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# Cooling tower chlorine mass balance

## Cooling tower chlorine mass balance

(OP)
Hello

I have a mass balance problem.

We have a cooling tower with two states: state A is witouth blowdown ( just evaporation ), state B ( with blowdown ) I will post next time.

In state A the volume flow of make up water is 2 m3/h with chlorine level 2 ppm ( free chlorine ). Evaporation is 2 m3/h. System volume is 30 m3.
Circulation pump rate ( water flow through the tower ) is 500 m3/h. Free chlorine flasoff ( stripoff ) through the tower is 1,5%. For the sake of calculation we will say that chlorine demand of system water and pipe walls is 0.

Circulating pumps are OFF, an we fill the system with the make up water ( 2 ppm Chlorine ).

So at time 0 the free chlorine level in system water is 2 ppm, than we turn on the circulating pumps.

How to calculate the free chlorine level after 1, 2,3... hours?

The only loss of chlorine is trough the tower, so if I calculate Circulation rate X concentration X flashoff factor, I get mass loss of chlorine per time, BUT the concentration is lower every next passage throuh the tower and I am not quite shure how to deal with that :)

Replies continue below

### RE: Cooling tower chlorine mass balance

The flashoff will be a function of ambient air conditions, water temperature, and Cl- concentration.
I have never seen a theoretical solution for this.
And in any real system the Cl-demand though the loop will usually be 80% of the consumption.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Cooling tower chlorine mass balance

Ed Stainless is correct in that flashoff is not the major source of demand. You also have to boost the chlorine level on the make up water eg: 2 m3/hr assuming it is not chlorinated.
How have you worked out that flashoff is 1.5%?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

### RE: Cooling tower chlorine mass balance

Yes, run this by your chemical treatment program manager.

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