Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSCOOLING TOWER AND CONDENSERWATER CONTROLCOOLING TOWER PERFORMANCECHARACTERISTICSThe cooling tower dissipates the heat collected from thebuilding by the chiller and chilled water system by cooling thecondenser water. Evaporatively cooled condenser water willcool refrigerant to within 7F of the outdoor air wet-bulbtemperature. Air cooled condensers will cool refrigerant towithin 20F of the outdoor air dry-bulb temperature. A coolingtower normally provides a refrigerant head about 30F lowerthan an air cooled condenser. This means an evaporative coolingtower provides a significantly lower cooling cost than an aircooled condenser.Figure 25 shows water-air counterflow in a cooling tower. Thefill increases the time that the water and air are in contact makingthe cooling tower more efficient. Fill is generally one of two types,either splash or film type. A splash type fill is a series of slats overwhich the water cascades. Film type fill causes the water to flowdown thin closely spaced vertical sheets of plastic.SPRAYDISTRIBUTIONAIR INWATERHEATEDAIR OUTFILLFig. 25. Cooling Tower, Showing Water-Air Counterflow.The range (inlet water temperature minus outlet watertemperature) at design load of a cooling tower is determinedby the amount of heat to be rejected and the water flow used tocarry this heat to the tower. The cooling capability is thenexpressed as the design approach (approach specifies how closeto the OA WB a cooling tower can cool water) at design range.Since most operation is at less than design load and/or designoutdoor air temperatures, partial load operating characteristicshave a strong influence on operating costs. Partial load operatingcharacteristics are also used to establish the cooling capacityand capability for free cooling cycles at low outdoor airtemperatures. Partial load characteristics for a tower at designflow rate are shown in Figure 26.AIRDRIFTELIMINATORSHOT WATERFROMCONDENSERAIR INTO CONDENSERWATER PUMPC2694WATER TEMPERATURE, °F1009080706050403026221814106TEMPERATURE RANGE, °F2824201612843030 40 50 60 70 80OUTSIDE AIR WET BULB TEMPERATURE, °FReprinted by permission from the ASHRAEHandbook–1996 Systems and EquipmentFig. 26. Typical Cooling TowerPartial Load Characteristics.In Figure 26 summer design conditions are approximately85F cold water leaving the tower, 95F water entering, and OAwet-bulb of 78F. This is 7F approach at 10F range. Notice thatthe point is plotted on the 10F range line. The same tower usedfor free cooling at an OA wet-bulb of 50F would provide 65Fleaving water (15F approach) at full load (10F range) or 58Fwater (8F approach) at 40 percent load (4F range).COOLING TOWER CAPACITY CONTROLSUMMER DESIGNCONDITIONS:ENTERING WATER, 95FLEAVING WATER, 85FOA WET BULB, 78FFREE COOLINGEXAMPLESC1479Fan control is the usual method of reducing tower capacityto maintain condenser water supply temperature in mildweather. A tower bypass valve is used to further limit coolingwhen the fans are off (Fig. 27). On-off fan control is verydifficult at light loads because at light load the OA WB is usuallywell below design, which increases the tower capacity,producing short fan “on” periods. Controlling the air volumewith dampers, blade pitch, or fan speed provides much closerand more stable control. A variable speed fan is more efficientthan a two-speed fan when varying tower capacity.Modulating tower water bypass for capacity control at lowambient temperatures can cause freeze-up in some tower designs.Since, use of a tower bypass mixing valve in the tower leavingwater can lower the pump suction pressure below the pumpsminimum NPSH, a diverting valve in the tower entering wateris generally used.ENGINEERING MANUAL OF AUTOMATIC CONTROL293