How many times the minerals in the make-up water are concentrated in a cooling tower, how blowdown controls it, and how to turn a target into a conductivity setpoint.
Cycles of concentration — cycles for short — is the ratio of a dissolved mineral in the system water to the same mineral in the make-up water. Conductivity tracks dissolved solids, so in practice:
Cycles = system conductivity ÷ make-up conductivity
Example. System conductivity is 2500 µS/cm and make-up conductivity is 500 µS/cm. The tower is running at 2500 ÷ 500 = 5.0 cycles.
Every mineral concentrates by the same factor. At 5 cycles, calcium, silica, chloride, and alkalinity are all about five times their make-up level — which is why cycles, not conductivity alone, is what the treatment program is built around.
Evaporation concentrates the water; blowdown removes concentrated water so fresh make-up can dilute it. The more you bleed, the lower the cycles.
Blowdown = evaporation ÷ (cycles − 1)
Make-up then equals evaporation plus blowdown (ignoring drift). For a tower evaporating 10 gpm:
| Cycles | Blowdown | Make-up | Water saved vs 2 cycles |
|---|---|---|---|
| 2 | 10.0 gpm | 20.0 gpm | — |
| 3 | 5.0 gpm | 15.0 gpm | 25 % |
| 4 | 3.3 gpm | 13.3 gpm | 33 % |
| 5 | 2.5 gpm | 12.5 gpm | 38 % |
| 6 | 2.0 gpm | 12.0 gpm | 40 % |
| 8 | 1.4 gpm | 11.4 gpm | 43 % |
| 10 | 1.1 gpm | 11.1 gpm | 44 % |
Going from 2 to 5 cycles saves over a third of the make-up water. Beyond about 6 cycles the savings flatten out while the scaling risk keeps climbing.
The target comes from the water treatment program, not the controller. It is the highest cycles the make-up water can reach before something in it causes trouble:
Hard or high-silica make-up allows only low cycles; soft, clean make-up allows high cycles. Softened or blended make-up changes the answer.
The InteliHive bleeds on system conductivity. Convert the target cycles into a conductivity setpoint:
Setpoint = target cycles × make-up conductivity
Example. Target 5 cycles, make-up conductivity 400 µS/cm. Setpoint = 5 × 400 = 2000 µS/cm. Set up a Setpoint Control mode on the system conductivity probe, Rising, driving the bleed valve, with a setpoint of 2000 µS/cm and a differential of about 2–5 % of the setpoint (40–100 µS/cm). The valve opens at 2000 and closes once conductivity has fallen back by the differential.
Some controllers — including the Pulsafeeder MC9300/9500 series — also read a make-up conductivity probe and calculate cycles live. They then hold two setpoints at once: a system conductivity setpoint, and a cycles setpoint. Whichever is reached first opens the bleed.
Example. System conductivity setpoint 2800 µS/cm; cycles setpoint 5.0.
| Condition | System | Make-up | Cycles | Bleed | Why |
|---|---|---|---|---|---|
| 1 | 2000 µS/cm | 500 µS/cm | 4.0 | Closed | Below both setpoints: 2000 < 2800, and 4.0 < 5.0 |
| 2 | 2600 µS/cm | 500 µS/cm | 5.2 | Open | Cycles setpoint exceeded (5.2 > 5.0); conductivity still below 2800 |
| 3 | 2000 µS/cm | 200 µS/cm | 10.0 | Open | Make-up fell, so cycles jumped to 10 — cycles setpoint exceeded |
| 4 | 3000 µS/cm | 750 µS/cm | 4.0 | Open | Conductivity setpoint exceeded (3000 > 2800), though cycles are only 4.0 |
Condition 3 shows why a cycles setpoint matters: when the make-up gets purer, a conductivity-only controller would let the tower run at 10 cycles; the cycles setpoint catches it. Condition 4 shows the reverse: when the make-up gets harder, the conductivity setpoint puts a ceiling on total dissolved solids.