During cooling and dehumidification across a cooling coil (surface below the incoming dew-point), the specific humidity (humidity ratio) of the leaving air will:

Introduction / Context:Cooling and dehumidification is the standard process in comfort air-conditioning. When the coil surface temperature lies below the incoming air’s dew-point, moisture condenses on the coil fins, altering the air’s moisture content. Knowing how the specific humidity changes is essential for load calculations and condensate management.

Given Data / Assumptions:

• Coil surface temperature lower than the entering air’s dew-point temperature.
• Steady flow, well-mixed outlet air, and negligible pressure change.
• Condensate drains away; no re-evaporation in the leaving airstream.

Concept / Approach:Specific humidity (also called humidity ratio w) is the mass of water vapour per unit mass of dry air. When condensation occurs, liquid water is removed from the airstream, reducing the mass of vapour per unit dry air, so w decreases. This differs from sensible-only cooling, where w remains essentially constant.

Step-by-Step Solution:

Verification / Alternative check:On the psychrometric chart, the state path slopes downwards and left toward the saturation curve, clearly showing a reduction in w.

Why Other Options Are Wrong:

• Remain constant: That would be sensible cooling only (no condensation).
• Increase: Moisture is removed, not added.
• First increase then decrease: Not for a standard steady cooling/dehumidifying coil.
• Oscillate with coil load: Process description is steady; oscillation is irrelevant here.

Common Pitfalls:Confusing relative humidity behaviour with humidity ratio; RH can rise even during sensible cooling, but w falls only when condensation occurs.

Final Answer:decrease