cooling tower question

[yazguli]

Hi everybody
In rating a cooling tower, dry bulb temperature of air is not considered. On the other hand the ratio of latent to sensible heat transfer is dependent on it. If the thermal performance of a cooling tower is considered as the sum of its ability to sensible heat transfer and latent heat transfer, when one of these is decreased due to a change in dry bulb temperature (at the same wet bulb temperature),the other one must be increased. For example,warmer air is drier and has more ability to absorb vapor but less ability for sensible heat transfer and in some cases (if only sensible heat transfer is regarded) it may even make water warmer .What is the mechanism or the equation by which this compensation is reached?

 

[moideen]

I think the dry bulb temperature is not a concern matter refer to the thermal performance of cooling tower. Always see the wet bulb condition. When wet bulb varies the tower’s thermal performance also varying. The indicator of thermal performance of tower is wet bulb approach. And the effectiveness of tower(efficiency) also related only to wet bulb temperature.

 

[yazguli]

Thank you for your reply,moideen; but my qusetion is :"Why it is not a concern?"
Suppose in a specific condition we have a 30 degrees centigrade dry bulb air.Later it get warmer up to 40 degrees.We loose some possibility of sensible heat transfer and gain some opportunity for more latent heat transfer.
But are they equal?How?

 

[Emrelug84]

Hello,
Cooling tower cools water by evaporation and evaporation occurs at any temperature. In detail, air flow evaporates some at the surface of water droplet and water temperature decreases. In that process, water temperature approaches to the wet bulb temperature (min. 5oC difference between water and wet bulb temperatures) since evaporated and cooled mediums are water. Evaporated water vapor temperature is the water vapor in that air mixture and its temperature is called wet bulb temperature. Cooled water temperature is therefore approaches that temperature (wet bulb temp)
Regards,

 

[lilliput1]

Typical duty rating of cooling towers is to cool 95F water to 85F with ambient air at 78F wet bulb. Ambient air then would be about 95F or higher so sensible cooling do not come in. Cooling is by evaporation of water which releases about 970.4 Btu/lb to saturate the incoming air stream.

 

[yazguli]

Thank you lilliput1 and Emrelug84 for your answers.
A portion of heat transfer in cooling towers is done by sensible heat transfer ; from water to air or in the opposite direction. Typical situation is not what happens all the time.There must be such relations between parameters that when latent heat transfer(mass transfer) is reduced due to wetter air,the sensible heat transfer compensate it to the same amount, if the rating of a cooling tower is not dependent on dry bulb temperature.I'm looking for those relations.In ASHRAE ,Systems and equipment a theory of Baker and Shryock is mentioned.Is it the answer?