Figure 13-3.Forming rectangular fiber glass ducts from duct board.
metal ducts. Therefore, fabric joints (usually
constructed of heavy canvas) are used to join the
branch ducts to the plenum.
SIZING DUCT SYSTEMS
There are numerous factors to be considered
when sizing duct systems. These factors cause you
to make modifications and adjustments through-
out the planning and installation process to
develop an efficient working system. First, you
must calculate the air volume required for heating
and for cooling the required space. This will assist
you in determining the necessary duct size, fan
size, fan speed, and so forth, that is needed to
circulate the conditioned air. While determining
the heating and the cooling factors, you should
think in terms of air circulation throughout the
building and in each individual room or space.
Remember, air movement is determined by the
type of return airflow that you use.
Four other important duct system components
are diffusers, grilles, registers, and dampers. Each
of these components has a direct correlation
between functional design, amount of air accom-
modated, and the air movement pattern.
The elbows within the duct system are a
major source of airflow restriction. Whenever
possible, you can gain efficiency by installing long
sweeping elbows. Short 90-degree elbows should
be used sparingly on long duct runs. However,
they can be used very effectively with a minimum
of air turbulence and airflow restriction when
installed just before diffusers, grilles, and
Your final duct calculations involve taking
unit pressure drops and total pressure drops
throughout the system. Some of the major con-
tributing factors to these pressure drops are as
. Length of duct
. Duct material and interior finish
. Changes in duct size
. Number of elbows