The Compressor That Ran Hot: Air Dryer, Ventilation, and Duty Cycle
A screw compressor in a small compressor room. It ran at 95°C discharge temperature. The alarm threshold was 100°C. The customer was one alarm away from a shutdown. The compressor was new. The room was 35°C in summer. The problem wasn’t the compressor — it was the room.
Why compressors overheat
A screw compressor generates heat during compression. The oil cools the air and the compressor element. The oil cooler rejects the heat to the ambient air. If the ambient air is too hot, the oil can’t cool down. The discharge temperature rises. At 100°C, the alarm trips.
| Ambient temp | Typical discharge temp | Status |
|---|---|---|
| 20°C | 75°C | Normal |
| 30°C | 85°C | Acceptable |
| 35°C | 95°C | Marginal (our problem) |
| 40°C | 105°C | Alarm / shutdown |
What we did
1. Ventilate the room. The compressor room had no exhaust fan. The heat from the compressor accumulated. We installed an exhaust fan (2000 m³/h) that pulls hot air out and draws cool air in through a louver. The room temperature dropped from 35°C to 28°C. The discharge temperature dropped to 85°C.
2. Clean the oil cooler. The cooler fins were dusty. We cleaned them with compressed air. The heat transfer improved. The discharge temperature dropped another 5°C.
3. Check the dryer. The refrigerated dryer was sized for 1000 L/min. The compressor produces 800 L/min. The dryer was fine. But in a hot room, the dryer’s condenser also runs hot. We added ventilation to the dryer as well.
The duty cycle
The compressor was running at 90% duty cycle (loaded 90% of the time). That means it’s almost always compressing, generating heat. If the demand is 700 L/min and the compressor is rated at 800 L/min, it’s oversized — it cycles more (loads and unloads), which generates less heat per hour. If it’s undersized (700 L/min demand on a 700 L/min compressor), it runs continuously and overheats. We checked: the compressor was sized correctly for the demand. The heat was a ventilation problem.
The room design
A compressor room needs: (1) cool air intake (from outside, not from the hot plant), (2) exhaust fan sized for the compressor’s heat rejection, (3) clearance around the compressor (1 m on all sides for airflow). The heat rejection of a 22 kW compressor is about 7 kW. The exhaust fan must move enough air to remove 7 kW of heat. At a 10°C temperature rise, that’s about 2000 m³/h. We installed that fan.
The compressor rule: A hot compressor is usually a hot room. Ventilate the compressor room. Clean the oil cooler. Make sure the intake air is cool (not recirculated hot air). If the room is 35°C, the compressor will run at 95°C regardless of how new it is. Move the heat out.
