Modern Engineering Thermodynamics

310 CHAPTER 9: Second Law Open System Applications
FINAL COMMENTS ON THE SECOND LAW
In Chapter’s 7, 8, and 9, we deal with the fundamentals of the second law of thermodynamics for nonequilibrium
systems. This law introduces the new thermodynamic property entropy, which is not conserved in any real engineering
process. The second law of thermodynamics states that entropy must always be produced in any irreversible
process; however, a positive entropy production does not mean that the net system entropy must necessarily
increase, because entropy may be transported out of a system faster than it is produced within the system; therefore,
the entropy level of the entire system can either increase or decrease in any real process.
In Chapter 8 we investigate closed system applications of the second law of thermodynamics for both reversible
and irreversible processes. In so doing, we expand the first law examples given in Chapter 5 to include an
entropy balance analysis. Since entropy production is a direct consequence of system losses that lead to diminished
operating efficiency, many of the examples in this chapter focus on determining the entropy production
or its rate to gain further insight into the causes of system inefficiency. This is done by using either the auxiliary
entropy production equations (the direct method) or an appropriate entropy balance equation (the indirect
method).
In this chapter, we use the second law of thermodynamics in the analysis of open systems. We use the entropy
balance to determine the entropy production rates for a variety of common engineering devices (nozzles, diffusers,
throttles, heat exchangers, etc.). We also look at how different processes that achieve the same end
states affect the amount of entropy produced by the system during those processes. This allows us to choose
the process or method of changing the state that is the least dissipative and consequently the most efficient.
Determining processes that minimize the entropy production produces economic and productivity rewards to
the user.
Problems (* indicates problems in SI units)
1. An inventor claims to have a steady state, steady flow system
with an isentropic flow stream (i.e., s in = s out ) that requires heat
addition. Show whether or not this system violates the second
law of thermodynamics.
2. The inventor in Example 9.1 now claims to have a system that
generates heat by pumping water through a pipe isothermally
(i.e., T in = T out ). Show whether or not this system violates the
second law of thermodynamics.
3.* Suppose the inventor in Example 9.1 filed a new patent claim
that said the heater was 20.0 kW and the heat transfer boundary
temperature was 100ºC. What are the water flow rate and entropy
production rate under these conditions?
4.* The inventor in Example 9.1 needs to know the heat transfer
rate and entropy production rate required to heat 5.00 kg/s from
15ºC to50ºC. You are the engineer, so what are the answers?
5.* The inventor in Example 9.1 now wants to patent a
water cooling system that cools 0.500 kg/s of water from 50ºC
to 15ºC by removing heat with a heat transfer boundary
temperature of 10.0ºC. What is (a) the heat transfer rate and
(b) the entropy production rate for this process.
6. An inventor reports that she has a refrigeration compressor that
receives saturated Refrigerant-134a vapor at 0.00°F and delivers
it at 150. psia and 120.°F. The compression process is adiabatic.
Determine whether or not this process violates the second law
of thermodynamics.
7.* A 1.00 MW steam power plant operates on the simple reversible
thermodynamic cycle shown Figure 9.23.
a. What is its thermal efficiency?
b. What is the steam mass flow rate in this system?
c. What is its thermal efficiency if it is operated on a Carnot cycle?
8.* A steam turbine is limited to a maximum inlet temperature of
800.°C. The exhaust pressure is 0.0100 MPa, and the moisture
in the turbine exhaust is not to exceed 9.00%.
T(K)
FIGURE 9.23
Problem 7.
700
650
600
550
500
1.0 1.1 1.2 1.3 1.4
s (kJ/kg •K)
a. What is the maximum allowable turbine inlet pressure if the
flow is adiabatic and reversible?
b. What is the maximum power output per unit mass flow rate?
9.* A steam turbine receives steam at 1.00 MPa and 700.°C and
exhausts at 0.100 MPa. If the turbine can be considered to
operate as a steady flow, reversible, adiabatic machine, what is
the work done per pound of steam flowing? Neglect any
changes in kinetic or potential energy.
10. Saturated mercury vapor enters a steady flow turbine of a highpressure
auxiliary power system at 600. psia and emerges as a
mixture of liquid and vapor at a pressure of 1.00 psia. What
must be the flow rate if the power output is to be 10.0 kW?
Assume the turbine is reversible and adiabatic, and neglect any
changes in kinetic or potential energy.
11.* In the year 2138, a law requires certain limits on the production
of entropy of any marketable piece of technology. This law is
similar in nature to the old air pollution laws of the 20th
century. It sets an upper limit of 1.00 × 10 –3 kJ/(kg · K · s) on the