Cooling Tower Water Treatment: Cycles, Scale and Blowdown

By SubcontractorHub Editorial Team·Published October 2026

Commercial water treatment dealers meet cooling towers on office buildings, hospitals, campuses and plants that already buy softeners or reverse osmosis for other uses. The tower is a different machine. It is open to the air, it runs warm, and the bill for water and sewer is tied directly to how many cycles the chemistry will allow. This guide explains the water balance, the three failure modes, and where makeup pretreatment fits. Broader commercial applications are in the commercial water treatment guide.

What the Tower Does to Water

Condenser water circulates through the tower. A fraction of it evaporates, which is how the heat leaves. Evaporation is nearly pure water, so dissolved minerals stay behind and the basin gets saltier every hour the tower runs. A smaller fraction leaves as drift, droplets blown out of the fan. The rest is blowdown, water bled to drain on purpose so the concentration stops rising.

Makeup is whatever you add to replace all three. If you bleed very little, you save water and you scale the tower. If you bleed constantly, the surfaces stay cleaner and the water bill climbs. Treatment exists to move that trade in the building's favor without pretending chemistry can replace a water balance.

Cycles of Concentration

Cycles are a ratio. Measure conductivity in the tower and conductivity in the makeup. Divide. Three cycles means the tower water holds about three times the dissolved solids of the city water feeding it. Some programs calculate cycles from a single ion, such as chloride, that does not drop out as scale. Conductivity is the everyday meter reading, and it is good enough to run the building if you also have a periodic lab sheet.

There is no universal “best” cycle count. Soft, low-silica makeup can run higher cycles than hard, high-silica makeup. A target copied from another city is how fill gets cemented into the tower. Start from the makeup analysis. The ions that hit a limit first, often calcium hardness, alkalinity, phosphate or silica, set the maximum cycles. Everything else in the program is there to push that limit a bit higher or to protect metal while you sit under it.

Blowdown, Evaporation and Makeup

The planning equations used on this site's calculator are straightforward. Recirculation is either measured in gallons per minute or estimated at about 3 gpm per ton of chiller capacity. Evaporation in gpm is an evaporation factor times recirculation times the temperature drop in degrees Fahrenheit. The default factor is 0.0008 per degree, a stand-in for most of the heat leaving as evaporation. Hot, dry climates can sit closer to 0.001.

Blowdown is then about evaporation divided by (cycles minus one), minus drift, because drift has already thrown some concentrated water away. Makeup is evaporation plus blowdown plus drift. Raise the cycles and blowdown falls. The water and sewer savings are real, and they stop being real the moment silica or hardness scale forces you back down.

Drift on a modern tower with eliminators is a small percentage of recirculation. Older towers drift more. Drift is also the path aerosols take off the site, which is why biology is not only a slime problem inside the fill. Run your own tons, range and cycles through the cooling tower blowdown calculator before you promise a water-savings number on a proposal.

Scale, Corrosion and Biofouling

These three fight each other. A program that drives pH up to hold corrosion down can precipitate calcium carbonate. A program that holds pH down to stop scale can rust steel. Biofilm hides under both and insulates heat-exchange surface so the chiller works harder. You treat all three, or the one you ignored becomes the callback.

Inhibitors, acid or alkali feed, and biocides are specified from the water and from the metals in the tower and the condenser. This page is not a chemical recipe. The dealer's job on the first visit is to identify which of the four rows is already happening, not to drop a drum of “tower chemical” and leave.

Silica and Other Limits

Silica scale is stubborn. Once it plates onto fill or a heat exchanger, mechanical cleaning is miserable and acid cleaning is not a casual service call. Field programs often hold recirculating silica near or below about 150 ppm as SiO2 unless the inhibitor supplier has documented a different ceiling for that chemistry. That 150 ppm figure is the planning default in the blowdown calculator. It is a ceiling to check, not a law.

Maximum cycles from silica alone are the ceiling divided by silica in the makeup. Makeup at 30 ppm silica and a 150 ppm ceiling allows about 5 cycles before silica, by itself, says stop. Calcium hardness or alkalinity may say stop sooner. If the building needs more cycles than the makeup allows, the makeup has to change. That is pretreatment: a softener on hardness, or reverse osmosis or deionization when silica or TDS is the wall. The deionized water system calculator is the sizing companion when the spec really is DI, which is a narrower case than “the tower is scaling.”

Makeup Water

Get a current analysis of whatever feeds the tower. City water changes by season. A well, a reclaim line or a blend of condensate and city water changes the whole program. Hardness, alkalinity, silica, pH, conductivity and a note about phosphate or other corrosion inhibitor already in the city water are the minimum. Without that sheet, a cycles target is a guess.

Softening makeup removes calcium and magnesium so calcium carbonate is less likely to set the cycle limit. It does not remove silica. It adds sodium. It can be the right project when the water bill is the customer's complaint and hardness is the ion in the way. It is the wrong project when the basin is full of mud and nobody has cleaned it. Commercial softener sizing, for this and for other process loads, belongs in the commercial guide and in a softener calculator fed by the actual gallons, not by a bathroom count.

Biology Is Part of the Program

Legionella associated with building water systems is why cooling towers are no longer a “keep the chiller happy” side task. ASHRAE Standard 188 is the published framework many facility programs use for risk management. A treatment contract that never mentions testing, biocide, or who is responsible for the basin cleaning is thin. So is a contract that quotes a kill claim without a sample plan.

Practically, the dealer logs conductivity, pH, inhibitor residual and a biological check on a schedule, and writes down blowdown setpoint changes. Drift eliminators have to be in place. Dead legs and idle towers in the shoulder season need a procedure, not a hope that the fan being off makes the basin safe. None of that requires an invented infection statistic to be worth putting in the scope.

What a Dealer Should Put on the Proposal

A useful cooling-tower scope has the makeup analysis, the current and target cycles, the blowdown and makeup the calculator produced, the limit that sets those cycles, the chemical or pretreatment that defends the limit, and the visit schedule. Water savings, if you state them, should be the calculator's output at the customer's rates, labeled as an estimate.

The same customer may already buy salt, filters or membranes for a kitchen, a lab or a boiler. Water treatment software is how those routes stay on one account instead of in a separate notebook for “the tower guy.” The tower is commercial work. Quote it like commercial work: with the numbers attached and a service cadence that survives the technician who sold it.

Frequently Asked Questions

Related Guides & Tools

• Cooling Tower Blowdown Calculator
• Commercial Water Treatment Guide
• Deionized Water System Sizing Calculator
• Commercial Water Softener Sizing Calculator
• → Water Treatment Contractor Software