would diminish proportional to the pressure and/or temp increase. It is impossible for the system to work as an evaporative cooler in a closed system. Now I do think it is possible, however impractical, for it to function well as a non-evaporative cooler in the same manner. That is to say it would not rely on evaporation to function. Basically what I am saying is it *may* be possible to cool the water by the same means as the radiator only directly. No interface material such as copper or aluminum. BUT like I said that would be very impractical and not as effective anyways. A better design would be to design an automatic refueling system. Depending on the rate of loss I experience in practice [once I build one a week from now] I may design such a system. For those interrested you can estimate loss as "loss = 0.00085*water flowrate(T1-T2)"

Also it is important to note that most the water they are complaining about losing is actually drift. Drift is actual water molecules entraped in water vapor escaping the system. Drift can account for a loss of 0.1~0.2% of supply.

I believe that atomizing the water might actually hurt the cooling characteristics of the tower. If the atomized water totally evaporated before reaching the water reservoir it could not cool down the temp of the water and it's loss would be in vein. It would however substantially lower the temperature of the air comming out the tower which is not the point. As always it is important to consider the exergy and anergy of the energy in the system and realize perpetual motion is not possible.