Cooling and dehumidification on the psychrometric chart: During a typical cooling-and-dehumidifying coil process, what happens to the dry-bulb temperature of the air?

Introduction / Context:
HVAC cooling coils frequently achieve both sensible cooling and moisture removal. Understanding how dry-bulb temperature (DBT) changes during such a process is essential for coil selection and load calculations.

Given Data / Assumptions:

• Air passes over a cooling coil colder than its dew point.
• Process pressure is approximately constant (near atmospheric).
• Condensate is removed, thus humidity ratio decreases.

Concept / Approach:
A cooling-and-dehumidification process on the psychrometric chart initially moves diagonally down-left (reducing DBT and humidity ratio) toward the saturation curve. Once on the saturation curve, further cooling continues along the curve to the apparatus dew point. The hallmark of the process is a drop in dry-bulb temperature due to heat extraction.

Step-by-Step Solution:

Verification / Alternative check:
Energy balance shows that coil heat extraction equals sensible plus latent loads; a nonzero sensible component necessarily lowers DBT when the coil is colder than the air.

Why Other Options Are Wrong:

• Remains constant: That describes adiabatic humidification or ideal evaporative cooling lines, not a cold coil below dew point.
• Increases: Opposite of cooling.
• Becomes equal to wet-bulb: Not generally; equality occurs only at saturation when DBT equals WBT equals dew point at a specific state.

Common Pitfalls:
Assuming dehumidification can occur without cooling; with cooling coils, condensation follows reduction below dew point and DBT falls.

Final Answer:
Decreases