Engineering Manual o.. - HVAC.Amickracing

MICROPROCESSOR-BASED/DDC FUNDAMENTALSNight PurgeThe night purge program uses cool, night outdoor air toprecool the building before the mechanical cooling is turnedon. Digital control systems often reduce fan capacity of VAVAHU systems during Night Purge to reduce energy usage.Outdoor temperature, outdoor RH or dewpoint, and spacetemperature are analyzed. One hundred percent outdoor air isadmitted under the following typical conditions:1. Outdoor air above a summer-winter changeover point,such as 50F.2. Outdoor temperature below space temperature by aspecified RH or determined differential.3. Outdoor air dewpoint less than 60F.4. Space temperature above some minimum for night purgesuch as 75F.EnthalpyThe enthalpy program (Fig. 8) selects the air source thatrequires the least total heat (enthalpy) removal to reach thedesign cooling temperature. The selected air source is eitherthe return air with a selectable minimum amount of outdoor airor a mixture of outdoor and return air as determined by localcontrol from discharge-air or space temperature measurement.Measurements of return-air enthalpy and return-air dry bulbare compared to outdoor air conditions and used as criteria forthe air source selection. A variation of this is comparing theoutside air enthalpy to a constant (such as 27.5 Btu per poundof dry air) since the controlled return air enthalpy is rather stable.OAh > RAhYESNOOADB>RADBYES1NOLoad ResetThe load reset program (Fig. 9) assures that only the minimumamount of heating or cooling energy is used to satisfy loadrequirements. Samples of zone demands are taken and the zonewith the greatest load is used to reset the temperature of theheating or cooling source.UPPERBOUNDLOWERBOUND100HOT DECKTEMPERATURE (F)60FULLHEATINGNOHEATINGZONE WITH GREATESTDEMAND FOR HEATINGHEATING LOAD RESETUPPERBOUNDLOWERBOUNDFig. 9. Typical Load Reset Schedules.C2435Load reset used with digital controllers is the application thatmost sets digital control apart from pneumatic and traditionalcontrol. The application uses VAV box loadings to determineVAV AHU requirements for air static pressure and temperature,uses reheat valve loadings to determine hot water plantrequirements for temperature and pressure, and uses chilledwater valve positions to determine chilled water plantrequirements for temperature and pressure. These adjustmentsto the various requirements reduce energy costs when all VAVdampers are less than 70 percent open and AHU designconditions (for example, 55F at two inch static pressure) arenot required.Without knowledge of the actual instantaneous demands ofthe loads that load reset controls, systems must run at thetheoretical worst-case setpoints for temperature and pressure.60COLD DECKTEMPERATURE (F)53NOFULLCOOLING COOLINGZONE WITH GREATESTDEMAND FOR COOLINGCOOLING LOAD RESET1SELECTMINIMUMOAOPTIONAL CONSIDERATIONSSELECTMINIMUMOALEGEND:OAh = OUTDOOR AIR ENTHALPYRAh= RETURN AIR ENTHALPYOA = OUTDOOR AIROADB = OUTDOOR AIR DRY BULBRADB = RETURN AIR DRY BULBFig. 8. Enthalpy Decision Ladder.SELECTLOCALCONTROLC2439As with most powerful application programs, load resetrequires a great depth of knowledge of the HVAC and controlprocess. The problem is that if load demands indicate that ahigher chilled water temperature would be appropriate and thetemperature increase is made, all loads are upset because ofthe chilled water temperature change. The loads must adjustand stabilize before another load reset adjustment is made.Monitoring and tuning is required to assure high performancecontrol and loop stability. Three parameters determine theperformance and stability of the system: the magnitude of theincremental corrections, the time interval between correction,and the magnitude of the hysteresis between raising andlowering the temperature.ENGINEERING MANUAL OF AUTOMATIC CONTROL131