DUCT AND VENTILATION SYSTEMS
As a Utilitiesman, you can expect to
become involved in the installation of duct
and/or ventilation systems designed to provide
conditioned air or to remove less desirable air
from a given space or facility. When sheet
metal is to be fabricated into system
components, the Steelworker provides the
expertise. When duct board is used,
fabrication and installation may be tasked to
the Utilitiesman exclusively.
This chapter provides some key
knowledge to aid you in the identification of
types of duct and ventilation systems, their
installation, and factors you must be aware of
in determining the sizes required to meet
specified building requirements. Keep in mind
that the term air conditioned refers to air that
has been cooled, heated, dehumidified or
humidified, or any combination of these.
To deliver air to the conditioned space,
you need air carriers. These carriers are called
ducts. They are made of sheet metal or some
structural material that does not bum
Duct systems are also classified as high-
-pressure or high-velocity ductwork and low--
pressure or low-velocity ductwork. The term
high-pressure or high-velocity ductwork
includes ductwork systems and plenums from
the fan discharge to the final high-velocity
mixing boxes, or other final pressure-reducing
devices or any air supply system served by a
fan operating with a static pressure range of 3
inches to 7 inches of water column (WC).
High-velocity or high-pressure systems with
fan static pressures of 3 inches WC or greater
are defined as high pressure. Usually the static
pressure is limited to a maximum of 7 inches
WC, and duct velocities are limited to 4,000
feet per minute (fpm). Systems requiring
pressures more than 7 inches WC are normally
unwarranted and could result in very high
operating costs. Systems with velocities more
than 4,000 fpm peforms satisfactorily when all
duct fittings are carefully designed and
installed. However, velocity pressure losses
are excessive and velocities more than 4,000
fpm are not recommended.
A high-velocity double-duct system begins
with a high-pressure fan of class II or III design
any conveys air through sound-treated
high-velocity ductwork connected to sound
and pressure-attenuating mixing units.
Connections to the outlets of the reduction
units are treated as low velocity.
Smaller sized ductwork, using higher
velocities, permits conveyance of air to areas
limited by construction and reduces
floor-to-floor height. Round ductwork
generally provides the greatest strength,
tightness, and economy. However, oval and
rectangular ducts can be used when large risers
A n e c e s s a r y c o m p o n e n t o f t h e
high-pressure system is the mixing box or unit.
Its function is to blend air at two different
temperatures for proper delivery to the rooms.
This requires special pressure-reducing air
valves at both hot and cold inlets, mixing
baffles to prevent stratification of air, and
sound attenuation treatment to absorb noise
generated by the air valves.