Handbook of air conditioning and refrigeration / Shan K

FIGURE 17.25 Rectangular supply duct with transversal slots.
Eq. (17.101), Wang et al. (1984) recommended the following formula to calculate v n�1, in ft/s
(m/s), based on the balanced total pressure before the transversal slots:
v (n�1) ��� (v2 2
on � vo(n�1) )Co �� Cc,bn � Cc,sn � fnL n
Dn AIR SYSTEMS: AIR DUCT DESIGN 17.67
� v n 2��
(17.102)
In Eq. (17.102), von and vo(n�1), in ft/s (m/s), are the supply air velocities at the slots. For a cold air
supply, usually vo(n�1) � von is desirable because of the effect of the duct heat gain.
For a free area ratio of slot area to duct wall area of 0.5, Co for a perforated plate including discharged
velocity pressure can be taken as 1.5. The vn, Ln, and Dn are known values during the calculation
of vn�1. Local loss coefficients Cc,bn and Cc,sn can be determined from the experimental curves
shown in Figs. 17.26 and 17.27. The coefficient Cc,b(n�1) can be assumed to have a value similar to
Cc,bn, and f can be calculated from Eq. (17.42). The mean air velocity in the duct at plane n � 1, denoted
by vn�1, can then be calculated from Eq. (17.102).
In Figs. 17.26 and 17.27, V˙ on indicates the volume flow rate of supply air discharged from the
slots per ft length of the duct, in cfm/ft (L/s�m); V˙ n represents the volume flow rate of the supply
air inside the duct at plane n, in cfm (m 3 /s).
Sizing of this kind of supply duct system starts from the duct section immediately after the
fan discharge. This section can be sized from Eqs. (17.90) and (17.93) based on life-cycle cost
1
C c,b(n�1) �� 0.5