Handbook of air conditioning and refrigeration / Shan K

17.66 CHAPTER SEVENTEEN
10. Because the local loss coefficient Cc,b for the branch stream from the 45° diverging wye at
junction 2 can be reduced to 0.45 if a conical branch is used, the critical path consists of duct sections
FO-1, 1-2, and 2-5. The total pressure loss of this supply duct system excluding the VAV box
and downstream flexible duct and diffuser, from Table 17.12, is
�pt � 0.116 � 0.0645 � 0.0847 � 0.2652 in. WC (66 Pa)
17.12 DUCT SYSTEMS WITH NEGLIGIBLE PRESSURE LOSS
AT BRANCH DUCTS
Supply Duct Systems
When supply outlets are directly mounted on the main duct without branch takeoffs, or the connecting
duct between the supply outlet and the main duct is about 2 ft or less, the total pressure loss of
the branch duct, excluding the supply outlet, is often very small or negligible. In such circumstances,
system balancing of the supply duct system depends mainly on the sizes of the successive
main duct sections. If volume dampers are installed just before the outlet or in the connecting duct,
the damper modulation will also vary the space diffusion airflow pattern.
Pressure Characteristics of Airflow in Supply Ducts
The rectangular supply duct with transversal slots shown in Fig. 17.25 is an example of a supply
duct system with a supply outlet directly mounted on the main duct. The volume flow per ft2 (m2 ) of
floor area of this type of duct system may sometimes exceed 8 cfm/ft 2 (40.6 L/s�m 2 ). This type of
supply duct system is often used in many industrial applications.
Consider two planes n and n � 1, with a distance of 1 unit length, say, 1 ft (0.305 m), between
them. If the space air is at atmospheric pressure, the total pressure loss of the supply air that flows
from plane n, turns a 90°, and discharges through the slots is
(17.99)
where �pc,bn, �pton � dynamic loss of branch stream when it makes a 90° turn and when it discharges
from slots, lb/ft 2 or in. WC (Pa)
Cc,bn � local loss coefficient of diverging branch stream with reference to velocity at
plane n
Co � local loss coefficient of transversal slots with reference to velocity at slot
vn, von � air velocity at plane n and at slot from plane n, ft/s (m/s)
Similarly, at plane n � 1 the total pressure loss of the branch stream when it discharges from the
slots is
pt(n�1) � (17.100)
The relationship of the total pressure between planes n and n � 1 along the airflow is given as
(C �ptn � ptn ��pc,bn ��pton �
2 2
c,b(n�1)v (n�1) � Covo(n�1) )�
2gc (C 2 2
c,bnvn � Covon)� 2gc p tn � p t(n�1) � ( f nL n/D n � C c,sn)�v n 2
(17.101)
In Eq. (17.101), f n, L n, D n, and C s,cn indicate the friction factor, length, circular equivalent, and C c,s
value between plane n and n � 1, respectively. Substituting Eqs. (17.99) and (17.100) into
2g c