PNEUMATIC CONTROL FUNDAMENTALS

1 PNEUMATIC CONTROL FUNDAMENTALS57 ENGINEERING MANUAL OF AUTOMATIC PNEUMATIC CONTROL ..61 Air Supply and Operation ..61 Restrictor ..62 Nozzle-Flapper Assembly ..62 Pilot Bleed System ..62 Signal Amplifier ..63 Feed and Bleed System ..63 Sensing Elements ..63 Bimetal ..63 Rod and Tube ..64 Remote Bulb ..64 Averaging Element ..64 Throttling Range Adjustment ..64 Relays and Switches ..65 Air Supply ..65 Air Compressor ..65 Air Drying Techniques ..66 General ..66 Dry Air Requirement ..66 Condensing Drying ..67 High-Pressure Drying ..67 Refrigerant Drying ..67 Desiccant Drying ..67 Pressure Reducing Valve Station ..68 Air Filter ..68 Pressure Reducing valves ..69 Single-Pressure Reducing Valve.

2 69 Two-Pressure Reducing Valve .. ControlFundamentalsPNEUMATIC CONTROL FUNDAMENTALSENGINEERING MANUAL OF AUTOMATIC ..70 Temperature Controllers ..71 Humidity Controllers ..71 Pressure Controllers ..71 Sensor-Controller Systems ..72 PNEUMATIC Controllers ..72 Proportional-Integral (PI) Controllers ..72 Controller Adjustments ..72 PNEUMATIC Sensors ..73 Velocity Sensor-Controller ..73 Actuators and Final CONTROL ..74 General ..74 Spring Ranges ..74 CONTROL valves ..75 Dampers ..76 Relays and Relay ..77 Snap Acting Relay ..78 Lockout Relay ..78 High-Pressure Selector Relay ..79 Low-Pressure Selector Relay ..79 Load Analyzer Relay ..79 Capacity Relay ..80 Reversing Relay.

3 80 Averaging Relay ..80 Positive-Positioning Relay ..80 Ratio Relay ..81 PNEUMATIC Potentiometer ..81 Hesitation Relay ..82 Electrical Interlocking Relays ..82 Electric- PNEUMATIC Relay ..82 PNEUMATIC -Electric Relay ..82 Electronic- PNEUMATIC Transducer ..83 PNEUMATIC Switch ..83 Manual Positioning Switch ..84 PNEUMATIC CONTROL ..84 Sequence CONTROL ..85 Limit CONTROL ..85 Manual Switch CONTROL ..86 Changeover CONTROL for Two-Pressure Supply System ..87 Compensated CONTROL System ..87 Electric- PNEUMATIC Relay CONTROL ..87 PNEUMATIC -Electric Relay CONTROL ..88 PNEUMATIC Recycling CONTROL ..88 PNEUMATIC CONTROL System CONTROL Sequence ..90 Supply Fan CONTROL Sequence.

4 92 Return Fan CONTROL Sequence ..92 Warm-up/Heating Coil CONTROL Sequence ..92 Mixing Damper CONTROL Sequence ..93 Discharge Air Temperature CONTROL Sequence ..94 Off/Failure Mode CONTROL Sequence ..94 PNEUMATIC CONTROL FUNDAMENTALS59 ENGINEERING MANUAL OF AUTOMATIC CONTROLINTRODUCTIONThis section provides basic information on PNEUMATIC controlsystems and components commonly used to CONTROL equipmentin commercial heating and air conditioning applications. Theinformation in this section is of a general nature in order toexplain the FUNDAMENTALS of PNEUMATIC CONTROL . Some termsand references may vary between manufacturers ( , switchport numbers). PNEUMATIC CONTROL systems use compressed air to operateactuators, sensors, relays, and other CONTROL controls differ from other CONTROL systems in severalways with some distinct advantages: PNEUMATIC equipment is inherently proportional but canprovide two-position CONTROL when required.

