Heating and humidification of air: The final relative humidity of the leaving air (after adding heat and moisture) is generally:

Introduction / Context:
Heating and humidification is common in winter air-conditioning, where dry cold air is warmed and moisture is added. Predicting the final relative humidity (RH) requires understanding of how both dry-bulb temperature and absolute moisture content change simultaneously.

Given Data / Assumptions:

• Humidifier adds water vapour (steam or evaporative), while a heater raises dry-bulb temperature.
• Process may be simultaneous (e.g., spray washer with heating) or sequential (heater then humidifier).
• Pressure remains near constant.

Concept / Approach:
RH is defined as p_w / p_ws(T). Adding moisture increases p_w; heating increases p_ws(T). Depending on the relative magnitudes of these changes, RH can rise or fall. On a psychrometric chart, heating shifts right; humidification shifts up; combined, the path can approach or recede from the saturation curve depending on rates of heat and moisture addition.

Step-by-Step Solution:

Verification / Alternative check:
Plotting initial and final states on the chart confirms that different heater/humidifier capacities produce different RH results for the same inlet state.

Why Other Options Are Wrong:

• Always higher / always lower: Over-generalizations; outcome depends on process intensity.
• Always 100%: Only occurs if the process reaches saturation, which is not guaranteed.
• Always unchanged: RH is sensitive to both T and moisture changes.

Common Pitfalls:
Assuming RH tracks moisture addition alone; ignoring the strong influence of temperature on saturation pressure.

Final Answer:
can be lower or higher than that of the entering air