Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSControl Strategies:1. Control valve Y 11 maintains the SSF temperature asdictated by TIC 13 which is reset by the outdoor airtemperature. Room temperature sensor TI 31 shifts thereset schedule up or down. Controller TC provides nightsetback and other unoccupied programs.2. Pump Y 12 provides constant SSF flow through the checkvalve.3. Summer operation shuts down the entire station.However, it is recommended that the summer functioninclude a function to exercise the pump and therefore allthe devices once a week.Indirect Heat Transfer SubstationsIndirect substations use heat exchangers and physicallydecoupled or independent heating circuits. Applications rangefrom small substations for a one family house to largesubstations for industrial types of networks. Three applicationsfor heat transfer substations with different heat exchangerconfigurations, primary flow and/or the differential pressure iscontrolled by control valves, heat flow controlled by modulatingcontrol valve to vary the primary flow, and circulating pumpsto provide the secondary supply and return flow, follow:Indirect heat transfer substations:– Transfer the required heat from the supply (primary) sideto the consumer (secondary) side.– Meter heat.– Provide safety functions to protect consumer andequipment against overheating, frost, and harmful agentsin hot tap water.– Provide optimization functions to reduce energyconsumption to the lowest possible level.– Provide hydraulic separation between the hightemperaturehigh-pressure system and the lowtemperaturelow-pressure system.The main parts of an indirect substation are:– Primary side.– Heat exchanger.– Circulating pumps.– Expansion and storage tanks.– Feed water facilities.– Secondary side.The primary supply flow (from the district heating network)enters the heat exchanger, transfers the heat to the secondary supplyflow, and returns to the heat source through the primary return.In large networks with distributed pressure and temperatureparameters the primary flow and/or the differential pressure iscontrolled by electric or self regulating control valves. Thisensures a constant inlet pressure differential regardless of theactual pressure differential pressure in the network. Separatecontrol valves or valves with combined functionality in thereturn or supply flow ensure this.PRIMARYSUPPLYPRIMARYRETURNM∆P=CONSTANTFig. 132. Primary Side of IndirectHeat Transfer Substation.SECONDARYSUPPLYSECONDARYRETURNThe heat exchanger (HEX) separates both circuits andmaintains the heat flow between primary and secondary side(Fig. 133). Both plate or tube type heat exchangers are used ineither parallel or series configurations. The heat flow iscontrolled by modulating the control valve position varyingthe primary flow.The secondary supply flow temperature setpoint is usuallyset from the outdoor air temperature during the day and setback during the night. A high limit safety device reduces thesupply flow temperature, if the return flow temperature exceedshigh limit setpoint.In addition HEXs are used for domestic hot water eitherdirectly or via a charge pump and hot water storage tanks.HEATEXCHANGERPRIMARY SUPPLYHEATEXCHANGERDIFFERENTIALPRESSURECONTROLSECONDARY SUPPLYSECONDARY RETURNM11440Functional principle:The primary side of the substation (Fig. 132) contains thedifferential pressure control and a normally closed temperaturecontrol valve with safety spring return, if the secondary mediumtemperature exceeds 212F. The differential pressure control isused in large networks where over time significant pressuredifferences exist.PRIMARY RETURNM11438Fig. 133. Heat Exchanger Portion ofIndirect Heat Transfer Substation.ENGINEERING MANUAL OF AUTOMATIC CONTROL363