By Jacob "Jake" Ashabraner, ASSE 12080 Certified, Legionella Certificate #64392, Glacier Laboratories
Key Takeaways
- Cycles of concentration (COC) is the ratio of dissolved solids in cooling tower water to those in the makeup. The higher the COC, the less water you waste to blowdown.
- Most towers run at COC 3–5 when they could safely run at 6–10, wasting 20–40% of blowdown water and chemistry.
- Every 1-cycle increase from 3 to 6 cuts blowdown roughly in half; gains diminish sharply above 8.
- Raising COC requires water chemistry that supports it, including proper hardness, alkalinity, silica, and inhibitor levels, plus a working conductivity controller.
- The path from 3 cycles to 6 cycles typically pays back in months through water bills, sewer fees, and chemistry savings.
What is Cycles of Concentration?
When cooling water evaporates from a tower, pure water leaves as vapor while dissolved materials, including calcium, magnesium, silica, sulfates, and chlorides, stay behind and concentrate. To keep those minerals from precipitating on your heat exchangers, you bleed off a portion of the water as blowdown and replace it with fresh makeup.
Cycles of concentration measures how many times the makeup water is "concentrated" before it is discharged. It is calculated as the ratio of a conservative ion (or conductivity) in the tower water vs. the makeup:
COC = Conductivity (tower water) / Conductivity (makeup water)
A COC of 5 means the water in your basin is 5x more concentrated than what you put in. Every doubling of COC roughly halves your makeup and blowdown volume1.
Why COC is the highest-leverage number in your program
Most industrial cooling towers operate between COC 3 and 5. The Compressed Air and Gas Institute reports that COCs "tend to range between 5 and 7 due to cost efficiency," with 10 being achievable on optimized systems2.
The dollar impact of moving up the cycle scale is dramatic:
| Moving from | Blowdown reduction |
|---|---|
| 2 → 4 cycles | ~67% less blowdown |
| 3 → 6 cycles | ~50% less blowdown |
| 4 → 8 cycles | ~35% less blowdown |
| 6 → 10 cycles | ~10% less blowdown |
The first few cycles above the current baseline deliver most of the savings. For a 500-ton tower running 3 cycles today, moving to 6 typically saves 400,000 to 600,000 gallons of water per year, plus the chemistry riding on that water and the sewer discharge fees.
How to measure COC accurately
- Use a handheld conductivity meter or, better, an in-line conductivity controller on the tower recirculation loop.
- Measure makeup water conductivity at least monthly (city water changes with source blending).
- If your tower is not equipped with a permanent conductivity controller and blowdown valve, that is the first capital item to fix. You cannot manage COC blind.
If your calculated makeup-to-blowdown ratio and your conductivity ratio disagree, you almost certainly have an unmeasured loss, such as a leak, drift beyond design, or a stuck blowdown valve.
What limits how high you can push COC
The ceiling on COC is set by the least-soluble species in your water:
- Calcium carbonate (CaCO₃): the classic scale former, managed with pH control and scale inhibitors such as phosphonates and polymers. Calcium carbonate is the largest COC limiters in the Southwest United States.
- Calcium sulfate (CaSO₄): hard to inhibit. If your makeup has high sulfate, this may cap you at 4 to 5 cycles regardless.
- Silica: precipitates above about 150 ppm as SiO₂. Makeup silica is your biggest COC limiter in the Pacific Northwest, where river-sourced water can run 20 to 40 ppm SiO₂.
- Chloride and sulfate: drive corrosion, especially on stainless steel. Even if scale is controlled, chloride above about 200 to 300 ppm can pit 304 stainless.
- Iron: accelerates biological fouling and stains fill.
A Langelier Saturation Index (LSI) or Ryznar Stability Index (RSI) calculation on your specific water gives the theoretical scale limit. From there, your inhibitor package sets how far past saturation you can safely operate.
The step-by-step path to higher COC
- Establish baseline. Measure makeup and blowdown conductivity, hardness, alkalinity, silica, chlorides, and sulfates for 2–4 weeks.
- Calculate current COC and blowdown rate. Compare to design.
- Run an LSI/RSI or ionic saturation analysis on the tower water at target COC (typically your current COC × 1.5).
- Verify that your chemistry supports the new target, including inhibitor dosage, dispersant, and biocide program.
- Adjust the conductivity setpoint on the controller in one-cycle increments. Never jump three cycles at once.
- Monitor tube surface temperatures and heat exchanger approach for 2–4 weeks after each step. Rising approach = scaling.
- Recalibrate the conductivity controller monthly. Probe fouling drifts readings and quietly drops your effective COC.
Red flags that you have pushed too far
- Rising condenser approach temperature without a load change.
- Visible white scale on fill or splash bars.
- Corrosion coupon rates climbing (carbon steel > 3 mpy, copper > 0.2 mpy).
- Biocide demand jumping because biofilm thrives at higher nutrient concentrations.
Back off one cycle, verify inhibitor residuals, and re-approach.
The bottom line
Cycles of concentration is the highest-ROI knob in a cooling water program. Most facilities have room to move up 2 to 3 cycles safely, and every cycle you gain compounds through less water, less chemistry, less blowdown to the sewer, and often lower Legionella nutrient loading. But it requires water chemistry that is actually characterized and a controller that is actually working.
If you are not sure where your tower is on this curve, Glacier's technicians can run a baseline assessment, model the LSI/RSI at higher setpoints, and stage the transition safely. Contact us at glacierlabs.com/contact/ or call (800) 637-6132.
