Thermodynamics
SECOND LAW OF THERMODYNAMICS
Solution:
where
= entropy added to the system
msin
p
msout
p
p
m (sout
sin)
sin
=
1.7088 Btu/lbm -°R (from steam tables)
sout
=
1.8158 Btu/lbm°R (from steam tables)
=
Btu/lbm-oR
p/
m
sout
sin
1.8158
1.7088
=
0.107 Btu/lbm -°R
p/
m
=
10,000 (0.107)
p
=
1070 Btu/lbm -°R. = entropy added to the system
p
It should always be kept in mind that the Second Law of Thermodynamics gives an upper limit
(which is never reached in physical systems) to how efficiently a thermodynamic system can
perform. A determination of that efficiency is as simple as knowing the inlet and exit
temperatures of the overall system (one that works in a cycle) and applying Carnot’s efficiency
equation using these temperatures in absolute degrees.
Diagrams of Ideal and Real Processes
Any ideal thermodynamic process can be drawn as a path on a property diagram, such as a T-s
or an h-s diagram. A real process that approximates the ideal process can also be represented
on the same diagrams (usually with the use of dashed lines).
In an ideal process involving either a reversible expansion or a reversible compression, the
entropy will be constant. These isentropic processes will be represented by vertical lines on
either T-s or h-s diagrams, since entropy is on the horizontal axis and its value does not change.
A real expansion or compression process operating between the same pressures as the ideal
process will look much the same, but the dashed lines representing the real process will slant
slightly towards the right since the entropy will increase from the start to the end of the process.
Figures 24 and 25 show ideal and real expansion and compression processes on T-s and h-s
diagrams.
Rev. 0
Page 77
HT-01
