Handbook of air conditioning and refrigeration / Shan K

FIGURE 17.17 Combination of flow resistances: (a) connected in series; (b) connected in parallel.
Flow Resistances Connected in Parallel
When a duct system has several sections connected in parallel, as shown in Fig. 17.17b, the total
volume flow rate of this duct V˙ that flows through cross-sectional planes 1 and 2 is the sum of the
individual volume flow rates , ,..., of each duct section, i.e.,
V˙ A
V˙ B
� √ �p 12
R A
where �p 12 � total pressure loss between planes 1 and 2, in. WC (Pa)
R A, R B,...,R n � flow resistance of duct sections A, B, ...,n, in. WC/(cfm) 2
(17.61)
As for the entire duct, V˙ � √�p12/R p.
Here Rp represents the flow resistance of the entire duct
whose sections are in parallel connection. Then
1
√R p
V˙ n
V˙ � V˙ A � V˙ B �� � ��V˙ n
� √ �p 12
R B
� 1
√R A
� 1
√R B
AIR SYSTEMS: AIR DUCT DESIGN 17.41
�� � ��√ �p 12
R n
�� � �� 1
√R n
(17.62)