Spray washer psychrometrics: If the spray water temperature is higher than the entering air dry-bulb temperature, how will the air state change across the spray washing chamber?

Introduction / Context:
Air–water contact devices (spray washers, air washers) can heat/cool and humidify/dehumidify air depending on the relative temperatures and vapour pressures. Knowing the direction of heat and mass transfer is crucial for process control.

Given Data / Assumptions:

• Spray water temperature > entering air dry-bulb temperature.
• Sufficient contact to exchange sensible heat and moisture.
• Negligible heat losses to surroundings for conceptual analysis.

Concept / Approach:
When water is warmer than air, heat flows from water to air, increasing the air's sensible temperature. Simultaneously, provided the water has a higher vapour pressure at its temperature than the partial pressure of vapour in air, evaporation occurs, adding moisture to air (humidification). Thus, the air is heated and humidified.

Step-by-Step Solution:

Verification / Alternative check:
Plot the process on a psychrometric chart from the inlet state toward the saturation curve at a higher enthalpy. The trajectory shows both DBT and humidity ratio increasing when water is hotter.

Why Other Options Are Wrong:

Heated and dehumidified: contradicts mass transfer direction; warm water adds, not removes, moisture.Cooled options: violate the given temperature relation (water hotter than air).Unchanged: ignores clear temperature and vapour pressure gradients.

Common Pitfalls:
Assuming every air–water contact is adiabatic saturation; the result depends on relative temperatures and flow rates, not a universal rule.

Final Answer:

heated and humidified