Modern Engineering Thermodynamics

152 CHAPTER 5: First Law Closed System Applications
EXAMPLE 5.3 (Continued )
Q B = 950. × 10 5 kJ/h
Boiler
Boiler
feed
pump
W p =−23.0 kW
Turbine
(W T ) net = ?
Condenser
Q C =−600. × 10 5 kJ/h
System
boundary
FIGURE 5.3
Example 5.3.
Step 6. Algebraically solve for the unknown(s). Algebraically solving for the net power of the turbine gives
ð _W T Þ net = _Q net = ð _Q boiler + _Q condenser Þ
Step 7. Calculate the value(s) of the unknowns. The calculations then give
ð _W T Þ net
= ½950: × 10 5 + ð−600: × 10 5 ÞŠ kJ/h = 350: × 10 5 kJ/h
= ð350: × 10 5 1h
kJ/hÞ = 9720 kW = 9:72 MW
3600 s
Step 8. A check of the algebra, calculations, and units shows that they are correct.
The positive sign tells us that the net power is coming out of the turbine. Since the turbine must also power the
boiler feed pump,
ð _W T Þ total = ð _W T Þ net + ð _W T Þ bioler feed pump = 9720 + 23 = 9740 kW ðto 3 significant figuresÞ
To simplify the example solutions from this point on, we omit the description of each step in the solution technique.
The steps are all there, but now the solutions flow in a more continuous manner.
5.5 INCOMPRESSIBLE LIQUIDS
Perhaps the auxiliary equations most often used in thermodynamic analysis are equations of state. The two most
common equations of state are those for ideal gases and incompressible liquids. Since most students are more familiar
with ideal gases than they are with incompressible liquids, we chose the next example to illustrate the latter case.
Note that this example could also be described as another illustration of the analysis of an electrical device.
EXAMPLE 5.4
A food blender has a cutting/mixing blade driven by a 0.250 horsepower (hp) electric motor. The machine is initially filled
with 1.00 quart of water at 60.0°F, 14.7 psia. It is then turned on at full speed for 10.0 min. Assuming the entire machine is
insulated and that the mixing takes place at constant pressure, determine the temperature of the water when the machine is
turned off.
Solution
First, draw a sketch of the system (Figure 5.4).