Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSNOTE: When using two-way AHU valves with thiscoupled chiller configuration, exercise care inoptimizing the chilled water temperature. Withboth chillers running, raising the chilled watertemperature results in greater flow and a smaller∆T which must be considered in the chillershedding strategy. For an example of a pressurebypass system, see DUAL PUMPS, DUALCHILLERS, PRESSURE BYPASS, 90PERCENT CHILLER FLOW, DIRECTRETURN.SpecificationThe chiller system operation shall be enabled anytime thetime of day is before 1545 and any AHU chilled water valve isopen greater than twenty percent for greater than three minutes.Anytime the chiller system is enabled, the lead chilled waterpump shall run.Anytime the lead chiller has run longer than 90 minutes, thechilled water temperature has been greater than 1 degree abovethe chilled water temperature setpoint for greater than 4 minutes,and the time is less than 1545, the off chilled water pump shallstart.Anytime both chillers are running and the chiller plant waterdifferential temperature has been less than 4.4F for greater than4 minutes, the chilled water pump with the longest “on” durationsince the last start shall stop and remain off at least 30 minutes.Anytime chilled water flow is proven via a chilled water pumpcurrent sensing relay, the respective chiller controls shall beenabled to operate under factory controls, subject to a chillersoftware ON-OFF-AUTO function (chilled water flow muststill be proven in the “ON” mode). Provide control and interlockwiring per the chiller manufacturers recommendation.Upon a call for chilled water, the chiller controls shall start thecondenser water pump and energize the cooling tower fan controls.When condenser water flow is proven via a condenser waterpump current sensing relay, the chiller shall start, operate, andload under chiller factory controls to maintain the chilled watertemperature setpoint, 44F at start-up.Anytime all chilled water valves are less than 80 percent open,the chilled water temperature setpoint shall be incremented ata rate of 0.3F every 5 minutes up to a maximum of 50F.Anytime any chilled water valve is full open, the chilled watertemperature setpoint shall be decremented at a rate of 0.3F every5 minutes down to a minimum of 42F.The maximum allowable percentage of chiller full loadelectrical current shall be commandable from the BMCS, andshall be 80 percent during all unoccupied periods of operation.SIMILAR MULTIPLE CENTRIFUGAL CHILLERS CONTROL APPLICATIONSEqual Sized Centrifugal Chillers ControlPRIMARY FLOW LOOPTEE AP S1CHILLER445.0T SP4 T SP3 T SP2 T SP1CHILLER345.0CHILLER245.0CHILLER1PP4 P P3 P P2 P P145.045.0T SPFORWARDFLOWBACKWARDFLOWT DDECOUPLERLINETEE B45.045.0T SSPS2SECONDARYFLOW LOOP45.8T 'SST S = SUPPLY TEMPERATURET R = RETURN TEMPERATURET SS = SUPPLY TEMPERATURE, SECONDARYT SP = SUPPLY TEMPERATURE, PRIMARYF = FLOWT D = DECOUPLER TEMPERATURET RP55.0 55.0T RSF S =F P∆ T P∆ T SWHERE F S = SECONDARY FLOWNOTE:F P = PRIMARY FLOW,∆ T P = DIFFERENTIAL TEMPERATURE, PRIMARY∆ T S = DIFFERENTIAL TEMPERATURE, SECONDARYF P = SUM OF PRIMARY PUMP FLOWSBASED UPON TEST-AND-BALANCEDATA AND PUMP ON-OFF STATUS.M10499Fig. 16. Multiple Equal Sized Chillers Control Graphic.ENGINEERING MANUAL OF AUTOMATIC CONTROL286