6-48
6-69 Steam is throttled from a specified pressure to a specified state. The quality at the inlet is 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.
Analysis There is only one inlet and one exit, and thus m 1 m 2 m . We take the throttling valve as the
system, which is a control volume since mass crosses the boundary. The energy balance for this steadyflow system can be expressed in the rate form as
E in E out
E in
m h1
h1
since Q # W
'E system Ê0 (steady)
0
E out
m h2
Throttling valve
Steam
2 MPa
h2
100 kPa
120qC
'ke # pe # 0 .
The enthalpy of steam at the exit is (Table A-6),
P2
T2
100 kPa ½
¾ h2
120qC ¿
2716.1 kJ/kg
The quality of the steam at the inlet is (Table A-5)
P2
2000 kPa
h1
h2
½
¾ x1
2716.1 kJ/kg ¿
h2 h f
h fg
2716.1 908.47
1889.8
0.957
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