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L. TESTING, ADJUSTING AND BALANCING (TAB)
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
Figure 4-3 SYMBOLS FOR VENTILATION & AIR CONDITIONING
(U.S. and/or Metric Units)
DUCT DESIGN FUNDAMENTALS
An HVAC air distribution system may consist simply
of a fan with ductwork connected to either the inlet or
discharge or to both. A more complicated system may
include a fan, ductwork, air control dampers, cooling
coils, heating coils, filters, diffusers, sound attenuation, turning vanes, etc. The fan is the component or
"air pump" in the system which provides energy to
the airstream to overcome the resistance to flow of
the other components. The discussion in this Section
A and the accompanying tables and figures on fan
and system curves were developed by the Air Moving
and Conditioning Association, Inc. and reprinted with
some minor editing with their permission. (AMCA
Publication 201--"Fans and Systems").
1. Component Losses
Each duct system has a combined set of pressure
resistances to flow which are usually different from
every other system and are dependent upon individual duct system components.
The amount of the total pressure drop or resistance
to flow for the individual duct system components can
be obtained from the component manufacturer. For
preliminary computations, some pressure data is
available in Chapter 9.
2. System Curves
At a fixed volume air flow rate through a given air
distribution system, a corresponding pressure loss or
resistance to this flow will exist. If the flow rate is
changed, the resulting pressure loss or resistance to
flow also will change. The relationship governing this
change is given by the following system equation:
Typical plots of the resistance to flow versus the airflow rate establish the system curves for three different and arbitrary fixed systems, (A, B and C), illustrated in Figure 5-1. For a fixed system, an increase
or decrease in the system airflow rate volume will
increase or decrease the system resistance along the
given system curve only.
Refer to System Curve A on Figure 5-1. Assume a
system design point at 100 percent volume and 100
percent resistance. If the airflow rate volume is increased to 120 percent of design volume, the system
resistance will increase to 144 percent of the design
resistance in accordance with the system equation.
A further increase in volume results in a corresponding increase in system pressure. A decrease in volume flow to 50 percent of design airflow volume would
result in a decrease to 25 percent of the design resistance.
Notice that on a percentage basis, the same relationships also hold for the System Curves B and C.
These relationships are characteristic of typical fixed
3. System Curve/Fan Curve
If the system curve, composed of the resistance to
flow of the system and the appropriate "System Effect
Factors," (discussed later in this section) has been
accurately determined, then it is assumed that the
fan selected will develop the necessary pressure to
meet the system requirements at the designed airflow
(cfm or l/s).
The point of intersection of the system curve and the
fan performance curve determines the actual airflow
volume. If the system resistance has been accurately
determined and the fan properly selected, their performance curves will intersect at the design airflow.
(See Figure 5-2). The normalized System Curve A
from Figure 5-1 has been plotted with a normalized
fan performance curve. The 100 percent design airflow volume of the system curve was arbitrarily selected to intersect at 60 percent of the free delivery
airflow volume of the fan.
The airflow rate volume through the system in a given
installation may vary from changes in the system re-
PERCENT OF DUCT SYSTEM AIRFLOW VOLUME (cfm or l/s)
Figure 5-2 INTERACTION OF SYSTEM CURVES AND FAN CURVE (1)
sistance, usually from fan dampers, duct dampers,
mixing boxes, terminal units, etc. Referring to Figure
5-2, the airflow volume rate may vary from 100 percent design airflow (Point 1, Curve A), to approximately 80 percent of the design airflow by increasing
the resistance to flow, thus changing the system
curve characteristic to Curve B. This results in fan
operation at Point 2 (the intersection of the fan curve
and the new System Curve B). Similarly, the airflow
rate can be increased to approximately 120 percent
of the design airflow volume by decreasing the resistance to flow, thus changing the system curve characteristic to Curve C. This results in fan operation at
Point 3 (the intersection of the fan curve and the new
System Curve C).
To review; when system losses have been estimated
accurately, when the duct systems have been fabricated and installed exactly as shown on the drawings
with specified components, then the design airflow
volume can be expected as illustrated in Figure 5-3
at Point 1.
