Condenser water flow, evaporation, makeup, blowdown, leaving-water temperature, cycles-of-concentration optimisation and annual water & energy cost — updated live as you type.
About the Cooling Tower Calculator
This professional tool sizes the water side of an open evaporative cooling tower and estimates its yearly running cost. From the heat your chiller plant rejects, it derives the circulating flow, evaporation, makeup and blowdown, the leaving-water temperature, and the annual water and pump-energy cost — and shows how cycles of concentration trade water against treatment risk.
It is built for HVAC and MEP engineers, water-treatment and facilities teams, and students who need a fast, transparent check before a manufacturer selection. Every output comes from standard ASHRAE and CTI relationships, listed in full below.
How a cooling tower balances heat and water
Warm condenser water is sprayed over the fill where a small fraction evaporates; that evaporation carries away the heat and cools the rest before it returns to the condenser. Because only pure water leaves as vapour, dissolved minerals concentrate over time, so a controlled blowdown is bled off and replaced with fresh makeup.
Methodology & formulas
1. Condenser water flow.
GPM = Heat rejected (BTU/hr) ÷ (500 × Range °F)
500 = 60 min/hr × 8.33 lb/gal × 1 BTU/lb·°F
The load entered is the chiller cooling load; condenser heat rejection per ton is taken from the editable “heat rejection per ton” field (default 15,000 BTU/hr·ton ≈ 1.25× cooling, giving ≈3 GPM/ton at 10 °F range). Enter 12,000 to use the cooling load directly with no gross-up.
2. Temperatures.
Cold (leaving) water = Wet-bulb + Approach
Hot (entering) water = Leaving water + Range
3. Water balance.
Evaporation ≈ 0.01 × GPM × (Range ÷ 10) (~1% per 10 °F)
Blowdown = Evaporation ÷ (COC − 1)
Drift = Drift% × GPM
Makeup = Evaporation + Blowdown + Drift
4. Annual cost & pump power.
Annual makeup (m³) = Makeup GPM × 0.22712 × Operating hours
Pump BHP = GPM × Head (ft) ÷ (3960 × η); kW = BHP × 0.7457
Energy cost = (Pump kW + Fan kW × cells) × hours × rate
Cycles-of-concentration optimisation
Because blowdown falls as 1 ÷ (COC − 1), the first few cycles save a lot of water and the curve flattens quickly. Going from 2 to 4 cycles roughly halves blowdown; beyond 6 the extra saving is small while scaling and corrosion risk keeps rising. The chart marks your setpoint so you can pick a defensible target with your water-treatment programme.
Input parameters explained
| Heat rejection load | Chiller plant cooling load (TR, kW or MMBTU/hr), grossed up by the heat-rejection-per-ton field. |
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| Range (ΔT) | Condenser water temperature drop across the tower. Typical 8–12 °F. Larger range ⇒ less flow. |
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| Approach | Closeness of cold water to wet-bulb. Typical 5–10 °F. Tighter approach ⇒ bigger tower. |
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| Wet-bulb | Site design ambient wet-bulb — the true driver of evaporative performance. Use the local 0.4% summer value. |
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| Cycles of concentration | Concentration ratio of circulating water vs makeup. Typical 3–6, limited by water quality. |
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| Cells / drift / heat-rejection | Number of tower cells (flow splits evenly), drift loss %, and the BTU/hr per ton used for heat rejection. |
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Worked example
500 TR · 10 °F range · 7 °F approach · 78 °F wet-bulb · COC 5 · 2 cells:
Flow = 500 × 15,000 ÷ (500 × 10)1,500 GPM
Flow = 1,500 × 0.22712340.7 m³/h
Per cell = 1,500 ÷ 2750 GPM
Evaporation = 1,500 × 0.0115 GPM
Blowdown = 15 ÷ (5 − 1)3.75 GPM
Cold water = 78 + 785 °F
Makeup = 15 + 3.75 + 1.520.25 GPM
Assumptions & limitations
- Evaporation uses the 1%-per-10 °F rule; actual evaporation varies with ambient conditions and load.
- The default 15,000 BTU/hr·ton assumes a typical water-cooled chiller — override it for high-efficiency, gas or process loads.
- Pump power is shaft (brake) power from your head and efficiency; add motor and VFD losses for input power.
- Fan power is the value you enter from the selected tower; the tool does not predict it.
- The maximum safe COC is set by makeup-water chemistry — confirm with a treatment specialist and scaling indices.
- This is a preliminary design aid, not a certified tower selection (do that per CTI Standard 201 from manufacturer data).
Frequently asked questions
What is the difference between range and approach in a cooling tower?
Range is the temperature drop of the water across the tower — hot water in minus cold water out (typically 8–12 °F). Approach is how close the cold leaving water gets to the ambient wet-bulb (typically 5–10 °F). Range is set by the load and flow; approach is set by the tower size. A smaller approach needs a larger, more expensive tower.
Why does the calculator use 15,000 BTU/hr per ton instead of 12,000?
A ton of refrigeration removes 12,000 BTU/hr at the evaporator, but the condenser must also reject the compressor heat — about 1.25× for a typical water-cooled chiller, giving ≈15,000 BTU/hr per ton and the well-known 3 GPM/ton at 10 °F range. The Pro calculator lets you override this “heat rejection per ton” field directly if you know the actual figure.
How is condenser water flow rate (GPM) calculated?
Flow (GPM) = Heat rejected (BTU/hr) ÷ (500 × Range °F). The 500 comes from 60 min/hr × 8.33 lb/gal × 1 BTU/lb·°F for water. At a 10 °F range this is about 3 GPM per ton of cooling.
How does increasing cycles of concentration save water?
Blowdown = Evaporation ÷ (COC − 1). Raising cycles from 2 to 4 roughly halves blowdown, and going beyond about 6 gives diminishing returns — exactly the curve shown in the optimisation chart. Higher cycles save makeup water but concentrate dissolved solids, so the practical limit is set by makeup-water quality and the treatment programme.
How much makeup water and how much does it cost per year?
Makeup = Evaporation + Blowdown + Drift. Multiply the makeup rate by your annual operating hours to get the yearly volume, then by your water tariff for cost. The Pro calculator does this automatically and also estimates pump energy cost, so you can compare total annual operating cost against different cycles of concentration.
How is the condenser pump power estimated?
Pump brake power (BHP) = GPM × Head (ft) ÷ (3960 × pump efficiency); multiply by 0.7457 for kW. Enter your design condenser loop head and pump efficiency and the tool returns shaft power, which feeds the annual energy-cost estimate.
Can the leaving water temperature be below the wet-bulb?
No. Evaporative cooling can approach but never reach (or beat) the ambient wet-bulb. Leaving water temperature = wet-bulb + approach.
Does this calculator select the physical tower?
No. It gives the thermal and water-balance and operating-cost quantities. Selecting fill, fan power, footprint and certified capacity is done from manufacturer data per CTI Standard 201 using your design wet-bulb, range and approach.
How do I convert flow to litres per second or m³/h?
1 US GPM = 0.0631 L/s = 0.22712 m³/h. The Pro calculator shows all three units side by side.
Last updated: 2026-01-03 · Preliminary design only — verify against project specifications, manufacturer data and local code.