Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSFig. 27. Cooling Tower Control.For minimum chiller energy usage, the condenser watertemperature setpoint should be as low as can be safely used bythe refrigeration system or as low as can be provided by outdoorair conditions. The approach value of the evaporative coolingtower indicates how close the cooled water temperature cancome to outdoor air wet-bulb temperature at design conditions.When outdoor air is at a lower wet-bulb temperature than design,the tower can cool water to a lower temperature, but never tothe wet-bulb temperature. Therefore, the controller setpointshould be at the lowest temperature attainable by the coolingtower to save chiller energy and not waste fan energy trying toreach an unobtainable value. Figure 28 is a reset schedule ofcondenser water setpoint as a function of outdoor air wet-bulbtemperature with the minimum safe temperature indicated.CONDENSER WATERSETPOINTMINIMUM SAFETEMPERATURECONDENSERWATERTEMPERATURECONTROL CWSFANS ON IN SEQUENCEON TEMPERATURE RISECOOLINGTOWERCONDENSERWATERTEMPERATURESETPOINTCWRBYPASSVALVEC2695OUTDOOR AIRWET-BULBTEMPERATUREMINIMUM TEMPERATUREDIFFERENCE THAT CAN BEATTAINED (APPROACH)OA WET BULB TEMPERATURECONDENDER WATER SETPOINT = OA WET BULB + MINIMUM DIFFERENCEFig. 28. Condenser Water TemperatureReset from Outdoor Wet-Bulb.C1480When wet-bulb reset is not used, a condenser watertemperature setpoint around midpoint between the design(usually 29.5°C) and the minimum value acceptable to thechiller is satisfactory.On-Off Cooling Tower Fan ControlOn-off fan control is satisfactory where the load is alwayshigh and where several towers are banked together formultistage control.On-off cooling tower control with a single setpoint for a PIcontrol function produces questionable performance. Apreferred solution starts the fan on a rise in condenser watertemperature to 27°C, stops the fan on a drop to 23°C, and limitson periods to a minimum of 5 minutes. The 5 minute minimumon time can still produce water too cool for the chiller duringlight loads and low OA WB temperatures. The use of amodulating valve can prevent condenser water temperature fromdropping below the lowest that the chiller can tolerate (valvecontrol would be PI with a single setpoint). During belowfreezing OA conditions throttling condenser water into the towercan cause icing, cycling the valve open and closed can reducefreezing. The specification following provides warmer icemeltingwater during colder weather.Specification:On a rise in condenser water temperature to 27°C the coolingtower fan shall start and on a drop in condenser watertemperature to 23°C the cooling tower fan shall stop. Whenstarted, the cooling tower fan shall run a minimum of fiveminutes. When the OA temperature is above freezing, the bypassvalve shall modulate if required to prevent the condenser waterfrom falling below 21.5°C. When the OA temperature dropsbelow 0°C, the bypass valve shall open to the bypass on a dropin condenser water temperature to a minimum acceptabletemperature and open to the tower upon a rise in temperature.The temperature rise shall vary between 2.5 and 6 kelvins asthe OA temperature varies from 0°C to –18°C.NOTE: The variable temperature rise allows warmer water toenter the tower during extreme weather to encouragemelting and reduce icing.Two-Speed Cooling Tower Fan ControlFor two-speed cooling tower fans, a single setpoint for a PIcontrol function also produces unpredictable performance. Abetter solution would be one of absolute setpoints similar tothat noted for on-off fans. If wet-bulb reset is used, the controlobjective is to turn the fan off at the optimum setpoint but at noless than the minimum temperature acceptable to the chillerplus 0.5 kelvins, and cycle to and from low and high speedsabove this value with minimum on times for each speed andwith no more than 7 speed changes per hour for fans under 3meters in diameter or as required by the tower manufacturer.The specification following is typical.ENGINEERING MANUAL OF AUTOMATIC CONTROL317