Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

In gas power plants, the working fluid (typically air) is compressed in the

gas phase, and a considerable portion of the work output of the turbine is

consumed by the compressor. As a result, a gas power plant delivers less net

work per unit mass of the working fluid.

364 | Thermodynamics

EXAMPLE 7–12 Compressing a Substance in the Liquid versus

Gas Phases

Determine the compressor work input required to compress steam isentropi-

cally from 100 kPa to 1 MPa, assuming that the steam exists as (a) satu-

rated liquid and (b) saturated vapor at the inlet state.

Solution Steam is to be compressed from a given pressure to a specified

pressure isentropically. The work input is to be determined for the cases of

steam being a saturated liquid and saturated vapor at the inlet.

Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential

energy changes are negligible. 3 The process is given to be isentropic.

Analysis We take first the turbine and then the pump as the system.Both

are control volumessince mass crosses the boundary. Sketches of the pump

and the turbine together with the T- sdiagram are given in Fig. 7–43.

(a) In this case, steam is a saturated liquid initially, and its specific volume is

which remains essentially constant during the process. Thus,

(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 know how vvaries

with Pto perform the integration in Eq. 7–53. This relation, in general, is not

readily available. But for an isentropic process, it is easily obtained from the

0.94 kJ>kg

 1 0.001043 m^3 >kg 2311000  1002 kPa 4 ¬a

1 kJ

1 kPa#m^3

b

wrev,in

2

1

v¬dPv 11 P 2 P 12

v 1 vf @ 100 kPa0.001043 m^3 >kg¬¬ 1 Table A–5 2

T

s

1

P 2 = 1 MPa 2

P 1 = 100 kPa P 1 = 100 kPa

P 2 = 1 MPa

(a) Compressing

a liquid

(b) Compressing

a vapor

PUMP

1

2

1 MPa

(a) 100 kPa

COMPRESSOR (b)

FIGURE 7–43

Schematic and T-sdiagram for
Example 7–12.