Question

The wet bulb lines are always .............. with constant enthalpy lines.

• A. Coincide
• B. Non coincide
• C. Inclined
• D. Declined

Hint:

This "coincidence" only holds true for air-water. For other systems (like drying with organic solvents), these lines would diverge significantly because the Lewis Number is not 1.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question relates to psychrometrics, specifically the geometric relationship between wet bulb temperature lines and constant enthalpy lines on a psychrometric chart for air-water systems.

Step 2: Detailed Explanation:

• The psychrometric chart is a graphical representation of the physical and thermodynamic properties of moist air. Key properties include dry bulb temperature, wet bulb temperature, humidity ratio, and enthalpy.

• Wet bulb temperature (\(T_{wb}\)) is measured using a thermometer with its bulb wrapped in a water-soaked wick. It represents the temperature reached through adiabatic evaporation.

• Enthalpy (\(H\)) represents the total heat content of the moist air.

• In theory, for an adiabatic saturation process, the cooling effect due to evaporation exactly matches the sensible heat lost by the air.

• For the specific case of the air-water vapor system at moderate temperatures and pressures, the Lewis Number (\(Le = Sc/Pr\)) is approximately equal to 1. This means that mass diffusivity and thermal diffusivity are nearly equal.

• Due to this "Lewis equality," the lines of constant wet bulb temperature and the lines of constant enthalpy on a standard psychrometric chart are nearly parallel and practically coincide.

• While they may deviate slightly in non-standard systems or other gas-liquid combinations, in the context of food drying and HVAC (air-water), they are traditionally treated as coinciding.

Step 3: Final Answer:
On a psychrometric chart for air-water, wet bulb lines and constant enthalpy lines essentially coincide.