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TRG-TRC016-EN 93
period four
Chiller-Plant Control
notes
chiller safeties must be capable of shutting the chiller down to avoid equipment
damage.
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.
Unload Before Start
head pressure
head pressure
system flow rate
system flow rate
2 pumps
2 pumps
1 pump
1 pump
system
system
curve
curve
870
870
gpm
gpm
[54.9 L/s]
[54.9 L/s]
610
610
gpm
gpm
[38.5 L/s]
[38.5 L/s]
Figure 101