Cooling Tower Makeup Water Calculation Guide

What makeup water includes

An open-recirculating cooling tower loses water through three main paths:

• Evaporation (E): water that changes phase while heat is rejected;
• Drift (D): liquid droplets carried out with the exhaust air; and
• Blowdown (B): intentional discharge used to control dissolved-solids concentration.

The steady-state balance is M = E + D + B, where M is makeup flow. Leaks, overflow, basin drawdown, intermittent operation, and water-treatment flushes are separate project inputs.

Step 1: estimate heat rejected and evaporation

For a preliminary water-side estimate, calculate rejected heat from circulating flow and the cooling range:

Q (kW) = G (m³/h) × 1000 / 3600 × cp (kJ/kg·K) × ΔT (°C)

Then use the selected latent heat of vaporization:

E (m³/h) = Q (kW) × 3600 / [hfg (kJ/kg) × 1000]

For water near ordinary cooling-tower temperatures, a transparent preliminary example may use cp = 4.187 kJ/kg·K and hfg = 2440 kJ/kg. Use equipment data or a detailed heat-and-mass-transfer model when the result will control tower selection or operating compliance.

Step 2: keep drift separate

Drift is a liquid loss, not evaporation. If the manufacturer provides a certified drift rate:

D (m³/h) = G (m³/h) × drift rate (%) / 100

Do not replace a certified product value with a generic percentage when sizing water treatment, discharge, or hygiene controls. The drift value should be traceable to the tower configuration and operating condition.

Step 3: calculate blowdown from cycles of concentration

Cycles of concentration (CoC) describe how concentrated dissolved solids become in circulating water relative to makeup water. Under a simple concentration balance that treats drift as a dissolved-solids loss:

B (m³/h) = (E + D) / (CoC − 1)

Higher CoC can reduce blowdown, but the practical limit depends on makeup-water chemistry, scaling, corrosion, biological control, equipment materials, and discharge requirements. CoC is a treatment decision, not a universal constant.

Worked example

Assume 355 m³/h of circulating water, a 5°C cooling range, cp = 4.187, latent heat of 2440 kJ/kg, CoC = 4, and certified drift of 0.003%.

This example is a water-balance estimate. It does not prove that the tower will reject the stated heat, that the water chemistry can sustain CoC = 4, or that the discharge is permitted.

Common mistakes

• Using a fixed evaporation percentage without checking heat load and cooling range.
• Counting drift as evaporation and then applying the concentration balance twice.
• Choosing CoC from a generic table without a water-treatment review.
• Using circulating flow when the input actually represents makeup or blowdown flow.
• Presenting a water-balance estimate as tower, pump, chemical-treatment, Legionella, or permit design.

Frequently asked questions

What is the cooling tower makeup water formula?

Makeup water equals evaporation plus drift plus blowdown. With a simple concentration balance, blowdown equals evaporation plus drift divided by cycles of concentration minus one.

How do you calculate cooling tower evaporation?

Estimate rejected heat from circulating flow, water specific heat, and cooling range, then divide by latent heat of vaporization. A rated tower or detailed model may require additional heat and mass transfer data.

What are cycles of concentration?

Cycles of concentration compare dissolved-solids concentration in circulating water with that in makeup water. Higher cycles can reduce blowdown, but water chemistry and treatment limits must be reviewed.

Does this calculate tower or pump size?

No. It estimates water balance only. Tower, pump, treatment, discharge, and hygiene decisions require equipment data, water chemistry, local requirements, and qualified review.

Sources reviewed 2026-10-04

• U.S. Department of Energy — Best Management Practice #10: Cooling Tower Management
• DOE — Cooling Towers: Water Use and System Calculations