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Cooling Tower Make-Up Water Tank Calculator

Cooling Tower is a specialized heat exchanger that utilises the evaporation of water to remove process heat or to cool down working fluid for various applications, such as chillers.

Due to evaporation, drift and blowdown, water from a cooling tower needs to be replenished from time to time, from a dedicated Cooling Tower Make-up Water Tank.

The size of this make-up water tank depends upon the rate of losses of water (through evaporation, drifting, blowdown) and the expected days of storage. It can be represented with the formula below.

Make-Up Water Volume

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Cycles of Concentration

The cycle of concentration is a dimensionless number representing the ratio of properties of the Cooling Tower Water to that of the Make-up water. For example, it can be derived from one of the following.

COC= Silica in Cooling Water / Silica in Makeup Water

COC = Mineral Hardness in Cooling Water/ Mineral Hardness in Makeup water

COC = Conductivity of Cooling Water / Conductivity of Makeup water


The cycle of concentration normally varies from 3.0 to 7.0. High CoC helps reduce the makeup water requirement of the cooling tower, but also increases the dissolved solids concentration in circulating cooling water which results in scaling and fouling of equipment.



Information such as the Evaporation loss, E, Drift loss, D, Blowdown loss, B and Cycle of Concentration, CoC shall best be obtained from the Cooling Tower specification or data sheet. If the information for E and D are not available, they can be estimated through a safe assumption represented by the formula below.


Evaporation Loss (Rate)

The evaporation rate (m3/h) from the cooling tower can be estimated as the product of 0.00085, Cooling tower (CT) flowrate and the difference in temperature of the CT entering and leaving water temperature.


Evaporation Loss (% of Flow, Q)

Therefore, the percentage of Evaporation loss over the CT flowrate can be simplified to equation below


Drift Loss (% of Flow, Q)

The drift loss can be generally taken as 0.02 % of the CT flowrate, Q


This is a rather safe assumption. Otherwise, the following guide may be applied

For Natural Draft Cooling Tower D = 0.003 to 0.01%

For Induced Draft Cooling Tower D = 0.001 to 0.003%

For Cooling Tower with Drift Eliminator D = 0.0001% (negligible)

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A downloadable Excel File for the computation of CT make up tank is available below.

Cooling Tower Make Up Tank Calculator (Excel)

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