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INTRODUCTION
This section provides basic information on pneumatic control
systems and components commonly used to control equipment
in commercial heating and air conditioning applications. The
information in this section is of a general nature in order to
explain the fundamentals of pneumatic control. Some terms
and references may vary between manufacturers (e.g., switch
port numbers).
Pneumatic control systems use compressed air to operate
actuators, sensors, relays, and other control equipment.
Pneumatic controls differ from other control systems in several
ways with some distinct advantages:
— Pneumatic equipment is inherently proportional but can
provide two-position control when required.
DEFINITIONS
Actuator: A mechanical device that operates a final control
element (e.g., valve, damper).
Authority (Reset Authority or Compensation Authority): A
setting that indicates the relative effect a compensa-
tion sensor input has on the main setpoint (expressed
in percent).
Branch line: The air line from a controller to the controlled device.
Branchline pressure (BLP): A varying air pressure signal from
a controller to an actuator carried by the branch line.
Can go from atmospheric to full main line pressure.
Compensation changeover: The point at which the
compensation effect is reversed in action and changes
from summer to winter or vice versa. The percent of
compensation effect (authority) may also be changed
at the same time.
Compensation control: A process of automatically adjusting
the control point of a given controller to compensate
for changes in a second measured variable such as
outdoor air temperature. For example, the hot deck
control point is reset upward as the outdoor air
temperature decreases. Also know as "reset control".
Compensation sensor: The system element which senses a
variable other than the controlled variable and resets
the main sensor control point. The amount of this effect
is established by the authority setting.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
PNEUMATIC CONTROL FUNDAMENTALS
— Many control sequences and combinations are possible
with relatively simple equipment.
— Pneumatic equipment is suitable where explosion hazards
exist.
— The installed cost of pneumatic controls and materials
may be lower, especially where codes require that low-
voltage electrical wiring for similar electric controls be
run in conduit.
— Quality, properly installed pneumatic equipment is
reliable. However, if a pneumatic control system requires
troubleshooting or service, most building-maintenance
people have the necessary mechanical knowledge.
Control point: The actual value of the controlled variable
(setpoint plus or minus offset).
Controlled variable: The quantity or condition that is measured
and controlled (e.g., temperature, relative humidity,
pressure).
Controller: A device that senses the controlled variable or receives
an input signal from a remote sensing element, compares
the signal with the setpoint, and outputs a control signal
(branchline pressure) to an actuator.
Differential: A term that applies to two-position devices. The
range through which the controlled variable must pass
in order to move the final control element from one to
the other of its two possible positions. The difference
between cut-in and cut-out temperatures, pressures, etc.
Direct acting (DA): A direct-acting thermostat or controller
increases the branchline pressure on an increase in the
measured variable and decreases the branchline pressure
on a decrease in the variable. A direct-acting actuator
extends the shaft on an increase in branchline pressure
and retracts the shaft on a decrease in pressure.
Discharge air: Conditioned air that has passed through a coil.
Also, air discharged from a supply duct outlet into a
space. See Supply air.
Final control element: A device such as a valve or damper
that acts to change the value of the manipulated
variable. Positioned by an actuator.
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