The project's purpose was to design and install a control panel, with user interface and operation buttons for a MYCOM N8WB reciprocating compressor. The machine is installed in parallel with the existing ones, to provide additional cooling capacity to a Refrigeration Cooling Storage facility.
It is a single stage/8-cylinders compressor with a refrigerant capacity of 100KW@-35/+35oC, driven by an ABB electromotor with a nominal power of 110KW@990rpm.
The PLC program design consists of four (4) individual tasks:
- 1. Safety controls/Alarm input sensors
- -Low/High (suction/discharge) pressure switch (DI)
- -Low pressure level (AI)
- -Oil pressure (AI)
- -Differential pressure
- -Compressor cooling water level (DI)
- -High pressure temperature (PT100)
- -Oil temperature (PT100)
- -Phase asymmetry sequence (DI)
- -Motor thermistors (PTC with Relay converter)
- -Motor thermal overload relay (DI)
- Of course, the EMERGENCY SWITCH will instantly power off the entire unit and cause a Master Alarm.
- 2. Operation inputs
- -Mode selection (AUTO-0-MANUAL)
- -Automatic mode START/STOP, according to DP
- -Manual mode, Motor and individual Cylinder Heads START/STOP
- -Set points adjustment - Admin level
- 3. Output signals
- -Unit Start/Stop (with OFF Delay) (DQ)
- -Individual cylinders and motor Start/Stop (manual mode only) (DQ)
- -Cooling water Open/Close (DQ)
- -Oil resistor On/Off (DQ)
- 4. Operation visualization and interaction with LOGO! TDE - Text Display
- -Manual/Auto screen operation data display
- -Real time Pressure and Temperature values
- -Troubleshooting and ALARMS decoding
- -Running/Service hours intervals
- -ALARM activation signaling
According to these, the PLC required should satisfy the below minimum characteristics:
- -Digital Inputs 8
- -Digital Outputs 7 (Relay Outputs)
- -Analog Inputs 4
- -Analog Outputs 0
- -PT100 Inputs 2
- -High Speed Counters 0
- -Ethernet I/O 0
For this application, we choose the SIEMENS LOGO!8 12/24RCE combined with LOGO! AM2 RTD with 2 PT100/PT1000 Inputs, LOGO! DM16 24R with integrated DI/DQ and LOGO! AM2 with 2XAI. The choice is made upon the requirements, flexibility of the CPU, programming environment, and both compact and reliable construction of the specific equipment. The Display is the 6-lines SIEMENS LOGO! TDE. At this application there was no requirement for a larger HMI - although it is possible. The advantage of this hardware selection is the low cost-to-capabilities ratio.
Operation - Automatic Mode When ALL Alarms are OFF, the PLC reads the Differential Pressure between Oil and Low side refrigerant. Settings points are DP=0.8bars - START and DP=0.2bars - STOP. Compressor Loading process is executed with Time Delay between motor and each Heads (1-3). The same Time Delay is applied to Unloding as well, with a LIFO sequence. Upon 100% unit Unload, the motor remains idle for additional 60sec and if no cooling demand is required, it then stops. There is also a 10minutes delay interval between STARTS, in order not to exceed 6-Motor STARTS per hour. In case of EMERGENCY ALARM the entire unit shuts down instantaneous, no time delay is applied.
- Manual Mode When ALL Alarms are OFF, The Operator can manually START the unit with the MOTOR-HEAD1-HEAD2-HEAD3 order and then manually STOP them in LIFO sequence. This is for testing purposes and maintenance ONLY. During the manual mode, ALL safety sensors are active. In this mode, no time delay restriction is applied to START-STOP sequence.
- Additional functions The operator can real time monitor the Pressure and Temperature values, both for Ammonia (NH3) and Oil and also adjust Set Points, check ALARMS Status, Total running hours and Service intervals. It is also possible to Data Log any information that is required and export to a .csv file, or even connect the PLC to a modem and operate the unit remotely through a PC or a mobile phone. For this application, remote control was considered to be non-secure procedure because of safety precautions. Local operator presence is necessary at all times to control and check the unit.
Text and images by george vakfaris.