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.
— 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.
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 compensation 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.
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.
Main line: The air line from the air supply system to controllers and other devices. Usually plastic or copper tubing.
Manipulated variable: Media or energy controlled to achieve a desired controlled variable condition.
Measuring element: Same as sensing element.
Mixed air: Typically a mixture of outdoor air and return air from the space.
Modulating: Varying or adjusting by small increments. Also called “proportional”.
Offset: A sustained deviation between the actual system control point and its controller setpoint under stable operating conditions. Usually applies to proportional (modulating) control.
Proportional band: As applied to pneumatic control systems, the change in the controlled variable required to change the controller output pressure from 3 to 13 psi. Usually expressed as a percentage of sensor span.
Reset control: See compensation control.
Restrictor: A device in an air line that limits the flow of air.
Return air: Air entering an air handling system from the occupied space.
Reverse acting (RA): A reverse-acting thermostat or controller decreases the branchline pressure on an increase in the measured variable and increases the branchline pressure on a decrease in the variable. A reverse-acting valve actuator retracts the shaft on an increase in branchline pressure and extends the shaft on a decrease in pressure.
Sensing element: A device that detects and measures the controlled variable (e.g., temperature, humidity).
Sensor: A device placed in a medium to be measured or controlled that has a change in output signal related to a change in the sensed medium.
Sensor Span: The variation in the sensed media that causes the sensor output to vary between 3 and 15 psi.
Setpoint: The value on the controller scale at which the controller is set (e.g., the desired room temperature set on a thermostat). The desired control point.
Supply air: Air leaving an air handling system.
Thermostat: A device that responds to changes in temperature and outputs a control signal (branchline pressure). Usually mounted on a wall in the controlled space.
Throttling range: Related to proportional band, and expressed in values of the controlled variable (e.g., degrees, percent relative humidity, pounds per square inch) rather than in percent.
ABBREVIATIONS
The following port abbreviations are used in drawings of relays and controllers:
B — Branch C — Common E — Exhaust
M — Main
O — Normally connected*
X — Normally disconnected*
P — Pilot (P and P for dual-pilot relays)
S — Sensor (S and S for dual-input controllers)
N.C. — Normally closed
N.O. — Normally open
* The normally connected and common ports are connected on a fall in pilot pressure below the relay setpoint, and the normally disconnected port is blocked. On a rise in pilot pressure above the relay setpoint, the normally disconnected and common ports are connected and the normally connected port is blocked. Refer to Figure 37 in RELAYS AND SWITCHES.
SYMBOLS
