Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSSpecification:Condenser water temperature setpoint shall be equal to the OAWB plus 7F, or the minimum temperature acceptable to thechiller, whichever is higher. Fan shall stop when condenserwater temperature drops to setpoint. Fan shall start at low speedwhen the temperature rises to the setpoint plus 6F, and shallnot stop for a minimum of 9 minutes. Fan shall transition fromlow to high speed anytime it has run at least 3 minutes at lowspeed and the temperature is above the low-speed start pointplus 2F or 85F, whichever is lower. Fan shall run a minimum of9 minutes on high speed, and shall transition from high speedto low speed if the temperature drops to a value equal to theswitch-to-high-speed value minus 6F.If required, use a valve control method similar to the oneused for single speed fan control.Variable Speed Cooling Tower Fan ControlFor variable speed cooling tower fans, an EPID controlfunction with WB optimized setpoint or fixed setpoint workswell until the fan drops to a minimum safe speed, below whichresults in motor overheating. If the tower capacity is stillexcessive at minimum fan speed, the fan control mode revertsto on-off. During this mode, disable the PI function or whenthe fan is restarted it will start at a speed greater than theminimum, although the minimum speed was too great. Thisproduces an undesirable short on time. The following is apossible application and specification:SINGLE COOLING TOWER VARIABLE SPEED FAN CONTROLFunctional Description177TOWER FAN CONDENSER WATER TEMPERATURESETPOINT EQUALS OA WET BULB TEMPERATURE PLUSOUTSIDE AIRTEMPERATURE747.0DEGREES71 788587 71DRYBULBWETBULBPERCENTOPEN TOTOWER100786MINIMUM CURRENT MAXIMUMLOW LOAD/FREEZINGCONTROL23TOWER FAN MINIMUMSPEED EQUALS256912854511PERCENT910CHILLER86M15046Fig. 29. Single Cooling Tower Variable Speed Fan Control GraphicItem No.Function1 Indicates percent maximum tower fan speed.2 Sets tower minimum speed.3 Current OA conditions.4 Sets minimum and maximum condenser watertemperature setpoints and WB approach.5 Icon to select tower low load control dynamicsequence display (Fig. 30).6 Condenser water control sensor.7 Displays valve position.8,9 Dynamic pump symbols denote pumpoperation.10,11 Operator information.12 Sets valve control setpoint.ENGINEERING MANUAL OF AUTOMATIC CONTROL295