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Sequence of Operation
the heat type installed. The minimum fan speed can be
100%. See Heat Types for more information.
As the Discharge Air Temperature Setpoint increases, the
heat capacity increases stages and/or modulates. The
control manages heating capacity to deliver a discharge air
temperature to the Discharge Air Temperature Setpoint
within the control deadband. If the unit discharge air
temperature reaches the Discharge Air Temperature
Maximum Heat Limit, heating capacity will hold and the
supply fan will further increase (if possible) to satisfy the
space heating demand.
Constant Volume — Space Temperature
Control
Constant Volume space temperature control units provide
heating and cooling in the same sequence of operation as
Single Zone VAV except the supply fan, when On, will
operate at its maximum speed. Heating and cooling have
the same constant volume, maximum speed.
Compressor Staging and Timers
There are timers associated with compressor staging
control to optimize refrigeration system performance:
•
Minimum OFF Time
•
Minimum ON Time
•
Inter-Stage Time
Minimum OFF Time — For each compressor (fixed or
variable speed), when the compressor has been turned
OFF for any reason, a timer will be started for that
compressor(s). The compressor(s) will not be allowed to be
turned back ON until their timer has expired.
Minimum ON Time — Once a compressor has been
turned ON a timer will be started for that compressor. The
compressor will not be allowed to be turned back OFF until
its timer has expired. A normal stop request (Front Panel
Stop, normal shutdown diagnostic, Off mode, etc...) will
honor the Minimum ON Time and keep the compressor on
until the timer expires
Important: Any Immediate stop request or Immediate
Shutdown diagnostic will override any
minimum on time and shut the compressor off
immediately.
Inter-Stage Time — For normal unit operation the timing
between stages will be maintained at a minimum of a fixed
Inter-stage Time. The compressor control algorithm may
request staging to occur beyond 3 minutes but the time
span from one stage turning ON(OFF) until the next
compressor stage turning ON(OFF) will be no less than the
3 minute inter-stage time. Rapid Restart and manual
compressor control can override this time.
Supply Fan Delay
A 60 second supply fan off delay is enforced after the last
compressor de-energizes.
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General Compressor Staging
Compressor staging will occur based on the unit's
algorithms cooling capacity request and the specific
Compressor Staging Table defined per the unit's
configuration. When the request is ADD, the next highest
stage sequence will identify which compressors will be
turned ON, or OFF if a 'Double Swap' is required. When the
request is SUBTRACT, the next lowest stage sequence will
identify which compressors will be turned OFF, or ON if a
'Double Swap' is required. Capacity Control limits may also
affect staging sequence.
Double Swap
To meet efficiency or optimized capacity requirements, it
will be necessary on some units, when staging up or down,
to turn ON one compressor and, for the same stage, to turn
OFF another compressor.
Compressor Proving
•
Fixed Speed Compressors: The compressor contactors
have an auxiliary contact that is wired to a compressor
proving input.
•
Variable Speed Compressors: The variable speed
compressor VFD has an integrated Run Inhibit binary
input wired to a second HPC (High Pressure Cutout)
switch. When this HPC switch opens the VFD will be
stopped and the run proving status will be
communicated back to the controller.
Variable Speed Compressor Optimal Start
This circuit-level feature applies to the variable speed
compressor and enforces a maximum speed limit for a
fixed time period following startup. During normal operation
the speed range limits are determined based on unit type,
efficiency, capacity stage and other limit controls. The
maximum speed limit applies to every startup to maintain
oil quality. When this function is active, an indication will be
made to the user interface
Evaporator Coil Surface Utilization (Two
Circuit Units Only)
All staging sequences performed by the algorithm energize
compressors on each circuit as soon as possible to
optimize cooling efficiency. If a circuit 1 compressor starts
first, a circuit 2 compressor starts second if it is available. If
a circuit 2 compressor starts first, a circuit 1 compressor
starts second if it is available.
Compressor Lockout and Inhibit
In all cases of Lockouts, Inhibits, and Limits (Compressor
Minimum OFF Time is not considered a lockout or inhibit)
Alternate Staging Sequences will be substituted.
Compressor Lockouts
Compressor lockouts are typically non-recoverable and
require user interaction at the User Interface to clear
associated diagnostics that return the compressor(s) to
operation. Other compressor lockouts will recover
automatically when the lockout command is removed.
RT-SVX063G-EN
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