Emerson E2 User Manual download pdf (Page 153)

Case Control Circuits Software Overview • 11-11

ming and viewing status. (The EC-2 29x version

controls the refrigeration solenoid valve to allow

the passage of refrigerant to the TXV valve,

whereas the 39x version controls a pulse valve on

the liquid side of the evaporator to regulate super-

heat.)

• CCBs - Version 2.3 and above of the E2 software is

backward-compatible with the CCB, an I/O Net-

work-based controller used primarily in old RMCC

installations. Like the CC-100 family, the CCB was

available in several different models that controlled

liquid pulse, liquid stepper, suction stepper, and

suction lineup.

11.4.2.1 Valve Control

The CC-100 is capable of supporting two types of

valves: pulse and stepper. The CC-100 uses PID control to

vary the aperture rates of both valves between 0% and

100% as required by their temperature control algorithms

(see Section 11.4.3, below).

Pulse Valves

A pulse valve is a device capable of being in only two

states: fully open or fully closed. To achieve the necessary

percentage of refrigerant flow, CC-100s repeatedly

“pulse” these valves open for a percentage of an interval

called the valve period (which defaults to six seconds).

For example, to achieve a 20% valve output in a CC-

100 with a valve period of six seconds, a pulse valve

would be opened for 20% of six seconds (or 1.2 seconds)

and closed for the remaining 80% of the valve period (4.6

seconds). This same six-second sequence will repeat for as

long as the CC-100 calls for a 20% valve output.

Stepper Valves

Stepper valves are devices that may opened to many

different positions between fully closed (0%) and fully

open (100%). Stepper valves usually have hundreds or

thousands of “steps” in between fully closed and fully

open. To achieve the desired opening percentage, the CC-

100 moves the valve the required number of steps.

To properly control a stepper valve, the CC-100 must

know the operating characteristics of the valve, such as its

maximum steps per second change rate, the total number

of steps between 0% and 100%, and its hysteresis rate (the

number of steps required for the valve to change direc-

tion).

11.4.3 Refrigeration Control

11.4.3.1 EEVs (Liquid Pulse and Liquid

Stepper)

In CC-100P, CC-100LS, EC2, and CCB (liquid and

pulse stepper) case controllers, there are two different con-

trol systems that work together to regulate refrigeration:

temperature control and superheat control.

Temperature Control

Temperature Control measures the case temperature

and turns refrigeration ON or OFF as required to keep the

case within a certain proximity of the user-specified tem-

perature setpoint.

The user supplies Temperature Control with the set-

point and a deadband, which is the range of case tempera-

tures equally above and below the setpoint within which

the case temperature will be considered acceptable. When

the case temperature is above the setpoint plus

one-half the

deadband, refrigeration will be turned ON. It will remain

ON until the temperature drops to below the temperature

setpoint minus

one-half the deadband, at which point

refrigerant flow will be turned OFF.

Case temperature may be supplied to Temperature

Control by a supply air sensor, a return air sensor, or a

mixture of both the supply and return air sensor values.

Temperature Control itself does not vary the opening

percentage of the pulse or stepper valve; it simply

addresses the case’s need for refrigerant flow to maintain

its setpoint. Once refrigeration is started, control of the

valve is handled by Superheat Control.

Superheat Control

The difference between the temperature of the refriger-

ant going in to the evaporator inlet (the coil in tempera-

ture) and the refrigerant leaving the evaporator outlet (the

coil out temperature) is called Superheat. When refriger-

ant is flowing through an evaporator, Superheat Control

uses PID Control to keep the Superheat at a user-defined

Superheat setpoint. Superheat Control positions the valve

to increase or decrease refrigerant flow in an effort to keep

the Superheat equal to a user-defined Superheat setpoint.

Recovery Mode

Recovery Mode is a special part of Superheat Control

that occurs at the beginning of every refrigeration cycle.

When refrigeration has been OFF and Temperature Con-

trol calls for refrigeration to be ON, a Recovery Mode

begins, during which the valve is fixed to a user-defined

percentage (usually 70%) for a fixed amount of time. This

floods the previously empty evaporator with refrigerant

and gradually establishes a differential between the coil in

and coil out temperatures. When the Recovery Mode ends,

the superheat will be relatively close to the setpoint; at this

NOTE: There are several variations of the

EC-2. Contact Retail Solutions at 1-800-829-

2724 for more information.

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Emerson E2 User Manual Page 153

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