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CSRA-SVX02A-EN
System Start-Up
Evaporator Freeze Protection Diagnostic
The evaporator freeze protection integrator (EFZ) will start integrating when
the leaving solution temperature is less than the low leaving solution
temperature cutout setpoint. Once EFZ integrates up to 30 Sec., the
following diagnostics will occur:
1. If all compressors are off, an auto diagnostic will occur. All of the com-
pressors will be prevented from operating until the LST is 4°F greater
than the LLSC. Once the LST is 4°F above the LLSC, the unit will allow
normal unit operation.
2. If any compressors are "On", a manual diagnostic will occur. All of the
compressors will be shutdown and locked out due to the violation of the
evaporator freeze protection.
Sequence of Operation
Chiller Control for Process Applications
Process applications are characterized as having fast changes in load and
these loads are not adequately controlled with the same scheme used for
comfort applications. To provide better control of these dynamic loads, a
Proportional + Integral "control to setpoint" (as opposed to "control to
deadband") control strategy is used. System reliability is inversely
proportional to compressor cycle rates so reliability is the driving factor for
determining maximum cycle rate which ultimately translates into cycle-to-
cycle leaving solution temperature swings.
To provide stable capacity control, a 1 minute minimum time between
compressor starts is required. The control will determine an estimate of
instantaneous load based on error from setpoint for the leaving solution
temperature and cycle the appropriate compressor stage to best match the
calculated load.
Determining instantaneous error:
Error = Chiller Solution Temp - Chiller Solution Setpoint.
PI Control Calculation:
The CGM calculates the Load Value by applying PI calculations to the
instantaneous error value. The Load Value consists of an integer part and a
fractional part. The integer part represents which compressor stages are
locked "On" and the fractional part represents the duty cycle required for the
next higher compressor stage. For a 4 compressor unit, the Load Value will
range from 0.0 to 4.0. For a 2 compressor unit, the Load Value will range
from 0.0 to 2.0.
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