Question

In a throttling process, which one of the following parameters remain constant?

• A. Temperature
• B. Pressure
• C. Enthalpy
• D. Entropy

Hint:

Remember: Throttling starts with T, but the parameter that stays the same starts with H (Enthalpy). Even though the pressure drops and the gas expands, the total "heat content" (Enthalpy) of the fluid remains unchanged.

Answer 1

The Correct Option is C

Throttling is a process where a fluid flows through a restricted passage, such as a partially opened valve, an orifice, or a porous plug. 1. Characteristics of Throttling:

• It is a highly irreversible process.

• There is a significant drop in pressure ($P_2 \lt P_1$).

• It occurs so rapidly or in such a small space that heat transfer is negligible ($Q = 0$).

• No useful work is done by or on the system ($W = 0$).

2. Applying the Steady Flow Energy Equation (SFEE): The SFEE for a unit mass is: $$h_1 + \frac{V_1^2}{2} + gZ_1 + Q = h_2 + \frac{V_2^2}{2} + gZ_2 + W$$ In a typical throttling process, changes in kinetic energy ($\Delta KE$) and potential energy ($\Delta PE$) are negligible. Combined with $Q=0$ and $W=0$, the equation simplifies to: $$h_1 = h_2$$

3. Conclusion: The initial enthalpy is equal to the final enthalpy. Therefore, a throttling process is also known as an

isenthalpic process. This principle is used extensively in refrigeration cycles where a refrigerant passes through an expansion valve.