Characteristics Of Airflow In Ducts; General; Pressure Changes Within A Duct - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
600
550
500
450
400
350
300
250
4
200
400
150
100
50
0
5
0
AIRFLOW m
CURVE A IS FOR ORIGINAL DUCT RESISTANCE.
CURVE B IS FOR HIGHER DUCT RESISTANCE DUE
TO VOLUME CONTROL DAMPERS CLOSING.
Fig. 7. Combination of Fan and System Curves.
The fan curves shown are for a fan running at two speeds,
400 rpm and 600 rpm. Also, two system curves, A and B, have
been plotted. The intersection of the system curves and the fan
curves indicate the quantities of air the fan will provide. With
System Curve A, if the fan is running at 600 rpm, it will deliver
3
10.75 m /s at 180 Pa (Point 1). With the same system curve
(A), if the fan is running at 400 rpm, it will deliver 7.4 m
75 Pa (Point 2).
System Curve B shows increased resistance of the duct
system due to dampers throttling or filters clogging. With
System Curve B, if the fan is running at 600 rpm, it will deliver
8.0 m 3 /s at 380 Pa (Point 3). With the same system curve (B), if
the fan is running at 400 rpm, it will deliver 5.1 m
(Point 4).
CHARACTERISTICS OF
AIRFLOW IN DUCTS

GENERAL

Supply and return ducts can be classified by application and
pressure (ASHRAE 1996 Systems and Equipment Handbook).
HVAC systems in public assembly, business, educational, general
factory, and mercantile buildings are usually designed as
commercial systems. Air pollution control systems, industrial
exhaust systems, and systems outside the pressure range of
commercial system standards are classified as industrial systems.
Classifications are as follows:
Residences— 125 Pa to 250 Pa.
Commercial Systems— 125 Pa to 2.5 kPa.
Industrial Systems—Any pressure.
B
5
3
600
RPM
A
1
RPM
2
10.0 15.0
7.4 10.75
3
/s
C4075
3
/s at 170 Pa
The quantity of air flowing in a duct can be variable or
constant, depending on the type of system. See TYPES OF
AIRFLOW SYSTEMS.

PRESSURE CHANGES WITHIN A DUCT

For air to flow within a duct, a pressure difference must exist.
The fan must overcome friction losses and dynamic (turbulent)
losses to create the necessary pressure difference. Friction losses
occur due to air rubbing against duct surfaces. Dynamic losses
occur whenever airflow changes velocity or direction. The
pressure difference required to move air must be sufficient to
overcome these losses and to accelerate the air from a state of
rest to a required velocity.
In HVAC systems, the air supplied by the fan includes two
types of pressures: velocity pressure and static pressure. Velocity
pressure is associated with the motion of air and is kinetic
energy. Static pressure is exerted perpendicularly to all walls
of the duct and is potential energy. Velocity and static pressure
are measured in pascals (Pa). Total pressure is the sum of the
static and velocity pressure and, therefore, is also measured in
pascals.
In an airflow system, the relationship between velocity
pressure and velocity is:
3
/s at
V = 1.3 VP
NOTE: See Velocity Pressure in DEFINITIONS for a
derivation of this formula.
If the velocity and duct size are known, the volume of airflow
can be determined:
Q = AV
Where:
Q = Airflow in cubic meters per second ( m
A = Cross-sectional area of duct in square meters
(m
V = Velocity in meters per second (m/s)
Examples of the relationships between total, velocity, and
static pressures are shown in Figure 8A for positive duct static
pressures and Figure 8B for negative duct static pressures. When
static pressure is above atmospheric pressure it is positive and
when below atmospheric pressure it is negative. The examples
use U-tube manometers to read pressure. The sensor connected
to the U-tube determines the type of pressure measured.
274 ENGINEERING MANUAL OF AUTOMATIC CONTROL
2
)
3
/s)