Humid air enthalpy trends: For a vapour–gas (e.g., water vapour–air) mixture, the total enthalpy can be increased by which change(s)?

Introduction / Context:
On a psychrometric chart and in energy balances, the enthalpy of moist air depends on both dry-bulb temperature and moisture content (humidity ratio). Understanding how each variable influences enthalpy is essential for HVAC calculations, dryers, cooling towers, and gas–vapour contacting equipment design.

Given Data / Assumptions:

• Mixture of a non-condensable gas (air) and a condensable vapour (water).
• Enthalpy expressed per unit mass of dry air: h ≈ h_air(T) + ω * h_vapour(T), where ω is humidity ratio.
• Constant pressure near atmospheric; ideal-mixture approximations.

Concept / Approach:
The mixture enthalpy increases with temperature due to sensible heating of both dry air and vapour, and increases with humidity because additional vapour carries substantial latent and sensible enthalpy. Therefore, raising temperature, raising humidity, or both will increase total enthalpy. This is visible on the psychrometric chart where lines of constant enthalpy slope upward with temperature and humidity ratio.

Step-by-Step Solution:

Verification / Alternative check:
Numerical examples using standard psychrometric formulas show monotonic enthalpy increase with either variable; tabulated charts confirm the same trend.

Why Other Options Are Wrong:

• Options (a), (b), (c) are individually correct but incomplete; only “All of the above” captures every way to increase enthalpy.
• “None of the above” contradicts thermodynamic definitions and empirical charts.

Common Pitfalls:
Assuming only temperature matters; neglecting the large latent enthalpy carried by water vapour; mixing per-mass-of-dry-air and per-mass-of-mixture bases.

Final Answer:
All of the above