Cooling Tower Makeup Water Calculator

Units Metric (m³/h, °C) US Customary (gpm, °F)

Circulation rate

Enter total circulating water flow.

Temperature range

Hot-to-cold water temperature difference.

Evaporation coefficient

Typical coefficient; adjust if your standard differs.

Drift loss (%)

Percent of circulation leaving as drift.

Cycles of concentration (COC)

Higher cycles usually reduce blowdown.

Other losses

Leakage, washdown, basin overflow, or intentional draw-off.

Safety allowance (%)

Optional margin for uncertainty and seasonal variation.

Calculate makeup Clear form

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Example data table

Formula used

The calculator estimates makeup water by adding evaporation, drift, other losses, and blowdown:

• E = k × Q × ΔT (evaporation loss)
• D = Q × (drift% ÷ 100) (drift loss)
• B = (E ÷ (COC − 1)) − (D + O) (blowdown from mass balance)
• M = E + D + O + B (makeup required)
• Mₛ = M × (1 + safety% ÷ 100) (makeup with allowance)

Assumption: drift and other losses leave at circulating concentration. If “other losses” are negligible or not at circulating concentration, set them to zero or adjust your method.

Professional guide: cooling tower makeup water on construction sites

Cooling towers are common on large construction projects that use temporary or permanent chilled-water systems. Makeup water planning is not only a utility question; it affects storage sizing, water treatment, logistics, and compliance. The tower continually loses water through evaporation as heat is rejected, plus small mechanical losses such as drift and operational losses such as basin overflow or washdown.

This calculator separates those components so you can build a realistic supply requirement. Evaporation is driven primarily by circulating flow and the tower temperature range. Drift is typically a very small fraction of circulation, but it can become important when the system runs continuously or when water quality limits force lower cycles of concentration. Blowdown is the controlled discharge used to keep dissolved solids from building up beyond the desired cycles.

For practical construction estimating, start with verified operating data from commissioning targets or equipment submittals. Select the unit system, then enter circulation rate and temperature range. Confirm the evaporation coefficient used by your organization, and keep drift realistic for the tower type. Next, choose cycles of concentration based on makeup quality and treatment strategy. Add any expected “other losses” (washdown, filter backwash, or chronic leakage). Finally, include a safety allowance if weather, load, or site water pressure varies.

Example (metric): A tower circulating 2500 m³/h with a 5 °C range, 0.02% drift, and COC 4 produces a makeup demand of roughly 22.4 m³/h. With a 10% allowance, plan for about 24.6 m³/h. This supports sizing of storage tanks, transfer pumps, and treatment capacity. If cycles are reduced, blowdown increases and makeup rises quickly, so always confirm water quality assumptions early.

Use the CSV/PDF exports as a record for submittals, method statements, and internal reviews. Keep field measurements and chemical control logs aligned with the chosen cycles.

FAQs

1) What is “makeup water” in a cooling tower?

Makeup water is the fresh supply added to replace evaporation, drift, blowdown, and other operational losses, keeping the basin level stable and the tower operating at target cycles.

2) Why do cycles of concentration affect blowdown?

As water evaporates, dissolved solids stay behind and concentrate. Higher cycles allow more concentration before discharge, reducing blowdown. Lower cycles require more blowdown and increase total makeup demand.

3) What drift percentage should I use?

Use tower-specific data when available. Modern drift eliminators can be very low, while older or damaged systems can be higher. If unsure, start conservatively and refine using vendor literature or field observations.

4) Can blowdown become negative in the calculation?

Yes, if assumed drift and other losses exceed the mass-balance blowdown requirement at the chosen cycles. The calculator sets blowdown to zero and flags a warning. Recheck cycles, losses, and assumptions.

5) Should “other losses” include leak repairs or abnormal events?

Include expected, repeatable losses such as routine washdown or known leakage. For abnormal events, handle them separately as contingency. You can also apply a safety allowance to cover uncertainty.

6) How do I use results for storage tank sizing?

Convert makeup flow to daily volume and add buffer for supply interruptions. Consider minimum run time, refilling logistics, and treatment dosing limits. Tank sizing often targets several hours to a day of demand.

7) Is this calculator suitable for final design?

It is suitable for estimating and early design checks. Final design should confirm vendor performance, site water quality, treatment requirements, and operating profiles. Validate with commissioning data when available.

How to use this calculator

1. Select your unit system and enter the circulating flow rate.
2. Enter the water temperature range across the tower.
3. Confirm the evaporation coefficient or use your standard value.
4. Add drift percentage and cycles of concentration (COC).
5. Include other losses and an optional safety allowance.
6. Press Calculate makeup to view results above the form.
7. Use the download buttons to export CSV or PDF reports.