Cooling Tower Cycles of Concentration & Blowdown Calculator

Cooling Tower Water Balance & Cycles of Concentration (COC)

Evaporative cooling towers reject building or process heat through the latent heat of vaporization of water (~1,000 BTU/lb). Because only pure water evaporates, dissolved minerals (calcium carbonate, silica, chlorides, sulfate) remain behind, progressively concentrating in the recirculating basin water.

1. The Water Balance Equation

Conservation of mass requires makeup water (M) to replace water lost to evaporation (E), intentional blowdown (B), and windage drift (D):

$$M = E + B + D$$

Evaporation rate is determined by the heat rejection load and thermal range (\Delta T = T_{\text{in}} - T_{\text{out}}):

$$E = \text{GPM} \times \Delta T \times 0.0008 \approx 3.0\ \text{GPM per 100 Tons}$$

2. Cycles of Concentration (COC) Formula

Cycles of concentration measures how many times dissolved solids concentrate in the basin relative to incoming municipal makeup water:

$$\text{COC} = \frac{\text{Chloride}_{\text{basin}}}{\text{Chloride}_{\text{makeup}}} = \frac{\text{Conductivity}_{\text{basin}}}{\text{Conductivity}_{\text{makeup}}} = \frac{M}{B + D}$$

Solving for required blowdown rate (B):

$$B = \frac{E}{\text{COC} - 1} - D$$

3. Diminishing Returns of Increasing COC

Increasing COC from 2 to 4 slashes blowdown volume by 66%, delivering massive water and sewer savings. However, increasing COC from 5 to 8 yields only modest marginal water savings while exponentially increasing scaling risk (calcium carbonate scaling and silica precipitation).