Question

Air blown out by an air cooler in a closed room approximately undergoes which process?

• A. Isothermal
• B. Adiabatic
• C. Isoenthalpic
• D. Isobaric
• E. None

Hint:

This concept is frequently tested. Associate evaporative coolers with the term "isoenthalpic" or "constant enthalpy" process. On a psychrometric chart, this process follows a line of constant wet-bulb temperature, which is nearly parallel to lines of constant enthalpy.

Answer 1

The Correct Option is C

Step 1: Understanding the Question: This question asks to identify the thermodynamic process for air as it is cooled by an evaporative air cooler. This is identical in concept to Q-10.

Step 2: Detailed Explanation: The process occurring in an air cooler is called evaporative cooling or adiabatic saturation.
• Hot, dry air is passed through pads saturated with liquid water.
• The water evaporates. The energy needed for this phase change (latent heat of vaporization) is taken from the air itself.
• This causes the air's sensible heat to decrease, resulting in a drop in its dry-bulb temperature.
• The water vapor is added to the air, increasing its humidity (and its latent heat content).
• The decrease in the air's sensible heat is balanced by the increase in its latent heat.
• This exchange ensures that the total heat content, or enthalpy, of the air-vapor mixture remains approximately constant. A process that occurs at constant enthalpy is called an isoenthalpic process. The other options are not the best description:
• Isothermal: False. The temperature of the air drops.
• Adiabatic: True, in the sense that there is no external heat added or removed from the air-water system. The process is one of internal energy conversion. However, isoenthalpic is a more specific and descriptive term for this particular type of adiabatic process.
• Isobaric: False. While the pressure change across the cooler pads is small, the primary characteristic is not constant pressure but constant enthalpy.

Step 3: Final Answer: The thermodynamic process that air undergoes in an air cooler is most accurately described as isoenthalpic.