Building Design and Construction Handbook - Merritt - Ventech!

13.80 SECTION THIRTEEN
After all discharge grilles and the air-handling unit are located, it is advisable
to make a single-line drawing of the duct run. The air quantities each line and
branch must carry should be noted. Of the few methods of duct-system design in
use, the equal-friction method is most practical. For most comfort cooling work, it
is considered good practice not to exceed 0.15-in friction per 100 ft of ductwork.
It is also well to keep the air below 1500 ft/min starting velocity.
If a fresh-air duct is installed the return-air duct should be sized for a quantity
of air equal to the supply air minus the fresh air.
FIGURE 13.36 Ductwork for cooling a store.
It is advisable, where physically possible,
to size the fresh-air duct for the
full capacity of the air-handling unit.
For example, a 10-ton system handling
4000 ft3 /min of supply air—1000 ft3 /
min fresh air and 3000 ft3 /min return
air—should have the fresh-air duct sized
for 4000 ft3 /min of air. A damper in the
fresh-air duct will throttle the air to
1000 ft3 /min during the cooling season;
however, during an intermediate season,
when the outside air is mild enough,
cooling may be obtained by operating
only the supply-air fan and opening the
damper, thus saving the operation of the
10-ton compressor-motor unit.
As an example of the method for sizing
an air-conditioning duct system, let
us determine the ductwork for the first
floor of the building in Fig. 13.36. Although
a load analysis shows that the air
requirement is 2979 ft3 /min, we must
design the ducts to handle the full capacity
of air of the packaged unit we
supply. Handling less air will unbalance
the unit, causing a drop in suction temperature,
and may cause freezing up of
the coil. If a 71 ⁄2-ton packaged unit is
used, for example, the ducts should have
a capacity at least equal to the 3000 ft3 /
min at which this unit is rated.
Table 13.17 shows the steps in sizing the ducts. The 3000 ft3 /min is apportioned
to the various zones in the store in proportion to the load from each, and the flow
for each segment of duct is indicated in the second column of the table. Next, the
size of an equivalent round duct to handle each airflow is determined from Table
13.8 with friction equal to 0.15 in per 100 ft. The size of rectangular duct to be
used is obtained from Table 13.6.
The preceding example of ductwork design falls into the category of lowpressure
duct systems. This type of design is used for most air-distribution systems
that are not too extensive, such as one- or two-floor systems, offices, and residences.
In general, the starting air velocity is below 2000 ft/min, and the fan static pressure
is below 3 in of water.
For large multistory buildings, high-velocity air-distribution duct systems often
are used. These systems operate at duct velocities well above 3000 ft/min and