About Cooling Tower Calculator
The cooling tower calculator evaluates the thermal performance and water balance of an evaporative cooling tower. From the circulating water flow, the hot and cold water temperatures, and the ambient wet-bulb temperature it returns the range, the approach, the heat rejected, and the thermal efficiency, then completes the water balance with the evaporation, drift, blowdown, and total makeup water required to maintain the recirculating system.
Enter the flow rate in cubic metres per hour, the hot and cold water temperatures and the wet-bulb temperature in degrees Celsius, the cycles of concentration, and the drift loss as a percent of flow. The tool applies the standard cooling-tower relations — range, approach, the sensible heat-rejection balance, the empirical evaporation rule, and blowdown from cycles of concentration — and charts how efficiency falls as the approach to the wet-bulb widens.
How It Works
- Enter the circulating water flow (m³/h) and the hot, cold and wet-bulb temperatures (°C).
- Set the cycles of concentration (COC) and the drift loss as a percent of the circulating flow.
- The tool computes range = hot − cold, approach = cold − wet-bulb, and heat rejection Q = ṁ·cp·range.
- It then finds evaporation ≈ 0.00085·1.8·flow·range, blowdown = evaporation/(COC−1), drift = drift%·flow, and makeup = the sum; efficiency = range/(range+approach).
Worked Example
A tower circulates 1000 m³/h, cooling water from 37°C to 27°C against a 21°C wet-bulb, with 4 cycles of concentration and 0.02% drift. The range is 37 − 27 = 10 K and the approach is 27 − 21 = 6 K, so the efficiency is 10 / (10 + 6) = 0.625 (62.5%). The heat rejected is ṁ·cp·range = (1000·1000/3600 kg/s)·4.186·10 = 11,628 kW. Evaporation ≈ 0.00085·1.8·1000·10 = 15.3 m³/h; blowdown = 15.3/(4 − 1) = 5.1 m³/h; drift = 0.02%·1000 = 0.2 m³/h; total makeup = 15.3 + 5.1 + 0.2 = 20.6 m³/h.
Formulas
- Range and approach
Range = T_hot - T_cold ; Approach = T_cold - T_wetbulb- Heat rejection
Q = m_dot * cp * Range , m_dot = flow * rho- Evaporation (rule of thumb)
Evaporation = 0.00085 * 1.8 * flow * Range- Blowdown, drift and makeup
Blowdown = Evaporation / (COC - 1) ; Drift = drift% * flow ; Makeup = Evaporation + Drift + Blowdown- Thermal efficiency
Efficiency = Range / (Range + Approach)
Standards & References
- CTI (Cooling Technology Institute) acceptance test code ATC-105
- ASHRAE Handbook — HVAC Systems & Equipment (cooling towers)
- Evaporation/blowdown rules of thumb for recirculating cooling water
- Cycles of concentration water-balance method
Frequently Asked Questions
What is the difference between range and approach?
Range is the temperature drop the tower achieves, hot water minus cold water. Approach is how close the cold water gets to the ambient wet-bulb, cold water minus wet-bulb. A smaller approach means a larger, more effective (and usually more expensive) tower.
Why does the wet-bulb temperature matter more than the dry-bulb?
Evaporative cooling is driven by the wet-bulb temperature, which is the lowest temperature water can reach by evaporation in the ambient air. The cold water can approach but never fall below the wet-bulb, so it sets the performance limit, not the dry-bulb.
How do cycles of concentration affect makeup water?
Cycles of concentration (COC) is the ratio of dissolved solids in the circulating water to the makeup water. Higher COC means less blowdown (= evaporation/(COC−1)) and lower total makeup, but it raises scaling and corrosion risk, so water treatment limits how high it can go.
Is the evaporation formula exact?
No. Evaporation ≈ 0.00085·1.8·flow·range is an industry rule of thumb that assumes most heat is rejected by evaporation. Actual evaporation varies with ambient humidity and the share of sensible cooling, but the rule is accurate enough for sizing makeup and blowdown.