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40 Chiller System Design and Control SYS-APM001-EN
System Design Options
and a more conservative zero condenser-water-pipe pressure drop, we can
examine the effect of reducing flow rates.
Figure 24. System energy consumption (no pipes)
Energy consumption for the chiller, condenser-water pump, and cooling-
tower fans is shown in Figure 24. Note that only at full load does the total
power of the chilled-water plant increase. Recall that this is with absolutely
no pressure drop through the condenser-water piping, valves, or fittings. It is
interesting to note that the break-even point at full load is approximately
8 feet of head (water) [23.9 kPa]. Also note that at all part-load conditions, the
total power of the low-flow system is less than that of the base system. It is
easy to see that even for short piping runs, reducing flow rates can improve
plant energy consumption.
Table 12. Reduced flow-rate effect
Condenser Water Pump Base Case Low Flow
Flow rate, gpm [L/s] 1350 [85.2] 900 [56.8]
System pressure drop, ft water [kPa] 0 0
Condenser bundle pressure drop, ft water [kPa] 19.9 [59.5] 9.6 [28.7]
Tower static lift, ft water [kPa] 19.1 [57.1] 12.6 [37.7]
Pump power output, hp [kW] 17.7 [13.2] 6.7 [5.0]
Pump fan electrical input, kW 14.2 5.4
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
25% 50% 75% 100%
System Load
System Energy Consumption (kWh)
3.0 gpm/ton 2.0 gpm/ton