Cooling Tower Blowdown & Makeup Water Calculator

Why Cooling Towers Need Blowdown

A cooling tower rejects heat by evaporating a small share of the water it circulates. The water leaves as vapor, but the calcium, magnesium, silica, alkalinity and other dissolved solids stay behind. Left alone, the tower water would keep concentrating until minerals dropped out as scale on the condenser tubes and fill, where even a thin layer costs chiller efficiency.

Blowdown, also called bleed-off, is the fix. The system drains some concentrated tower water to the sewer and replaces it with fresher makeup water, holding the dissolved solids at a set level. Three streams leave the tower and makeup replaces all of them:

• Evaporation (E) is set by the heat load: about 0.0008 × gpm × range in °F. Treatment cannot change it.
• Drift (D) is fine droplets carried out with the air. Modern drift eliminators hold it near 0.005% of the recirculation rate.
• Blowdown (B) is the water drained on purpose: E ÷ (C − 1) − D. This is the stream the treatment program controls, and the one the calculator prices.

What Cycles of Concentration Mean

Cycles of concentration compare the tower water with the makeup water. If the makeup reads 500 µS/cm conductivity and the tower reads 1,500, the tower is running at 3 cycles. Chloride, or another ion that does not scale out, gives a cross-check. Higher cycles mean less blowdown, but the savings flatten quickly:

Figures are for a 1,500 gpm tower with a 10°F range (12 gpm evaporation, 0.075 gpm drift). Most of the saving comes from getting a tower off 2 or 3 cycles. Past 6 to 8 the gains are small and the scaling risk climbs, which is why the ceiling is set by chemistry, not by how much water you want to save. Silica is a common limit: the calculator divides a tower silica limit (150 ppm is a common guideline) by the makeup silica to find the maximum cycles.

Worked Example: A 500-Ton Chiller Plant

An office building runs a 500-ton chiller with a 10°F range and modern drift eliminators. The tower runs 12 hours a day for 180 days. It currently runs at 3 cycles. Makeup silica is 20 ppm. Water costs $5 and sewer $6 per 1,000 gallons.

• Flow: 500 tons × 3 gpm per ton = 1,500 gpm.
• Evaporation: 0.0008 × 1,500 × 10 = 12.0 gpm.
• Drift: 1,500 × 0.005% = 0.075 gpm.
• At 3 cycles: blowdown = 12 ÷ 2 − 0.075 = 5.925 gpm; makeup = 12 + 5.925 + 0.075 = 18.0 gpm.
• At 6 cycles: blowdown = 12 ÷ 5 − 0.075 = 2.325 gpm; makeup = 12 + 2.325 + 0.075 = 14.4 gpm.
• Saved: 3.6 gpm of makeup and 3.6 gpm of blowdown. Run time is 12 h × 60 × 180 days = 129,600 minutes, so each is 466,560 gallons a year.
• Savings: 466.56 thousand gallons × ($5 water + $6 sewer) = about $5,132 a year.
• Silica check: 150 ÷ 20 = 7.5 maximum cycles, so 6 cycles is inside the limit.

Annual makeup drops from 2,332,800 to 1,866,240 gallons and blowdown from 767,880 to 301,320 gallons. Many utilities also credit the sewer charge on water that evaporated rather than went down the drain, measured by meters on the makeup and blowdown lines. That credit can be worth more than the cycles increase, but it depends on the local utility, so the calculator leaves it out.

Cooling Tower Water Treatment Chemicals and Controls

Higher cycles are only safe with a cooling tower chemical treatment program built for the water. A typical cooling tower water treatment system combines:

• Scale inhibitors that keep calcium and other minerals in solution at concentrations where they would otherwise deposit. Scaling indices such as the Langelier Saturation Index (LSI) help set how far the water can be pushed.
• Corrosion inhibitors matched to the metals in the loop, often steel and copper, with corrosion coupons to confirm they work.
• Biocides: an oxidizing biocide, commonly a chlorine or bromine product, fed continuously or on a schedule, and often a non-oxidizing biocide rotated in to control slime and organisms the oxidizer misses.
• A conductivity controller that opens the blowdown valve when the tower water reaches the set point and paces chemical feed to makeup or blowdown. The set point is how the target cycles are actually held.

The specific chemistry belongs to the water treatment provider. For the wider picture across boilers, process water and pretreatment, see the commercial water treatment guide. Where hard makeup water limits cycles, a softener on the makeup line is one option; size it with the commercial water softener sizing calculator, and have the treatment provider adjust the corrosion program, because softened water behaves differently.

Legionella and Building Water Management

Cooling towers are warm, wet and throw aerosols into the air, which makes them a known source of Legionella, the bacteria behind Legionnaires' disease. ASHRAE Standard 188 calls for a building water management program for buildings with risk factors like cooling towers: a hazard analysis, control measures such as biocide residuals and cleaning, monitoring, corrective actions and records. Some states and cities add their own cooling tower registration, sampling or inspection rules. Check the current standard and local requirements for the specifics rather than relying on a summary.

For HVAC and Water Treatment Contractors: Sell It as a Monthly Service

A cooling tower treatment account is recurring work by nature. The chemistry has to be tested, the controller checked, the chemicals refilled, and the results logged every visit, because the test logs are what the building owner shows an auditor or a water management team. That makes it a natural monthly service contract: a site survey and water analysis, a program proposal with the savings from this calculator, then scheduled visits with conductivity, biocide residual and inhibitor readings recorded each time.

That revenue only holds if every visit gets scheduled and logged. HVAC contractor software keeps the proposal, the service agreement and the visit history together, and software for water treatment companies covers the same work from the water side. See what else SubcontractorHub does for HVAC contractors.