During a purely sensible cooling process (cooling without condensation), which of the following holds true for the moist air stream?

Introduction / Context:
Sensible cooling is a foundational psychrometric process used in HVAC coil analysis. When air is cooled without removing moisture, designers must know which properties change and which remain essentially unchanged to predict relative humidity shifts and to avoid unintended condensation.

Given Data / Assumptions:

• No moisture removal (coil surface temperature above the dew-point of entering air).
• Air pressure near atmospheric and essentially constant.
• Well-mixed air at outlet; adiabatic external surroundings aside from the cooling coil.

Concept / Approach:
In sensible cooling, the humidity ratio w (and equivalently the partial pressure of water vapour p_w) stays nearly constant because no water is added or removed. As temperature drops, saturation vapour pressure p_ws(T) decreases, so relative humidity RH = p_w / p_ws increases. Wet-bulb temperature decreases since the air’s capacity to accept moisture increases as it gets cooler relative to its moisture content.

Step-by-Step Solution:

Verification / Alternative check:
On a psychrometric chart, sensible cooling is a horizontal move to the left (constant w). Lines of constant RH slope upwards to the left, confirming RH rises. Lines of constant T_wb also slope such that T_wb falls with leftward movement at constant w.

Why Other Options Are Wrong:

• Relative humidity remains constant: False; RH increases as temperature decreases at constant w.
• Wet-bulb temperature increases: Opposite of actual behaviour; T_wb decreases.
• Specific humidity increases: No moisture is added; w remains essentially constant.
• Humidity ratio increases sharply: Again, no moisture exchange means w stays constant.

Common Pitfalls:
Thinking RH is a conserved property; forgetting that RH is a ratio involving saturation pressure, which is temperature dependent.

Final Answer:
Partial pressure of water vapour remains approximately constant