Question

In the expansion valve of a refrigerator the following property remains constant
1. Entropy
2. Enthalpy
3. Internal energy
4. product Pv

Answer 1

Correct Answer - Option 2 : Enthalpy

Concept:

Expansion Valve:

The expansion valve is an adjustable valve that controls the refrigerant flow through the system and the pressure difference between the two sides. The inlet side of the valve is at high pressure and the refrigerant is at room temperature liquid at this location.

In the expansion process, the pressure drops rapidly to the low-pressure value and the refrigerant is superheated in this new pressure condition.

The refrigerant material, then undergoes a partial change of phase, using its internal energy to provide the heat of transformation from the liquid to vapor phase.

The reduction in internal energy is associated with a decrease in temperature of the two-phase refrigerant material exiting from the expansion valve.

Explanation:

Expansion valve follows the throttling process so for throttling process, specific enthalpy of the process will remain constant.

⇒ h₁ = h₂       … (1)

Since, h = u + pv

Where, u = Specific internal energy, pv = Specific flow energy

So, using equation (1)

⇒ u₁ + p₁v₁ = u₂ + p₂v₂

∵ Specific flow energy at the entry and the exit of the expansion valve will not be the same,

⇒ u₁ ≠ u₂

⇒ The internal energy of the system will not be the same.

During the mass transfer, entropy changes so the entropy of the system will not remain constant.