Handbook of air conditioning and refrigeration / Shan K

FIGURE 17.28 A return duct system with short connecting ducts for a textile mill.
The width of the duct is
Return or Exhaust Duct Systems
W � 10.78
3
� 3.59 ft (1.095 m)
AIR SYSTEMS: AIR DUCT DESIGN 17.71
Return duct systems with very short connecting ducts between the return inlet and the main duct
are often used in industrial applications. They are often used in large workshops and need nearly
equal return volume flow rate for each branch intake. To provide a better system balance, it is necessary
to reduce the total pressure difference between the branch inlets at the two ends of the
main duct.
For a conditioned space where noise is not a major problem, one effective means of reducing the
total pressure loss of a long return main duct is to have a higher branch duct velocity than the velocity
in the main duct. Because a total pressure difference between the branch inlets and the main
duct at two ends is inevitable, the area of the return grille can be varied along the main duct to provide
a more even return volume flow rate. Unlike the supply outlets, the variation of the area of the
return grille has a minor effect on the space air diffusion.
A return duct system with short connecting ducts is shown in Fig. 17.28 for a textile mill.
Twelve floor grilles are connected to the return main duct at the lower floor. The connecting duct
has a floor to main return duct difference of 2.6 ft (0.79 m). The sizes of the branch ducts vary from
16 in. � 16 in. (406 mm � 406 mm) at the remote end to 12 in. � 8 in. (305 mm � 203 mm) near
the fan end. The main duct varies from 16 in. � 32 in. (406 mm � 813 mm) to 60 in. � 32 in.
(1524 mm � 813 mm). In Fig. 17.28, the variations in total pressure p t and static pressure p s along
the main duct are shown by the upper and middle curves, in in. WG (Pa); and the velocity in main
return duct v, in fpm (m/s), is shown by the lower curve. The velocity in the branches varies from
700 fpm (3.56 m/s) at the remote end to 2000 fpm (10.16 m/s) at the fan end. Note that p t at grille
6 is � 0.111 in. WG (� 27.6 Pa), � 0.105 in. WG (� 26.1 Pa) at grille 5, and � 0.114 in. WG