Question

Refrigerant-134a at 800 kPa and 25°C is throttled to a temperature of -20°C. Determine the pressure and the internal energy of the refrigerant at the final state. 

Answer 1

Refrigerant-134a is throttled by a valve. The pressure and internal energy after expansion are to be determined.

Assumptions:

1. This is a steady-flow process since there is no change with time. 
2. Kinetic and potential energy changes are negligible.
3. Heat transfer to or from the fluid is negligible.
4. There are no work interactions involved.

Properties: The inlet enthalpy of R-134a is, from the refrigerant tables (Tables A-11 through 13)

Analysis: There is only one inlet and one exit, and thus \(\overset • m_1 = \overset • m_2 = \overset • m\). We take the throttling valve as the system, which is a control volume since mass crosses the boundary. The energy balance for this steady- flow system can be expressed in the rate form as

Obviously h_f < h₂ < h_g, thus the refrigerant exists as a saturated mixture at the exit state, and thus