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Refrigeration Control Systems

Compressor Control Systems

Wizard Parallel Compressor Control System

Plus Seven Stages

 

Features

 

The Wizard Compressor Control - Plus - Seven Stage is designed to operate a parallel, multiple staged compressor rack or group of parallel reciprocating or scroll compressors on a single suction group.

 

Suction Pressure Control

 

The WCC-P7 will maintain the suction pressure as close as the Suction Pressure Setpoint by turning on/off up to eleven compressor and / or unloader stages on a single suction pressure refrigeration system.

 

The WCC-P7 will initially start the compressors in the fully unloaded (lowest capacity) configurations. Should additional capacity be required, the WCC-P7, after a calculated delay, will turn off (add capacity) the next available unloader. Each unloader will be staged sequentially, in a First On-Last Off (FILO) sequence.  If all the unloaders assigned to the running compressors have been turned off, then the WCC-P7 will turn on the next compressor.

 

If the suction pressure is lower than setpoint, after a calculated delay, the WCC-P7 will turn on (reduce capacity) the next unloader assigned to the running compressor. If all the unloaders on the running compressors have been turned on, then it will turn off the next compressor. The delay time is calculated automatically using the suction differential and the rate of change of the suction.

 

A minimum runtime can be adjusted at the Compressor Cycling Speed menu.

 

When the suction pressure is higher than the suction pressure high alarm limit, the WCC-P7 will turn off/on the next unloader/compressor step with no time delay.

 

When the suction pressure is lower than the suction pressure low alarm limit, the WCC-P7 will turn on/off the next unloader/compressor step with no delay.

 

When the Defrost Status Input is closed, the WCC-P7 will maintain at lease one compressor running, providing enough refrigerant for cool gas or hot gas defrost. The use of this Defrost Input is optional and is application dependent.

 

The WCC-P7 will go into alarm stage and turn off the alarm relay when the suction pressure reaches its high alarm limit for more than 30 minutes or the suction pressure is below its low alarm limit for more than 30 minutes.

 

Oil Pressure Monitoring

 

The WCC-P7 can monitor up to four oil pressure transducers or digital inputs.  The Oil Pressure control scheme can be assigned to either activate alarms, or to shut down specific compressors should the oil controls indicate a Low Oil condition.

 

The Oil Pressure Monitor can be configured as a pressure sensor or a digital input.

 

Pressure - The WCC-P7 monitors an oil pressure transducer and calculates the pressure differential. An Oil Pressure Differential Setpoint can be assigned.  The WCC-P7 can shutdown a specific compressor or the entire system and activate an alarm when the oil pressure differential is below the Low Oil Pressure Differential Alarm Setpoint for a user defined delay period.

 

Digital - The digital signal from a oil control monitor such as a Copeland Centronics Module is monitored by the WCC-P7.  The WCC-P7 can shutdown a specific compressor or the entire system and activate an alarm when the digital signal is closed.

 

Receiver Level Monitoring

 

The WCC-P7 can monitor an analog receiver level sensor or a digital float and activate alarms should the receiver level be below a user configurable level.  If the sensor is an analog sensor, the WCC-P7 can be configured with two alarm levels, one for alarm indication, a lower level for system shutdown.

 

The Receiver Sensors can be configured for a wide variety of sensor:  including: 4-20ma, 0-5VDC, 1-2VDC, 1-6VDC.

 

Booster Compressor (Satellite) Control

 

Satellite control is optional. The WCC-P7 is capable of controlling a single Booster Compressor (Satellite Compressor) based upon a temperature sensor and a control setpoint/differential.  The WCC-P7 will cycle the Booster Compressor as needed and will go into alarm stage and activate the System Alarm Relay when the room temperature exceeds a high alarm limit for 30 minutes or the room temperature is below its low alarm limit for more than 30 minutes.

 

Suction Pressure Reset (SPR) - Load Shedding

 

The WCC-P7 can raise the Suction Setpoint up to eight pounds based upon the temperature of the SPR (Room 1) Temperature Probe.  When the room temperature is satisfied and the Suction Pressure is below the Suction Pressure Setpoint, the WCC-P7 will increase the Suction Pressure Setpoint by increments of one pound every few minutes until it reaches a maximum offset of eight pounds.

 

Should the room temperature increase above the SPR setpoint, the WCC-P7 will restore the original Suction Pressure Setpoint.

 

Monitor Head Pressure

 

Head pressure monitoring is optional. The WCC-P7 will monitor a 0-500PSIG Head/Discharge Pressure transducer.  The WCC-P7 will go into alarm stage and turn off the alarm relay when the head pressure exceeds a high alarm limit for more than 90 seconds or the head pressure is below its low alarm limit for more than six minutes.

 

The Head Pressure input can be used as high side suction input on split suction system with high side and low side interlock.

 

Compressor/Unloader Staging Methods (up to Eleven Stages)

 

Sequential - The stages will be activated and deactivated in a First In and Last Out method. The first compressor will have the longest runtime and the last compressor has the least runtime. The method is suitable for those rack design requires first compressor running most of time for proper oil distribution.

 

Round Robin - The stages will be activated and deactivated in a First In and First Out method. These This will maintain even run times on participate compressors. It also prevent the lubrication oil from migrating out of the compressors.

