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Sequence of Operations
Device Sequences
The chiller uses the following devices as part of the chiller
process.
1. EXV device.
2. Fan Bank device.
3. Economizer device.
4. Low-Lift Refrigerant Pump.
5. Hot Gas Load Balancing device.
6. Pumping.
7. Fluid and Refrigerant isolation device.
All devices in the chiller control system have Hand-Off-Auto
functionality:
•
Hand – user takes control of the device from the HMI or
equivalent interface.
•
Off – the device is inactive regardless of the state of the
chiller control system:
•
Auto – device is available to the chiller control
sequences.
Note: All devices must be in auto mode for proper chiller
operation.
Electronic Expansion Valve (EXV) Sequence
The EXV is a per refrigeration circuit device and operates
when any compressor on the circuit is running and this
software handles up to four independent refrigeration
circuits. The software includes a selection of algorithms to
control the EXV. The factory and Trane Technologies
technicians set the algorithm to best suit your conditions
and configuration. The algorithms for control are:
•
Suction Superheat (SSH) – can work with sensors on
the evaporator or the internal sensors on the
compressors. If no barrel sensors are available, the
system defaults to compressor internals. This is default
for TCAA chillers. Setpoint is a function of capacity.
•
Discharge Superheat (DSH) – like SSH, this algorithm
works with compressor internal or external sensors.
Very stable method for chiller control but cannot be
used in conjunction with economizers. Setpoint is a
constant.
•
SubCooling (SC) – uses the liquid line temperature
sensor with either compressor saturated discharge
temperature of external pressure sensor converted to
temperature. Setpoint is a function of capacity.
•
Liquid Level – can work with either high pressure side
r low pressure side with appropriate level sensor. This
algorithm is mostly for backward compatibility with 1
generation TCAA units.
The following is the sequence of operations for an EXV.
1. All Compressors on refrigeration circuit Idle, Valve
Closed.
2. Any Compressor Starting – Valve to Start Position
(Settable Parameter).
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3. All compressors on refrigerant circuit to Operational –
Valve PID controlled to setpoint. Valve position subject
to Max and Min position parameter settings.
4. If during EXV normal function, a low SST signal occurs,
the valve will rapidly open to avoid Fault.
PID and all other parameters are configurable with
appropriate credentials (Tech).
Fan Bank Sequence
The fans on a TCAA chiller are arranged by fan bank,
where a fan bank serves an individual refrigerant circuit.
This software does not control individual fans. Like the
EXV, this software can control up to four fan banks and
does include several different algorithms designed to get
the best from your chiller. The fan control algorithms are:
•
Ambient – Fan SDT setpoint is a function of Ambient
temperature with an approach setting. The SDT
setpoint value is subject to the Max and Min SDT
setpoint parameters.
•
Capacity – Fan SDT setpoint is a function of capacity
within the bounds of Max and Min SDT setpoint
parameters.
•
Ambient with Capacity – (Default) SDT setpoint is a
function of both Ambient Temperature with approach
and capacity within the bounds of Max and Min SDT
setpoint parameters.
•
Speed – SDT Setpoint is a function of compressor
relative speed to Surge and Choke. It is design to keep
the compressor at its most efficient.
•
Low-Lift – SDT setpoint is a function of pressure ratio
(PR) between a Max and Min PR parameter. This is not
a selectable function but engages automatically at low-
lift conditions.
The following is the sequence of operations for a fan bank.
1. Circuit Idle (all compressors on the circuit are Idle), Fan
Bank speed value is 0 percent - Off.
2. Any Compressor on circuit Starting, Fan Bank speed
set to Start Speed parameter.
3. All compressors on circuit Operational, fan bank speed
is PID controlled to the SDT setpoint.
4. Circuit Stopping going to Idle, Fan Bank speed set to
Fans Off Speed parameter. Fan Bank stays a Fans Off
Speed for the duration of the Fans Off Delay parameter
then Fan Bank speed to 0 percent - Off.
PID and all other parameters are configurable with
appropriate credentials (Tech).
st
Economizer
The software provides the economizer function on a per
compressor basis and is therefore, capable of six
independent economizer controls. The software can control
the various economizer options following:
1. Flash tank – controls a solenoid per compressor and a
circuit shared flash tank level.
ARTC-SVX013A-EN
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