Cooling tower, dry warm air or cold wet air? 1

[hades1490]

Hello everyone,

I am presently analizing a cooling tower energy performance, looking mainly at its fans energy consumption.
The cooling tower air intake is installed very close to the opening of a building, basically taking air warmer than the ambient, initially with the same specific humidity (lower relative humidity) than the outside air.

The question that arises is:
Is the performance of the cooling tower better than the case of taking outside air?

In my opinion, there are two opposed effects:
The warmer the air, the worse is the convective heat transfer between water and air.
The dryer the air (in relative terms) allows the air to absorb more water per kg of dry air, and therefore the heat transfer due to the latent evaporation heat of the water increases.

In terms of water and air operating points:

The water is quite warm,in the region of 60 C (I still do not know why a cooling tower is required, when an water-air HX would be neough, but.....anyway....)

The outside air temperature and humidity although obviously vary along the date and time, correspond to a mild and humid climate.

Any idea or suggestion that you may have would be helpful.

Thanks.

 

[katmar]

It is difficult to give a generic answer to this question, but it would be relatively easy for you to do the actual calculations.

A useful article is available at

http://www.cheresources.com/ctowerszz.shtml

Towards the end of the article there is a link to download a program that does the detailed calculations. Its an old DOS program but is easy to use. Use it to calculate the performance factor of your tower as it is (i.e. calculate KaV) and then once you have this you can ring the changes on the air temperatures and humidity to see what the effects on performance are.

I have used this program for predicting summer performance from winter data, and the results were very good.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[hades1490]

Thanks for your reply Katmar, actually, the program is very useful (as well as other program installed on this pc called Wasp, I think you have heard of it )

I have a doubt about the program: it does not include the option to input the dry bulb temperature, what makes me think that the actual dry bulb temperature does not matter very much... I have tested a little bit the program and the result is that for higher wet bulb temperatures in this case, the results are better (would indicate that the warmer-dryer the air is, the better) however, I am still not completely convinced beacuse of this feature that the program lacks, which is actually the point in question.

Regards

 

[katmar]

If you have a look at the theory on the page I referred you to earlier, the important factor for the air is the enthalpy per mass. Now, if you look at your psychrometric chart you will see that the wet bulb lines are parallel to the enthalpy lines. So if you fix the wet bulb temperature (which is one of the inputs for the program) then you have fixed the enthalpy of the air. It is easier to do it this way than to enter the dry bulb temperature and the absolute humidity because it is easier to measure the wet bulb temperature than the absolute humidity.

For a given absolute humidity, as you increase the dry bulb temperature you increase the wet bulb temperature as well. So heating the incoming air is generally not a good thing to do. But as I said before, run the numbers and see what the actual effects are.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[imok2]

Cooling tower efficiency is based on the wet bulb. Dry bulb is of not much consequence. Your efficiency is based on the amount of evaporation taking place as the water is cooled. like for every lb of water evaporated will remove ~ 970 btu of heat from the water remaining

 

[hades1490]

Imok, thanks for answering. In relation to the wet bulb temperature. If this is higher, initially, the performance would be better or worse?

Thinking of the extreme case, where the intake air is fully saturated. Assuming adiabatic process, this air could not evaporate water and therefore, it would not be able to cool the water. In this case, correct me if I am wrong, in theory, an infinite amount of air would need to be circulated.

On the other hand, warmer air, with the same specific humidity and higher wet bulb temperature could absorb certain amount of water, cooling it, and therefore requiring less air (and less consumption on the fans) for the same amount of heat to be removed.

I ran the software, and increasing the value of the wet bulb temperature, makes the KaV/L value increase. As far as I understand the meaning of KaV/L, the higher is the value, higher will be the heat transfer. Please remember that this analysis is for an existing cooling tower, there is no design involved.

Regards, and thanks very much for your comments.

 

[katmar]

I ran the software, and increasing the value of the wet bulb temperature, makes the KaV/L value increase.

This is correct. What the software is saying is that if your tower is able to achieve the cooling you measured, even though the wet bulb temperature is higher, then the towers cooling capacity (i.e. KaV/L) is higher.

That is true in rating mode. On the other hand, if you increase the wet bulb temperature in design mode where your KaV/L is fixed then you will get less cooling.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[hades1490]

Understood