However, when the duct systems have not been estimated accurately or installed as shown, a higher
pressure loss causes the fan to operate at Point 2 of
Figure 5-3, and a lower system pressure loss at Point
3. Again note that the interaction of the installed duct
system curve and the fan curve from actual operating
conditions determine the duct system airflow volume
4. Fan Speed Change Effects
A change in fan speed will alter the airflow volume
rate through a given system as shown by Equation
Airflow rate = cfm (l/s)
Fan Speed = rpm (rad/s)
Figure 5-4 illustrates the increase in system airflow
when the fan speed is increased 10 percent. Any
change in fan speed creates a new fan curve. The
system operating point then moves along the system
curve from Point 1 to Point 2. The 10 percent increase in airflow extracts a severe fan power
penalty. According to the fan laws, the fan power
output must then increase 33 percent.
Figure 5-3 DUCT SYSTEM NOT AT
DESIGN POINT (1)
PERCENT OF DUCT SYSTEM AIRFLOW VOLUME (cfm or I's)
Figure 5-4 EFFECT OF 10 PERCENT INCREASE IN FAN SPEED (1)
Using Equation 5-3:
Fan Power = HP (kW or W)
Fan Speed = rpm (rad/s)
Example 5-1 (U.S.)
A 10 HP fan runs at 500 rpm.
Calculate the HP at 550 rpm.
Using Equation 5-3:
Example 5-1 (Metric)
A 75 kW fan runs at 50 rad/s.
Calculate the fan power at 55 rad/s.
Frequently, the extra horsepower (Watts) is not available from the existing fan motor, and the motor power
wiring is too small to add a larger motor.
This fact is often startling to the system designer who
finds the system short of air. Only 10 percent more air
is needed, but the selected motor horsepower is not
capable of a 33 percent increase in load. The increased power requirements are the result of increased work done. The greater volume flow rate of
air moved by the fan against the resulting higher system resistance to the flow, causes increased work to
be done. In the same system, the fan power increases as the cube of the speed ratio, and fan efficiency remains the same at all points on the same
system curve. (See HVAC Fan Equations in Chapter
Increasing the fan speed also may create problems
for the fan by putting it and possibly the ductwork into
a higher pressure classification. Be sure to review
the fan rating table for pressure class limits or contact
the fan manufacturer to determine if the fan speed
may be increased safely.
5. Air Density Effects
when selecting fans from manufacturers' catalogs or
curves (fan airflow volume is constant).
The resistance of a duct system is dependent on the
density of the air (or gas) flowing through the system.
Air at standard conditions has a density of 0.075 Ib/
cu.ft. (1.204 kg/m3). Figure 5-5 illustrates the effect
on the fan performance of a density variation from
this standard value.
The fan pressure and horsepower vary directly as the
ratio of the gas density at the fan inlet to standard
density. This density ratio must always be considered
d = Density-lb/cu.ft. (kg/m3)
TP = Total pressure-in. w.g. (Pa)
Fan Power = bhp (kW)
6. "Safety Factor" Cautions
System designers sometimes add "Safety Factors"
to their estimate of the system resistance to compensate for unknown field conditions. These "Safety Fac-
PERCENT OF DUCT SYSTEM AIRFLOW VOLUME (cfm or I/s)
Figure 5-5 DENSITY EFFECT (1)
tors" may compensate for resistance losses that
were overlooked and the actual system will deliver
design flow (Point 1, Figure 5-3). Occasionally, however, the estimated system resistance, including the
"Safety Factors," is in excess of the actual installed
system conditions. Since the fan has been selected
for design conditions (Point 1), it will deliver more air
(Point 3) because the actual system resistance at the
design flow rate is less than design (Point 4).
This result may not necessarily be an advantage because the fan will usually be operating at a less efficient point on the performance curve and may require
more horsepower than at design flow. Under these
conditions, it may be necessary to reduce the fan
speed or to adjust a damper to increase the actual
system resistance (Curve C) to the original design
characteristic (Curve A).
AFFECTING DUCT SYSTEM
1. System Effect
A "derating" of the HVAC system fan, called "System
Effect" must be taken into account by the system
designer if a realistic estimate of fan/system performance is to be made. It must be appreciated that the
System Effect Factors given in Chapter 6 of this manual are intended as guidelines and are, in general,
approximations. Some have been obtained from research studies, others have been published previously by individual fan manufacturers, and many represent the consensus of engineers with considerable
experience in the application of fans.
