Modern Engineering Thermodynamics

9.6 Heat Exchangers 289
Exercises
10. If the surface temperature of the valve in Example 9.4 is decreased from 60.0°F to 40.0°F, resolve part a to find
the new entropy production rate of the valve. Keep the values of all the other variables the same as they are in
Example 9.4. Answer: _S P = 6:35 × 10 −4 Btu/(s · R).
11. If the mass flow rate through the expansion valve discussed in Example 9.4 is increased from 0.100 lbm/s to 0.500 lbm/s,
determine the new entropy production rate for parts a and b of the example. Keep the values of all the other variables the
same as they are in Example 9.4. Answer: _S P a = 0:0191 Btu/(s·R), _S P
b = 2:35 × 10−3 Btu(s·R).
12. Resolve part a of Example 9.4 if the R-134a exits the valve at 80.0% quality rather than 53.0%, determine the new
entropy production rate for the system. Keep the values of all the other variables (except _Q and s out ) the same as they
are in Example 9.4. Answer: _S P
a = 1:16 × 10−3 Btu/(s·R).
9.6 HEAT EXCHANGERS
Heat exchangers are discussed briefly in the energy balance examples of Chapter 6. This section expands the
earlier material on this subject by introducing some of the basic concepts of heat exchanger design and analysis.
Heat exchangers are normally classified as either parallel flow, counterflow, orcross flow, as shown in Figure 9.5. If
both fluids flow in the same direction, it is said to be a parallel flow heat exchanger; if they flow in opposite
directions, it is said to be a counterflow heat exchanger; and if they flow at right angles to each other, it is said
to be a cross flow heat exchanger.
The two most common types of heat exchangers are shell and tube and plate and tube. The simplest type of
shell and tube heat exchanger is the double-pipe system shown in Figures 9.1a and 9.1b. Figure 9.1c illustrates
a simple plate and tube geometry. The efficiency of a shell and tube heat exchanger can be improved
THE WORLD’S LARGEST HORIZONTAL SHAFT HEAT EXCHANGER
Removing sulfur and nitrogen oxides from the combustion products of large industrial or electrical power plant furnaces is
important for the preservation of the environment. If the catalytic reduction of nitrogen oxides in the furnace exhaust gas
occurs after the desulfurization process, the exhaust gas must be reheated to a temperature of about 320°C. However, most
of the thermal energy used to reheat the gas can be recovered using a heat exchanger so that a temperature differential of
only about 30°C needs to be produced with an auxiliary heater.
In 1988, the largest horizontal shaft heat exchanger then existing was installed at the Heilbronn electrical power plant in
Germany. It has a rotor diameter of 15.5 m, is about 46 m long, and weighs 870 tons. It is a counterflow heat exchanger
that handles about 900,000 m 3 /h of exhaust gas.
m B
m A
m A
(a)
(b)
m B
m B
m A
(c)
FIGURE 9.5
Single-tube, single-pass heat exchanger geometries: (a) parallel flow; (b) counterflow; (c) cross flow.