Handbook of air conditioning and refrigeration / Shan K

Return or Exhaust Duct Systems
AIR SYSTEMS: AIR DUCT DESIGN 17.63
Behls (1978) compared four designs of a bunker ventilation duct system with 10 risers connected to
a main exhaust duct. The results of these four designs are as follows:
System total
pressure loss, Riser unbalance,
in. WC (Pa) in. WC (Pa)
Fixed-diameter riser, tee/diffuser � 7.39 (� 1837) 6.55 (1628)
Variable-diameter riser, tee/diffuser � 5.89 (� 1464) 1.77 (440)
Fixed-diameter riser, conical tee � 5.29 (� 1315) 4.06 (1009)
Variable-diameter riser conical tee � 4.03 (� 1002) 1.62 (403)
Using a greater branch duct velocity than the main duct, and varying the sizes of the risers, select
proper duct fittings to decrease system total pressure loss and system unbalance.
Example 17.2. For a round supply duct system made of galvanized steel with spiral seams, as
shown in Fig. 17.20, its operational and constructional characteristics are as follows:
Supply air temperature
Kinematic viscosity of air
Density of supply air
Absolute roughness
Local loss coefficients in section FO-1
Fan total efficiency, average
Motor efficiency
Electrical energy cost
Installation cost of duct
CRF
Annual operating hours
60°F (15.6°C)
1.59 � 10 �4 ft 2 /s (1.46 � 10 �5 m 2 /s)
0.075 lb/ft 3 (1.20 kg/m 3 )
0.0003 ft (0.09 mm)
0.5
0.7
0.8
$0.08/kWh
$3.5/ft 2
0.10
3000
The total supply volume flow rate at the fan discharge is 3000 cfm (1.415 m 3 /s), and the adjusted
volume flow rates because of duct heat gain for each of the branch takeoffs are illustrated in
Fig. 17.20. First, size this supply duct system. Then if each branch takeoff is connected to a VAV
box that needs a total pressure loss of 0.75 in. WC (186 Pa) at its inlet, calculate the total pressure
loss required for this supply duct system excluding the VAV box and downstream flexible duct and
diffuser.
Solution:
1. For the first iteration, start with the round duct section 2-5, which may be the last section of
the design critical path. Assume a diameter of 12 in., or 1 ft, and an air velocity of 1200 fpm, or 20
ft/s. Assume also that the local loss coefficient of the straight-through stream of the diverging tee
C c,s � 0.15, and for the elbow C o � 0.22. The Reynolds number based on diameter D is
Re D � vD
n �
20 � 1
� 125,786
�4 1.59 � 10