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In cases where manufacturer's performance ratings
do not include the effect of the bearings and supports,
it will be necessary to compensate for this inlet restriction, if possible by use of the fan manufacturer's
allowance for bearings in the fan inlet.
If no better data is available, an approximation may
be made as described under "Obstructed Inlets" in
subsection B of this section.

2. Drive Guards
Most fans may require a belt drive guard in the area
of the fan inlet. Depending on design, the guard may
be located at the plane of the inlet, along the casing
side sheet or it may be "stepped out" due to "stepped
out" bearing pedestals.
In any case, depending on the location of the guard
and on the inlet velocity, the fan performance may be
significantly affected by this obstruction.
It is desirable that a drive guard located in this position be furnished with as much opening as possible
to allow maximum airflow to the fan inlet. However,
the guard design must comply with any Occupational
Health and Safety Act requirements or any other applicable codes.
If available, use the fan manufacturer's allowance for
drive guards obstructing the fan inlet. System Effect
Curves for drive guard obstructions situated at the
inlet of a fan may be approximated using Figures 621, 6-22, and Table 6-3.
Where possible, open construction on guards is recommended to allow free air passage to the inlet.






Guards and sheaves should be designed to obstruct
as little of the inlet as possible and in no case should
the obstruction be more than 1/3 of the inlet area.

3. Belt Tube in Axial Fans
With a belt-driven axial flow fan, it is usually necessary that the fan motor be mounted outside the fan
To protect the belts from the airstream and also to
prevent any leakage from the fan housing, manufacturers, in many cases, provide a belt tube.
Most manufacturers include the effects of this belt
tube in their rating tables; however, in cases where
this is not reflected, the appropriate System Effect
Curves obtained from Table 6-3 may be used.

4. Factory Made Inlet Boxes
The "System Effect" of fan inlet boxes can vary
widely, depending upon the design. This data should
be available from the fan manufacturer. In the absence of fan manufacturer's data, a well designed
inlet box should approximate System Effect Curves
"S" or "T" of Figure 6-1.
Inlet box dampers may be used to control the airflow
volume through the system. Either parallel or opposed blade types may be used.
The parallel blade type is installed with the blades
parallel to the fan shaft so that, in a partially closed
position, a forced inlet vortex will be generated. The
effect on the fan characteristics will be similar to that
of inlet vane control.




The opposed blade type is used to control airflow
volume by changing the system by the addition of the
pressure loss created by the damper in a partially
closed position.
If possible, complete data should be obtained from
the fan manufacturer giving the "System Effect" or
pressure loss of the inlet box and damper over the
range of application. If data is not available, System
Effect Curves "S" or "T" from Figure 6-1 should be
applied in making the fan selection.

5. Inlet Vane Control
To maintain fan efficiency at reduced flow conditions,
airflow quantity is often controlled by variable vanes
mounted in the fan inlet (see Figure 6-24).
These are arranged to generate a forced inlet vortex
which rotates in the same direction as the fan impeller.
Inlet vanes may be of two different basic types:
1. Integral (built-in)
2. Cylindrical (add on).
The "System Effect" of a wide open inlet vane must
be accounted for in the original fan selection. This
data should be available from the fan manufacturer.
If not, the System Effect Curves of Table 6-4 should
be applied in making the fan selection using Figure



The HVAC system designer is responsible for the layout of the equipment room and the equipment duct
connection configuration. Therefore System Effect
Factors can be noted and included in the system total
pressure loss/fan capacity calculations.
Using a fan similar to that in the duct system example
in Figures 7-2 or 8-2 of Chapters 7 or 8, the fan is in
a plenum having adequate clearance for air entry to
the fan inlet. However, the fan contains integral inlet
vanes. With the blades wide open (Table 6-4), Sys-









