Quantifying What Module 5.1 Described
Module 5.1 explained that evaporation concentrates dissolved solids in recirculating cooling water. Cycles of concentration (COC) is how that concentration is actually measured and controlled — directly analogous to the blowdown mass balance from Module 2.5, but applied to a system with an extra loss stream evaporation doesn't have on the boiler side.
Three Ways Water Leaves a Cooling Tower
Recirculating cooling water is lost from the system three ways: evaporation (pure water vapor, leaving dissolved solids behind — the concentrating mechanism itself), drift (small liquid droplets physically carried out of the tower by airflow, carrying whatever concentration the bulk water currently has), and blowdown (a deliberate, controlled discharge specifically intended to limit concentration, directly parallel to boiler blowdown's purpose in Module 2.5). Makeup water must replace the total of all three losses to maintain system water balance.
Calculating Cycles of Concentration
Cycles of Concentration
COC = TDS(recirculating water) ÷ TDS(makeup water)
A COC of 5 means the recirculating water's dissolved solids concentration is five times that of the makeup water feeding the system — the direct, measurable consequence of evaporation repeatedly removing pure water while blowdown controls how far that concentration is allowed to climb before being checked. Typical cooling tower operation runs somewhere around 3 to 7 cycles, though this varies significantly with makeup water quality and the specific scale/corrosion control program in place.
The Blowdown Rate / Cycles Relationship
Blowdown rate and cycles of concentration are directly linked: increasing blowdown rate lowers cycles of concentration (more frequent solids removal keeps concentration lower), while decreasing blowdown raises cycles (less removal allows more concentration before equilibrium). This is the same core relationship as Module 2.5's boiler blowdown mass balance, just expressed through a different formula because of the extra evaporation loss term unique to cooling towers.
Why plants don't simply maximize cycles of concentration: Higher cycles mean less makeup water consumption and less blowdown discharge — genuine water conservation and cost benefits. But higher cycles also mean higher concentrations of scale-forming minerals (Module 5.3) and corrosive species (Module 5.4), narrowing the margin before scale or corrosion problems appear. Cycles of concentration is fundamentally a tradeoff between water conservation and chemistry risk, not a number to simply maximize.
Using Cycles as a Diagnostic Tool
- Cycles of concentration can be tracked using any conservative (non-reactive) dissolved ion present in both makeup and recirculating water — chloride is commonly used since it doesn't precipitate or react chemically the way calcium or phosphate-based treatment chemicals might.
- A cycles of concentration reading significantly lower than expected given the blowdown rate can indicate unaccounted water loss elsewhere in the system (a leak) or higher-than-expected makeup consumption.
- A cycles of concentration reading significantly higher than expected can indicate blowdown control isn't functioning as intended — worth checking before assuming a scale or corrosion inhibitor dosing problem, similar in spirit to Module 3.3's caution about checking mechanical causes before chemical ones.
Field note: Cycles of concentration is the single number that ties together water conservation, blowdown control, and the scale/corrosion risk covered in the next two modules. Tracking it consistently — using a stable, conservative tracer ion — is one of the most useful ongoing cooling water chemistry habits a plant can maintain.
Cycles of Concentration (COC)
The ratio of dissolved solids concentration in recirculating cooling water to that in makeup water, quantifying how much evaporative concentration has occurred.
Drift
Small liquid water droplets physically carried out of a cooling tower by airflow, a water loss mechanism distinct from evaporation since it carries dissolved solids at bulk water concentration.
Conservative Tracer Ion
A dissolved ion (such as chloride) that does not react or precipitate under normal system conditions, used to reliably calculate cycles of concentration.
Water Balance
The accounting of all water inputs (makeup) and outputs (evaporation, drift, blowdown) in a recirculating cooling system, which must balance for stable operation.