Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSCHILLED WATER SYSTEMCHILLED WATER SETPOINT4546 52MINIMUM CURRENT MAXIMUMANYTIME ANY AHU VALVE IS OPEN GREATER THAN 20 % FOR MORE THAN 3MINUTES AND THE TIME IS BEFORE 1545 THE CHILLED WATER PUMP STARTS ANDENABLES THE CHILLER SYSTEM CONTROLS.AT CHILLER START-UP, THE CHILLED WATER TEMPERATURE SETPOINT IS 46 .ANYTIME ALL AHU CHILLED WATER VALVES ARE LESS THAN 85 % OPEN, THESETPOINT INCREMENTS UP AT THE RATE OF 0.3 DEGREES EVERY 5.5MINUTES UP TO A MAXIMUM OF 52 DEGREES.ANYTIME ANY AHU CHILLED WATER VALVE IS FULL OPEN, THE CHILLED WATERTEMPERATURE SETPOINT DECREMENTS DOWN AT THE SAME RATE TO A MINIMUMOF 45 DEGREES.Fig. 10. Single Chiller Control Dynamic Sequence Display.M10495Features1. Automatic start-stop and setpoint optimization of chiller.2. User friendly monitoring and adjustment.3. Optimized unoccupied operation.4. Chiller cannot start late shortly before the unoccupiedperiod begins.Conditions For Successful Operation1. Control network, software, and programming adviseschiller controller of AHU chilled water demands.2. Interlock and control wiring coordinated with the chillermanufacturer.3. Appropriate cooling tower and control.4. For single-chiller systems without primary-secondarypumping, three-way air handling unit valves may be usedfor 80 to 85 percent of the chilled water flow (Smallvalves, up to 15 to 20 percent total flow, may be twoway,which are simpler to pipe.) If all two-way valvesare provided on single pump systems, chilled water flowor pressure controls (See DUAL CENTRIFUGALCHILLERS) are provided to maintain the required flow(varies with chiller manufacturers) through the chiller.Do not use three-way valves when diversity is used inthe chiller system design.NOTE: Little pumping energy can be saved on asingle-pump single-chiller system by usingtwo-way AHU control valves since the chillerusually requires high flow anyway.5. During the unoccupied period the 80 percent load limitingparameter (see SPECIFICATION following) is based onthe assumption that AHUs are VAV and are operatingunder a reduced maximum cfm setpoint during allunoccupied cooling modes (see the Air Handling SystemControl Applications) to save fan energy and place thechiller operation in the maximum efficiency range.6. Chilled water temperature reset from AHU chilled watervalve position requires:a. No valve always full open.b. Maximum of 30 to 40 valves. With too many valvesthe probability that one valve will always be open isvery great.c. Zone setpoint discipline is maintained. Loweringsetpoints to resolve complaints may result in one ormore valves being always open.7. Chilled water temperature reset timing increments arecompatible with valve control response. If the temperaturereset is too fast, the valve cannot adjust to the newtemperature, resulting in instability.SpecificationThe chiller system operation shall be enabled anytime thetime of day is less than 1545 and any AHU chilled water valveis open greater than twenty percent for more than three minutes.Anytime the chiller system is enabled, the chilled water pumpshall run.ENGINEERING MANUAL OF AUTOMATIC CONTROL281