Psychrometrics check: “Dew-point temperature is the temperature at which moisture in the air just begins to condense.” Evaluate the statement.

Introduction / Context:
Dew-point temperature (DPT) is a key property in air-conditioning, drying, and condensation control. It defines when condensation will occur on a surface and guides coil surface temperature selection in cooling/dehumidification.

Given Data / Assumptions:

• Fixed total pressure (near atmospheric) during consideration.
• Air–vapour mixture with partial pressure of water vapour p_v.
• Pure water condensation assumption.

Concept / Approach:
The dew-point temperature is the saturation temperature corresponding to the partial pressure of water vapour in the mixture. When air is cooled at constant moisture content (no removal yet), reaching DPT means the air becomes saturated and any further cooling causes condensation.

Step-by-Step Explanation:

Verification / Alternative check:
On a psychrometric chart, move horizontally left (constant humidity ratio) to intersect the saturation curve; the temperature at that point is the dew-point.

Why Other Options Are Wrong:

• “Incorrect” conflicts with the thermodynamic definition.
• “Only at 100% RH” confuses condition with definition; DPT is defined for any state and equals DBT only at 100% RH.
• Below 0°C or pressure specifics are not necessary to define the concept, though they influence numeric value.

Common Pitfalls:
Assuming dew-point equals wet-bulb; they coincide only at saturation. Surface temperatures must be kept above DPT to avoid condensation.

Final Answer:
Correct