Sterlco GC Series Central Chiller - Manual (Page 10)

GC Series Central Chillers Chapter 2: Functional Description 10 of 79
is off. There is a “compressor anticyle” timer that will delay the time between compressor
starts to prevent short cycling of the compressor. This is a start to start timer set at 5 minutes.
For example, if the compressor has been running for 5 minutes and shuts off, then the
compressor can start immediately if the demand is there. If it has only been running for 2
minutes, then it would not be able to start again for 3 minutes.
In a single module system, the second compressor or lag compressor starts when the lead
compressor is on, the hot gas is at 0%, and the leaving fluid temperature is above the setpoint
plus the “compressor on differential”. There is also an adjustable “lag compressor on delay”
timer. Before starting either compressor, the compressor with the least amount of runtime
hours is considered the lead compressor and the first to start. When both compressors are
running, the compressor with the most hours is considered the lead compressor and is the first
to stop. The lead compressor will shut off if the leaving fluid temperature drops below the
“compressor off differential” and the hot gas valve is at 100% for the “lead compressor off
delay” time. The compressor anti-cycle timers are active with the two compressor units.
Multiple Module Operation
Each individual module is controlled independently by its pCO control using its leaving fluid
temperature as the process variable. (The program to control compressor staging, the
optional hot gas valve and the alarms is identical to the single module control.)
Each module has a flow switch to verify flow before staging the compressor The lead module
that is started is determined by which module has the least amount of hours. An average
temperature of the leaving water temperature determines if additional modules are required.
The average temperature is calculated based on the leaving temperature of each enabled
module. The sequence of module staging is determined by the runtime hours. Subsequent
modules are started based on the previous module’s hot gas and compressor staging. For
each module, only one compressor will be on before staging the next module. This is to try
to keep equal capacity on each module so that the overall leaving fluid temperature can be
met. Each individual module will maintain its compressor lead lag sequence to determine
which compressor stages first on the module.
Each refrigeration module has a cooling enable/disable. If the module is disabled, a display
message pops up that states “When disabling a module on a chiller with multiple modules,
the fluid flow to that module must be shut off”. The flow switch status will determine if this
was done and a reminder will display every five minutes. The module enable/disable will
determine if that module will be allowed to start in the sequence.
If any module is in an alarm condition, that module will be taken out of the sequence. Once
the alarm is cleared, it will be brought back in the sequence based on its run time hours. In
other words, a module that is in an alarm condition will not prevent the other modules from
staging on. All alarms including the flow alarm will be disabled when the module is
disabled.
Within all of the modules there will be two PID controls running simultaneously for
determining the position of the optional hot gas valve. One PID output will be based on
leaving fluid temperature and the other based on a minimum saturated suction temperature for
freeze protection. The hot gas valve position will be determined by which output is greater.
If it is controlling to the saturated suction temperature to prevent freezing, a warning will
display on the screen that the hot gas is in this mode. Once the control goes back to setpoint
control, the warning will disappear. The saturated suction temperature is calculated from the
suction pressure and the type of refrigerant.
The discharge pressure is controlled using an analog output signal to drive a fan for air-
cooled units or a water regulating valve for water-cooled units. The output signal is