Why does a screw compressor trip on high discharge temperature?

What the trip is protecting

In an oil-injected screw compressor, the oil does three jobs: it seals the rotor clearances, lubricates the bearings and, most importantly, carries away the heat of compression. Air leaving the air end normally sits around 80–95 °C. Most controllers alarm near 105 °C and trip near 110–115 °C, because above that the oil starts to oxidise quickly, varnish forms on the rotors and the risk of an oil-vapour fire in the separator rises.

So a high discharge temperature trip is almost never a fault of the sensor alone. It is telling you that heat is being generated faster than the oil circuit can remove it. The job of the engineer is to find which part of that heat balance has changed.

Check the cooling side first

In our experience more than half of all trips are caused by poor heat rejection. Start at the oil cooler before opening anything.

• Air-cooled units: look for a blocked cooler core (cotton fluff, dust, oil mist). Clean from the fan side outwards with low-pressure air.
• Check the room. If hot discharge air recirculates to the intake, the cooling air can be 15 °C hotter than ambient. Duct the hot air out.
• Water-cooled units: measure cooling-water inlet and outlet temperatures. A rise of more than 10 °C or an inlet above 35 °C points to a cooling-tower or scaling problem.
• Confirm the cooling fan runs in the correct direction after any motor rewind.

Then check the oil circuit

If the cooler is clean, follow the oil. The thermostatic (thermal bypass) valve sends oil around the cooler while the machine warms up. If its element sticks, hot oil bypasses the cooler permanently. A simple test: with the machine loaded, the cooler oil outlet pipe should be clearly cooler than the inlet. If both are the same temperature, the valve is suspect.

Low oil level is the next cause — read the sight glass with the machine stopped and depressurised. Also check the oil filter differential pressure, a blocked oil stop valve, and the age of the oil. Degraded oil loses its heat capacity and its film strength; if it is dark and smells burnt, change it along with the separator element.

Don't forget the sensor and load

Compare the controller reading with an infrared gun on the discharge pipe. A drifting PT100 or a loose terminal can cause nuisance trips. Finally, review load: running continuously at 100 % in summer, or at a pressure setpoint higher than the design, adds heat. Each 1 bar of extra pressure costs about 7 % more power, and most of that energy ends up in the oil.

Quick checklist

• Cooler clean and fan running correctly
• Room ventilation and no hot-air recirculation
• Thermal bypass valve opening
• Correct oil level, fresh oil, clean oil filter
• Sensor reading verified independently
• Pressure setpoint not higher than required