Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALS–PATHOFIONSAIRFLOW+–+ALTERNATEPLATESGROUNDEDWIRESAT HIGHPOSITIVEPOTENTIALAIRFLOW–+–INTERMEDIATEPLATESCHARGEDTO HIGHPOSITIVEPOTENTIALPOSITIVELY CHARGEDPARTICLES+–THEORETICALPATHS OFCHARGES DUSTPARTICLESSOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOKFig. 18. Electrostatic Filter.C2714CONTROL SYSTEM CHARACTERISTICSAutomatic controls are used wherever a variable conditionmust be controlled. In HVAC systems, the most commonlycontrolled conditions are pressure, temperature, humidity, andrate of flow. Applications of automatic control systems rangefrom simple residential temperature regulation to precisioncontrol of industrial processes.CONTROLLED VARIABLESAutomatic control requires a system in which a controllablevariable exists. An automatic control system controls thevariable by manipulating a second variable. The second variable,called the manipulated variable, causes the necessary changesin the controlled variable.In a room heated by air moving through a hot water coil, forexample, the thermostat measures the temperature (controlledvariable) of the room air (controlled medium) at a specifiedlocation. As the room cools, the thermostat operates a valvethat regulates the flow (manipulated variable) of hot water(control agent) through the coil. In this way, the coil furnishesheat to warm the room air.CONTROL LOOPIn an air conditioning system, the controlled variable ismaintained by varying the output of the mechanical equipmentby means of an automatic control loop. A control loop consistsof an input sensing element, such as a temperature sensor; acontroller that processes the input signal and produces an outputsignal; and a final control element, such as a valve, that operatesaccording to the output signal.The sensor can be separate from or part of the controller andis located in the controlled medium. The sensor measures thevalue of the controlled variable and sends the resulting signalto the controller. The controller receives the sensor signal,compares it to the desired value, or setpoint, and generates acorrection signal to direct the operation of the controlled device.The controlled device varies the control agent to regulate theoutput of the control equipment that produces the desiredcondition.HVAC applications use two types of control loops: open andclosed. An open-loop system assumes a fixed relationshipbetween a controlled condition and an external condition. Anexample of open-loop control would be the control of perimeterradiation heating based on an input from an outdoor airtemperature sensor. A circulating pump and boiler are energizedwhen an outdoor air temperature drops to a specified setting,and the water temperature or flow is proportionally controlledas a function of the outdoor temperature. An open-loop systemdoes not take into account changing space conditions frominternal heat gains, infiltration/exfiltration, solar gain, or otherchanging variables in the building. Open-loop control alonedoes not provide close control and may result in underheatingor overheating. For this reason, open-loop systems are notcommon in residential or commercial applications.A closed-loop system relies on measurement of the controlledvariable to vary the controller output. Figure 19 shows a blockdiagram of a closed-loop system. An example of closed-loopcontrol would be the temperature of discharge air in a ductdetermining the flow of hot water to the heating coils to maintainthe discharge temperature at a controller setpoint.ENGINEERING MANUAL OF AUTOMATIC CONTROL18