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EVAPORATIVE COOLING TOWERS

Core Definition & Principle

An Evaporative Cooling Tower is a heat rejection device that uses the principle of evaporative cooling to lower the temperature of a water stream by rejecting waste heat to the atmosphere. It is the most common type of cooling tower for industrial and large-scale HVAC applications.

Fundamental Principle: When water is exposed to moving air, a small portion evaporates. This phase change from liquid to vapor requires latent heat of vaporization, which is drawn from the remaining water mass, thereby lowering its temperature.

How It Works: The Thermodynamic Process

1. Hot Water Inlet: Warm water from an industrial process or HVAC condenser (typically 35-45°C) is pumped to the top of the tower.
2. Distribution & Exposure: Water is distributed and broken into droplets or thin films over a “fill” or “packing” medium to maximize its surface area exposed to air.
3. Air Contact: Air is moved through the tower (naturally or mechanically).
4. Evaporation & Heat Transfer: A small percentage (1-2%) of the circulating water evaporates into the passing air stream. The latent heat required for this evaporation is extracted from the remaining water.
5. Cooled Water Collection: The now-cooled water (typically 25-35°C) falls into a cold water basin and is recirculated back to the process.
6. Warm, Moist Air Exhaust: The air, now warmer and saturated with water vapor, is exhausted to the atmosphere, often visible as a plume under the right conditions.

Key Components

1. Fill/Packing: Increases contact surface and time between air and water.
   1. Splash Fill: Breaks water into droplets (good for dirty water).
   1. Film Fill: Spreads water into thin sheets (higher efficiency, prone to fouling).
2. Distribution System: Nozzles or gravity basins to distribute water evenly over the fill.
3. Drift Eliminators: Remove entrained water droplets from the exhaust air to conserve water.
4. Cold Water Basin: Collects cooled water for recirculation.
5. Air Moving Device:
   1. Fans (Mechanical Draft)

• Hyperbolic Stack (Natural Draft)
• Casing/Structure: Houses all components (FRP, concrete, wood, steel).
• Louvers: Direct airflow and contain water splash.

Types of Evaporative Cooling Towers

Classification Basis	Types
By Air Flow Generation	1. Natural Draft (Hyperbolic, large scale) 2. Mechanical Draft (Fan-driven)
Mechanical Draft Sub-Types	a. Induced Draft (Fan at top – most common) b. Forced Draft (Fan at bottom)
By Air/Water Flow Direction	1. Cross Flow (Air horizontal, Water vertical) 2. Counter flow (Air vertical up, Water vertical down)
By Construction Material	FRP, Concrete, Wood, Galvanized Steel
By Shape/Form	Package Type (Factory-built), Field-Erected

Key Performance Parameters

1. Range: T_hot(inlet) – T_cold(outlet) – The temperature drop of the water. Indicates heat load rejected.
2. Approach: T_cold(outlet) – T_wet_bulb(ambient) – How close the cooled water gets to the ambient wet-bulb temperature. Lower approach = Higher tower efficiency.
3. Wet-Bulb Temperature: The theoretical limit for cooling. Water cannot be cooled below the ambient wet-bulb temperature.
4. Cycle of Concentration (COC): Ratio of dissolved solids in blowdown water to make-up water. Higher COC = less water wasted but greater scaling risk.
5. Evaporation Loss: ~1% of circulation flow for every 7°C (12.5°F) of range.
6. Drift Loss: Tiny droplets lost with exhaust air (0.001%-0.2%).
7. Blowdown: Intentional purge of concentrated water to control scaling.

Advantages

• High Efficiency: Uses latent heat of vaporization (≈ 1000 BTU/lb), making it very effective.
• Energy Efficient: Consumes significantly less water than once-through systems and less electrical energy than dry coolers for the same duty.
• Lower Condensing Temperatures: Enables more efficient operation of chillers and process equipment.

• Compact Footprint: Compared to air-cooled (dry) systems for the same capacity.
• Proven & Reliable Technology: Widely used for decades.

Applications

• Power Generation: Condenser cooling for thermal and nuclear power plants.
• HVAC: Cooling for large building chillers and district cooling plants.
• Heavy Industry: Refineries, petrochemical plants, steel mills, manufacturing.
• Process Cooling: Plastics, food & beverage, pharmaceuticals.

Critical Operational Considerations

1. Water Treatment: Mandatory to control scale, corrosion, and biological contamination.
2. Legionella Risk Management: Requires robust biocide treatment and regular monitoring.
3. Winter Operation: Needs strategies to prevent freezing (e.g., basin heaters, variable fan speed, reversing fan cycles).
4. Material Selection: Must suit water chemistry and atmospheric conditions.