14 Cooling Tower

Updated on 01/04/26 by Mananjay MahatoShare on WhatsApp

1. Fundamental Concept

  • Purpose: Rejects waste heat from the water-cooled condenser to the atmosphere.

  • Working Principle: Based on Evaporative Cooling. A small portion of water (approx. 1%) evaporates, absorbing latent heat from the remaining water, thereby cooling it.

  • Heat Transfer: Occurs through both Sensible Heat (temperature difference) and Latent Heat (evaporation).

    • Exam Fact: About 75–80% of the cooling in a tower is due to evaporation.

2. Key Terminology (Very Important for Exams)

  • Range: The difference between the temperature of hot water entering and cold water leaving.

    • $$Range = T_{in} – T_{out}$$

  • Approach: The difference between the temperature of cold water leaving and the Ambient Wet Bulb Temperature (WBT).

    • $$Approach = T_{out} – WBT$$

    • Note: The WBT is the theoretical limit to which water can be cooled

  • Efficiency ($\eta$): The ratio of the actual cooling to the maximum possible cooling.

    $$Efficiency (\%) = \frac{Range}{Range + Approach} \times 100$$

  • Drift: Water droplets carried away by the air stream (lost water). Drift Eliminators are used to minimize this.

  • Blowdown (Bleed-off): Removal of a portion of circulating water to prevent the buildup of dissolved solids and scale.

3. Types of Cooling Towers

Type Air Flow Mechanism Key Characteristics
Natural Draft Natural Convection (Stack effect) Large, hyperbolic concrete structures; used in power plants. No fans.
Forced Draft Fan at the Air Inlet (Bottom/Side) High-velocity air enters; prone to recirculation (hot air being sucked back in).
Induced Draft Fan at the Air Outlet (Top) Sucks air through the tower. More efficient; prevents recirculation. Most common in HVAC.
Cross-Flow Air flows horizontally Water falls vertically; air moves at 90°. Lower pressure drop.
Counter-Flow Air flows vertically upward Air moves opposite to falling water. Best heat transfer efficiency.

4. Major Components

  • Fill (Packing): Increases the surface area and contact time between water and air.

    • Types: Splash fill (breaks water into droplets) and Film fill (spreads water into a thin layer).

  • Nozzles/Sprinklers: Distribute hot water evenly over the fill.

  • Basin: Collects the cooled water at the bottom.

  • Make-up Water Valve: A float valve that adds water to replace losses from evaporation, drift, and blowdown.

5. Maintenance & Troubleshooting (RAC Mechanic Focus)

Problem Possible Cause Remedy
High Water Temp Clogged nozzles or fouled fill Clean/descale components.
High Water Loss Damaged drift eliminators Repair or replace eliminators.
Excessive Noise Fan imbalance or bearing failure Realign fan or lubricate/replace bearings.
Scale Formation High Concentration of Solids Increase Blowdown rate and check water treatment.
Algae/Slime Biological growth Add biocides (Chlorine) to the water.

  • Theoretical Limit: A cooling tower can never cool water below the Wet Bulb Temperature of the entering air.

  • Psychrometrics: As air passes through a cooling tower, its Dry Bulb Temperature (DBT) and Wet Bulb Temperature (WBT) both increase, and its Relative Humidity approaches 100%.

  • Capacity: Usually measured in “Nominal Tons.” One cooling tower ton is the rejection of 15,000 BTU/hr (higher than a standard refrigeration ton of 12,000 BTU/hr to account for compressor heat).

14 Cooling Tower