cooling towers 2

[karim1982]

good evening,

can anybody tell me why in a cooling tower the fluid that cool down is the water and the air is warmed up.

becuse the process of a cooling tower is the adiabatic cooling, and in the adiabatic coolers the fluid that cools is air and the water is heated.

best regards.
karim.

 

[bimr]

Wet cooling towers (or open circuit cooling towers) operate on the principle of evaporative cooling.

http://www.youtube.com/watch?v=z9-cVGrR9OE

 

[25362]

If the entering air is unsaturated some water evaporates to saturate the gas. If the incomig air is already saturated, then it heats up (sensible heat transfer) and becomes unsaturated such that evaporation takes place.

In short, the evaporative (mass transfer) driving force is the difference in moisture content of the air.

 

[quark]

Good question and great reply by 25362. Hits the nail on the head, as usual.
Karim, only part of the cooling tower process is adiabatic and process continues beyond that. Once air goes to saturation point (similar to adiabatic cooler this far), it picks up sensible heat from air and further increases its tendency to absorb moisture. This all happens along the saturation curve.

 

[karim1982]

Thanks guys,

So this means that if the air is not saturated , the air will cool down through the tower till it is saturated then only after that the cooling of the water take place?

Regards.
Karim.

 

[chicopee]

It seems to me that somebody was asleep in their thermo. and heat transfer classes.

 

[bimr]

Ignoring any negligible amount of sensible heat exchange that may occur through the walls (casing) of the tower, the heat gained by the air must equal the heat lost by the water.

The purpose of the cooling tower is to cool the water, nobody is especially concerned about heating the air until there is a loss of efficiency.

 

[rotw]

In a plant usually the cooling water cools down a dozen of equipment, for example cooling water enters heat exchangers it does at certain temperature and then exits at a temperature higher (say 10K difference). If you sum up all the flows through out all equipment in the plant and multiply by Cp times supposedly same Delta T it often is a huge amount of heat that needs to be removed from water. I deem this is the purpose of cooling tower...

If you heat up (increase temperature) of wet air you decrease its relative humidity so you increase its capability to absorb moisture or we can also say its capability to dry. This is how work solar dryers: the air is heated thanks to the sun radiation/convection so this increases the capacity of the air to absorb moisture and it does absorb moisture until it reaches saturation.

Same principle (evaporative cooling) for chillers that are also used at the inlet of gas turbines...the colder is the air admitted to a gas turbine the better is the shaft power output (less head and less actual volume flow to the axial compressor for same kg/h). So water is evaporated at the turbine admission and in return it decreases the dry temperature of the air. For water to evaporate substantially the humidity of ambient air, sucked through inlet duct, needs to be low initially (like in desert).

 

[racookpe1978]

If water ever went out to the "cooling tower" when the saturated air entering the cooling tower was "hotter than the water" entering the cooling tower then, yes, of course the water would cool down the air.

But, even in the deepest of water - or perhaps, most especially in the deepest of winter, the entering air is almost never saturated. So, as mentioned, the evaporation losses almost always cool the air -> giving up that heeat of evaporation to the air inside the bottom of the cooling tower.

Also, the heat MUST BE REMOVED from the process fluid - from the water needed to cool the process fluids and process equipment. If it did not need to be removed, then the company would never go to the expense of building and running a cooling tower!