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Topic: Evaporation rate in cooling tower  (Read 684 times)

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Offline daf44

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Evaporation rate in cooling tower
« on: December 21, 2018, 04:24:55 AM »
Hello,

Cooling towers need make-up water to counter, among others, water evaporation. I'm trying to figure out what the difference will be between the evaporation rate in the summer and in the winter.

Assumptions:
Water temperature to be cooled: 35 degrees Celsius
Winter period: Wet bulb temperature = 5 degrees Celcius, Relative humidity = 85%
Summer period: Wet bulb temperature = 21 degrees Celcius, Relative humidity = 75%

I was told that because the wet bulb temperature in the summer is higher than in the winter, the water in the summer can be cooled down to ~25 degrees, while in the winter it can go to ~17 degrees (cooling tower is operating at max. capacity, no temperature regulation). Because evaporation rate = flowrate*heat capacity*temperature difference over cooling tower / latent heat of evaporation, the evaporation rate will be higher in the winter (flow rate, heat capacity and latent heat of evaporation stay constant).

On the other side, the rel. humidity in the winter is higher than in the summer, so the air can take up less water in the winter, which would mean less evaporation compared to the summer period, no?

Can somebody help with this problem i'm facing?

Offline Enthalpy

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Re: Evaporation rate in cooling tower
« Reply #1 on: December 21, 2018, 08:13:56 AM »
You must also compare how much vapour capacity the air gains between entering and exiting the tower. From your example figures, air contains more relative moisture in winter, but it warms more too.

One other point is that the air throughput must change depending on the weather. Heating makes air in the tower lighter than the surrounding one, but the vapour content lightens it much, while condensation increases the density.

One simpler, semi-qualitative comparison would be: if (the same amount of) air exited saturated at the same temperature, 85%RH at +5°C lets less vapour enter the tower than 75%RH at +21°C, so for the same dissipated power and evaporation, the air can exit less warm in winter. It also lets cooler water exit the tower, so the work vapour condenses at a lower temeprature hence pressure, and the turbine is more efficient.

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