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Inverter Chiller Below 50% Part-Load — Efficiency Drop and Surge Risk

Likely causes

Sudden cooling load drop forces operation below 40% — COP drops sharply

Check: Check current load ratio (%) on BMS or control panel. Monitor if chiller rated COP 5.5 drops below COP 3.0 at 40% load. Verify if operating without relaxing setpoint at night or seasonal transitions

What to do: Stop one of two parallel chillers — remaining unit runs at 70~80% load for high efficiency. Enter chiller COP vs load curve into BMS for automatic staging control before entering <40% load. Relax nighttime setpoint by 1~2°C (e.g., 6°C → 8°C) to suppress part-load ratio

Centrifugal compressor surging — IGV below minimum opening (15~20%)

Check: Monitor compressor suction/discharge pressure ratio — pressure ratio below 2.0 signals surge boundary. Check for high-frequency impact noise ('bang-bang' sounds). Verify if inverter frequency drops below 30 Hz

What to do: Set inverter lower frequency limit to 30~35 Hz (per manufacturer surge map). Install hot-gas bypass valve — automatic bypass prevents surging at minimum load (30~40%). When surging occurs, immediately increase IGV opening or manually open hot-gas bypass

Field tip

Pressure ratio below 2.0 + high-frequency vibration noise = surge entry signal. Immediately open guide vane or hot-gas bypass. Ignoring surging leads to impeller damage.

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