Engineering Manual o.. - HVAC.Amickracing

PNEUMATIC CONTROL FUNDAMENTALSMain line: The air line from the air supply system to controllersand other devices. Usually plastic or copper tubing.Manipulated variable: Media or energy controlled to achievea desired controlled variable condition.Measuring element: Same as sensing element.Mixed air: Typically a mixture of outdoor air and return airfrom the space.Modulating: Varying or adjusting by small increments. Alsocalled “proportioning”.Offset: A sustained deviation between the actual systemcontrol point and its controller setpoint under stableoperating conditions. Usually applies to proportional(modulating) control.Proportional band: As applied to pneumatic control systems,the change in the controlled variable required to changethe controller output pressure from 20 to 90 kPa.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 theoccupied space.Reverse acting (RA): A reverse-acting thermostat or controllerdecreases the branchline pressure on an increase in themeasured variable and increases the branchline pressureon a decrease in the variable. A reverse-acting valveactuator retracts the shaft on an increase in branchlinepressure and extends the shaft on a decrease in pressure.Sensing element: A device that detects and measures thecontrolled variable (e.g., 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 medium.Sensor Span: The variation in the sensed media that causesthe sensor output to vary between 20 to 100 kPa.Setpoint: The value on the controller scale at which thecontroller is set (e.g., the desired room temperatureset on a thermostat). The desired control point.Supply air: Air leaving an air handling system.Thermostat: A device that responds to changes in temperatureand outputs a control signal (branchline pressure).Usually mounted on a wall in the controlled space.Throttling range: Related to proportional band, and expressedin values of the controlled variable (e.g., degrees, percentrelative humidity, kilopascals) rather than in percent.ABBREVIATIONSThe following port abbreviations are used in drawings ofrelays and controllers:B — BranchC — CommonE — ExhaustM — MainO — Normally connected*X — Normally disconnected*P — Pilot (P 1and P 2for dual-pilot relays)S — Sensor (S 1and S 2for dual-input controllers)N.C. — Normally closedN.O. — Normally open* The normally connected and common ports are connected ona fall in pilot pressure below the relay setpoint, and the normallydisconnected port is blocked. On a rise in pilot pressure abovethe relay setpoint, the normally disconnected and common portsare connected and the normally connected port is blocked.Refer to Figure 38 in RELAYS AND SWITCHES.60ENGINEERING MANUAL OF AUTOMATIC CONTROL