Preventing Chiller Surge Through Maintenance

Chiller surge is a condition in which refrigerant flows in reverse from the condenser back to the compressor, which can lead to severe damage.

What is chiller surge?

Each chiller system has a maximum lift (head pressure), which is the difference between condenser refrigerant pressure and evaporator refrigerant pressure. Lift can also be measured by the difference between the leaving chilled-water temperature (LCHWT) and the entering condenser-water temperature (ECWT). If this differential increases to where it exceeds the system’s pumping capacity, refrigerant will flow backward through the compressor wheel every few seconds. Pressure builds up in the compressor, causing the refrigerant to surge forward again, and the cycle repeats. This reverse-load can damage the thrust assembly, bearings and gearbox, and can cause a rupture in the safety disc or blow the relief valve. Surge is identifiable by its loud, distinctive sound, sometimes likened to an elephant’s squeal or a surging jet engine, as well as a fluctuation in compressor amperage.

What causes chiller surge?

Chillers are designed to meet specific system requirements and operating conditions. With proper maintenance and operation within design intent, a properly selected chiller will not surge. Changes in operating conditions, however, can cause surge to occur, particularly under low load conditions. Surge can be attributed to maintenance issues including fouled tubes, low refrigerant charge, or non-condensables in the refrigerant. It can also be caused by poor control of water flow rates and condenser water temperatures. Low load issues (surge issues) can be avoided or corrected with hot gas bypass or VFDs.