Cooling Towers
Evaporative heat rejection — where plant heat meets the atmosphere.
Cooling towers reject waste heat to the atmosphere by evaporating a small fraction of circulating water. Types: induced/forced draft, cross-flow/counter-flow, natural draft (hyperbolic).
How it works
- 1
Hot water from condensers is distributed over the fill.
Hot condenser water (37 °C) enters the header and is spread evenly over the fill by spray nozzles (counter-flow) or a gravity basin with metering orifices (cross-flow). Even distribution is critical — dry patches on the fill destroy thermal performance far more than a dirty fan.
- 2
Air is drawn by fans, and a portion of the water evaporates.
Induced-draft fans on top (or forced-draft blowers at the side) pull ambient air through the fill counter to, or across, the falling water. Roughly 1.5–2 m³/s of air per m³/h of water gives the design L/G ratio; airflow is trimmed by fan pitch or a VFD.
- 3
Latent heat of vaporization cools the remaining water.
About 1 % of the circulating water evaporates for every 5.5 K of cooling. Because evaporation removes latent heat (~2260 kJ/kg), a small loss of water cools a large mass of it — this is why a tower can approach the wet-bulb temperature rather than the dry-bulb.
- 4
Cooled water collects in the basin and returns to the process.
Cooled water (32 °C) collects in the basin, passes through a strainer and returns to the condensers. Dissolved solids left behind by evaporation concentrate, so continuous blowdown plus make-up holds the cycles of concentration at 3–6, and dosing (scale inhibitor, corrosion inhibitor, biocide) keeps the loop clean and Legionella-free.
Key components
Film fill (PVC sheets, 12–19 mm flute) for clean water, or splash bars/low-clog fill for dirty or high-suspended-solids duty. Provides the huge wetted surface where heat and mass transfer occur; scaling or collapse here shows instantly as a rising approach.
Multi-pass PVC blades that force the exiting air to change direction so entrained droplets are thrown out and returned. Limit drift to <0.005 % of circulation — important for water loss, plume, and for containing Legionella-bearing aerosol.
Large-diameter axial fan (FRP blades) driven through a right-angle gearbox and drive shaft, or a belt drive on small towers. Monitor gearbox oil level and condition, vibration switch operation, and blade pitch — mismatched pitch is a common cause of low airflow and motor overload.
Non-clog spray nozzles or gravity orifices. Check with the fan off for blocked or missing nozzles; a handful of blocked nozzles can cost 1–2 K of approach.
Collection sump with strainer, level control (float or level transmitter) for make-up, an overflow, a drain and a side-stream filter connection. Basin sludge is the main breeding ground for bacteria — sweep and clean at least annually.
Typical operating parameters
Problem solving videos
Safety notes
- •Legionella risk — dosing & routine testing mandatory.
- •Confined space entry procedures for basin cleaning.
- •Fan LOTO before entering plenum.
Maintenance schedule
- •Daily: check make-up, blowdown, chemical dosing.
- •Monthly: fill and drift eliminator inspection.
- •Annual: gearbox oil, fan balance, structural inspection.
Problem solving matrix
| Issue | Likely cause | Fix |
|---|---|---|
| Poor approach | Fouled fill / low airflow | Clean fill, check fan pitch |
| High make-up | Excess blowdown / drift | Tune CoC, inspect eliminators |
| Algae growth | Poor biocide dosing | Review water treatment program |
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