Question

A well-insulated valve is used to throttle steam from 8 MPa and 500°C to 6 MPa. Determine the final temperature of the steam.

Answer 1

Steam is throttled by a well-insulated valve. The temperature drop of the steam after the expansion 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.

Properties: The inlet enthalpy of steam is (Tables A-6)

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