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36 Chiller System Design and Control SYS-APM001-EN
System Design Options
Same tower, larger chiller
One retrofit option that benefits many building owners is installing a new,
larger chiller selected for a lower flow rating and re-using the existing cooling
tower, condenser-water pump, and condenser-water pipes. In many cases,
this allows the building owner to increase the chilled-water-system capacity
for an expansion, with a limited budget. An example can easily demonstrate
this.
A hospital presently has a 450-ton [1,580-kW refrigeration] chiller that needs
to be replaced. The condenser water flow is 1,350 gpm [85.2 L/s]. The present
cooling-tower selection conditions are summarized in Table 11. Recently, the
cooling-tower fill was replaced. The tower, condenser water piping, and
pump are in good condition. The hospital is planning an addition with 50
percent more load for a total of 675 tons [2,370 kW]. Must the hospital replace
the condenser water system? The answer is “no,” as long as the chiller is
selected properly.
How is this possible? As long as the new chiller’s condenser-water pressure
drop is at or below that of the present chiller, the same amount of water can
still be pumped. With the same flow rate, a 675-ton [2,370-kW] chiller may be
selected with a condenser-water-temperature rise of approximately 15°F
[8.3°C]. Using the cooling-tower manufacturer’s selection software, the same
cooling tower can be selected at the elevated temperature difference. As
shown in Figure 22, the new selection point will be:
• entering water temperature: 103°F [39.4°C]
• leaving water temperature: 88°F [31.1°C]
• ambient wet bulb: 78°F [25.6°C]
Table 10. Same tower, smaller approach
Present
Same tower,
smaller
approach
Capacity, tons
[kW refrigeration]
450 [1,580] 450 [1,580]
Cooling tower
Approach, °F [°C] 7 [3.8] 5.5 [3]
Flow rate, gpm [L/s] 1350 [85.2] 900 [56.8]
Entering temperature, F [C] 94.3 [34.6] 97.6 [36.4]
Leaving temperature, F [C] 85 [29.4] 83.5 [28.6]
Ambient wet-bulb
temperature, F [C]
78 [25.6] 78 [25.6]