Trane SYS-APM001-EN manual System Configurations

System Configurations

Alternatively, the operating chiller can be reset to produce a lower supply temperature at this condition. In this way, the mixed system supply-water temperature may be maintained at a more acceptable temperature. This complicates the control system and presents the possibility of increasing chiller energy consumption due to the requirement for lower-temperature water. There will also be a low limit to this water temperature, dependent on the chiller’s low pressure cut-out control, low evaporator-refrigerant- temperature limits, or low leaving chilled-water limits. The more chillers in the system, the worse the problem becomes. For this reason, this configuration is seldom used in systems with more than two chillers.

Additionally, ASHRAE/IESNA Standard 90.1–2007 (Section 6.5.4.2) prohibits this type of system when the pump is larger than 10 hp [7.5 kW]. The standard requires that, in systems that contain more than one chiller piped in parallel, system water flow must be reduced when a chiller is not operating.

	Figure 26. Parallel chillers with separate, dedicated chiller pumps
	Off
		42°F
		[5.6°C]
	On
	54°F
	[12.2°C]	60% to 70% of
		system flow
	Coil starved for flow
	If separate, dedicated chiller pumps are used (Figure 26), a chiller–pump pair
	can be cycled together. This solves the flow mixing problem described
	above, but presents a new problem. Below 50-percent load, only one chiller
	and one pump are operating. The total water flow in the system decreases
	significantly, typically 60 to 70 percent of full system flow, according to the
	pump–system curve relationship.
	Ideally, at this part-load flow rate, all of the coils will receive less water,
	regardless of their actual need. Typically, however, some coils receive full
	water flow and others receive little or no water. In either case, heavily-loaded
	coils or the loads farthest from the pump will usually be “starved” for flow.
	Examples of spaces with constant heavy loads that may suffer include
	computer rooms, conference rooms, photocopy rooms, and rooms with high
	solar loads.
SYS-APM001-EN	Chiller System Design and Control	43