Cooling towers are among the largest water users in a central air-conditioning system. Underestimate makeup water and the basin level drops too low, so pumps may draw air; overestimate it and money is wasted on water and treatment chemicals. Whether you are a designer, an operator or an owner, this calculation is worth getting right. Makeup water has three components: evaporation loss, blowdown loss and drift loss.
1. Evaporation loss: set by temperature range and circulating flow
A cooling tower rejects heat mainly by evaporating water. Evaporating 1 kg of water absorbs about 2430 kJ, while cooling 1 kg of circulating water by 1°C releases 4.187 kJ, which gives the common estimate:
E = L × Δt × 4.187 ÷ 2430 ≈ 0.0017 × L × Δt
Here E is the evaporation rate (m³/h), L is the circulating water flow (m³/h) and Δt is the temperature difference between water entering and leaving the tower (°C). By this formula, with a 5°C range the evaporation is about 0.85% of the circulating flow.
The coefficient should be adjusted by season: 0.0017 at full load in summer; in winter part of the heat leaves as sensible heat, so the coefficient can fall to 0.0010–0.0014. Δt should also be a measured or design-condition value. At part load the range is smaller, so the 5°C on the nameplate should not be applied blindly.
2. Blowdown and drift: set by cycles of concentration and the drift eliminator
Water keeps evaporating but dissolved salts stay in the system, so blowdown is needed to control water quality. Cycles of concentration N is the ratio of dissolved-solids concentration in the circulating water to that in the makeup water. A mass balance gives:
Blowdown + drift = E ÷ (N − 1)
Drift loss W is water droplets carried out of the tower by the airflow; it depends on tower type and drift eliminator quality. With eliminators in good condition it is typically taken as 0.005%–0.01% of circu