During humidification of air by steam injection at constant pressure (typical HVAC process), how does the dry-bulb temperature (DBT) of the air change?

Introduction / Context:
Humidification processes in air-conditioning can be achieved by steam injection or by adiabatic water spray (evaporative cooling). The direction of change of dry-bulb temperature (DBT) depends on the method. This question targets steam humidification at essentially constant pressure, widely used for precise humidity control.

Given Data / Assumptions:

• Process: direct steam injection into the airstream.
• Pressure is approximately constant (duct/room pressure).
• No significant heat loss to surroundings during injection.

Concept / Approach:
When saturated steam is injected, its latent and sensible energy are transferred to the air–vapour mixture. Both humidity ratio and mixture enthalpy rise. On the psychrometric chart, steam injection moves the state up and to the right, increasing moisture content and DBT simultaneously (unless specifically arranged for adiabatic saturation with water only).

Step-by-Step Solution:

Verification / Alternative check:
Compare to evaporative (adiabatic) humidification where adding liquid water causes DBT to drop toward wet-bulb; the opposite trend confirms that steam injection increases DBT.

Why Other Options Are Wrong:

• Remains constant: true only for specific adiabatic processes with water spray, not steam injection.
• Decreases: that describes evaporative cooling, not steam humidification.
• Equality to wet-bulb/dew-point: not a general outcome of steam injection.

Common Pitfalls:
Mixing up steam humidification with evaporative cooling; always identify the humidification method before predicting DBT changes.

Final Answer:
Increases