Engineering Manual o.. - HVAC.Amickracing

PNEUMATIC CONTROL FUNDAMENTALSReset authority, also called “reset ratio”, is the ratio of theeffect of the reset sensor compared to the primary sensor.Figure 22 shows the effect of authority on a typical resetschedule. The authority can be set from 10 to 300 percent.The low-pressure sensor measures duct static pressure anddifferential pressure. When the duct static pressure or thepressure differential increases, branchline pressure increases.DA TEMPERATURECONTROL POINT (°F)130300OUTDOOR AIR TEMPERATURE (°F)60COMPENSATIONSTART POINTFig. 22. Typical Reset Schedule forDischarge Air Control.C1094The integral control reset time determines how quickly thePI controller responds to a change in the controlled variable.Proportional correction occurs as soon as the controlledvariable changes. The integral function is timed with the resettime adjustment. The reset time adjustment is calibrated from30 seconds to 20 minutes. The proper setting depends on systemresponse time characteristics.PNEUMATIC SENSORSPneumatic sensors typically provide a direct acting 3 to 15 psipneumatic output signal that is proportional to the measuredvariable. Any change in the measured variable is reflected as achange in the sensor output. Commonly sensed variables aretemperature, humidity, and differential pressure. The sensorsuse the same sensing elements and principles as the sensors inthe controllers described earlier, but do not include setpointand throttling range adjustments. Their throttling range is thesame as their span.VELOCITY SENSOR-CONTROLLERThe velocity sensor-controller combines a highly sensitiveair velocity sensor with a pneumatic controller to detect andcontrol airflow regardless of system static pressure. It is usedin air terminal units and other air handling systems. Reverseanddirect-acting models are available for normally closed andnormally open dampers.The velocity sensor measures actual velocity and does notrequire the conversion of velocity pressure to velocity. Althoughthe sensor is typically used in duct air velocity applications, itcan accurately sense velocities as low as 100 feet per minute.Flow-limiting orifices inserted into the sensor sampling tubecan measure velocity ranges up to 3,500 feet per minute.Figure 23 shows the operation of a velocity sensor. A restrictorsupplies compressed air to the emitter tube located in the airstream to be measured. When no air is flowing in the duct, thejet of air from the emitter tube impinges directly on the collectortube and maximum pressure is sensed. Air flowing in the ductblows the air jet downstream and reduces the pressure on thecollector tube. As the duct air velocity increases, less and lessof the jet enters the collector tube. The collector tube isconnected to a pressure amplifier to produce a usable outputpressure and provide direct or reverse action.MAIR FLOWGAPEMITTER TUBECOLLECTORTUBEA gage connected to the sensor output can be used to indicatethe temperature, humidity, or pressure being sensed. The gagescale is calibrated to the sensor span.Temperature sensors may be vapor-filled, liquid-filled,averaging capillary, or rod-and-tube. The controller usuallyprovides restricted air to the sensor.Humidity sensors measure the relative humidity of the air ina room (wall-mounted element) or a duct (insertion element).Nylon is typically used as the sensing element. Humidity sensorsinclude temperature compensation and operate on a forcebalanceprinciple similar to a wall thermostat.TO PRESSUREAMPLIFIERC2610Fig. 23. Velocity Sensor Operation.A controller connected to the pressure amplifier includessetpoints for maximum and minimum dual air velocity limits.This allows the air volume to be controlled between the limitsby a thermostat or another controller.Two models of the controller are available. One modeloperates with a one-pipe, bleed-type thermostat, and the otherwith a two-pipe thermostat. The two-pipe model also allowssequencing for reheat applications.ENGINEERING MANUAL OF AUTOMATIC CONTROL 71