Question

When an air cooler is used in a closed room, air is pulled through the cooler pads. Which thermodynamic process does the air approximately undergo?

• A. Isothermal
• B. Adiabatic
• C. Isochoric
• D. Isoenthalpic
• E. Isentropic

Hint:

In psychrometrics (the study of air-water vapor mixtures), the process of evaporative cooling is represented by a straight line along a constant wet-bulb temperature line on a psychrometric chart.
Lines of constant wet-bulb temperature are also very nearly lines of constant enthalpy. Thus, adiabatic saturation is considered an isoenthalpic process.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the thermodynamic process that approximates the change in air properties as it passes through the wet pads of an air cooler.

Step 2: Key Formula or Approach:
The process is evaporative cooling. In this process, the energy (latent heat) required to evaporate the water is taken from the sensible heat of the air. We need to analyze how thermodynamic properties like temperature, enthalpy, and entropy change.
The first law of thermodynamics for an open system (steady flow) is (Delta h + Delta KE + Delta PE = q - w).
For the air passing through the pads, there is negligible heat transfer with the surroundings ((q approx 0)), no work done ((w=0)), and negligible change in kinetic and potential energy. The process is thus approximately adiabatic ((q approx 0)).
However, this process is specifically called "adiabatic saturation," where the enthalpy of the air-vapor mixture remains constant.

Step 3: Detailed Explanation:
When air passes over the wet pads, two things happen:
1. The air's temperature (dry-bulb temperature) decreases as it gives up sensible heat to evaporate the water.
2. The air's humidity increases as it absorbs the water vapor. The energy used for evaporation becomes latent heat in the water vapor that is now part of the air.
This exchange—a decrease in sensible heat balanced by an increase in latent heat—results in the total enthalpy of the air remaining nearly constant. This is the definition of an isoenthalpic process ((Delta h = 0)).
Let's look at the other options:

• Isothermal: Temperature is not constant; it decreases.

• Adiabatic: This is a good approximation as there's little heat exchange with the outside. However, isoenthalpic is more specific and accurate for this particular process. Adiabatic saturation is an isoenthalpic process.

• Isochoric: Volume is not constant as the air is flowing and its density changes.

• Isentropic: Entropy is not constant. The evaporation of water is an irreversible process, which leads to an increase in the total entropy of the system.
Therefore, the most accurate description of the process is isoenthalpic.

Step 4: Final Answer:
The process of air passing through the pads of an evaporative cooler is best described as an isoenthalpic process, where the total enthalpy of the air remains constant.