340
Heating-Ventilating-Cooling: Organization
The depth of S3 will be governed by the requirements of the main supply
and return/exhaust ducts. This depth can be expected to be of the order
of 36". (This figure is obtained from the duct size calculation described in
the Section 344.2 below.) The ducts will be hung directly below the
beams.
The S4 subzone will contain the branch ducts which will dictate its vertical
dimension. This can be expected to be of the order of 16" including
clearance around the ducts. As far as possible, only one branch duct
should be run between any pair of ceiling hangers (the hangers placed on
beam center lines could be from 4'6" to 7'6" apart.) This will improve
access for ceiling penetration below the ducts for other services.
Offsets from the branch ducts to diffusers will be made by flexible duct
connections. Regulations limit the length of these connections to twelve
feet, which should easily prove sufficient. These flexible ducts would run
in the S5 subzone, and if the latter is dimensioned at about 12" in height,
will allow ducts up to 10" in diameter with their plenum boxes.
Branches for any special exhaust ducts should also follow the same
principle and run between separate hangers from supply branches, and
using flexible ducts for offsets.
If a plenum return/exhaust system is used, return air boots will be required
at the diffusers. With fire dampers, the boots will project 17" above the
bottom face of the ceiling, for an 18" x 18" unit. But as the boots replace a
ducted system they can be lined up clear of the supply branches as
though a return duct were present.
343.1.2
Channels
The supply duct mains will be divided generally into two in each service
zone to reduce the problem of duct size and reduce the lengths of the
branch ducts. The two channels for these ducts will be approximately at
the third points of the cross section, with the subzones for exhaust/return
ducts usually between them. (See Figure 340-3.) The supply ducts and
the terminal heaters with supply pipes, or the mixing boxes, will be in the
HVC supply channels at the S3 level.
The equipment will be placed on the same side of the ducts at each
branch take-off to facilitate installation and maintenance and, with the
terminal heaters, allow straight runs of supply and return pipes. (See
Figure 340-2.)
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