Unformatted text preview: plant delivers less net work
per unit mass of the working fluid.
EXAMPLE 7–12 Compressing a Substance in the Liquid vs. Gas Phases Determine the compressor work input required to compress steam isentropically
from 100 kPa to 1 MPa, assuming that the steam exists as (a) saturated liquid
and (b) saturated vapor at the inlet state. SOLUTION We take the turbine and then the pump as the system. Both are
control volumes since mass crosses the boundary. Sketches of the pump and
the turbine together with the T-s diagram are given in Fig. 7–43.
Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible. 3 The process is given to be isentropic.
Analysis (a) In this case, steam is a saturated liquid initially, and its specific
υ1 υf @ 100 kPa 0.001043 m3/kg (Table A–5) which remains essentially constant during the process. Thus,
2 wrev, in 1 υ dP υ1(P2 P1) (0.001043 m3/kg)[(1000 100) kPa] 1 kJ
1 kPa · m3 0.94 kJ/kg
(b) This time, steam is a saturated vapor initially and remains a vapor during the
entire compression process. Since the specific volume of a gas changes considerably during a compression process, we need to k...
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