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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 PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and educators for course preparation. If you are a student using this Manual, you are using it without permission.