Emerson E2 Software Overview, Suction Groups, Introduction, Standard Suction Group Application

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.

11.1.2The (Standard) Suction Group Application

11.1.2.1Overview 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.

11.1.2.2Variable-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.

11.1.2.3Floating 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.

11.1.3The 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

Suction Groups Software Overview • 11-1