Modern Engineering Thermodynamics

182 CHAPTER 6: First Law Open System Applications
2.0
1.8
p = 1 atm
CO 2
Joule−Thomson coefficient, °C/Atm
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
p = 1 atm
p = 100 atm
p = 200 atm
p = 20 atm
p = 60 atm
Air
−0.2
−0.4
−0.6
−150 −100 −50 0 50 100 150 200 250 300
Temperature, °C
FIGURE 6.6
The variation in the Joule-Thomson coefficient of air and carbon dioxide with pressure and temperature.
6.7 THROTTLING CALORIMETER
A throttling calorimeter is a device that expands (i.e., throttles) a mixture of liquid plus vapor into the superheated
vapor region. Under the vapor dome, temperature and pressure are not independent properties; therefore, their
measurement alone cannot be used to fix the thermodynamic state of a substance. If, however, the thermodynamic
state can be moved into a region where pressure and temperature are independent properties, then its
state can be determined from a pressure gauge and a thermometer reading. This is the purpose of a throttling
calorimeter, as illustrated in the following example.
EXAMPLE 6.3
Wet (i.e., a mixture of liquid plus vapor) steam flows in a pipe at 2.00 MPa. An insulated throttling calorimeter is attached
to the pipe and a small portion of the steam is withdrawn and throttled to atmospheric pressure. The temperature and
pressure of the throttled steam in the calorimeter are 150.°C and 0.100 MPa. Determine the quality of the wet steam in the
pipe and estimate its Joule-Thomson coefficient μ J
.
Solution
First, draw a sketch of the system (Figure 6.7).
p 1 = 2.00 MPa
x 1 = ?
Highpressure
steam
pipe
Throttling
valve
1
p 2 = 0.100 MPa
T 2 = 150.°C
Throttling
calorimeter
2
Exhaust
steam
FIGURE 6.7
Example 6.3.