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Operating Principles
Control Sequences of Operation
Occupied/Unoccupied Switching
There are three ways to switch occupied/unoccupied:
1. Field-supplied contact closure (hardwired binary
input to field wiring terminal block (2TB3)) — This
input accepts a field supplied switch or contacts
closure such as a time clock. Open Contacts =
Unoccupied and Closed Contacts = Occupied.
2. BACnet® MSTP control system can control the
occupied/unoccupied request of the unit.
3. Through the TD7 Display via the on-board schedule.
Compressor Lead/Lag
Compressor lead/lag is present in the controls for all
units with two or more fixed speed compressors. Only
Compressors 2 and 3 rotate. Balanced run hours is
achieved by changing the fixed speed compressors
sequence number when commanded to rotate.
Variable speed compressor A is always the first stage
on and last stage to turn off for mechanical cooling.
Emergency Stop Input
A binary input is provided on the unit's field wiring
terminal block (2TB3) for installation of a field-provided
switch or contacts to immediately shutdown all unit
functions. An open contact will indicated an emergency
shutdown condition exist and shut down the unit and
generate a manual reset alarm. A closed set of contacts
will indicate a normal condition. After the contacts are
closed, the manual reset alarm must be cleared to
resume unit operations.
External Auto/Stop Input
A binary input is provided on the unit's field wiring
terminal block (2TB3) for installation of a field-provided
switch or contacts to safely shutdown all unit functions.
An Open contact will indicate a Stop condition exist
and the unit will shutdown and generate an automatic
reset alarm. A Closed contact will indicate an Auto
condition and allow the unit to operate.
Local Auto Stop
A binary value is located on the TD7 which allows the
user to start and stop the unit in a safe manner for
maintenance. On the TD7 home screen in the upper
right hand corner the unit can be put into Auto to
perform normally or Stop to perform a controlled
shutdown and keep the unit off.
Water Flow Control
With compatible piping configurations, the unit can be
configured to provide variable water flow with the
48
water regulating valve option only, which maximizes
energy saving by unloading the water pumping
system.
Head Pressure Control
Water-cooled condensers: Units that are ordered with
the optional flow control valves and configured for
variable water flow will modulate a water valve to
maintain a user-defined condensing pressure setpoint
and compressor refrigerant pressure differential.
Condensing pressure and pressure differential will be
referenced utilizing factory installed suction and
discharge pressure sensors located at each condenser.
Water Purge
This user-definable feature allows the user to select a
purge schedule to automatically circulate water
through the economizer and condensers periodically
during non-operational times. This allows fresh
chemicals to circulate in waterside heat exchangers.
Supply Air Temperature Control
Unit Sequence of Operation
Discharge Air High Temp Limit Software
Alarm (Software)
When the Discharge Air Temperature is above the
Discharge Air High Temperature Cutout setpoint of 100
degrees for 5 minutes, the unit will perform a
controlled shutdown. A Manual Reset alarm will be
generated and need to be cleared before the unit can
resume operating.
Condenser Water Low Temperature
Alarm (Software)
When either the entering or leaving condenser water
temperature is below the Condenser Water Low
Temperature cutout setpoint of 35 degres for 5
minutes, the unit will perform a controlled shutdown.
All Condenser water valves will be commanded open if
installed, and a manual reset diagnostic will be
generated and need to be cleared before the unit can
resume operation.
Leaving Air Low Temperature Alarm
When the Discharge Air Temperature is below the
Discharge Air Temperature Cutout for 5 minutes, the
unit will perform a controlled shutdown. A Manual
Reset alarm will be generated and need to be cleared
before the unit can resume operation.
PKG-SVX027C-EN
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