Cooling Tower Thermal Performance
The lungs of the industrial plant. Learn how evaporation carries away gigajoules of heat and why 'Wet Bulb' temperature is the absolute limit of physics.
The Power of Evaporation
Cooling towers are specialized heat exchangers that reject process heat into the atmosphere. They work primarily by Evaporative Cooling. When a small amount of water evaporates, it absorbs a massive amount of "Latent Heat" from the remaining water, cooling it down. This is the same principle that keeps humans cool through sweat.
Heat Load Formula
Range, Approach, and Effectiveness
- Range: The temperature difference between hot water in and cold water out. It represents the "Heat Load."
- Approach: The difference between cold water out ($T_2$) and the Ambient Wet Bulb ($T_{wb}$). A smaller approach means a bigger, more expensive tower. You can never cool water below the wet-bulb temperature.
- Effectiveness (Efficiency): Ratio of actual cooling to theoretically possible cooling. $\eta = \text{Range} / (\text{Range} + \text{Approach})$.
Drift and Makeup Water
Because the tower works by evaporation, it constantly loses water.
1. Evaporation Loss: Typically $1\%$ of flow for every $7$°C of cooling.
2. Drift Loss: Tiny droplets carried away by the wind.
3. Blowdown: Water drained to remove concentrated minerals.
The sum of these three is the Makeup Water requirement of the plant.
Frequently Asked Questions (FAQ)
What is 'Wet Bulb' temperature?
The Wet Bulb temperature is the lowest temperature that can be reached by evaporating water into the air. It depends on humidity. If the air is $100\%$ humid, Wet Bulb equals Dry Bulb (normal temp), and the cooling tower will stop working entirely because no more water can evaporate.