WaterFurnace Envision2 NXW Chiller - Manual (Page 21)

21
ENVISION 2 NXW REVERSIBLE CHILLER INSTALLATION MANUAL
Control Features
Advanced Freeze Detection Operation:
“Pre Freeze” Alarm
Predictive freeze condition detection:
If the refrigerant temperature is within 7.2°F of the freeze
detection setpoint, the predictive freeze detection
algorithm is enabled, and if the logic determines that
a freeze condition is likely to happen based on current
conditions, the compressor of the involved refrigerant
circuit is immediately stopped. Three (3) such events
in 60 minutes will trigger a compressor lockout that
requires a manual reset. In the absence of such a
condition, the compressor is allowed to operate so that
the refrigerant temperature may eventually be at the
threshold of the freeze detection setpoint. NOTE: This
Lockout produces a “Pre Freeze” detection error on
the MUI display.
Capacity Limiting
If the leaving water temperature drops to 1.8°F above the
freeze detection setpoint, the lead compressor is halted.
When the leaving water temperature rises to 3.6°F above
the freeze detection setpoint, it will be allowed to resume
operation. This limiting is allowed to repeat indefinitely. This
causes “COMP1 Low Limit” to be displayed on the MUI.
If the leaving water temperature drops to the freeze
detection setpoint, the lag compressor is halted. When the
leaving water temperature rises to 1.8°F above the freeze
detection setpoint, it will be allowed to resume operation.
This limiting is allowed to repeat indefinitely. This causes
“COMP2 Low Limit” to be displayed on the MUI.
Compressor Current Switch (AI-3 EXP and AI-4 EXP)
The compressor current switch is designed to insure that
the compressor is on when the compressor output is
energized. This switch is normally open and closes when
current is flowing to the compressor. If the compressor fails
to start the switch will open. The switch must be open for a
continuous 15 seconds for a fault to occur. After 3 faults in
60 minutes the control will put the unit into an alarm state.
Flow Proving Switch (DI-1 and DI-10)
The load and source flow-proving switches are optional and
can be field installed. These switches shall be normally open
flow switches that will close when the water flow through
the heat exchangers reach an acceptable level. The flow-
proving switches must be closed 15 seconds prior to enabling
either compressor output (DO-1 and DO-2). If the load flow-
proving switch opens at any time both compressor outputs
(DO-1 and DO-2) must be disabled immediately.
High Pressure (DI-11 and DI-12)
The high-pressure switches shall be a normally closed (NC)
switch that monitors the systems compressor discharge
refrigerant pressures. There shall be an individual high
pressure switch for each circuit. If the input senses the
high-pressure switch is open during the period that the
compressor output is enabled, it must shut down the
compressor immediately and count the fault. The compressor
Advanced Freeze Detection System
The source and load heat exchangers are protected by a
multi-sourced temperature logic strategy. The temperature
logic is based upon the refrigerant temperature sensed as
the refrigerant is about to enter the heat exchanger; while
entering and leaving water temperatures are being used as
correlating factors. The detection scheme is shown as basic
and advanced algorithms.
Basic Freeze Detection Operation: “Comp1 or
Comp2 Freeze” Alarm
This alarm can be triggered by one of two detection
schemes.
Hard Limit Freeze Detection
If the refrigerant temperature drops below the freeze
detection setpoint by 1.8°F, the associated compressor
is locked out immediately regardless of any other
factors and requires a manual reset. NOTE: This Lockout
produces a “Comp 1 or Comp 2 Freeze” error on the
MUI display.
Freeze Detection
The refrigerant temperature is compared to the freeze
detection setpoint (15°F [antifreeze] or 33°F [water]
field selectable), and if the temperature falls below the
setpoint for 30 continuous seconds, the associated
compressor will be halted. This function becomes
enabled after the first two minutes of compressor
operation. Three such events in 60 minutes will trigger a
compressor lockout that requires a manual reset. NOTE:
This Lockout produces a “Comp 1 or Comp 2 Freeze”
error on the MUI display.
In addition to the above:
Entering Water Temperature Influence
If the entering water temperature of the evaporative
heat exchanger is within 10°F of the freeze setpoint,
the previously mentioned two minute delay will be
eliminated. This allows the freeze detection to operate
immediately when the compressor starts based on
entering water temperature.
Leaving Water Temperature Influence
If the leaving water temperature of the evaporative
heat exchanger is within 10°F of the freeze setpoint, the
previously mentioned 30 second delay will begin to be
proportionately reduced, ending at a 1 second delay
when the leaving water temperature is 1.5°F above the
freeze setpoint.
Dual Circuited Heat Exchanger Protection
A low temperature condition on either refrigerant circuit
will prevent the start of both compressors. If the low
temperature condition exists for 5 minutes when both
compressors are off, a lockout is triggered for both
compressors. However, if –for instance-both compressors
are operating and circuit 1 experiences a refrigerant
temperature below the freeze detection setpoint such
that compressor 1 is halted, compressor 2 will not be
halted as a result.