Table of Contents
Advertisement
A wide proportional band setting (10 times the maximum
duct static pressure at the fan discharge) on the fan control is a
good starting point and ensures stable fan operation. Integral
action is necessary to eliminate offset error caused by the wide
proportional band. Integral times should be short for quick
response. The use of inverse derivative, which essentially slows
system response, does not produce the combination of stability
and fast response attainable with wide proportional band and
integral control modes. (See the Control Fundamentals section
for more information on proportional band and integral action.)
Inlet vane dampers, variable pitch blades (vane axial fans),
or variable speed drives are used to modulate airflow (both
supply and return). Actuators may require positive positioning
to deal with nonlinear forces. Variable speed drives, especially
variable frequency, provide excellent fan modulation and
control as well as maximum efficiency.
Duct Static High-Limit Control
High-limit control of the supply fan duct static should be
used to prevent damage to ducts, dampers, and air terminal
units (Fig. 25). Damage can occur when fire or smoke dampers
in the supply duct close or ducts are blocked, especially during
initial system start-up. Fan shut-down and controlling high-
limit are two techniques used to limit duct static. Both
techniques sense duct static at the supply fan discharge.
SUPPLY
FAN
DUCT STATIC
HIGH-LIMIT SENSOR
Fig. 25. Duct Static High-Limit Control.
Fan shut-down simply shuts down the fan system (supply
and return fans) when its setpoint is exceeded. High-limit control
requires a manual restart of the fans and should be a discrete
component separate from the supply fan primary control loop.
The fan shut-down technique is lowest in cost but should not
be used with smoke control systems where continued fan
operation is required.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
AIR TERMINAL
UNITS
C2620
281
A controlling high-limit application is used when the fan
system must continue to run if duct blockage occurs, but its
operation is limited to a maximum duct static. For example, a
fire or smoke damper in the supply duct closes causing the
primary duct static pressure sensor to detect no pressure. This
would result in maximum output of the supply fan and
dangerously high static pressure if the controlling high pressure
limit is not present. A controlling high-limit control will
modulate the fan to limit its output to the preset maximum duct
static (Fig. 26).
DUCT
STATIC
HIGH-LIMIT
SENSOR
SMOKE DAMPERS
SUPPLY
FAN
SMOKE DAMPERS
Fig. 26. Controlling Static High-Limit.
RETURN FAN CONTROL FOR VAV SYSTEMS
Return fan operation influences building (space)
pressurization and minimum outdoor air. There are four
techniques to control the return fan: open loop, direct building
static, airflow tracking, and duct static.
Open Loop Control
Open loop control (Fig. 27) modulates the return fan without
any feedback. This type of control presumes a fixed relationship
between the supply and return fans, controls the return fan in
tandem with the supply fan, and changes the output of the return
fan without measuring the result. Open loop control requires
similar supply and return fan operating characteristics.
Therefore, a careful analysis of the supply and return fan
operating curves should be done before selecting this technique.
Also, accurate balancing is essential to ensure proper
adjustment at maximum and minimum operating points.
Mechanical linkage adjustments or other means are used to
adjust the differential between the two fans for desired flows
and to minimize tracking errors at other operating points. With
digital control, software can be used to align the fan loading
relationships, to vary exhaust effects, and to offset dirty filter
effects to minimize flow mismatches.
DUCT STATIC
PRESSURE SENSORS
DUCT STATIC
PRESSURE SENSORS
AIR TERMINAL UNITS
C2621
Advertisement
Chapters
Table of Contents
