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FLEX DUCT INAIR SERIES — Flexible duct is often used
in air series to connect Moduline units. In calculating pres-
sure requirements for units in air series, use the following
guidelines:
1. For close-coupled units (either continuous diffuser or units
in adjoining ceiling modules) where the flexible duct used
is 6 to 8 in. in length, add 0.1 in. wg to unit requirement
for minimum static pressure at control unit (Table 10).
For example, consider three 37HS2 units in air series, in
adjoining modules, with 200 cfm per unit (Fig. 23). The
minimum static pressure for multiple units is equal to 0.75
in. wg (from Table 10). Adding 0.1 in. wg for flexible
duct, the total pressure downstream of run-out duct equals
0.85 in. wg.
2. For units in air series with extended flexible duct lengths,
add ⌬P from Table 11.
For example, consider four 37HS2 units in air series,
200 cfm per unit (Fig. 24). The units are in alternate
series modules; 8-in. duct; 9- x 9-in. plenum. The
minimum static pressure for multiple units is equal to 0.75
in. wg. Three lengths of flex duct at 4.5 ft equal 13.5 ft.
From Table 11, read with 8-in. duct diam, ⌬P is equal to
0.16 in. wg for each 10 ft. of flex duct. Therefore, for
13.5 ft, ⌬P is equal to 0.16 + 0.06 in. wg. The total ⌬P
downstream of run-out duct is equal to 0.75 + 0.22
= 0.97 in. wg.
3. Flexible duct, unsupported, tends to sag; this deflection
increases the pressure drop. Whenever possible, support
the duct with wire between connections. Sagging will in-
crease the ⌬P as much as 0.06 in. wg in a 10-ft length;
therefore rigid duct is recommended for lengths greater
than 5.5 ft.
4. The use of flexible duct in air series run-out when 90 de-
gree turns are required will also add ⌬P. Table 12 pro-
vides the additional pressure drop.
For example, refer to Fig. 25. The ‘‘straight’’ line ⌬Pis
equal to 0.97 in. wg. If a bend is added to the run-out, the
total pressure drop becomes:
0.97 + 0.16 = 1.13 in. wg.
Fig. 22 — Air Series of Units of the Same Size and Capacity
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