Cooling tower performance: To what theoretical limit can circulating water be cooled in a standard evaporative cooling tower operating with ambient air?

Introduction / Context:
Cooling towers reject heat by partial evaporation of water into an airstream. Understanding the limiting cold-water temperature is essential for plant performance estimates and chiller selection.

Given Data / Assumptions:

• Evaporative cooling tower operating at near-atmospheric pressure.
• Air–water contact allows heat and mass transfer (sensible and latent).
• Negligible heat sources other than process heat load.

Concept / Approach:
The fundamental limit is the entering air wet-bulb temperature (WBT). In an ideal tower with infinite size and perfect contact, the cooled water approaches the WBT asymptotically; it cannot be cooled below the WBT because equilibrium between water temperature and the adiabatic saturation temperature of the air defines the driving potential.

Step-by-Step Solution:

Verification / Alternative check:
Psychrometric analysis: adiabatic saturation process aligns with wet-bulb lines; at equilibrium, water temperature equals the air’s adiabatic saturation (wet-bulb) temperature.

Why Other Options Are Wrong:

• Dew point: lower than WBT only at very dry conditions; towers cannot reach dew point by evaporative contact.
• Dry-bulb: generally higher than WBT; not the limiting temperature in evaporative cooling.
• “Ambient air temperature” is ambiguous and often equals dry bulb; it is not the limit.

Common Pitfalls:
Confusing dew point with wet bulb; dew point governs condensation processes, not evaporative cooling limits.

Final Answer:
Wet-bulb temperature of air