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INTRODUCTION
This section explains the need for airflow control in a central
air handling system, describes the various means of airflow
measurement, provides fan and duct characteristics, and
discusses suggested means of airflow control. The final control
system design depends, of course, on specific job requirements.
There are several types of airflow control that relate directly
to the control of airflow in a central air handling system. These
types of airflow control include space pressurization, zone
pressurization, and exhaust air control. Space or zone
pressurization is used when an enclosed area within a building
(e.g., a clean room, hospital space, laboratory, fire and smoke
control area) must be kept at a positive or negative pressure so
DEFINITIONS
Airflow: The rate at which a volume of air moves through a
duct. In this section, airflow is denoted Q and is
measured in cubic meters per second (m
is derived:
Q = A x V
AVG
Where:
Q = Airflow in m
A = Cross-sectional area of duct in
square meters (m
V
= Average velocity
AVG
Axial fan: A propeller type fan where airflow within the wheel
is substantially parallel to the shaft and in-line with
the duct. Axial fan airflow can be controlled by speed,
variable inlet vanes, or variable pitch blades depending
on the fan type.
Centrifugal fan: A fan where airflow within the wheel is
substantially radial to the shaft and the air must make
two turns before being expelled from the fan housing.
Centrifugal fan airflow can be controlled by speed,
variable inlet vanes, or less commonly by dampers.
Constant Air Volume (CAV) system: A central fan system in
which airflow in the duct is maintained at a constant
volume.
Differential: The difference between supply and return airflows
necessary to maintain a positive or a negative pressure
in an area. For example, if supply airflow is 1.0 m
and return airflow is 0.8 m
3
is 0.2 m
/s. The 0.2 m
through exhaust fans or vents and exfiltration.
Duct: A circular or rectangular tube for conveying air.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
3
/s). Airflow
3
/s
2
)
3
3
/s, the differential (positive)
3
/s surplus leaves the building
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
contaminated air does not migrate to unwanted areas. Basic
types of space pressure control are static pressure, airflow
tracking, and constant airflow. Exhaust air control regulates
the amount of air exhausted to keep it at the minimum safe
level. Space pressure control is generally required with exhaust
air control, and control of airflow in a central air handling system
is generally required with space pressure control and/or exhaust
air control.
For information on air terminal units used in building airflow
control system applications, refer to the Individual Room
Control Applications section.
Duct cross-sectional area: For round ducts, the duct cross-
sectional area is r
rectangular ducts, the duct area is X times Y, where X
and Y are the height and width dimensions. In this
section, a duct cross-sectional area is measured in
square meter (m
Duct diameter: For round ducts, the diameter is twice the radius
(2r). For rectangular ducts, an equivalent diameter is
derived: 2XY (X + Y), where X and Y are the height
and width.
Fan surge: A condition that occurs when air passing over the fan
blades causes a stall. A fan surge causes a fluctuation in
duct static pressure and an increase noise level.
Flow Measuring Station (FMS): A device containing multiple
static pressure sensors and multiple total pressure
sensors manifolded separately for instantaneously
measuring average pressures across the face of a duct.
Impact tube: A sensing device with a single opening that points
directly into the airstream for measuring total pressure.
Manometer: An instrument for measuring low pressure such
as static pressure.
Pitot tube: A sensing device containing both an impact tube
and a static pressure tube in a single probe.
Static pressure: The pressure created by air (whether in motion
or not) confined in an enclosed area such as a duct or
/s
building due to its potential energy. Static pressure,
denoted SP, is exerted perpendicularly on all interior
walls of the enclosure (duct or building) with respect to
a reference pressure outside the enclosure. When static
pressure is above atmospheric pressure it is positive and
when below atmospheric pressure it is negative.
265
2
, where r is the radius. For
2
).
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