Trane SYS-APM001-EN manual System Design Options, Same tower, larger chiller

System Design Options

Table 10. Same tower, smaller approach

				Same tower,
		Present		smaller
				approach
Capacity, tons		450	[1,580]	450 [1,580]
[kW refrigeration]
	Approach, °F [°C]	7	[3.8]	5.5 [3]
Cooling tower	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		78	[25.6]	78 [25.6]
temperature, °F [°C]

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]

36 Chiller System Design and Control SYS-APM001-EN