Psychrometrics: During a dehumidification process (moisture removal from air at otherwise similar conditions), how does the specific humidity (humidity ratio) change?

Introduction / Context:Specific humidity (also called humidity ratio, w) is the mass of water vapor per unit mass of dry air. Dehumidification is a core HVAC process where moisture is removed, typically by cooling air below its dew point so that water condenses out.

Given Data / Assumptions:

• Air stream is contacted with a cold surface (cooling coil) or a desiccant to remove moisture.
• No external steam or water injection during the dehumidification step.
• Pressure is near atmospheric and approximately constant.

Concept / Approach:On the psychrometric chart, dehumidification at roughly constant pressure moves the state leftward (lower humidity ratio) and, for cooling-coil processes, also downward in dry-bulb temperature until the apparatus dew point is reached. Removing vapor mass per unit dry air necessarily reduces the specific humidity.

Step-by-Step Solution:

Verification / Alternative check:Mass balance on water vapor: m_dot_v,out = m_dot_v,in − m_dot_condensed. With constant dry-air flow, w_out = w_in − (m_dot_condensed / m_dot_dry_air) < w_in.

Why Other Options Are Wrong:

• Remains constant: Only true for sensible cooling or heating without condensation or moisture addition.
• Increases: Would require humidification, not dehumidification.
• Indeterminate: The sign is definite given moisture removal.

Common Pitfalls:Confusing sensible-only cooling with cooling plus dehumidification; condensation changes humidity ratio while sensible processes do not.

Final Answer:Decreases