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.WARMERWhere: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”.CONTROL POINTSETPOINTOFFSET0%LOAD50%LOADOFFSET100%LOADCompensation ControlGENERALCOOLERFig. 33. Relationship of Offset toLoad (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 kelvins for temperature, in percentagesfor relative humidity, and in pascals or kilopascals for pressure.For some controllers, throttling range is referred to as“proportional band”. Proportional band is throttling rangeexpressed as a percentage of the controller sensor span:Proportional Band =“Gain” is a term often used in industrial control systems forthe change in the controlled variable. Gain is the reciprocal ofproportional band:Gain =Throttling RangeSensor Span100Proportional Bandx 100C2096The output of the controller is proportional to the deviationof the control point from setpoint. A proportional controllercan be mathematically described by:Compensation is a control technique available in proportionalcontrol in which a secondary, or compensation, sensor resetsthe setpoint of the primary sensor. An example of compensationwould be the outdoor temperature resetting the dischargetemperature of a fan system so that the discharge temperatureincreases as the outdoor temperature decreases. The samplereset schedule in Table 2 is shown graphically in Figure 34.Figure 35 shows a control diagram for the sample reset system.DISCHARGE AIRTEMPERATURE SETPOINT (°C)ConditionOutdoor designtemperature40(FULL RESET)20Table 2. Sample Reset Schedule.Outdoor AirTemperature(°C)Discharge AirTemperature(°C)–20 40Light load 20 20V = KE + M-20(FULLRESET)OUTDOOR AIR TEMPERATURE (°C)20(RESETSTART)C3984Fig. 34. Typical Reset Schedule for Discharge Air Control.22ENGINEERING MANUAL OF AUTOMATIC CONTROL