Question

Heat energy required to increase the temperature of unit mass of gas and its accompanying vapour by one degree at constant pressure is known as______

• A. Enthalpy
• B. Specific heat
• C. Humid heat
• D. Grosvenor humidity

Hint:

For the air-water system, the specific heats are approximately \( C_{p, \text{air}} \approx 1.005 \text{ kJ/kg}\cdot^\circ\text{C} \) and \( C_{p, \text{water}} \approx 1.88 \text{ kJ/kg}\cdot^\circ\text{C} \).
This gives the common formula for humid heat:
\[ C_s \approx 1.005 + 1.88 \cdot Y \]

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the definition of the heat energy required to increase the temperature of a unit mass of dry gas along with its accompanying vapor by one degree at constant pressure.
This is a fundamental definition in psychrometrics and humidification.

Step 2: Key Formula or Approach:
For a gas-vapor mixture (such as air-water vapor), the humid heat (\( C_s \)) is defined based on a unit mass of dry gas:
\[ C_s = C_{p,g} + Y \cdot C_{p,v} \]
where \( C_{p,g} \) is the specific heat of the dry gas, \( C_{p,v} \) is the specific heat of the vapor, and \( Y \) is the humidity (mass of vapor per unit mass of dry gas).

Step 3: Detailed Explanation:

• Humid Heat Concept: In air-conditioning and drying calculations, the dry gas mass remains constant throughout the process, while the vapor content changes.
Therefore, thermal properties are defined per unit mass of dry gas.
Humid heat represents the heat capacity of \( 1 \text{ kg} \) of dry air plus the \( Y \text{ kg} \) of water vapor accompanying it.

• Specific Heat: This is the heat capacity of a pure substance per unit mass of that substance, and does not account for the dry gas basis of a mixture.

• Enthalpy: This is a thermodynamic state function representing total heat content, rather than a rate of change with temperature.

• Grosvenor Humidity: This is another name for absolute humidity (\( Y \)), which is a mass ratio rather than a thermal property.


Step 4: Final Answer:
The thermal property defined in the question is known as the humid heat.