Handbook of air conditioning and refrigeration / Shan K

17.38 CHAPTER SEVENTEEN
Entrances, Exits, Enlargements, and Contractions
Entrances and exits are the end openings mounted on a duct or a duct wall, as shown in Fig. 17.14.
Because of the change in direction of the streamlines at the entrance, eddies and large-scale
turbulences develop along the duct wall when the airstream passes through the entrance. Generally,
the total pressure drop �p t of the airstream before it enters the entrance is negligible. A sharp-edge
entrance may have a C o � 0.9, whereas for an entrance flush-mounted with the wall, C o reduces to
0.5. An entrance with a conical converging bellmouth may further reduce C o to 0.4. If an entrance
with a smooth converging bell mouth is installed, C o could be as low as 0.1.
When the air flows through a wall outlet or an exit, flow separation occurs along the surface of
the vanes, and wakes are formed downstream, so there is a drop in total pressure. The velocity of
the airstream reaches its maximum value at the vena contracta, where the cross section of airflow is
minimum and the static pressure is negative. The total pressure loss at the outlet always includes the
velocity pressure of the discharge airstream.
Various types of return inlets, such as grilles and louvers, and supply outlets, such as diffusers,
are discussed in Chapter 18.
When air flows through an enlargement or a contraction, flow separation occurs and produces eddies
and large-scale turbulences after the enlargement, or before and after the contraction. Both cause
a total pressure loss �p t, as shown in Fig. 17.15. To reduce the energy loss, a gradual expansion, often
called a diffuser, or converging transition is preferred. An expansion with an including angle of enlargement
� � 14°, as shown in Fig. 17.15a, is ideal. In actual practice, � may be from 14° to 45° because
of limited space. For converging transitions, an including angle of 30° to 60° is usually used.
17.7 FLOW RESISTANCE
FIGURE 17.15 Enlargements and contractions: (a) abrupt enlargement; (b) sudden contraction.
Flow resistance is a property of fluid flow that measures the characteristics of a flow passage which
resist the fluid flow in that passage with a corresponding total pressure loss at a specific volume