Emerson E2 - Manual (Page 143)

Suction Groups Software Overview • 11 - 1
11 Software Overview
11.1 Suction Groups
11.1.1 Introduction
The E2 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 applica-
tions:
• Suction Group - The “standard” suction group
application that has existed since the first version of
E2 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 E2 support the standard Suction Group
application.
• Enhanced Suction Group - Introduced in E2 version
2.30F01, the Enhanced Suction Group has all of the
important features of the older Suction Group appli-
cation, 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 Suc-
tion Group is easier to configure and more conser-
vative 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.
11.1.2 The (Standard) Suction
Group Application
11.1.2.1 Overview of PID Control Strat-
egy
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 per-
centage 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 percent-
age and determines what combination of compressors
should be switched ON or OFF to best fulfill the require-
ment. 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 E2 will try to
switch on compressors totaling 30 HP.
For more information on PID, see Appendix D: PID
Control.
11.1.2.2 Variable-Speed Compressors
Suction Group applications are compatible with vari-
able-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 bet-
ter 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 vari-
able-speed compressor will be the first compressor on and
the last compressor off.
11.1.2.3 Floating Setpoint Control
The Floating Setpoint strategy within the E2 RX pro-
vides a method for varying the suction setpoint of the
group based on the temperature within a circuit. When
Floating Setpoint Control is enabled, the E2 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 E2
RX is instructed to make a one PSI adjustment to the suc-
tion pressure setpoint to either reduce or increase the case
temperature. If the temperature continues to remain out-
side of the range for a user-defined period of time, the E2
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 E2 RX is able
to ensure product integrity while achieving maximum rack
efficiency.
11.1.3 The Enhanced Suction
Group Application
A Suction Group application looks at suction pressure
or suction temperature and compares it to the suction set-
point to determine whether compressor stages should be