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# Cooling Tower - Evaporation Loss Calculation

## Cooling Tower - Evaporation Loss Calculation

(OP)
Hi All,

I am calculating the cooling tower evaporation loss for our cooling tower as follows:

We = 0.00085 x Wc x ( T1 - T2 )

Wc = Circulation flow rate at Tower Inlet, US gpm
T1 = Tower Inlet Temperature, Deg F
T2 = Tower Outlet Temperature, Deg F
We = Evaporation Loss, US gpm.

Please see the attached drawing. Our cooling tower is supplied with large excess of water as makeup water.
At the cooling tower outlet the excess water over flows to other sources as mentioned in the drawing. For example if the pump flow rate is 3051 gpm, 3000 gpm only return back to tower. The remaining 51 gpm is sent to these other sources I mentioned above.

1) Now my question is to calculate the evaporation losses, I want to confirm that circulation flow rate, Wc in the above formula is the water returning to the tower and not the pumping rate?. That means I need to consider the 3000 gpm for calculation and not the 3051 gpm in the above example.

2) My second question is about the Cooling Water outlet temperature, T2. Is this the temperature if the water falling down in the tower before it mixes with the make-up water or is it after the mixing. The temperature of the water falling down can be calculated depending on the tower efficiency and approach. I just want to know which temperature to use even though they may very close.

Thanks and Regards,
Pavan Kumar

### RE: Cooling Tower - Evaporation Loss Calculation

Do the other inflows go straight into the well or over the fill?
If they go into the well then use the 3000.
Are you sure that the 0.00085 is correct for your local weather (temp and RH)?
More than half of all cooling towers built do not meet their design point, measure and test carefully.

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P.E. Metallurgy, consulting work welcomed

### RE: Cooling Tower - Evaporation Loss Calculation

(OP)
Hi EdStainless,

All the other flows go into the well. What is the correct equation interms of the local temperature and RH?.

Thanks and Regards,
Pavan Kumar

### RE: Cooling Tower - Evaporation Loss Calculation

Should be possible to do this from first principles ?

Sensible heat loss in recirculating cooling water = Heat transferred to ID fan air + heat gain by evaporated water.

Assuming that exit air is say at 5degC dT approach to incoming hot water ?

### RE: Cooling Tower - Evaporation Loss Calculation

Pavan,

This can be derived from the actual operating point and local conditions of the cooling tower. Please note though, that the material and energy balance here will under-predict losses if there is any entrainment of water droplets into the cooling tower.

### RE: Cooling Tower - Evaporation Loss Calculation

You can go the the Cooling Tower Institute for a lot of information on this.

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P.E. Metallurgy, consulting work welcomed

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