Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSIn Figure 34, condenser and chilled water flows are divertedfrom the chiller to a heat exchanger.COOLINGTOWERCOOLINGTOWERTOWER HEATER(OPTIONAL)NOTE:INDICATES NO FLOWCONDENSERWATER PUMPTOWER HEATER(OPTIONAL)FILTERCONDENSERWATERPUMPCONDENSERCOMPRESSOREVAPORATORHEATEXCHANGERLOADCONDENSERCOMPRESSOREVAPORATORCHILLEDWATERPUMPWATER FLOW DURING CHILLER COOLINGLOADC2698COOLINGTOWERNOTE:INDICATES NO FLOWFig. 35. Free Cooling by InterconnectingWater Circuits.TOWER HEATER(OPTIONAL)CONDENSERWATERPUMPCONDENSERCOMPRESSOREVAPORATORHEATEXCHANGERLOADWhen the chiller is shut down during free cooling operation,the normal chilled water supply temperature control is bypassedand the chilled water supply temperature is determined by thetower water temperature. The normal cooling tower watercontrol cycles tower fans and modulates the tower bypass valveto maintain a setpoint in the 65 to 85F range but cooling towerwater is controlled at 35 to 45F when used directly for cooling.WATER FLOW DURING FREE COOLINGFig. 34. Free Cooling UsingAuxiliary Heat Exchanger.C2699In Figure 35, condenser and chilled water circuits areinterconnected bypassing the chiller. In this configurationproviding tower water treatment and filtering and pumpingcapacity for proper cooling tower flow are major considerations.During free cooling the fans may be manually turned on ifthere is no danger of supply water being too cold or too muchenergy being used by a winterized cooling tower. However, ifeither condition exists, the tower water temperature controlshould be in operation at all times to maintain a free coolingtemperature of 35 to 45F.The changeover from free cooling to chiller operation maybe manual or automatic. Changeover sequence should changethe tower water controller setpoint first, then change the heatexchanger valves when temperatures are proper for the newmode of operation.Following is a tower-free-cooling example using plate-frameheat exchangers for the dual chiller system shown in the DUALCENTRIFUGAL CHILLER example. The DUAL COOLINGTOWER VARIABLE SPEED FAN CONTROL example is alsorelevant.ENGINEERING MANUAL OF AUTOMATIC CONTROL301