tem Effect Curve "Q" will be used in Figure 6-1 to
determine the static pressure loss. The manufacturer's literature indicates that the selected 48 inch
(1220 mm) SWSI fan has an inlet and outlet area of
13.1 square feet (1.22 m2) each. At 20,000 cfm
(10,000 I/s) and 2.4 in. w.g. (600 Pa) static pressure,
the velocities are 1527 fpm (776 m/s). From Figure
6-1, reading up from 1527 fpm (776 m/s) to the "Q"
curve gives a System Effect Factor of 0.23 in. w.g.
(57 Pa) for the inlet side of the fan. This becomes
part of the static pressure derating of the fan.
The fan discharge size for this example is 43 inches
(1092 mm) wide by 44 inches (1118 mm) high and the

blast area ratio is 0.8. The 1.5 R/W elbow (the duct
size is the same as the fan discharge size) is located
30 inches (760 mm) from the fan discharge, which
would result in an approximately "25% effective duct"
in position A (see Figures 6-2 and 6-3). From Table
6-2, the System Effect Factor Curve "T" or "U" is
selected to be used in Figure 6-1. At 1527 fpm (776
m/s), both curves are off the graph, so no System
Effect Factor would be added for the discharge side
of the fan. Therefore the fan would be rated at 2.17
in. w.g. (2.4-0.23) or 543 Pa (600-57) static pressure.
In many cases, a duct transition is used at the fan
discharge connection (normally made with a flexible
connection). Then the velocity in the duct has no relationship with the fan discharge velocity unless it
falls within the parameters discussed earlier in "Outlet Ducts" of Subsection A.
It is important to note again that System Effect cannot
be measured in the field by testing and balancing
technicians. Therefore the system 'designer should
deduct System effect from the fan capacity rather
than adding it to the total pressure loss of the HVAC



For duct sizing procedures using S.I. units or the
metric system, see chapter 8.
1. The total pressure (TP) at any location within a
system is the sum of the static pressure
(SP)and the velocity pressure (Vp).
2. Total pressure always decreases algebraically
in the direction of airflow (negative values of
return air or exhaust systems increase in the
direction of airflow, and positive values of supply
air systems decrease in the direction of airflow).
See Figure 5-10 and the text on page 5.11.
3. The losses in total pressure between the fan
and the end of each branch of a system are the
4. Static pressure and velocity pressure are mutually convertible and either can increase or decrease in the direction of flow.


1. Design the duct system to convey the design
airflow from the fan to the terminal devices in
the most efficient manner as allowed by the
building structure.
2. Consider energy conservation in the fan selection, duct configuration, duct wall heat gain or
loss, etc.
3. Special consideration should be given to the
need for sound attenuation and breakout noise.
4. Testing, adjusting and balancing equipment and
dampers should be shown on the drawings.
5. Locations of all life safety devices such as fire
dampers, smoke dampers, etc. should be
shown on the drawings.
6. The designer should consider the pressure
losses that occur from tie rods and other duct
7. If the ductwork is well designed and con-

structed, at least 75 to 90 percent of the original
velocity pressure can be regained.
8. Round ducts generally are preferred for higher
pressure systems.
9. Branch takeoffs and fittings with low loss coefficients should be used. Both 900 and 450 duct

takeoffs can be used. However, the use of conical tees or angular takeoffs can reduce pressure losses.
10. Use of the SMACNA Duct Design Calculators
would aid the duct design process, especially
when making changes in the field.



1. Introduction
The "equal friction" method of duct sizing probably
has been the most universally used means of sizing
low pressure supply air, return air and exhaust air
duct systems and it is being adapted by many for use
in medium pressure systems. It normally has not
been used for sizing high pressure systems. This
design method "automatically" reduces air velocities
in the direction of the airflow, so that by using a reasonable initial velocity, the chances of introducing airflow generated noise from high velocities are reduced
or eliminated. When noise is an important consideration, the system velocity readily may be checked at
any point. There is then the opportunity to reduce
velocity created noise by increasing duct size or adding sound attenuation materials (such as duct lining).
The major disadvantages of the equal friction method
are: (1) there is no natural provision for equalizing
pressure drops in the branches (except in the few
cases of a symmetrical layout); and (2) there is no
provision for providing the same static pressure behind each supply or return terminal device. Consequently, balancing can be difficult, even with a considerable amount of dampering in short duct runs.
However, the equal friction method can be modified
by designing portions of the longest run with different
friction rates from those used for the shorter runs (or
branches from the long run).



