Johnson Controls YORK YVAM 0350, Operation & Maintenance Manual

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Reduced speed also reduces the available capacity of the chiller, when speed reduction is possible.
If speed is reduced, the chiller power use is reduced. As a result, at reduced capacity requirements
where condenser pressure is also reduced, the motor speed is reduced as much as possible
while maintaining chilled liquid temperature and sufficient lift. When the speed cannot be further
reduced due to lift required for the specified leaving chilled water temperature setting and
available cooling to the condenser and capacity must be further reduced, a mechanism called
variable geometry diffuser (VGD) at the exit of the impeller is used to reduce refrigerant gas flow.
The VGD not only controls capacity, but serves to mitigate stall. Stall is an effect caused by slow
refrigerant gas passing through the compressor at reduced flow rates needed for low capacity
operation.
The YVAM chiller uses these mechanisms in a controlled order to maintain best efficiency.
The YVAM chiller controls capacity by adjusting the compressor VGD position, the compressor
motor variable speed drive, and optional hot gas bypass valve (HGBP) position, if equipped, in
a specific sequence depending on 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 to achieve the best chiller efficiency is as follows:
• Conditions that require capacity increase: HGBP, if present, is driven toward closed. When
the HGBP is fully closed, the VGD is driven toward open. Then when the VGD is fully open, the
speed of the variable speed drive is increased.
• Conditions that require capacity decrease: The drive speed is decreased to the minimum
speed for the lift required. When the drive speed is at the minimum limit to avoid surge for
the lift, the VGD is driven toward closed. When the VGD reaches closed or the minimum
position allowed for the operating condition, 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
variable speed drive) to mitigate the condition to keep the chiller online. When any of these physical
thresholds are approached, the control proportionally limits the amount of capacity increase
permitted. If exceeded, it 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 can 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 that cannot be eliminated immediately. As a result, 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 occurs are evaluated and the shutdown is based on accumulated time below the
freeze temperature.
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