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Circuit Functions
CIRCUIT STATES
The circuit will always be in one of four states:
Off - The circuit is not running.
Pre-open - The circuit is preparing to start up Running - The
circuit is running
Pump off - The circuit is performing a normal shutdown.
The transitions between these states are shown in the following
diagram.
Figure 18. Circuit states.
T1 - At pre-opening
• No compressor is running and any compressor in the circuit
is commanded to start.
T2 - Pre-open to run
• 5 seconds have elapsed in pre-opening state.
T3 - Run to pump down
Any of the following is required:
• Last compressor in the circuit is commanded to stop.
• Unit status = Pump stopped.
• Circuit breaker is open.
• Circuit mode is disabled.
• Circuit breaker is open -Circuit mode is disabled -Pump down
alarm is active.
T4 – Pumping down on Off
Any of the following is required:
• Evaporator pressure < Pump downstream pressure value.
• Unit status = Off.
• Unit status = Off -Quick circuit shutdown alarm active.
T5 - Run to Off
Any of the following is required:
• Unit status = Off.
• Fast circuit shutdown alarm is active.
• A low temperature start attempt failed.
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T6 - Pre-open to Off
Any of the following is required:
• Unit status = Off.
• Unit status = Pump off.
• Circuit breaker is open.
• The circuit mode is deactivated.
• The circuit quick stop alarm is activated.
• The pumping alarm is activated.
COMPRESSOR CONTROL
Compressors should operate only when the circuit is in the
operating or pumping state. They should not operate when the
circuit is in any other state.
Compressor start-up
A compressor must start if it receives a start command from the
unit capacity control logic.
Compressor shutdown
A compressor must be shut down if any of the following situations
occur:
• The unit's capacity control logic commands it to shut down.
• A discharge alarm occurs and sequencing requires this
compressor to be the next compressor to shut down.
• The circuit status is pumping and sequencing requires this
compressor to be the next compressor to shut down.
CONTROLLER CALCULATIONS
A minimum time between compressor starts and a minimum time
between compressor stop and compressor start will apply. The
time values are determined by the start timer and stop timer set
points. These cycle timers should not be applied by power cycling
the chiller. This means that if the power is cut off, the cycle timers
should not be active. These timers can be cleared by a setting on
the controller.
CONDENSER FAN CONTROL
The condenser fan control shall start the fans as required
whenever the compressors are running in the circuit.
All fans and solenoid valves shall be off when the circuit is in the
off and pre-open state.
The digital outputs of the condenser fans will turn on or off
immediately for condenser stage changes.
The outputs of the capacitor solenoid valves will turn on
immediately when a step-up stage requires the output to turn on,
but will have a delay to turn off during a step-down stage.
This delay is 20 seconds. If the circuit is turned off, the capacitor
solenoid valve outputs will turn off without delay.
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