Air-conditioning process selection on the psychrometric chart Outside air at 45 °C dry-bulb and 70% RH must be conditioned to a room state of 25 °C dry-bulb and 50% RH. Which practical treatment best achieves this target?

Introduction / Context:
Determining the correct HVAC air-treatment sequence is a core psychrometrics skill. We compare initial hot-humid outdoor air with the desired cooler, moderately humid room condition.

Given Data / Assumptions:

• Outdoor state: T_db = 45 °C, RH = 70% (high moisture and temperature).
• Room state: T_db = 25 °C, RH = 50%.
• Standard atmospheric pressure and well-mixed air streams.

Concept / Approach:
Moving from a hot, very humid point to a cooler, less humid point generally requires cooling air below its dew point, condensing moisture, then reheating if needed to the final dry-bulb. That combined action is called cooling and dehumidification (often via a cooling coil with surface temperature below dew point).

Step-by-Step Solution:

Verification / Alternative check:
On the chart, a path leftward and downward (cooling and dehumidification) reaches a state near the target; simple dehumidification without cooling is not physically achievable for air without a desiccant process.

Why Other Options Are Wrong:

Dehumidification only: air temperature and humidity ratio cannot both be reduced without cooling below dew point or chemical drying.Cooling and humidification: humidity ratio would rise, not desired.Dehumidification then sensible heating: final 25 °C requires prior cooling below dew point; however, the net description is still “cooling and dehumidification.”Adiabatic humidification: adds moisture, opposite of need.

Common Pitfalls:
Stopping at sensible cooling only; this raises RH and can approach saturation, but does not lower humidity ratio to the required value.

Final Answer:

cooling and dehumidification