Modern Engineering Thermodynamics

6.6 Throttling Devices 179
These assumptions imply a negligible aerodynamic drag on the water stream and negligible viscous dissipation within the
stream itself. When these conditions are applied to the modified energy rate balance for system b, we obtain
_Q ↘0
− _W ↘0
+ _m½h in − h out + ðV 2 in − 0Þ/2g c + ð0 − Z out Þg/g c Š b = 0
or
ðZ out Þ b = ½ðg c /gÞðh in − h out Þ + V 2 in /2gŠ b
The change in specific enthalpy for this example is again given by Eq. (6.19), and under the assumptions listed for system b,
it is clear that this change is zero. Then, our working modified energy rate balance reduces to
or
ðZ out Þ b =
ðZ out Þ b = ðV 2 in Þ b/2g
ð109 ft/sÞ 2
2ð32:174 ft/s 2 = 185 ft
Þ
Note that our calculations for both (V out ) a
and (Z out ) b in this example gave numbers somewhat higher than we would
observe if we measured these values in an experiment. This is because we ignore the viscous dissipation effects in the water
and surrounding air. Dissipation effects are considered in more detail in the next chapter, on the second law of
thermodynamics.
Exercises
4. Determine the exit diameter of the garden hose nozzle used in Example 6.2. Assume the water is incompressible so that
v in = v out . Answer: (D out ) a = 0.147 in.
5. Determine the height to which the stream of water rises when the garden hose in Example 6.2 is pointed straight up and
the nozzle removed. Answer: (Z out ) b = 1.03 in.
6. If the exit diameter of the garden hose nozzle used in Example 6.2 is reduced to 0.100 in, determine the exit velocity and
the height to which the stream of water rises when pointed straight up. Answers: (V out ) a = 235 ft/s, and (Z out ) b = 858 ft.
6.6 THROTTLING DEVICES
Throttling device is the generic name of any device or process that simply dissipates pressure energy _mpv by
irreversibly converting it into thermal energy. Unlike nozzles and diffusers, throttling devices provide no form of
useful energy recovery. They merely convert pressure energy into thermal energy through dissipative viscous flow
(usually turbulent) processes. In fact, any device that incurs a large irreversible pressure drop can be thought of
as a throttling device. Figure 6.5 schematically illustrates a variety of common throttling devices.
p 1 p 2 < p 1 p 1
p 2 < p 1
(a) Orifice plate
(b) Porous plug
p 1 p 2 < p 1 p 1
p 2 < p 1
(c) Butterfly valve
(d) Any type of flow or
pressure control valve
FIGURE 6.5
Some common throttling devices.