Static regain (or loss) due to velocity changes, has
been added to the equal friction design procedure
by using fitting pressure losses calculated with new
loss coefficient tables in Chapter 14. Otherwise, the
omission of system static regain, when using older
tables, could cause the calculated system fan static
pressure to be greater than actual field conditions,
particularly in the larger, more complicated systems.
Therefore, the "modified equal friction" low
pressure duct design procedure presented in
this subsection will combine the advantages of
several design methods when used with the
loss coefficient tables in Chapter 14.

2. Modified Equal Friction Design
"Equal friction" does not mean that total friction remains constant throughout the system. It means that
a specific friction loss or static pressure loss per 100
equivalent feet of duct is selected before the ductwork
is laid out, and that this loss per 100 feet is used
constantly throughout the design. The figure used for
this "constant" is entirely dependent upon the experience and desire of the designer, but there are practical limits based on economy and the allowable velocity range required to maintain the low pressure
system status.
To size the main supply air duct leaving the fan, the
usual procedure is to select an initial velocity from
the chart in Figure 14-1. This velocity could be selected above the shaded section of Figure 14-1 if
higher sound levels and energy conservation are not
limiting factors. The chart in Figure 14-1 is used to
determine the friction loss by using the design air
quantity (cfm) and the selected velocity (fpm). A friction loss value commonly used for lower pressure
duct sizing is 0.1 in. of water (in.w.g.) per 100 equivalent feet of ductwork, although other values, both
lower and higher, are used by some designers as
their "standard" or for special applications. This
same friction loss "value" generally is maintained
throughout the design, and the respective round duct
diameters are obtained from the chart in Figure 14-1.
The friction losses of each duct section should be
corrected for other materials and construction methods by use of Table 14-1 and Figure 14-3. The correction factor from Figure 14-3 is applied to the duct
friction loss for the straight sections of the duct prior
to determining the round duct diameters. The round
duct diameters thus determined are then used to se-






lect the equivalent rectangular duct sizes from Table
14-2, unless round ductwork is to be used.
The flow rate (cfm) in the second section of the main
supply duct, after the first branch takeoff, is the original cfm supplied by the fan reduced by the amount
of cfm into the first branch. Using Figure 14-1, the
new flow rate value (using the recommended friction
rate of 0.1 in. w.g. per 100 ft.) will determine the duct
velocity and diameter for that section. The equivalent
rectangular size of that duct section again is obtained
from Table 14-2 (if needed). All subsequent sections
of the main supply duct and all branch ducts can be
sized from Figure 14-1 using the same friction loss
rate and the same procedures.
The total pressure drop measured at each terminal
device or air outlet (or inlet) of a small duct system,
or of branch ducts of a larger system, should not differ
more than 0.05 in. w.g. If the pressure difference
between the terminals exceeds that amount, dampering would be required that could create objectionable air noise levels.
The modified equal friction method is used for sizing
duct systems that are not symmetrical or that have
both long and short runs. Instead of depending upon
volume dampers to artificially increase the pressure
drop of short branch runs, the branch ducts are sized
(as nearly as possible) to dissipate (bleed-off) the
available pressure by using higher duct friction loss
values. Only the main duct, which usually is the longest run, is sized by the original duct friction loss value.
Care should be exercised to prevent excessively high
velocities in the short branches (with the higher friction rates). If calculated velocities are found to be too
high, then duct sizes must be recalculated to yield
lower velocities, and opposed blade volume dampers
or static pressure plates must be installed in the
branch duct at or near the main duct to dissipate the
excess pressure. Regardless, it is a good design
practice to include balancing dampers in HVAC duct
systems to balance the airflow to each branch.

3. Fitting Pressure Loss Tables
Tables 14-10 to 14-18 contain the loss coefficients for
elbows, fittings, and duct components. The "loss
coefficient" represents the ratio of the total pressure
loss to the dynamic pressure (in terms of velocity

pressure). It does not include duct friction loss
(which is picked up by measuring the duct sections
to fitting center lines). However, the loss coefficient
does include static regain (or loss) where there is a
change in velocity.