Psychrometrics — During sensible cooling of moist air (no condensation), which property(ies) remain constant at fixed total pressure?

Introduction:
Sensible cooling means the temperature of moist air is reduced without changing its moisture content (no dehumidification). Recognizing which properties remain constant under this condition is fundamental to psychrometric chart work and HVAC calculations.

Given Data / Assumptions:

• No condensation occurs (final dry-bulb temperature remains above the dew point).
• Total pressure remains constant (e.g., 1 atm).
• Air behaves as an ideal gas mixture; water vapor content is unchanged.

Concept / Approach:
The specific humidity (humidity ratio) w is proportional to the vapor partial pressure p_v via w = 0.622 p_v / (P − p_v). If w is constant and the total pressure P is constant, p_v must also remain constant. Meanwhile, relative humidity increases as temperature falls because the saturation vapor pressure decreases with temperature, narrowing the gap between p_v and p_sat(T).

Step-by-Step Solution:
Impose constraint: no phase change → water mass fraction constant → w constant.At fixed P, solve w relation → p_v remains constant.Conclude: both w and p_v are unchanged; only relative humidity changes.

Verification / Alternative check:
Plotting a sensible-cooling line on a psychrometric chart progresses horizontally to the left (constant humidity ratio), intersecting higher relative-humidity curves as temperature decreases.

Why Other Options Are Wrong:

• (a) or (b) alone is incomplete; both remain constant.
• (d) contradicts the definition of sensible cooling.
• (e) Wet-bulb temperature also decreases during sensible cooling without moisture removal.

Common Pitfalls:
Confusing relative humidity (which rises) with specific humidity; assuming p_v changes because temperature changes, forgetting composition is unchanged.

Final Answer:
Both (a) and (b)