Introduction; Definitions - Honeywell AUTOMATIC CONTROL Engineering Manual

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 feet per minute (cfm). Airflow is
derived:
Q = A x V
AVG
Where:
Q = Airflow in cfm
A = Cross-sectional area of duct in
square feet (ft
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 1800 cfm
and return airflow is 1500 cfm, the differential
(positive) is 300 cfm. The 300 cfm surplus leaves the
building through exhaust fans or vents and exfiltration.
Duct: A circular or rectangular tube for conveying air.
2
)
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 πr
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
2
square feet (ft
NOTE: If duct dimensions are in inches (in.) and
the result of the duct area is in square
inches (in
to obtain square feet (ft
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.
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ENGINEERING MANUAL OF AUTOMATION CONTROL
2
, where r is the radius. For
).
2
), divide the result by 144 in
2
).
2 2
/ft