whether loading or unloading is required to keep Leaving Chilled Liquid Temperature at setpoint.
Motor speed is additionally and simultaneously adjusted as necessary to maintain the minimum
compressor lift required to prevent surge. The sequence for operation of the control devices is as
follows to achieve the best chiller efficiency:
• Conditions require capacity increase: HGBP (if present) is driven toward closed. When the HGBP
is fully closed, the VGD is driven toward open. When the VGD is fully open, the VSD speed is
increased.
• Conditions require capacity decrease: VSD speed is decreased. When the VSD speed is at the
minimum limit to avoid surge for the lift, the VGD is driven toward closed. When the VGD
reaches closed, the HGBP (if present) is driven toward open.
Also, High Condenser Pressure, Low Evaporator Pressure, High Motor Current, and High Input
Current limits and overrides limit or reduce the output to the appropriate devices (HGBP, VGD, or
VSD) to mitigate the condition to keep the chiller online. As any of these physical thresholds are
approached, the control proportionally limits the amount of capacity increase permitted and if
exceeded issues unloading into the capacity control command.
Anti-surge minimum frequency
In order to maintain sufficient compressor lift to overcome condenser pressure and prevent surge
throughout operation, the control maintains and continuously updates a minimum limit for VSD
speed. This limit is the active anti-surge minimum frequency. It is calculated and applied to the
speed each cycle of the capacity control routine.
Smart freeze
The smart freeze feature prevents nuisance chiller shutdowns due to brief periods of chilled liquid
flow fluctuations or other brief operating conditions that would normally cause low evaporator
pressure safety shutdowns. With this feature enabled and activated, the chiller is permitted to
ride through these temporary conditions. Also, this feature allows the Leaving Chilled Liquid
Temperature Setpoint to be set as low as 36.0ºF (2.2ºC). Smart freeze protection can be enabled or
disabled at the keypad, by a service technician. It cannot be used in brine cooling mode.
Smart freeze protection uses the evaporator refrigerant temperature as one of the variables to
determine when freezing is imminent. The basis of this feature is that the chilled liquid contains an
amount of heat, which cannot be eliminated immediately. Therefore, it requires a certain amount of
time for the liquid to change to a solid. During this period of time, those parameters that determine
when solidification will occur are evaluated and the shutdown is based on accumulated time below
the freeze temperature.
Surge protection
The surge protection feature detects surge events. It provides a running count of the surges
detected over the lifetime of the chiller. It allows the user to define how many surges are excessive
and how the control will react to an excess surge condition. When excessive surging is detected,
this feature can shutdown the chiller.
Surge events are detected by monitoring the relationship between the condenser pressure
and evaporator pressure while the chiller is running. An excess surge condition is detected by
comparing the number of surge events that occur in a selectable time period to a selectable
threshold.
If the number of surge events (surge window count) detected in the time period programmed as
the Count Window setpoint (1 to 5 minutes; default 5) exceeds the threshold programmed as the
Count Limit setpoint (4 to 20; default 4) an excess surge condition is detected.
Unless the shutdown features have been enabled, as explained below, the chiller continues to
run under the same conditions displaying WARNING – EXCESS SURGE DETECTED. This message
displays until a manual reset with the Warning Reset key at the operator access level. If the
YMC
2
Mod B with OptiView Control Center
18
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