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

576 | Thermodynamics

Thus,

and

Discussion This problem was worked out in Example 10–4 for the same pres-

sure and temperature limits with reheat but without the regeneration process.

A comparison of the two results reveals that the thermal efficiency of the cycle

has increased from 45.0 to 49.2 percent as a result of regeneration.

The thermal efficiency of this cycle could also be determined from

where

Also, if we assume that the feedwater leaves the closed FWH as a satu-

rated liquid at 15 MPa (and thus at T 5 
342°C and h 5 
1610.3 kJ/kg), it

can be shown that the thermal efficiency would be 50.6.

10–7 ■ SECOND-LAW ANALYSIS

OF VAPOR POWER CYCLES

The ideal Carnot cycle is a totally reversible cycle,and thus it does not

involve any irreversibilities. The ideal Rankine cycles (simple, reheat, or

regenerative), however, are only internally reversible,and they may involve

irreversibilities external to the system, such as heat transfer through a finite

temperature difference. A second-law analysis of these cycles reveals where

the largest irreversibilities occur and what their magnitudes are.

Relations for exergy and exergy destruction for steady-flow systems are

developed in Chap. 8. The exergy destruction for a steady-flow system can

be expressed, in the rate form, as

(10–18)

or on a unit mass basis for a one-inlet, one-exit, steady-flow device as

xdest
T 0 sgen
T 0 asesi (10–19)

qout

Tb,out



qin

Tb,in

b¬¬ 1 kJ>kg 2

X

#

dest
T 0 S

#

gen
T 01 S

#

outS

#

in^2 
T 0 aa

out

m#s

Q

#

out

Tb,out

a

in

m#s

Q

#

in

Tb,in

b¬¬ 1 kW 2

wpump,in
 11 yz 2 wpump I,in 11 y 2 wpump II,in 1 y 2 wpump III,in

wturb,out
 1 h 9 h 102  11 y 21 h 11 h 122  11 yz 21 h 12 h 132

hth

wnet

qin

wturb,outwpump,in

qin

hth
 1 

qout

qin

 1 

1485.3 kJ>kg

2921.4 kJ>kg

0.492 or 49.2%

1485.3 kJ>kg

 11 0.17660.1306 21 2335.7191.81 2 kJ>kg

qout
 11 yz 21 h 13 h 12

2921.4 kJ>kg

 1 3583.11089.8 2 kJ>kg 11 0.1766 21 3674.93155.0 2 kJ>kg

qin
 1 h 9 h 82  11 y 21 h 11 h 102