Chapter 21: Entropy Considerations and Analysis¶

ILLUSTRATIVE EXAMPLE 21.1, Page number: 460¶

In :
from __future__ import division
from math import log

#Variable declaration:
m = 1                           #Mass flowrate (lb)
cP = 1                          #Heat capacity (Btu/lb.°F)
#From figure 21.3:
T1 = 300                        #Temperature of hot fluid leaving exchanger (°F)
T2 = 540                        #Temperature of hot fluid entering exchanger (°F)
T3 = 60                         #Temperature of cold fluid leaving exchanger (°F)
T4 = 300                        #Temperature of cold fluid entering exchanger (°F)

#Calculation:
DSh = m*cP*log((T1+460)/(T2+460))      #Entropy for hot fluid (Btu/°F)
DSc = m*cP*log((T4+460)/(T3+460))      #Entropy for cold fluid (Btu/°F)
DSa = DSh+DSc                   #Entropy for one exchanger (Btu/°F)
DSt = DSa*2                     #Total entropy change (Btu/°F)

#Result:
print "The entropy chage is :",round(DSt,4)," Btu/°F ."
if (DSt>0):
print "There is a positive entropy change."
else :
print "There is a negative entropy change."
The entropy chage is : 0.2101  Btu/°F .
There is a positive entropy change.

ILLUSTRATIVE EXAMPLE 21.2, Page number: 461¶

In :
from __future__ import division
from math import log

#Variable declaration:
#From example 21.1:
DSh = -0.2744                   #Entropy for hot fluid (Btu/°F)
DSc = 0.3795                    #Entropy for cold fluid (Btu/°F)
m = 1                           #Mass flowrate (lb)
cP = 1                          #Heat capacity (Btu/lb.°F)
#From figure 21.4:
DT = 0                          #Temperature difference driving force (°F)
DS_D = 0                        #Entropy for D exchanger (Btu/°F)

#Calculation:
DS_C = DSh+DSc                  #Entropy for C exchanger (Btu/°F)
DSt = DS_C+DS_D                 #Total entropy change of exchangers (Btu/°F)

#Result:
print "The total entropy change is :",DSt," Btu/°F ."
The total entropy change is : 0.1051  Btu/°F .

ILLUSTRATIVE EXAMPLE 21.3, Page number: 462¶

In :
from math import log

#Variable declaration:
#From figure 21.5:
m = 2                           #Mass flowrate (lb)
cP = 1                          #Heat capacity (Btu/lb.°F)
DS1 = -0.2744                   #Entropy for hot fluid for E exchanger (Btu/°F)
T1 = 180                        #Temperature cold fluid entering the E exchabger (°F)
T2 = 60                         #Temperature cold fluid leaving the E exchabger (°F)

#Calculation:
DS2 = m*cP*log((T1+460)/(T2+460))   #Entropy for cold fluid for E exchanger (Btu/°F)
DS_E = DS1+DS2                  #Entropy for E exchanger (Btu/°F)
DS_F = DS_E                     #Entropy for F exchanger (Btu/°F)
DSt = DS_F+DS_E                 #Entropy change in exchangers E and F (Btu/°F)

#Result:
print "The entropy change in exchangers E and F is :",round(DSt,4)," Btu/°F ."
The entropy change in exchangers E and F is : 0.2818  Btu/°F .