The amount of chlorine needed to destroy the biofouling organisms in the water and oxidize organic and inorganic matter is called the "chlorine demand" of the system. The chlorine feed rate depends upon the degree of biofouling expected under the worst conditions, the non biological chlorine demand in the water, the holding time in the system, and the discharge limitations. It is difficult to give an exact chlorine demand curve because it depends on the local geographical conditions - summer, winter, temperatures, etc. It's a matter of judgment. The followings graph is the typical demand residual curve in cooling tower waters.

1. You decide at what level of demand you wish to start shock dosing.
2. The length of shock dosing is dependent on time for water to go from point of Cl₂ injection till it reaches the sample point on the return to cooling tower. (circuit time)
3. Time to reach zero residual depends on site conditions.
4. More important is the level of demand that does not cause problems before starting shock dosing cycle.

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What the above means is you find the circuit time of the system, set the shock dosing for this time (period), then you set amount of Cl₂ based on 5 mg/liter shock dose as follows:

M³ of water flow through circulating pumps multiplied by dosage (5 mg/liter) divided by 1000 - this equals kg/hr to which you must set the rotometer.

Kg/hr = M³/hr x dosage rate (mg/liter) / 1000

You check period by measuring the Cl₂ residual at the sample point in the return line, at about 0.5 mg/liter. Free chlorine (HOCl) is when you stop the period, adjust the timer if necessary.