©2025 Copeland LP.
026-1803 R13 Supervisor I&O User Guide 9 - 9
9.7 Suction Groups
9.7.1 Introduction
The Supervisory Controller RX refrigeration controller uses
suction group applications to cycle compressors in an effort
to maintain a constant suction pressure or temperature in a
refrigeration system. A suction group may contain up to 16
compressor stages (standard, variable-speed, or unloader).
There are two different types of suction group applications:
• Suction Group - The “standard” suction group
application that has existed since the first version of the
Supervisory Controller firmware. The Suction Group uses
PID Control and with proper optimization it can achieve
very tight suction pressure or temperature control, but
compressor cycling is not based on load analysis and
can be inefficient, especially in larger racks. All versions of
Supervisory Controller support the standard Suction
Group application.
• Enhanced Suction Group - Introduced in Site
Supervisor version 2.30F01, the Enhanced Suction
Group has all of the important features of the older
Suction Group application, but instead of employing
user-configured PID constants to optimize operation, the
Enhanced Suction Group determines optimum control
by “learning” the effect each compressor and circuit load
has on the suction input. The Enhanced Suction Group is
easier to configure and more conservative with
compressor cycling than the standard Suction Group. In
addition, the Enhanced Suction Group also supports use
of the Copeland Digital Scroll and Copeland Digital
Discus compressors.
9.7.2 The (Standard) Suction Group
Application
9.7.2.1 Overview of PID Control Strategy
A Suction Group application looks at suction pressure or
suction temperature to determine how many compressor
stages should be ON or OFF. The application compares the
suction pressure or temperature to its control setpoint using
PID control. The result of this comparison is a percentage
from 0% to 100% that corresponds to the total capacity of the
compressor rack that should be active.
The Suction Group application then takes that percentage
and determines what combination of compressors should be
switched ON or OFF to best fulfill the requirement. For
example, if the application says that 60% of the total
compressor rack’s power should be active, and the rack has
compressors totaling 50 HP, then the supervisory controller
will try to switch on compressors totaling 30 HP.
9.7.2.2 Variable-Speed Compressors
Suction Group applications are compatible with variable-
speed compressors. VS compressors allow Suction Group
applications to “fine-tune” the amount of total active rack
horsepower to react to small changes in suction pressures or
temperatures. As a result, the rack does a better job of
controlling suction, requiring less compressor switches.
When a variable-speed compressor is present, it will
generally be treated as the primary pressure controlling
device, and any other standard compressors in the rack are
secondary devices used only if the VS compressor is unable
to fully handle the required horsepower. The variable-speed
compressor will be the first compressor on and the last
compressor off.
9.7.2.3 Floating Setpoint Control
The Floating Setpoint strategy within the supervisory
controller RX provides a method for varying the suction
setpoint of the group based on the temperature within a
circuit. When Floating Setpoint Control is enabled, the
Supervisory Controller RX monitors either a circuit
temperature or a case temperature from a CC-100 and
adjusts the suction setpoint if the temperature is too low or
too high.
The user establishes a range outside of which the
Supervisory Controller RX is instructed to make a one PSI
adjustment to the suction pressure setpoint to either reduce
or increase the case temperature. If the temperature
continues to remain outside of the range for a user-defined
period of time, the Supervisory Controller RX continues to
make pressure setpoint adjustments until the temperature is
within the established range.
By varying the suction pressure setpoint to match the
temperature requirements of the circuit, the Supervisory
Controller RX is able to ensure product integrity while
achieving maximum rack efficiency.
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