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G. DUCT HEAT GAIN OR LOSS
low, the designer should check that airflows are proportioned properly. The air in the ductwork still gets
warmer or cooler as it passes through the conditioned
space, thus decreasing the temperature difference.
As a result, less air is required to supply the outlets
at the start of the supply run and more is required at
Naturally, when a duct or plenum carrying conditioned
air is located outside of the conditioned space, the
heat gain or loss must be accounted for in both the
design air quantity and total sensible load. This system load must be calculated by the designer when
running conditioned air ductwork through boiler
rooms, attics, outdoors, or other unconditioned
spaces. Alternate routing might be more desirable
than increasing the system load.
In several places in this manual, life cycle costing is
discussed and it is suggested that semi-extended
plenums could reduce first cost and operating cost.
However, the designer must also realize if the velocities are reduced too much as a result of this, duct
wall heat transfer increases, indicating that additional
duct insulation might be required.
The use of additional insulation on duct work is becoming more universal with increased energy costs,
as evidenced by the fact that ASHRAE energy standards require certain ducts and plenums to be insulated or lined. This greatly reduces the impact of the
duct heat gain or loss.
With the design for the duct system approaching the
final stages, an analysis must now be made to deter-
mine if acoustical treatment is necessary. The addition of sound traps, duct liner or vibration isolation
might be required due to conducted or generated
noise and vibration in certain critical areas. Chapter
11 contains data and methods needed to make this
determination. Some duct resizing may be required
at this stage to incorporate the necessary acoustical
treatment into the duct system design.
It is beneficial to all concerned to have the designer
indicate all ductwork static pressure classification
changes on the drawings. For clear interpretation of
the requirements for ductwork and economical attainment of performance objectives, it is essential that
the contract plans depict the portion of each duct
system to be constructed for a particular static pressure classification (see Table 4-1). These static pressure rating changes are shown by "flags" at each
point where the duct static pressure classification
changes, with the number on the "flag" indicating the
pressure class of the ductwork on each side of the
dividing line (see Figure 4-1). Also see the sample
duct layouts for the duct design examples shown in
Chapters 7 and 8.
Special consideration must be given to the pressure
classes of ductwork used for some variable air volume (VAV) systems. It is possible for these supply
duct systems to experience the total fan pressure at
the most distant VAV box under minimum airflow conditions. Under these conditions, the maximum duct
construction classification should remain the same
Table 4-1 HVAC DUCT PRESSURE-VELOCITY CLASSIFICATION
SAMPLE SITUATION: WITH A TERMINAL REQUIRING 015 in.wg STATIC PRESSURE,
A BRANCH DAMPER REQUIRING 0.15 in.wg. SP. DUCT DESIGNED FOR 01 inw.g.
(SP) LOSS per 100 ft. AND FITTING LOSSES EQUAL TO THE STRAIGHT DUCT LOSS, THE
CIRCUIT CAN BE 100 LINEAL FEET LONG BEFORE 0.5 in.w.g. LOSS IS EXCEEDED
Figure 4-1 DUCT PRESSURE CLASS DESIGNATION
throughout the supply duct system upstream of the
Special consideration also must be given to emergency mode operations such as when smoke control
systems go into operation or fire dampers close
against full system airflow. Select duct pressure classifications that will handle the sudden pressure
changes without damage to the duct distribution system.
Until recent joint research projects between ASHRAE
and SMACNA on duct leakage, HVAC system designers arbitrarily established percentage leakage rates
for duct systems, that were impossible to attain by
the installing contractor. The anticipated amount of
duct system leakage may now be calculated once the
duct pressure and seal classifications are determined. These leakage rates, in terms of cfm/100 sq.
ft. (I/s per m2) of duct surface may also be expressed
in percentage of total system airflow. Seal class and
duct leakage class tables and charts, along with examples of use may be found in Chapter 5. The
amount of system leakage which varies with the average pressure of the system, must be added to the
total airflow capacity of the HVAC system fans(s).
With the various elements of the HVAC duct system
selected, the duct system laid out, and the sizing
finalized, the designer now must calculate the total
pressure of the systems which the fan(s) must overcome. In Chapters 7 and 8, there is a detailed description of how to determine the friction losses in
ductwork and the dynamic losses through fittings.
These, in combination with the pressure loss data for
duct system components and apparatus listed in
Chapter 9, enable the designer to sum up the total
pressure requirements for the fan(s). Estimated system air leakage must be added to the system airflow
at this time.
TESTING, ADJUSTING &
A very important step in HVAC duct system design is
to provide the proper physical layout for testing, adjusting and balancing the airflow in the system after
the building is completed. It is essential that sufficient
length of straight duct be provided in an accessible
area so that the TAB personnel can perform their
function properly to determine the total system airflow
with a reasonable degree of accuracy. This same
thought also applies to critical branch ducts of the
supply air system. This subject will be discussed in
more detail in Chapter 10; however, it is important
that the designer indicate all necessary balancing
dampers and devices on the drawings.
Assuming that all of the steps mentioned earlier in
this chapter have been followed, the final plans can
be drawn and the specifications completed. Using the
pre-liminary design (usually a single line drawing) as
a guide, a double line duct system is shown on the
final mechanical drawings, employing the symbols
commonly used for ventilation and air conditioning
(see Figures 4-1 to 4-3). Adequate detail must be
employed to accurately convey to the installing HVAC
contractor what types of fittings are required and the
locations of equipment, ductwork, fire and smoke
dampers, balancing dampers, etc., so that the installed system will function within the design parameters and meet applicable code requirements.
SAMPLE SITUATION: WITH A TERMINAL REQUIRING 40Pa STATIC PRESSURE.
A BRANCH DAMPER REQUIRING 40 Pa SP DUCT DESIGNED FOR 0.8Pa/m
SP LOSS AND FITTING LOSSES EQUAL TO THE STRAIGHT DUCT LOSS. THE
CIRCUIT CAN BE 30 METRES LONG BEFORE 125 Pa LOSS IS EXCEEDED.
Figure 4-2 DUCT PRESSURE CLASS DESIGNATION