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

238 | Thermodynamics

Analysis We take the turbineas the system. This is a control volumesince

mass crosses the system boundary during the process. We observe that there

is only one inlet and one exit and thus m. 1 m. 2 m.. Also, work is done by

the system. The inlet and exit velocities and elevations are given, and thus

the kinetic and potential energies are to be considered.

(a) At the inlet, steam is in a superheated vapor state, and its enthalpy is

At the turbine exit, we obviously have a saturated liquid–vapor mixture at

15-kPa pressure. The enthalpy at this state is

Then

(b) The energy balance for this steady-flow system can be expressed in the

rate form as

Rate of net energy transfer Rate of change in internal, kinetic,

by heat, work, and mass potential, etc., energies

Dividing by the mass flow rate m

.

and substituting, the work done by the turbine

per unit mass of the steam is determined to be

(c) The required mass flow rate for a 5-MW power output is

Discussion Two observations can be made from these results. First, the

change in potential energy is insignificant in comparison to the changes in

enthalpy and kinetic energy. This is typical for most engineering devices.

Second, as a result of low pressure and thus high specific volume, the steam

velocity at the turbine exit can be very high. Yet the change in kinetic energy

is a small fraction of the change in enthalpy (less than 2 percent in our

case) and is therefore often neglected.

m# 

W

#

out

wout



5000 kJ/s

872.48 kJ/kg

5.73 kg/s


[
887.3914.95
0.04] kJ/kg872.48 kJ/kg

wout
c1h 2 
h 12 

V 22 
V 12

2

g 1 z 2 
z 1 2d
 1 ¢h¢ke¢pe 2

m

#

ah 1 

V 12

2

gz 1 bW

#

outm

#

ah 2 

V 22

2

gz 2 b¬¬ 1 since Q

#

 02

E

#

inE

#

out

E

#

in
E

#

out^ ^ dEsystem>dt^ ^0

¢peg 1 z 2 
z 12  1 9.81 m>s^22316 
 102 m4a

1 kJ>kg

1000 m^2 >s^2

b0.04 kJ/kg

¢ke

V^22 
V^21

2



1 180 m>s 22 
 1 50 m>s 22

2

¬a

1 kJ>kg

1000 m^2 >s^2

b14.95 kJ/kg

¢hh 2 
h 1  1 2361.01
3248.4 2 kJ>kg887.39 kJ/kg

h 2 hfx 2 hfg[225.94(0.9)(2372.3)] kJ/kg2361.01 kJ/kg

P 1 2 MPa

T 1  400    C

f h 1 3248.4 kJ/kg (Table A–6)

0 (steady)

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