720
Design Rationale: Services
In addition to this, the accumulation of dust can be cleaned up
periodically since a man can comfortably reach every corner of the
service zone.
Con:
1. Special precautions must be taken for the control of smoke in case of a
fire.
2. Special devices must be used to control noise transference through the
return air boots.
724.1.6
Exhaust Shaft to the Roof
Expelling exhaust air at the roof eliminates cross-contamination of the
fresh air supply. If, for reasons of economy on a particular project, the
expulsion of general exhaust air is desired on a per floor basis, it must be
shown to be safe by wind studies based upon local conditions.
Special exhaust air (from toilets, isolation rooms, fume hoods and other
heat and odor-producing functions) is always expelled at the roof through
a separate shaft or shafts.
724.1.7
Adaptability
1. Supply and Return/Exhaust: Main Distribution
The return/exhaust main is the largest duct. As other components
pass over it, under it and beside it, and as it tends to be located toward
the center of the service zone, it would be extremely difficult to remove
and replace if its size had to be changed. It is therefore considered to
be a permanent component. As such, this main duct must be sized for
the maximum loading anticipated in the life of the building. This
increases first cost somewhat, but actually reduces the fan horsepower
required, as friction losses are diminished in an oversized duct.
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