AC Chiller Control Upgrade

Control system for AC Compressors with Electronic Expansion Valve Control

P & S Automation have designed and successfully commissioned several AC Control systems, the latest being an upgrade for Carrier 30HT, HQ Water Cooled twin circuit, reciprocating liquid chillers.

Using a modern Programmable Logic Controller (PLC) and Touchscreen Operator Interface (HMI) we have designed a control system that can be retrofitted to chillers which can still be mechanically maintained.

The original Electronic Expansion Valves (EXV) can be retained as this has the potential to provide fine control of the Superheat temperature.

When commissioned the new controls proved to enable the chiller to run with far greater efficiency cutting down the number of compressors normally in operation from 8 to 5.

The use of a Modern HMI provides instant clarity of the system status, provides trend graphs to assist with tuning and allows simple and intuitive adjustment of parameters.

The PLC holds the operating program and maintains safe constant control, the stepper motor within the EXV is driven directly by solid state PLC outputs thus keeping hardware to a minimum. A Battery Backed supply which is built into the panel ensures that in the event of power loss which causes the compressors to stop the EXVs will close so stopping the possibility of liquid collecting at the inlet to the compressors.

Additional Chillers can be networked together to provide further control options.

Although the PLC controls the system, the interlocks are hardwired into the compressor starters thus ensuring safe operation of the compressors even in the unlikely event of the PLC failing.