Cooling towers and spray ponds — effect of ambient wet-bulb. How does available cooling capacity change as the ambient air wet-bulb temperature decreases?

Introduction / Context:
Evaporative devices such as cooling towers and spray ponds reject heat by sensible and latent transfer to ambient air. Their ultimate sink temperature is limited by the air wet-bulb temperature. Understanding this dependency is critical for plant performance predictions and seasonal capacity planning.

Given Data / Assumptions:

• Counterflow or crossflow tower rejecting heat to ambient air.
• Approach is defined relative to the air wet-bulb temperature.
• Range and water flow kept comparable.

Concept / Approach:
A lower ambient wet-bulb temperature increases the temperature driving force for evaporative cooling at a fixed water inlet temperature. This yields greater potential heat rejection and allows colder outlet water or, alternately, greater load at the same outlet setpoint.

Step-by-Step Solution:
Define approach = T_cold,out − T_wb,ambient.For a given tower, a lower T_wb increases approach margin for the same cold-water setpoint or permits a lower cold-water temperature.Therefore, capacity (maximum heat that can be rejected) increases as wet-bulb decreases.

Verification / Alternative check:
Seasonal performance curves for towers show higher tons of refrigeration available in winter (lower T_wb) versus summer (higher T_wb), confirming the trend.

Why Other Options Are Wrong:
(b) contradicts evaporative cooling physics. (c) ignores the dominant role of wet-bulb. (d) and (e) add behaviour not observed in properly operating towers.

Common Pitfalls:
Confusing wet-bulb with dry-bulb; for evaporative devices, wet-bulb sets the practical lower limit, not dry-bulb.

Final Answer:
increases