Chiller tripping on high head pressure: causes and fixes

Understanding head pressure

Head pressure is the refrigerant pressure in the condenser. It is set by the temperature at which the refrigerant can reject heat. If heat cannot leave the condenser, the saturation temperature and therefore the pressure rise until the high-pressure switch stops the compressor. A healthy water-cooled chiller usually runs with a condensing temperature 1–3 °C above the leaving condenser-water temperature; this difference is called the approach.

The approach temperature tells the story

Log leaving condenser water temperature and condenser saturation temperature from the panel. If the approach has grown from 2 °C to 5 °C or more over months, the tubes are fouled with scale or biological slime. Mechanical brushing of the tubes and better water treatment at the cooling tower is the cure.

If the approach is normal but both temperatures are high, the problem is outside the chiller: the cooling tower is not cooling. Check tower fans, fill media, nozzles and the water flow.

Flow and non-condensables

Measure condenser water pressure drop across the chiller and compare with the design sheet. Low flow, from a choked strainer or a partly closed valve, raises the leaving water temperature and the head pressure together.

Air or moisture inside the refrigerant circuit — called non-condensables — collect at the top of the condenser and raise pressure without adding cooling. A sign is a condenser pressure higher than the saturation pressure corresponding to the liquid temperature. Low-pressure centrifugal chillers have a purge unit for this; on others, recover the refrigerant and evacuate properly.

Air-cooled units

• Clean the finned coils and straighten bent fins
• Check that every condenser fan runs and none are reversed
• Look for hot-air recirculation between closely spaced units
• Confirm the refrigerant charge is not excessive after a service