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.
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Abetween the incoming dry-bulb and wet-bulb temperatures
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Bbetween the incoming wet-bulb and dew-point temperatures
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Clower than the incoming dew-point temperature
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Dequal to the adiabatic saturation temperature
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Ehigher than the incoming dry-bulb temperature
Answer
Correct Answer: lower than the incoming dew-point temperature
Explanation
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:
Find the incoming dew-point T_dp from the psychrometric state.Set coil surface temperature T_s < T_dp to initiate condensation.Result: both sensible cooling and latent removal occur.If T_s > T_dp, only sensible cooling occurs and humidity ratio remains unchanged.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