5 Many CONTROL sequences and combinations are possiblewith relatively simple equipment. PNEUMATIC equipment is suitable where explosionhazards exist. The installed cost of PNEUMATIC controls and materialsmay be lower, especially where codes require that low-voltage electrical wiring for similar electric controls berun in conduit. Quality, properly installed PNEUMATIC equipment isreliable. However, if a PNEUMATIC CONTROL system requirestroubleshooting or service, most building-maintenancepeople have the necessary mechanical : A mechanical device that operates a final controlelement ( , valve, damper).Authority (Reset Authority or Compensation Authority):A setting that indicates the relative effect acompensation sensor input has on the main setpoint(expressed in percent).

6 Branch line: The air line from a controller to the pressure (BLP): A varying air pressure signalfrom a controller to an actuator carried by the branchline. Can go from atmospheric to full main changeover: The point at which thecompensation effect is reversed in action and changesfrom summer to winter or vice versa. The percent ofcompensation effect (authority) may also be changedat the same CONTROL : A process of automatically adjustingthe CONTROL point of a given controller to compensatefor changes in a second measured variable such asoutdoor air temperature. For example, the hot deckcontrol point is reset upward as the outdoor airtemperature decreases. Also know as reset CONTROL .

7 Compensation sensor: The system element which senses avariable other than the controlled variable and resetsthe main sensor CONTROL point. The amount of thiseffect is established by the authority point: The actual value of the controlled variable(setpoint plus or minus offset).Controlled variable: The quantity or condition that is measuredand controlled ( , temperature, relative humidity,pressure).Controller: A device that senses the controlled variable orreceives an input signal from a remote sensingelement, compares the signal with the setpoint, andoutputs a CONTROL signal (branchline pressure) to : A term that applies to two-position devices. Therange through which the controlled variable must passin order to move the final CONTROL element from one tothe other of its two possible positions.

8 The differencebetween cut-in and cut-out temperatures, pressures, acting (DA): A direct-acting thermostat or controllerincreases the branchline pressure on an increase inthe measured variable and decreases the branchlinepressure on a decrease in the variable. A direct-actingactuator extends the shaft on an increase in branchlinepressure and retracts the shaft on a decrease air: Conditioned air that has passed through a , air discharged from a supply duct outlet into aspace. See Supply CONTROL element: A device such as a valve or damperthat acts to change the value of the manipulatedvariable. Positioned by an CONTROL FUNDAMENTALSENGINEERING MANUAL OF AUTOMATIC CONTROL60 Main line: The air line from the air supply system to controllersand other devices.

9 Usually plastic or copper variable: Media or energy controlled to achievea desired controlled variable element: Same as sensing air: Typically a mixture of outdoor air and return airfrom the : Varying or adjusting by small increments. Alsocalled proportional .Offset: A sustained deviation between the actual systemcontrol point and its controller setpoint under stableoperating conditions. Usually applies to proportional(modulating) band: As applied to PNEUMATIC CONTROL systems,the change in the controlled variable required tochange the controller output pressure from 3 to 13psi. Usually expressed as a percentage of sensor CONTROL : See compensation : A device in an air line that limits the flow of air: Air entering an air handling system from theoccupied acting (RA): A reverse-acting thermostat or controllerdecreases the branchline pressure on an increase inthe measured variable and increases the branchlinepressure on a decrease in the variable.

10 A reverse-actingvalve actuator retracts the shaft on an increase inbranchline pressure and extends the shaft on adecrease in element: A device that detects and measures thecontrolled variable ( , temperature, humidity).Sensor: A device placed in a medium to be measured orcontrolled that has a change in output signal relatedto a change in the sensed Span: The variation in the sensed media that causesthe sensor output to vary between 3 and 15 : The value on the controller scale at which thecontroller is set ( , the desired room temperatureset on a thermostat). The desired CONTROL air: Air leaving an air handling : A device that responds to changes in temperatureand outputs a CONTROL signal (branchline pressure).