Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSLOADEXPANSION TANKA converter (heat exchanger) shown in Figure 89 can be usedin large or high rise buildings to reduce the zone temperature/pressure requirements from those of the mains.LOADLOADLOADLOADLOADLOADLOADEXPANSION TANKLOADLOADLOADLOADLOADBOILERORCHILLERLOADRETURNSUPPLYFig. 87. Two-Pipe Direct-Return System.C2835HIGH OR MEDIUMTEMP MAINSCONVERTERRETURNSUPPLYC2915-1Primary-Secondary PumpingPrimary-secondary pumping allows water temperature andon/off times of each secondary zone to be independentlycontrolled. The main supply pump uses less power in thisarrangement since it is sized to handle only main pressure losses.Secondary pumps handle only the zone piping losses.Figure 88 shows common configurations for primarysecondarypumping. The three-way valve configurationprovides more positive control than the two-way valve.ZONE PUMPFig. 89. Converter Used to SupplyZones from a Larger System.CONTROL OF HOT WATER SYSTEMSGeneralHeating (terminal) units used in hot water heating systems are:— Radiant Panels— Radiators or finned tubes— Forced air heating coilsBOILERBALANCINGVALVELOADTROOMTHERMOSTATZONE WITH 3-WAY VALVEZONE PUMPControl of heat output from a heating unit can use one or acombination of the following methods:— On-off control, by starting and stopping a pump oropening and closing a valve— Modulating flow control— Supply water temperature controlPRIMARYPUMPLOADModulating Flow ControlFig. 88. Piping for Primary-Secondary Pumping.TROOMTHERMOSTATZONE WITH 2-WAY VALVEOTHERZONESM10485Varying water flow to finned tube radiation or a heating coil,each supplied with constant temperature hot water, is showngraphically in Figures 90 and 91.In both cases, reducing flow 75 percent reduces the heatoutput only 30 percent because as flow is reduced, more heat isextracted from each gallon of water. At low flows leaving watertemperature decreases sharply (Fig. 91). At light loadsmodulating flow control by itself is not the best means ofcontrolling heat output.ENGINEERING MANUAL OF AUTOMATIC CONTROL337