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

cycle and thus the pumps and the turbines are isentropic; there are no pres-

sure drops in the boiler, process heater, and condenser; and steam leaves

the condenser and the process heater as saturated liquid.

The work inputs to the pumps and the enthalpies at various states are as

follows:

(a) The maximum rate of process heat is achieved when all the steam leaving

the boiler is throttled and sent to the process heater and none is sent to the

turbine (that is, m

.

4 
m

.

7 
m

.

1 
15 kg/s and m

.

3 
m

.

5 
m

.

6 
0). Thus,

The utilization factor is 100 percent in this case since no heat is rejected in

the condenser, heat losses from the piping and other components are

assumed to be negligible, and combustion losses are not considered.

(b) When no process heat is supplied, all the steam leaving the boiler passes

through the turbine and expands to the condenser pressure of 5 kPa (that is,

m

.

3 
m

.

6 
m

.

1 
15 kg/s and m

.

2 
m

.

5 
0). Maximum power is produced

in this mode, which is determined to be

Thus,

That is, 40.8 percent of the energy is utilized for a useful purpose. Notice

that the utilization factor is equivalent to the thermal efficiency in this case.

(c) Neglecting any kinetic and potential energy changes, an energy balance

on the process heater yields

u

W

#

netQ

#

p

Q

#

in

1 19,971 02 kW

48,999 kW

0.408 or 40.8%

Q

#

in
m

#

11 h 1 h 112 
^1 15 kg>s^231 3411.4144.78^2 kJ>kg^4 
48,999 kW

W

#

net,out
W

#

turb,outW

#

pump,in
^1 20,076^1052 kW
19,971 kW
20.0 MW

W

#

pump,in
^1 15 kg>s^21 7.03 kJ>kg^2 
105 kW

W

#

turb,out
m

#

1 h 3 h 62 
 1 15 kg>s 231 3411.42073.0 2 kJ>kg 4 
20,076 kW

Q

#

p,max
m

#

11 h 4 h 72 
^1 15 kg>s^231 3411.4640.09^2 kJ>kg^4 
41,570 kW

h 10 
h 7 wpump II,in
 1 640.097.10 2 kJ>kg
647.19 kJ>kg

h 9 
h 8 wpump I,in
 1 137.757.03 2 kJ>kg
144.78 kJ>kg

h 8
hf (^) @ (^) 5 kPa
137.75 kJ>kg
h 7
hf (^) @ (^) 500 kPa
640.09 kJ>kg
h 6
2073.0 kJ>kg
h 5
2739.3 kJ>kg
h 1
h 2
h 3
h 4
3411.4 kJ>kg

7.10 kJ>kg
wpump II,in
v 71 P 10 P 72
1 0.001093 m^3 >kg 2317000  5002 kPa4a
1 kJ
1 kPa#m^3
b

7.03 kJ>kg
wpump I,in
v 81 P 9 P 82
1 0.001005 m^3 >kg 2317000  52 kPa4a
1 kJ
1 kPa#m^3
b
582 | Thermodynamics