Trane SYS-APM001-EN manual Unequal Chiller Sizing, Chilled-Water System Variations, Evaporators

Chilled-Water System Variations

series. The left half of Figure 50 shows a modularized configuration where series chiller modules are placed in parallel with each other, so that any upstream chiller’s valves could be “paired” with virtually any downstream chiller by opening the appropriate valves. The condenser side in a counterflow arrangement is shown in the right half of Figure 50. The advantages of this system for large chilled water systems include highest efficiency, scalability as the project grows, and high redundancy without a significant investment in extra equipment.

Figure 50. Series arrangement of evaporators and condensers

55.0°F		Evaporators			98.9°F		Condensers
[12.8°C]					[37.2°C]
	Circuit 1		Circuit 2			Circuit 1		Circuit 2
	Circuit 1		Circuit 2			Circuit 1		Circuit 2
	Circuit 1		Circuit 2			Circuit 1		Circuit 2
3 Chiller Modules				45.1°F	3 Chiller Modules				91.3°F
				[7.3°C]					[32.9°C]
	Circuit 2		Circuit 1			Circuit 2		Circuit 1
	Circuit 2		Circuit 1			Circuit 2		Circuit 1
	Circuit 2		Circuit 1			Circuit 2		Circuit 1
37.0°F					85.0°F
[2.8°C]					[29.4°C]

Unequal Chiller Sizing

Many designers seem to default to using the same capacity chillers within a chilled-water plant.25, 26 There are benefits to using unequally-sized chillers to meet the system loads. One is that when a chiller is brought online, so is its ancillary equipment, thus increasing system energy consumption. In general, the smaller the chiller, the smaller the ancillary equipment. Another is being able to ensure that chillers are efficiently loaded. Many times this can be accomplished by using chillers that do not have the same capacity. Examine the use of 60/40 splits (one chiller at 60 percent of system capacity, the other at 40 percent) or 1/3–2/3 splits (one chiller at 1/3 of system capacity the other at 2/3). The benefit is that the system load can be more closely matched with the total chiller capacity, increasing total system efficiency by eliminating the operation of chillers and ancillary equipment for more hours of the year. One caveat for variable-primary-flow systems is that if the pressure drops are different across the unequally sized chillers, they will load even more unequally without the use of pressure-reducing valves that require extra pump energy, as shown in Table 15 on page 61.

78 Chiller System Design and Control SYS-APM001-EN