Modern Engineering Thermodynamics

312 CHAPTER 9: Second Law Open System Applications
heat exchanger while it absorbs heat from the environment. At
the outlet, the pressure is 1.00 MPa. Determine (a) the heat
transfer rate, and (b) the entropy production rate. (c) Show
whether this device violates the second law of thermodynamics.
Assume that the system boundary temperature is isothermal
at 200.°C.
27. A contact feedwater heat exchanger for heating the water going
into a boiler operates on the principle of mixing steam with
liquid water. For the steady flow adiabatic process shown in
Figure 9.25, calculate
a. The rate of change of entropy of the entire heater.
b. The rate of entropy production inside the heater.
p = 100. psia
x = 0.98
p = 100 psia
T = 80.0°F
FIGURE 9.25
Problem 27.
Steam
Water
Feedwater
heater
Water
m = 25.0 × 10 3 lbm/h
p = 100. psia
T = 80.0°F
28. As an engineering consultant, you are asked to review a design
proposal in which an electric resistance heater is to be used in
conjunction with a precision air bearing (Figure 9.26). The
heater uses 100 W of electrical power and the air bearing has a
constant surface temperature of 160.°F. The heater is well
insulated on the outside and air enters the bearing at 40.0°F,
35.0 psia and exits at 80.0°F , 40.0 psia. The bearing is a steady
flow, steady state device with an air flow rate of 35.55 lbm/h.
Determine
a. Whether this device violates the first law of thermodynamics.
b. Whether this device violates the second law of
thermodynamics.
Air inlet
40.0°F
35.0 psia
FIGURE 9.26
Problem 28.
160.°F
Air outlet
80.0°F
40.0 psia
100. watt
Resistance heater
29.* A slide projector contains a 500. W lightbulb cooled by an
internal fan that blows room air across the bulb at a rate of
1.00 kg/min. If the equilibrium surface temperature of the
bulb is 350.°C and the inlet temperature (the room air) is at
20.0°C, then determine (a) the outlet temperature of the
cooling air and (b) the rate of entropy production in the air
passing through the projector. Assume the air is an ideal gas
with constant specific heats and that it undergoes an aergonic
process.
30. Determine the total entropy production rate for the heat
exchanger shown in Figure 9.27. In addition to the air-water
heat transfer within the heat exchanger, the air also loses an
unknown amount of heat to the surroundings while the water
receives an additional 10.0 Btu/s from the surroundings. Assume
the internal air-water interface is isothermal at 100.°F and the
outer surface of the heat exchanger is isothermal at 70.0°F.
Neglect all flow stream pressure losses.
140.°F
Air
35.0°F
Water
FIGURE 9.27
Problem 30.
100. °F
T b = 70.0°F
Q air = ?
Air−water
interface
Q water = 10.0 Btu/s
110.°F
78.5°F
m air = 100. lbm/s
m water = 16.3 lbm/s
31.* A new Yo Yo Dyne propulsion system has three flow streams,
as shown in Figure 9.28. It mixes 0.500 kg/s of saturated water
vapor at 100.°F with 0.200 kg/s of saturated liquid water at
100.°C in a steady flow, steady state, isobaric process. This
system is cheaply made and uninsulated; consequently, it loses
heat at the rate of 75.0 kJ/s to the surroundings. Assuming the
system boundary temperature is isothermal at 100.°C, determine
a. The quality of the outlet mixture.
b. The entropy production rate of the system.
Sat. liq.
0.200 kg/s
at 100.°C
Sat. vap.
0.500 kg/s
at 100.°C)
FIGURE 9.28
Problem 31.
Vapor−mixer
incorporated
W = 1.00 hp
(power to the mixing blades)
Q = 75.0 kJ/s
Mixture
x =?
32.* In a steady flow, adiabatic, aergonic desuperheater (a kind of
mixing heat exchanger), water is sprayed into superheated steam
in the proper amount to cause the superheated steam to become
saturated.
a. Calculate the mass flow rate of water necessary for
desuperheating.
b. What is the entropy production rate of this system?
c. Show whether this process violates the second law of
thermodynamics.
Given that
■ Steam mass flow rate = 200. kg/h.
■ Steam entering state = 10.0 MPa, 600.°C.
■ Water entering state = 10.0 MPa, 100.°C.
■ Steam outlet state = 10.0 MPa, saturated vapor.
33. A steady state, steady flow steam mixer consists of a box with
two inlet pipes and one outlet pipe. One inlet pipe carries
saturated water vapor at 50.0 lbm/s and 20.0 psia. The other
inlet pipe carries saturated liquid water at 10.0 lbm/s and 20.0
psia. The mixing process is isobaric. In addition, 9602 Btu/s of