Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSThe difference between repeatability and static error is thatrepeatability is the ability to return to a specific condition,whereas static error is a constant deviation from that condition.Static error (e.g., sensor error) does not interfere with the abilityto control, but requires that the control point be shifted tocompensate and maintain a desired value.The dead zone is a range through which the controlledvariable changes without the controller initiating a correction.The dead zone effect creates an offset or a delay in providingthe initial signal to the controller. The more slowly the variablechanges, the more critical the dead zone becomes.CAPACITANCEFigure 45 shows a high-velocity heat exchanger, whichrepresents a process with a small thermal capacitance. The rateof flow for the liquid in Figure 45 is the same as for the liquidin Figure 44. However, in Figure 45 the volume and mass ofthe liquid in the tube at any one time is small compared to thetank shown in Figure 44. In addition, the total volume of liquidin the exchanger at any time is small compared to the rate offlow, the heat transfer area, and the heat supply. Slight variationsin the rate of feed or rate of heat supply show up immediatelyas fluctuations in the temperature of the liquid leaving theexchanger. Consequently, the process in Figure 45 does nothave a stabilizing influence but can respond quickly to loadchanges.LIQUID INHEATINGMEDIUM INCapacitance differs from capacity. Capacity is determinedby the energy output the system is capable of producing;capacitance relates to the mass of the system. For example, fora given heat input, it takes longer to raise the temperature of acubic foot of water one degree than a cubic foot of air.When the heat source is removed, the air cools off morequickly than the water. Thus the capacitance of the water ismuch greater than the capacitance of air.A capacitance that is large relative to the control agent tendsto keep the controlled variable constant despite load changes.However, the large capacitance makes changing the variable toa new value more difficult. Although a large capacitancegenerally improves control, it introduces lag between the timea change is made in the control agent and the time the controlledvariable reflects the change.Figure 44 shows heat applied to a storage tank containing alarge volume of liquid. The process in Figure 44 has a largethermal capacitance. The mass of the liquid in the tank exerts astabilizing effect and does not immediately react to changessuch as variations in the rate of the flow of steam or liquid,minor variations in the heat input, and sudden changes in theambient temperature.LIQUIDINSTEAMINLIQUID OUTHEATINGMEDIUM OUTC2076Fig. 45. Typical Process with Small Thermal Capacitance.Figure 46 shows supply capacitance in a steam-to-waterconverter. When the load on the system (in Figure 44, cold air)increases, air leaving the heating coil is cooler. The controllersenses the drop in temperature and calls for more steam to theconverter. If the water side of the converter is large, it takeslonger for the temperature of the supply water to rise than ifthe converter is small because a load change in a process witha large supply capacitance requires more time to change thevariable to a new value.VALVESTEAMCONVERTERCONTROLLERHOT WATER SUPPLY(CONSTANT FLOW,VARYINGTEMPERATURE)TANKLIQUIDOUTCONDENSATERETURNC2075Fig. 44. Typical Process with Large Thermal Capacitance.CONDENSATERETURNPUMPHOT WATERRETURNCOLD AIR(LOAD)STEAMTRAPHEATINGCOILHOT AIR(CONTROLLEDVARIABLE)Fig. 46. Supply Capacitance (Heating Application).C2077ENGINEERING MANUAL OF AUTOMATIC CONTROL31