Compressor Trips on High Discharge Temperature in Summer
A rotary screw air compressor rated 22 m3/min at 800 kPa runs in a plant room that reaches 41 C in summer. It trips on high discharge temperature at 105 C, which is the shutdown setpoint. Normal running discharge temperature is 88 C in winter. The oil cooler is a fin-and-tube unit with a fan; the fins show dust loading on the inlet side. Oil level is at the minimum mark. The oil filter differential is 1.8 bar against a 2.0 bar changeout limit. Ambient air for the compressor is drawn from the plant room, which also houses two other compressors. The machine ran 6,000 hours since last oil change, with a 4,000-hour recommended interval.
1. Rank the likely causes of the high discharge temperature and justify the ranking.
Read the worked answer
Ranking by likely contribution and ease of correction:
- Fouled oil cooler. Dust on the fins reduces heat rejection directly. This is the most common cause of a summer-only trip, and the inlet-side loading confirms it. Cleaning is low cost and high impact.
- High ambient intake temperature. The compressor draws 41 C room air, and two other machines add heat. Inlet air temperature directly raises the compression temperature rise. Winter running at 88 C versus summer tripping is consistent with this.
- Oil level at minimum. Less oil means less heat removal from the airend and higher oil temperature. Topping up is simple.
- Oil change overdue at 6,000 hours against 4,000. Degraded oil loses heat transfer and lubricity.
- Oil filter differential at 1.8 bar, near the 2.0 bar limit. Restricted flow reduces oil to the airend and cooler performance.
Less likely but possible: a failing thermostatic control valve stuck in the wrong position, or a worn airend. These should be checked after the simple items are corrected.
2. Propose a corrective plan and a monitoring method to prevent recurrence.
Read the worked answer
Corrective plan:
- Clean the oil cooler fins and check fan operation and airflow path. Consider relocating or ducting cooler intake air from outside or from a cooler zone.
- Top up oil to the correct level and change oil and oil filter now, given the 6,000-hour run and near-limit differential.
- Improve plant room ventilation. Add exhaust or intake fans sized for the total heat load of all three compressors. Target room temperature below 35 C.
- Check the thermostatic oil control valve and the discharge temperature sensor calibration.
- Review the 4,000-hour oil interval against actual oil analysis results and set a condition-based schedule.
Monitoring: log discharge temperature, oil level, filter differential, and room temperature weekly. Set an alarm at 98 C so action is taken before the 105 C trip. Trend oil analysis at each change to catch degradation early. Record ambient temperature alongside discharge temperature so summer trends are visible before the trip point is reached.