ENGINEERING MANUAL OF AUTOMATIC CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
266
• Peak Velocity—The greatest air velocity occurring in an
increment of a duct cross-section. Peak velocity is
denoted V
PK
.
• Velocity Pressure: The pressure created by air moving
at a velocity due to its kinetic energy. Velocity pressure,
denoted VP, is always exerted in the direction of airflow
and is always a positive value. Velocity pressure and
velocity are related by the equation:
Where:
V = Velocity in m/s
VP = Velocity pressure in pascals (Pa)
Da = Density of the air flowing in the
duct measured in kilograms per
cubic meter (kg/m
3
)
The density of air (Da) is 1.2 kg/m
3
(at 20°C, 101.325
kPa atmospheric pressure, and 50 percent relative
humidity). With this data, the relationship of velocity
to velocity pressure is simplified:
This equation reduces to:
See General Engineering Data section for Velocity vs.
Velocity Pressure table.
Static pressure sensor or tube: A sensing device with several
holes perpendicular to an airstream for measuring
static pressure.
Total pressure: The algebraic sum of Velocity Pressure (VP) plus
Static Pressure, denoted TP. Total pressure is derived:
TP = VP + SP
Turndown: The relationship, in percent, between the maximum
minus the minimum airflow to the maximum airflow.
For example, in a system with a maximum airflow of
1 m
3
/s and minimum airflow of 0.2 m
3
/s, the turndown
is 80 percent.
Variable Air Volume (VAV) system: A central fan system in
which airflow in the duct varies depending on the
instantaneous load requirements of the connected VAV
terminal units.
Velocity: The speed or rate of flow of the air stream in a duct.
In this section, velocity is denoted V and is measured
in meters per second (m/s). See General Engineering
Data section.
• Average Velocity—The sum of the air velocities from
equal area increments of a duct cross-section divided by
the number of increments. Average velocity, denoted
V
AV G
, is derived:
Where
N = Number of duct increments
Turndown % =
()
Max Flow – Min Flow
Max Flow
x 100
V
AVG
= ∑ (V
1
+ V
2
+ V
3
+…… + V
N
) / N
V = VP
2
D
V = VP
2
1.2
V = 1.3 VP
AIRFLOW CONTROL FUNDAMENTALS
NEED FOR AIRFLOW CONTROL
Proper control of airflow is important to physiological
principles including thermal and air quality considerations. Air
distribution systems, containment pressurization, exhaust
systems, and outdoor air dilution are examples of airflow control
systems used to meet ventilation requirements. Life safety
requirements are also met with fire and smoke control systems
using airflow control functions. Therefore, an understanding
of airflow control is required to provide the various locations
in a building with the necessary conditioned air.
One means of maintaining indoor air quality is to dilute
undesirable materials (e.g., carbon dioxide, volatile organic
compounds) with outdoor air. It is important to understand the
control of outdoor air airflow rates in order to:
— Increase outdoor airflow rates when needed for dilution
ventilation
— Prevent excessive building and space pressurization
— Minimize outdoor airflow rates when possible to limit
energy costs
WHAT IS AIRFLOW CONTROL
In HVAC systems, a well designed combination of fans, ducts,
dampers, airflow sensors, static pressure sensors, air terminal
units, and diffusers is necessary to provide conditioned air to
the required spaces. The function of airflow control is to sense
and control the static pressures and airflows of the building.
The static pressures occur in ducts and building spaces; airflows
occur in ducted air supplies, returns, and exhausts.
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