Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSAn example of offset would be the proportional control of achilled water coil used to cool a space. When the cooling loadis 50 percent, the controller is in the middle of its throttlingrange, the properly sized coil valve is half-open, and there isno offset. As the outdoor temperature increases, the roomtemperature rises and more cooling is required to maintain thespace temperature. The coil valve must open wider to deliverthe required cooling and remain in that position as long as theincreased requirement exists. Because the position of the finalcontrol element is proportional to the amount of deviation, thetemperature must deviate from the setpoint and sustain thatdeviation to open the coil valve as far as required.Figure 33 shows that when proportional control is used in aheating application, as the load condition increases from 50percent, offset increases toward cooler. As the load conditiondecreases, offset increases toward warmer. The opposite occursin a cooling application.WARMERCONTROL POINTWhere:VKEM= output signal= proportionality constant (gain)= deviation (control point - setpoint)= value of the output when the deviation iszero (Usually the output value at 50 percentor the middle of the output range. Thegenerated control signal correction is addedto or subtracted from this value. Also called“bias” or “manual reset”.)Although the control point in a proportional control systemis rarely at setpoint, the offset may be acceptable. Compensation,which is the resetting of the setpoint to compensate for varyingload conditions, may also reduce the effect of proportional offsetfor more accurate control. An example of compensation isresetting boiler water temperature based on outdoor airtemperature. Compensation is also called “reset control” or“cascade control”.SETPOINTOFFSET0%LOAD50%LOADOFFSET100%LOADRESET CONTROLGENERALCOOLERFig. 33. Relationship of Offset to Load(Heating Application).The throttling range is the amount of change in the controlledvariable required for the controller to move the controlled devicethrough its full operating range. The amount of change isexpressed in degrees Fahrenheit for temperature, in percentagesfor relative humidity, and in pounds per square inch or inchesof water for pressure. For some controllers, throttling range isreferred to as “proportional band”. Proportional band isthrottling range expressed as a percentage of the controllersensor span:Proportional Band =Proportional band and throttling range, in digital controlsystems, are often expressed in degrees.“Gain” is a term often used in industrial control systems forthe change in the controlled variable. Gain is the reciprocal ofproportional band:Gain =100Proportional BandThe output of the controller is proportional to the deviationof the control point from setpoint. A proportional controllercan be mathematically described by:V = KE + MThrottling RangeSensor SpanC2096x 100Reset is a control technique available in proportional controlin which a secondary, or compensation, sensor resets the setpointof the primary sensor. An example of reset would be the outdoortemperature resetting the discharge temperature of a fan systemso that the discharge temperature increases as the outdoortemperature decreases. The sample reset schedule in Table 2 isshown graphically in Figure 34. Figure 35 shows a controldiagram for the sample reset system.Outdoor Air Discharge AirTemperature TemperatureCondition (F) (F)Outdoor designtemperature0 100Light load 70 70DISCHARGE AIRTEMPERATURE SETPOINT (°F)100(FULL RESET)700(FULLRESET)Table 2. Sample Reset Schedule.OUTDOOR AIR TEMPERATURE (°F)70(RESETSTART)C2719Fig. 34. Typical Reset Schedule for Discharge Air Control.ENGINEERING MANUAL OF AUTOMATIC CONTROL25