 

Combination of Sequential and Round-Robin - For parallel racks with two different sizes of compressors or one compressor with an unloader. Will activate the sequential compressors via the FILO format, then will activate the round-robin compressors randomly. The sequential compressors will be de-activated last.

 

Mix Match - On parallel racks with different compressor sizes. The user sets the order of which the compressors will activate and de-activate in order to smooth the rack operating step sizes.

 

Unloaders - The WCC-P7 can operate up to six unloaders per rack. Each unloader can be assigned to any of the parallel compressors. The unloaders will operate on an accelerated algorithm as compared to the compressor algorithm.  When a compressor with an assigned unloader is activated, the compressor will initially start unloaded. If the WCC-P7 requires an increase in the rack load, the compressor will then be fully loaded before an additional compressor is activated.

 

Compressor 1 Swing Staging - When the rack is set up to run in a combination of one sequential compressor and more than one round-robin compressor mode, the first sequential compressor will operate as the swing compressor. This compressor, usually a smaller compressor, will swing in / swing out (turned on / turned off) between activation of the round-robin (bigger) compressors. The smaller compressor will act as a half step, smoothing out the performance of the rack.

 

Unloader 1 Swing Staging - The first unloader can be activated as a Swing Unloader. It will swing in / swing out (Load / Unload) between compressor staging steps, effectively adding a half step to the staging, smoothing out the performance of the rack.

 

Air Conditioning Mode

 

Select this option if this control is used for controlling the air conditioning rack. When this option is selected, the Defrost Status Input (dry contact) is used to put this control into and idle mode. When the Defrost Status Input is closed, this control will turn off all the compressors and enter into the idle mode. The message, "A/C OFF" will flash on the display. The suction pressure will still be monitored. If the suction pressure drifts up to 50 LBS, the compressors will turn on and start pumping down the suction pressure until it reaches 5 LBS. At that time all compressors will be turned off. When the defrost input is opened, normal compressor cycling will resume.

 

Mechanical Backup

 

Mechanical pressure controls can be installed on the rack and operate in two different methods.

 

Series Switchback - The WCC-P7 is configured to operate the compressors by de-energizing the control relays (Select Compressor Relay N.C. mode in SYSTEM CONFIG MENU). The mechanical low pressure control is wired in series with the WCC-P7. If the Control fails or in suction high/low alarm, all compressor relays will be activated and the low pressure mechanical controllers will cut out the compressors when the suction goes below their settings. NOTE: This method is only possible with the addition of a timing relay in series with the compressor contactor coil voltage to prevent short cycling on start-up.

 

Parallel Switchback - The WCC-P7 is configured to operate the compressors by energizing the control relays (Select Compressor Relay N.O. mode in SYSTEM CONFIG MENU). The mechanical low and high pressure controls are wired in parallel to the WCC-P7 with the line voltage to the backup wire through a "Switchback" time delay relay controlled by the Alarm Relay. If the Control fails or in suction high/low alarm, all control relays are de-energized, the Switchback relay is energized and power is switched to the backups. The mechanical backup controls now operate the compressors based upon their Cut-In and Cut-Out settings. With this method, the backup settings can be set to run the rack as closely to the suction set point as possible.

 

Monitoring and Alarming

 

The WCC-P7 monitors six separate sensors and activates alarms based upon user definable alarm setpoints. These alarms include:

 

Low Suction Pressure - The WCC-P7 will shutdown all compressor operations and activate alarms when the suction pressure is below the Low Suction Alarm Setpoint for a user defined delay period

 

High Suction Pressure - The WCC-P7 will shutdown all compressor operations and activate alarms when the suction pressure exceeds the High Suction Alarm Setpoint for a user defined delay period

 

Low Oil Pressure Differential - The WCC-P7 monitors an oil pressure transducer and calculates the pressure differential. The WCC-P7 will shutdown the compressor operation and activate an alarm when the oil pressure differential is below the Low Oil Pressure Differential Alarm Setpoint for a user defined delay period

 

High Head Pressure - The WCC-P7 monitors the Head/Discharge Pressure Transducer and will shutdown the compressor operation and activate an alarm when the head pressure exceeds the High Head Pressure Alarm Setpoint for a user defined delay period

 

Low Head Pressure - The WCC-P7 monitors the Head/Discharge Pressure Transducer and activates an alarm when the head pressure is below the Low Head Pressure Alarm Setpoint for a user defined delay period

 

High/Low Room Temperature - The WCC-P7 can monitor up to two temperature sensors located in a refrigerated spaces.   The WCC-P7 activates an alarm when the space temperatures exceeds the High  Temperature Alarm Setpoint for a user defined delay period or goes below the Low Temperature Alarm Setpoint for a user defined delay period

 

Low Receiver Level - The WCC-P7 monitors receiver level indicator, either a digital signal from a float switch or an analog signal from a float level sensor.  The WCC-P7 activates an alarm when the receiver level is below the Low Receiver Level Setpoint.  Should the analog sensor level drop below a Shutdown Level, the WCC-P7 will shutdown operation of the compressors.

 

Clark Johnson Company, Inc.       2016