Cooling Tower Make-Up Water Calculator (Evaporation, Blowdown, Cycles of Concentration)
Water a cooling tower evaporates, bleeds off and must be topped up with, from the circulating flow, the cooling range and the cycles of concentration.
Results
The dashed line marks the value you entered.
What this tool does
A cooling tower cools mostly by evaporating part of the water, so it loses roughly 1 % of its circulating flow for every 5.5 °C of cooling range; the heat it rejects divided by the latent heat of water gives the exact figure. The evaporated water leaves its dissolved salts behind, so some water must be bled off to keep them in check: the more cycles of concentration allowed, the less blowdown and make-up are needed, with diminishing returns as the chart shows. The limit on cycles comes from the water chemistry, such as scaling and corrosion, not from the formula.
Formula
E = Q̇ ÷ h_fg = L × 4.186 × range ÷ h_fg · B = E ÷ (C − 1) − D · make-up M = E + B + D
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ql | Circulating water flow | m³/h |
rn | Cooling range (hot water − cold water) | °C |
cy | Cycles of concentration | — |
dr | Drift loss (% of circulating flow) | % |
hf | Latent heat of evaporation of water | kJ/kg |
M | Make-up water needed | m³/h |
E | Evaporation | m³/h |
B | Blowdown (bleed-off) | m³/h |
ES | Evaporation as a share of the circulating flow | % |
Q | Heat rejected | kW |
Worked example
- Circulating water flow500 m³/h
- Cooling range (hot water − cold water)5.5 °C
- Cycles of concentration4
- Drift loss (% of circulating flow)0.005 %
- Latent heat of evaporation of water2430 kJ/kg
- Make-up water needed6.32 m³/h
- Evaporation4.74 m³/h
- Blowdown (bleed-off)1.55 m³/h
- Evaporation as a share of the circulating flow0.95 %
- Heat rejected3198 kW
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.