The question
What are cycles of concentration, how do you work out what yours actually are, and how high can you safely run?
What the number actually is
Cycles of concentration is the ratio between the concentration of a dissolved species in the circulating water and its concentration in the make-up. Run at four cycles and the circulating water carries roughly four times the dissolved solids of the water you feed in.
The same ratio describes the water balance. At four cycles, one quarter of the water leaving the system leaves as blowdown and three quarters as evaporation. Raise the cycles and the blowdown share falls; lower them and it rises sharply.
Working out what yours really are
The practical method is the ratio of system conductivity to make-up conductivity. It is quick, it can be read continuously, and it is what most controllers actually use.
A chemical tracer such as chloride gives a cross-check, because it is conservative — it is not consumed, precipitated or added by the treatment programme. Where the conductivity ratio and the chloride ratio disagree significantly, something is entering or leaving the system that has not been accounted for.
That disagreement is worth investigating. Unmetered make-up, an unnoticed overflow, drift losses higher than design, or a process leak into the circuit all show up here first.
What sets the ceiling
The limit is not a general number. It is whichever species in your particular make-up water reaches its practical limit first as concentration rises.
- Calcium and alkalinity together set the carbonate scaling limit — usually the first constraint on a hard supply.
- Silica has a hard practical ceiling and is difficult to inhibit, so a high-silica supply often limits cycles before hardness does.
- Chloride and sulphate drive corrosivity rather than scaling, and can become the constraint where stainless steel is present in the circuit.
- The inhibitor programme raises the achievable ceiling — that is precisely what threshold inhibitors are for.
Because those constraints differ by supply and by metallurgy, the achievable ceiling is specific to your system. It is established from an analysis, not from a rule of thumb.
Running near the limit safely
Higher cycles reduce water use, chemical discharge and the energy spent treating replacement water. They also reduce the margin between normal operation and precipitation, which makes control quality the deciding factor.
A system with reliable conductivity measurement, verified chemical feed and a logged history can be operated closer to its limit than one running on a timer, because a deviation will be visible while there is still time to correct it.
The practical approach is to move the setpoint in steps, monitoring at each one, so the limit is approached deliberately rather than discovered by finding deposits.
What to check
- 01Measure conductivity in both the circulating water and the make-up, and calculate the ratio.
- 02Cross-check with a conservative tracer such as chloride.
- 03Investigate any significant disagreement between the two ratios.
- 04Confirm the make-up supply is metered.
- 05Check for overflow, excessive drift or an unrecorded bleed path.
- 06Compare the calculated cycles with what the current water analysis would support.
When to get technical support
Raising cycles changes the scaling and corrosion balance at the same time — the two move in opposite directions, and the safe ceiling depends on the metallurgy in the circuit as much as on the water. Establish the achievable limit from a current analysis and the inhibitor programme before moving the setpoint, and move it in monitored steps rather than in one adjustment.
Talk to an EngineerFrequently asked
No. The achievable figure depends entirely on the make-up water chemistry, the metallurgy in the circuit and the inhibitor programme. Any general number quoted without an analysis behind it is a guess.