Dehumidification trend: In a cooling and dehumidification process across a coil, how does the relative humidity of the air typically change?

Introduction / Context:
Air-handling coils often cool air below its dew point so that moisture condenses on the fins. Understanding how relative humidity (RH) changes through this process is essential for comfort control and latent load calculations.

Given Data / Assumptions:

• Coil surface temperature is below the air dew point.
• Air is cooled sensibly and then along the saturation curve as condensation begins.
• Process is steady with negligible heat leaks.

Concept / Approach:
As the air is cooled at nearly constant humidity ratio until it reaches dew point, RH rises. Once dew point is met, further cooling removes moisture; the air state moves along a path close to the saturation curve, where RH approaches 100%. Therefore, dehumidification commonly results in an overall increase in RH, despite a decrease in humidity ratio.

Step-by-Step Solution:

Verification / Alternative check:
Plot on a psychrometric chart: the process line trends toward and often along the saturation curve, confirming higher RH at the outlet.

Why Other Options Are Wrong:

• “Remains constant” applies only to special controlled reheat or desiccant cases.
• “Decreases” contradicts the basic cooling-to-saturation behavior.
• “Indeterminate” is incorrect with stated conditions.

Common Pitfalls:
Confusing relative humidity (a ratio) with humidity ratio; RH can increase even while total moisture per kg dry air decreases.

Final Answer:
Increases