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PNEUMATIC CONTROL FUNDAMENTALS
Figure 25 shows a typical application of a thermostat and
velocity controller on a Variable Air Volume (VAV) terminal
unit with hot water reheat. The thermostat senses a change in
room temperature and resets the velocity setpoint of the velocity
controller. The controller repositions the VAV damper to
increase or decrease airflow accordingly. If a change in duct
static pressure modifies the flow, the controller repositions the
actuator to maintain the correct flow. The reheat valve operates
only when the thermostat has reset the velocity setpoint down
to minimum airflow and the thermostat calls for heating.
ACTUATORS AND FINAL CONTROL ELEMENTS
A pneumatic actuator and final control element such as a valve
(Fig. 26) or damper (Fig. 27) work together to vary the flow of
the medium passing through the valve or damper. In the actuator,
a diaphragm and return spring move the damper push rod or valve
stem in response to changes in branchline pressure.
DIAPHRAGM
BRANCH
LINE
SPRING
VALVE
STEM
INLET
FLOW
Fig. 26. Pneumatic Actuator and Valve.
DAMPER
ROLLING
DIAPHRAGM
PISTON
AIRFLOW
SPRING
PUSH
ROD
Fig. 27. Pneumatic Actuator and Damper.
VALVE
ACTUATOR
OUTLET
VALVE
FLOW
M10361
BRANCH
LINE
DAMPER
ACTUATOR
C2611
M
VAV BOX
DAMPER
REHEAT VALVE
ACTUATOR
ROOM
THERMOSTAT
Fig. 25. VAV Box Velocity Controller Control System.
ACTUATORS
GENERAL
Pneumatic actuators position damper blades and valve
stems. A damper actuator typically mounts on ductwork or on
the damper frame and uses a push rod and crank arm to position
the damper blades (rotary action). A valve actuator mounts on
the valve body and positions the valve stem directly (linear
action) for a globe valve or rotary action via linkage for a
butterfly valve. Valve actuator strokes typically are between 6
and 38 millimeters. Damper actuator strokes range from one
to four inches (longer in special applications). In commercial
pneumatic actuators, air pressure positions the actuator in one
direction and a spring returns it the other direction.
Valve actuators are direct or reverse acting. Damper actuators
are direct acting only. A direct-acting actuator extends on an
increase in branchline pressure and retracts on a decrease in
pressure. A reverse-acting actuator retracts on an increase in
branchline pressure and extends on a decrease in pressure.
Pneumatic valve and damper actuator assemblies are termed
"normally open" or "normally closed." The normal position is
the one assumed upon zero actuator air pressure. Three-way valves
have both normally open (N.O.) and normally closed (N.C.) ports.
SPRING RANGES
Springs used in valve and damper actuators determine the
start pressure and pressure change required for full movement
of the actuator from open to closed, or from closed to open.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
74
VELOCITY
CONTROLLER
VELOCITY
SENSOR IN
DUCTWORK
VAV BOX
DAMPER
M10296
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