Emerson E2 operation manual Valve Control, EEVs Liquid Pulse and Liquid Stepper

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.

NOTE: There are several variations of the EC-2. Contact Retail Solutions at 1-800-829- 2724 for more information.

11.4.2.1Valve 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

11.4.3.1EEVs (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

Case Control Circuits Software Overview • 11-11