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2. Direct Expansion with TXV – controls a solenoid per
compressor.
3. Direct Expansion with EXV – controls a solenoid per
compressor and EXV per circuit with Suction Superheat
control.
4. Direct Expansion with no solenoid – Assume DX
with EXV per compressor. EXV controlled to Suction
Superheat setpoint.
The Economizer function has the following algorithms.
1. Speed – uses relative speed of the compressor to
decide when to engage the economizer.
2. Pressure Ratio (Default) – uses Pressure Ratio to
decide when to engage.
3. Capacity – uses capacity calculation to decide when to
engage.
The following is the sequence of operations for the
Economizer (ECO) function: Control signal is based on
Algorithm selected.
1. Compressor Idle, ECO is Off, all valves are closed.
2. Compressor Operational and controlling signal rises
above the engage threshold, ECO system is engaged.
a. ECO solenoid is open and EXV is controlled by PID
loop to appropriate Setpoint (Flash Tank Level –
default 25 percent; DX SSH – Default 9 R.
Note: Flash Tank program includes tank over full
setting which automatically closes solenoid to
prevent liquid refrigerant being introduced into
interstage of the compressor.
3. Compressor Operational and controlling drops below
the disengage threshold, ECO system is disengaged.
4. Compressor Stopping, Fault, Offline, Absent – ECO
System is Off.
PID and all other parameters are configurable with
appropriate credentials (Tech).
Low-Lift Refrigerant Pump
The system includes optional liquid refrigerant pump to
operate during conditions of low-lift to provide motor
cooling. Under normal conditions, the compressor relies on
lift (PR > 1.5) to drive motor cooling but with the assist of
the refrigerant pump the chiller can run at PR < 1.5, leading
to greater efficiencies.
The low-lift liquid refrigerant pump has the following
algorithms.
1. Motor (Default) – the system looks at compressor
inverter and cavity temperatures with pressure ratio
(PR) as a permissive.
2. Pressure Ratio – for split systems, pump is controlled
by PR exclusively to provide motor cooling and keep
liquid refrigerant flowing to EXV.
The following is the sequence of operations for the low-lift
liquid refrigerant pump:
1. Compressor Operational.
ARTC-SVX013A-EN
2. If Pressure Ratio (PR) drops below Engage Threshold,
System is considered in Low-Lift condition. For PR
mode, pump always runs while low-lift is true.
3. If low-lift condition true and if either temperature is
greater than the compressor cooling valve open
temperature (typically 111 °F) then run pump.
4. Pump runs until both temperatures are below the
compressor cooling valve close temperature (typically,
108 °F) then pump stops.
5. The system includes a (default 30 second) anti-cycle
timer for the pump.
6. If the PR rise above disengage threshold, the system is
no longer considered in low lift.
PID and all other parameters are configurable with
appropriate credentials (Tech).
Hot Gas Load Balancing
In low-load conditions, the system can keep the chiller from
cycling by introducing hot gas into the evaporator, giving
the chiller a false load. Care must be taken when setting
the parameters for the Hot Gas Load Balancing (HGBV)
function to engage the HCBV system before the conditions
for shutdown are met.
The following is the sequence of operations for HGBV.
1. Chiller operational with only one compressor running.
2. LCHWT drops less that setpoint and a small offset less
than LUZ stage out offset. HGBV system is engaged:
a. Running compressor's HGBV solenoid opens, and
the appropriate modulating valve opens and
controls to setpoint with PID Loop.
3. Any other chiller conditions – the HGBV system is
disengaged, and all valves are closed.
PID and all other parameters are configurable with
appropriate credentials (Tech).
Pumping
The system includes pumping control. It allows for duty/
stand-by control of up to two VFD pumps with automatic
switch-over and rotation.
This pump system can be applied to chiller pumping (all
chillers) and condensing pumping (water-cooled chillers).
Pumping is enabled to the following signals.
•
System On.
•
Cooling Enable.
•
Compressor Run (condensing pumping only).
A VFD pump can be controlled with different signals as
well.
•
Flow.
•
Differential Pressure (DP).
•
Delta Temperature (DT) .
•
2 -Speed control – accommodates multiple fluid
circuits.
Sequence of Operations
71
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