Cooling and dehumidification requirement: To cool and dehumidify a moist air stream across a coil, the coil surface temperature must be set __________ the incoming air’s dew-point temperature.

Introduction / Context:
Dehumidification requires condensing moisture from air. The condition for condensation is governed by the dew-point temperature of the incoming stream and the coil surface (or apparatus dew-point).

Given Data / Assumptions:

• Uniform coil surface temperature.
• Well-mixed outlet air leaving the coil.
• Steady operation near atmospheric pressure.

Concept / Approach:
Condensation begins when a surface is at or below the dew-point temperature of the adjacent moist air. For continuous cooling and dehumidification, the coil must be colder than the incoming air’s dew-point so that water vapour condenses on the fins/tubes.

Step-by-Step Solution:

Verification / Alternative check:
On the chart, cooling and dehumidification follows a line trending down and left toward the saturation curve, which only happens when the coil is below T_dp.

Why Other Options Are Wrong:
Temperatures between DBT and WBT may not be low enough to reach dew-point; adiabatic saturation temperature is unrelated to coil surface requirement.

Common Pitfalls:
Assuming any cooling causes dehumidification; unless T_s ≤ T_dp, no condensation occurs.

Final Answer:
lower than the incoming dew-point temperature