In HVAC psychrometrics, what happens to the specific humidity (humidity ratio, kg of water vapour per kg of dry air) during a combined heating and humidification process of air?

Introduction / Context:
In air-conditioning processes, “specific humidity” (also called humidity ratio) indicates the mass of water vapour per unit mass of dry air. Understanding how it changes during common processes such as heating, cooling, humidification, and dehumidification is crucial for load calculations and correct coil selection.

Given Data / Assumptions:

• The process under consideration is heating combined with humidification.
• Air is contacted with a moisture source (steam injection or water spray) while sensible heat is added.
• Steady flow with negligible heat loss to surroundings outside the intended process.

Concept / Approach:
Heating changes the dry-bulb temperature (sensible effect). Humidification adds moisture to the air (latent effect), which increases the humidity ratio. On a psychrometric chart (for a fixed barometric pressure), humidification moves the state upward (higher humidity ratio), while heating moves it to the right (higher dry-bulb temperature). A combined heating-and-humidification step therefore tends to move diagonally up-right.

Step-by-Step Solution:

Verification / Alternative check:
Trace the process on a psychrometric chart: starting state 1 moves horizontally right to state 2 (heating at nearly constant w), then moves vertically or diagonally up-right to state 3 (moisture addition). The vertical component confirms an increase in w.

Why Other Options Are Wrong:

Remains constant: only true for pure sensible heating or cooling with no moisture addition.Decreases: opposite of what humidification does.First decreases then increases: no inherent mechanism in standard heating + humidification for an initial decrease in w.Becomes zero: impossible unless all moisture is removed, which contradicts humidification.

Common Pitfalls:
Confusing relative humidity with humidity ratio. Relative humidity may rise or fall with heating depending on moisture addition, but humidity ratio increases whenever water vapour is added.

Final Answer:

Increases