Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSControl Strategies1. Primary Flow Loop Differential Pressure Control:Provides a constant value pressure drop across the heatexchangers. Actuator control valves (Y17, Y18) open orclose sequentially to maintain the pressure drop measuredby PDIC 14.2. Secondary Loop Supply Flow Temperature Control: TheSSF temperature is reset from outdoor air temperature(Fig. 137). The actual SSF temperature is measured bysensors TIC 28 and TIC 38. The regulation of the heatflow through the HEX is provided by the actuator controlvalves Y 23 and Y 33 in the primary return flow line ofeach heat exchanger.3. Secondary Flow Loop Differential Pressure Control:Circulating pumps Y 48 and Y 49 are switched off or onand the speed varied to maintain the differential pressuresetpoint. A frequency converter (fc) controls pump speed.4. SSF Loop Pressure Control: Maintains the pressuresensed by transmitter PIC 57 within a defined range toavoid too low pressure (vaporization) and too highpressure (pump overload). Vent valve Y 54 maintains thehigh limit and compressor Y 55 maintains the low limit.5. Expansion Tank Water Level Control (WLC) Loop: LIC56 maintains the expansion tank water level. Normally PRFwater through Valve Y 51 provides make-up water, in casesof a high quantity water loss, Pump Y 64 pumps make-upwater from the water supply tank. The WLC protects thecirculating pumps against running without water.6. Water Supply Tank Water Level Control: LIC 63) controlsValve Y 61, providing make-up water from the cold water.7. Secondary Return Flow (SRF) and the Primary ReturnFlow (PRF) temperatures are limited to provide a hightemperature drop between supply and return flow.SSF SETPOINT TEMPERATURE (F)356320284248212176LIMIT SSF SETPOINTTEMPERATURE ISUNLIMITED-13 -6 1 9 16 23 30 37 45 52 59 66OUTSIDE AIR TEMPERATURE (F) M11439Fig. 137. Adjustment Of Temperature Curves.DDC CONTROL OF A SMALL HOUSE SUBSTATIONDISTRIBUTION SUBSTATION CONSUMERDDCTI01TI31OUTSIDE AIRROOMTZA+21TIC22Y23SSFPSFHEATTRANSFERSTATIONTI12Y11MHEX 1TIC24SRFPRFM11436Fig. 138. DDC Control of a Small House Heat Transfer Substation.ENGINEERING MANUAL OF AUTOMATIC CONTROL367