Cycles of Concentration Control: Boilers & Cooling Towers

One of the blowdown control strategies available on the 1575e & NexSys platforms is “Cycles of Concentration.”  We get many questions regarding how it works.

In simple terms, it represents how long fresh make-up water can be circulated in the loop before TDS build-up requires system blow-down.

In control terms, cycles of concentration calculate the tower conductivity setpoint as a multiple of your make-up water conductivity.  For example, if make-up water is 500 µS and the tower conductivity setpoint is 2000 µS, the number of cycles is 4 (500 µS X 4 = 2000 µS).  In cycles operation, you set the desired number of cycles, not the tower water setpoint. In the above example, you set the cycles at 4, and the makeup conductivity is measured at 500 µS, so the tower setpoint is 2000 µS. If the makeup water conductivity increases to 600 µS, the tower water setpoint automatically increases to 2400 µS. If the makeup water conductivity drops to 400 µS, the tower setpoint drops to 1600 µS.

In practical terms, cycles operation is a way to optimize water and chemical usage while adapting to changes in makeup water composition.  A higher number of cycles translates to a higher system setpoint, which reduces blow-down quantity and, in turn, make-up water (and chemical) requirements.  In the end, setting the number of cycles sets a maximum on the buildup of scaling contributors as well as inhibitors.  Confusing? One of our customers wrote a nice article on this too.

It is very important to monitor your make-up water when using cycles of concentration.  As make-up conductivity varies, tower conductivity will vary accordingly.  This control scheme is great when you are in an area where the water source for makeup changes significantly, sometimes with no notice. The NYC area changes water sources from at least three different rivers/watersheds – changing the makeup conductivity from 90 to 390 µS or a blend. Running a cycles control scheme would automatically adjust the tower conductivity when the makeup water changes.

Even more dramatic changes occur in the Phoenix area, where the water source changes from surface water brought by the Salt River Project (Salt and Verde Rivers), the Central Arizona Project (Colorado River), or well water which can exceed 1000 µS.

And you can even set a minimum makeup conductivity level so that if unforeseen high purity makeup water occurs, your setpoint will not be too low causing too much bleed and chemical usage. On the high end, max conductivity alarms can prevent too high a setpoint caused by high conductivity makeup water.

The “Cycles of Concentration” method can be used with:

1575e controller for boilers and cooling towers with system conductivity sensor and make-up conductivity sensor/transmitter.

1575e cooling tower schematic

NexSys Controllers for boilers and cooling towers are almost the same, just using NCON nodes for the makeup water conductivity sensor.

NexSys Controller with cycles of concentration control


If you are managing water treatment in an area with variable makeup water characteristics, consider using this excellent feature of Lakewood Instruments controllers.