Fans of different types and even fans of the same
type, but supplied by different manufacturers, will not
necessarily react with the system in exactly the same
way. It will be necessary, therefore, to apply judgement based on actual experience in applying the System Effect Factors.
Figure 5-6 illustrates deficient fan/system performance resulting from undesirable flow conditions. It
is assumed that the system pressure losses have
been accurately determined (Point 1, Curve A) and a
suitable fan selected for operation at that point. However, no allowance has been made for the effect of
the system connections on the fan's performance. To
compensate for this "System Effect" and to explain
how it works, it will be necessary to add a "System
Effect Factor" to the calculated system pressure
losses to determine the actual system curve. The
System Effect Factor for any given configuration is
dependent on the airflow velocity at that point.
In the example illustrated on Figure 5-6, the point of
intersection between the fan performance curve and
the actual system curve is Point 4. The actual airflow
volume will, therefore, be deficient by the difference
from 1 to 4. To achieve the design airflow volume, a
System Effect Factor equal to the pressure difference
between Points 1 and 2 should have been added to
the calculated system pressure losses and the fan
selected to operate at Point 2. Note, that because
the System Effect is velocity related, the difference
represented between Points 1 and 2 is greater than
the difference between Points 3 and 4.
Chapter 6--"Fan-Duct Connection Pressure
Losses" contains the necessary data, charts and tables needed to determine the System Effect Factors
required by duct connections to HVAC system fans.
The System Effect Factor is given in inches of water
gauge (Pascals) and may be added to the total system pressure losses as shown on Figure 5-6. However, System Effect can not be measured in the field
when the system is being tested and balanced. It can
only be calculated using the data in Chapter 6.
Therefore the HVAC system designer should derate
the HVAC system supply fan by deducting the System Effect Factor from the fan rated capacity (in. w.g.
The velocity figure used in entering the chart will be
either the inlet or the outlet velocity of the fan. This
will be dependent on whether the configuration in
question is related to the fan inlet or the outlet. Most
catalog ratings include outlet velocity figures, but for
centrifugal fans, it may be necessary to calculate the
inlet velocity. The necessary inlet dimensions usually
are included in the fan catalog.
2. Wind Effect
With few exceptions, building intakes and exhausts
cannot be located or oriented for a prevailing wind to
assure HVAC system operation. Wind can assist or
hinder supply air and exhaust air fans depending on
their position on the building, but even in locations
with a predominant wind direction, the ventilating system must perform adequately for all other directions.
Airflow through a wall opening results from positive
or negative external and internal pressures. Such differential pressures may exceed 0.5 in. w.g. (125 Pa)
Figure 5-6 CHANGES FROM
"SYSTEM EFFECT" (1)
during high winds. Supply and exhaust systems, and
openings, dampers, louvers, doors, and windows
make the building flow conditions too complex for
most calculation. The opening and closing of doors
and windows by building occupants add further complications.
Mechanical HVAC systems are affected by wind conditions. A low-pressure wall exhaust fan, 0.05 to 0.1
in w.g. (12 to 25 Pa) can suffer a drastic reduction in
capacity. Flow can be reversed by wind pressures on
windward walls, or its rate can be increased substantially when subjected to negative pressures on
the lee and other sides, Clarke (1967) when measuring HVAC Systems operating at 1 to 1.5 in. [w.g.
(250 to 375 Pa), found flow rate changes of 25 percent for wind blowing into intakes on an L-shaped
building compared to the reverse condition. Such
changes in flow rate can cause noise at the supply
outlets and drafts in the space served.
For mechanical systems, the wind can be thought of
as producing a pressure in series with a system fan,
either assisting or opposing it (Houlihan 1965).
Where system stability is essential, the supply air and
exhaust air systems must be designed for higher
[pressures about 3 to 4 in. w.g. (750 to 1000 Pa)] to
minimize unacceptable variations in flow rate. To conserve energy, the system pressure selected should
be consistent with system needs.
Where building balance and minimum infiltration are
important, consider the following:
a) Fan system design with pressure adequate to
minimize wind effects.
b) Controls to regulate flow rate or pressure or
c) Separate supply and exhaust systems to serve
each building area requiring control or balance.
d) Doors (possibly self-closing) or double-door air
locks to non-controlled adjacent areas, particularly outside doors.
e) Sealing windows and other leakage sources
and closing natural vent openings.