Cycles of concentration is one of the most important parameters in cooling tower operation. It determines how much mineral content builds up in the circulating water relative to the makeup water, and it directly controls both water consumption and the risk of scale and corrosion. Most operators know they should monitor it. Far fewer understand what happens when it drifts too high — or that the consequences go well beyond what the conductivity reading alone will tell them.
What cycles of concentration means
A cooling tower loses water continuously through evaporation. As pure water evaporates, the dissolved minerals, salts, and ions that were in that water remain in the recirculating system. Over time, their concentration increases relative to the original makeup water. Cycles of concentration (CoC) is the ratio of the concentration of dissolved solids in the circulating water to the concentration in the makeup water.
At 3 cycles of concentration, the dissolved solids in the system are 3 times more concentrated than in the makeup water. At 5 cycles, they are 5 times more concentrated. The higher the cycles, the less makeup water is consumed and discharged — which is why operators are incentivised to run at the highest cycles of concentration the chemistry allows.
The limit on how high cycles can be pushed is set by the solubility of the mineral species in the water. When concentration exceeds the saturation limit of calcium carbonate, calcium sulphate, silica, or other scale-forming compounds, precipitation occurs on heat transfer surfaces. The system has gone too far.
How to calculate cycles of concentration
Cycles of concentration can be calculated from conductivity: divide the conductivity of the circulating water by the conductivity of the makeup water. If the makeup water measures 300 µS/cm and the circulating water measures 1,500 µS/cm, the system is running at 5 cycles of concentration.
This calculation assumes the makeup water conductivity is stable. Where makeup water quality varies — as it does with surface water sources — cycles of concentration calculated from a fixed makeup water conductivity value will drift from the actual ratio as the source water changes. More precise calculation uses a conservative tracer ion — chloride or silica, which do not precipitate under normal cooling water conditions — rather than total conductivity.
What happens when you run above the limit
The consequences of exceeding the maximum allowable cycles of concentration are not immediate or dramatic. They develop progressively, which is what makes them easy to miss until the damage is significant.
Scale deposition on heat exchanger surfaces. When calcium carbonate or calcium sulphate saturation is exceeded, precipitation occurs preferentially on hot metal surfaces — the boiler tubes and heat exchanger plates where temperature drives the saturation limit lower. Scale buildup is a thermal insulator. Even 0.5 mm of calcium carbonate scale reduces heat transfer efficiency by approximately 10%, forcing the chiller to work harder to achieve the same cooling output. Energy consumption increases without any change in cooling load.
Scale also reduces the flow cross-section of heat exchanger passages, increasing the pressure drop across the system and reducing flow rates. As scale thickness increases, the flow reduction and thermal resistance compound — at some point requiring either chemical cleaning or mechanical descaling to restore performance.
Pitting corrosion from elevated chloride concentration. This consequence is less well understood than scale, but often more damaging. As cycles of concentration increase, chloride concentration in the circulating water increases proportionally. Chlorides are aggressive to the passive oxide film that protects stainless steel and copper alloys in cooling systems. When chloride concentration exceeds the threshold for the metallurgy — which varies by alloy, temperature, and pH — pitting corrosion initiates at the metal surface.
Pitting corrosion in stainless steel heat exchanger tubes is particularly insidious. Pits are small, localised, and invisible from the outside. They penetrate progressively through the tube wall under stress. The first visible indication is often a leak — at which point the pit has already penetrated the full tube wall thickness. By the time one tube leaks, neighbouring tubes are typically in a similar state of degradation.
The maximum allowable chloride concentration in the circulating water should be calculated from the makeup water chloride content and the target cycles of concentration, and this should set the upper limit for cycles — not just the general conductivity setpoint.
Silica scale formation. Silica is a particularly problematic scale compound because it forms hard, glassy deposits that are extremely difficult to remove chemically once established. Unlike calcium carbonate scale, which responds to acid cleaning, silica scale typically requires mechanical removal or specialised high-pH chemical treatment under controlled conditions. The solubility of silica in cooling water is approximately 150 ppm at neutral pH, rising at higher pH. If makeup water silica content is 30 ppm and cycles of concentration are pushed to 6, circulating water silica reaches 180 ppm — above the precipitation threshold at neutral pH.
How biocide demand increases with cycles
This is the least discussed consequence of high cycles of concentration but one of the most operationally significant.
As dissolved solids concentration increases, the demand for oxidising biocides increases. Chlorine and bromine react with ammonia and organic nitrogen compounds in the circulating water to form chloramines and bromamines — combined forms of the biocide that have significantly lower bactericidal activity than free chlorine or free bromine. At high cycles of concentration, the nitrogen compound concentration increases, and a larger fraction of the dosed biocide is consumed in forming less effective combined species rather than remaining as free biocide.
The result is that the same biocide dose that maintained effective microbial control at 3 cycles of concentration may be insufficient at 6 cycles — not because the biocide was dosed incorrectly, but because the circulating water chemistry changed. ORP or free chlorine monitoring would detect this; a conductivity controller on its own would not.
Setting the correct cycles of concentration limit
The maximum allowable cycles of concentration for a specific system is determined by the makeup water chemistry, the system metallurgy, the biocide programme, and the inhibitor capability. It is not a universal number.
The starting point is a Langelier Saturation Index (LSI) or Puckorius Scaling Index (PSI) calculation at the intended cycles of concentration, using the actual makeup water analysis. For systems with stainless steel heat exchangers, a separate chloride limit calculation sets a ceiling based on the alloy specification. For systems with significant organic load, biocide demand at elevated cycles should be assessed.
Running higher cycles of concentration saves water and reduces chemical consumption per unit of water discharged. But it narrows the safety margin for every other parameter in the programme. The correct cycles limit is the highest value at which every water chemistry parameter — scale index, chloride concentration, silica concentration, biocide demand — remains within its safe operating range simultaneously.
Autoflo Technology supplies cooling tower water treatment controllers and monitoring systems for industrial and commercial cooling applications across Malaysia. Contact us at info@autoflotechnology.com to discuss your system parameters.