If you want to know how much water a cooling tower wastes, start with one number: its cycles of concentration. Evaporation is set by the heat the tower rejects. Everything the tower uses beyond evaporation is set by cycles.
What it means
Pure water evaporates; dissolved minerals don't. As the tower runs, the recirculating water gets more concentrated than the makeup water feeding it. Cycles of concentration is that ratio: how many times more concentrated the tower water is than the makeup. FEMP defines it as the ratio of dissolved solids in the blowdown to those in the makeup, approximately equal to the ratio of makeup volume to blowdown volume.
To stop the minerals from scaling, the tower bleeds off concentrated water (blowdown) and replaces it with makeup. Makeup has to cover evaporation, blowdown and drift (mist carried off by the air).
The formulas
With evaporation E and cycles C:
- Makeup = E × C / (C − 1)
- Blowdown + drift = E / (C − 1)
Evaporation itself is about 0.00085 × circulating flow (gpm) × range (°F) for average conditions. The water balance calculator runs these for your plant.
How much raising cycles saves
Because evaporation doesn't change, raising cycles only cuts the part you control:
| Cycles | Makeup (× evaporation) | Blowdown (× evaporation) |
|---|---|---|
| 2 | 2.00 | 1.00 |
| 3 | 1.50 | 0.50 |
| 4 | 1.33 | 0.33 |
| 6 | 1.20 | 0.20 |
| 10 | 1.11 | 0.11 |
Going from 3 to 6 cycles cuts makeup by 20% and blowdown by 60%. FEMP rounds the blowdown figure to about 50%. It also notes that many systems operate at two to four cycles while six or more may be possible. The biggest gains come early: 2 to 4 cycles saves far more than 6 to 10. See the cycles savings calculator.
How to measure it
- Conductivity. Dissolved solids track electrical conductivity closely. Tower water conductivity divided by makeup conductivity gives cycles. Most tower controllers already measure tower conductivity to trigger blowdown; many sites never log the makeup side.
- Water meters. Makeup volume divided by blowdown volume over the same period. It only works if both are metered and the meters are read on the same dates.
The two methods should roughly agree. When they don't, the usual suspects are an unmetered bleed, a stuck blowdown valve, a leaking overflow, or drift much higher than it should be.
What limits cycles
Chemistry, not arithmetic. As cycles rise, calcium, alkalinity, silica and other species concentrate until the water will deposit scale or corrode metal. How high a tower can safely run depends on the makeup water's quality and on the treatment program: scale inhibitors, pH control and, where it pays, softening or other pretreatment of the makeup.
Scale matters beyond the tower. The same concentrated water flows through the chiller's condenser tubes, and scale there insulates the tubes, raises the condenser approach and costs chiller energy. The right cycles target balances water saved against energy risked, which is why it should be set with both sides' data in view.
A compliance note for New York City
Under NYC's cooling tower rules (24 RCNY Chapter 8, as amended in 2026), a failure of conductivity controls to maintain proper cycles of concentration is one of the events that requires additional emergency Legionella sampling. In NYC, a cycles problem is also a compliance problem (NYC rules explained).