Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSWhen two chillers of equal size and similar characteristics areused, the point at which the second chiller is activated is usuallywhen the first chiller reaches 100 percent load. This isdemonstrated in Figure 13 which plots kilowatt per kilowatt versuspercent load for one chiller and two chillers at various temperaturedifferences between condenser and chilled water temperatures.Curves vary slightly for different temperature differences so amicroprocessor-based control system is used for maximumefficiency. The microprocessor checks chilled and condenser watertemperature, looks up chiller efficiency at those temperatures, andcalculates the optimum changeover point.Curves A and B in Figure 13 illustrate that for the chillersoperating at design condition with a 24 kelvins temperaturedifferential (∆T) between chilled and condenser water thesecond chiller must be added when the first chiller reaches 100percent load (50 percent of chiller system capacity). The nextset of curves (C and D) show the chiller is more efficient becauseof the smaller ∆T (17K) and that the first chiller can be loadedto 110 percent (55 percent of system load) before the secondchiller is added. The third set of curves shows the extremelyefficient operation with a 10.5 kelvins ∆T.0.32DESIGN A24 KELVINSBA 1 CHILLER - 24 KELVINS (29.5°C - 5.5°C)B 2 CHILLERS - 24 KELVINSC 1 CHILLER - 17 KELVINS (24°C - 7°C)D 2 CHILLERS - 17 KELVINSE 1 CHILLER - 10.5 KELVINS (18.5°C - 8°C)F 2 CHILLERS - 10.5 KELVINS0.29MAXIMUMCAPACITY C17 KELVINSDONE CHILLER0.26kW/kW INPUT0.23TWO CHILLERS0.20EMINIMUMCAPACITY10.5 KELVINSFADD ONECHILLER0.170.1412.5 25 37.5 50 62.5 75 87.5 100 112.5LOAD IN PERCENT OF CHILLER SYSTEM CAPACITYC4084Fig. 13. Efficiency–Two Equal Size Chillers.306ENGINEERING MANUAL OF AUTOMATIC CONTROL