Enter the conductivity of the system water and the conductivity of the makeup into the calculator to determine the cycles of concentration.
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Cooling Tower Cycles of Concentration Formula
The following equation is used to calculate the Cooling Tower Cycles of Concentration.
COC = Csystem / Cmakeup
- Where COC is the Cooling Tower Cycles of Concentration (unitless)
- Csystem is the conductivity of the circulating (system) water
- Cmakeup is the conductivity of the makeup water
To calculate the cooling tower cycles of concentration, divide the conductivity of the system by the conductivity of the makeup.
In practice, cycles of concentration can be estimated using conductivity or another relatively conservative dissolved constituent measured in both the circulating water and the makeup water (often chloride). Results may not be identical between different tracers, and silica is not reliably conservative because it can be limited by scaling/precipitation.
What is a Cooling Tower Cycles of Concentration?
Definition:
Cooling tower cycles of concentration (COC) is the ratio of the concentration of dissolved solids (or a chosen tracer such as conductivity or chloride) in the recirculating/circulating water to that in the makeup water. It describes how much the makeup water has been concentrated by evaporation in the tower.
This is important because, if there are high levels of dissolved solids in the water, there will be scaling or fouling of the heat transfer surfaces which reduces efficiency.
COC is often approximated using conductivity, or by using lab measurements such as chloride or total dissolved solids (TDS) on both makeup and circulating samples and taking the ratio. For conductivity-based calculations, readings should be temperature-compensated (conductivity changes with temperature); pressure is not typically a meaningful factor for these measurements in normal cooling tower sampling.
Common ways to determine cooling tower cycles of concentration include (1) the concentration-ratio method (circulating-to-makeup ratio of conductivity, chloride, or another conservative constituent) and (2) a water/solute mass balance using measured makeup, blowdown, and (if significant) drift losses.
For best accuracy, use paired makeup and circulating samples and a lab analysis of a conservative constituent (for example, chloride) or TDS measured by a standard gravimetric residue method. Field conductivity meters are widely used for day-to-day control, but the inferred cycles can shift if treatment chemicals or other changes substantially affect conductivity.
Operating at higher cycles generally reduces blowdown and makeup water usage, but it increases the risk of scale formation, corrosion, or deposition unless water chemistry and treatment are managed appropriately. Practical cycle limits are usually set by scaling indices and specific constituent limits (for example, silica, calcium hardness, alkalinity, and temperature).
Example: if makeup conductivity is 300 µS/cm and circulating conductivity is 1500 µS/cm, then COC = 1500 ÷ 300 = 5. As a rule of thumb, if drift is small, blowdown is approximately evaporation ÷ (COC − 1), so at 5 cycles the blowdown rate is about one-quarter of the evaporation rate.
