Trane TRG-TRC016-EN manual Unload Before Start

period four

Chiller-Plant Control

notes

chiller safeties must be capable of shutting the chiller down to avoid equipment damage.

Unload Before Start
	2pumps
headpressure	1pump
	system
	curve
	system flowrate	610gpm	870gpm	Figure 101
		[38.5L/s]	[54.9L/s]

The next control function is to partially unload the operating chillers before an additional chiller and pump are turned on. Depending on the system configuration, there can be very rapid variations in water flow through the chiller evaporator when a pump is turned on or off, or when a control valve is opened or closed. Partially unloading the chiller prior to such variations allows the chiller to continue to operate without interruption.

This can be explained by looking at a flow diagram for a chilled-water system with multiple pumps (see Figure 36). This diagram shows that, with one pump and chiller operating, the flow rate through the chiller is 610 gpm [38.5 L/s].

When the second, same-size pump and chiller are turned on, the flow rate through the system increases to 870 gpm [54.9 L/s], but the flow through each chiller drops to 435 gpm [27.4 L/s]. This is an instantaneous reduction of

175 gpm [11 L/s], or 30 percent, through the first chiller.

The temperature of the water leaving the chiller and the temperature of the refrigerant in the evaporator drop as a result of this drastic flow reduction. New, advanced chiller controls may allow the refrigerant temperature to drop below the fluid’s freezing point for a brief period of time while the compressor unloads. The evaporator low-temperature safety may, however, turn off the chiller if the controls and compressor cannot react quickly enough.

The “unload-before-start” function partially unloads the operating chillers, raising the refrigerant temperature in the evaporator, before the flow reduction occurs. The chillers are allowed to reload as soon as the additional chiller is turned on.

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