Start with the conclusion: the trip is a result, not a cause
A screw compressor's discharge temperature protection is typically set somewhere between 100 and 120 °C — the exact figure varies by manufacturer and model, so always defer to the equipment manual. When it triggers, the unit unloads and stops, the plant loses compressed air, and the line stops with it.
That trip is almost never sudden. The sequence we see on site is far more predictable: heat rejection degrades first, oil circulation follows, the oil starts to break down, the temperature ramp gets steeper, and finally the unit tops out during one high-load run.
The problem is that the gauge on the machine tells you what the temperature is now, not how far above its own normal it has drifted. In most cases the signals were in the data for a week — nobody was looking at them as a trend.
Eight signals that show up in the week before a trip
1. Temperature rise matters more than absolute temperature
Plot discharge temperature minus ambient temperature. That curve is far more comparable than raw temperature. If, at the same load factor, temperature rise drifts from a stable 45 °C to 52 °C, the cooling side is already in trouble even though the absolute reading is still below the alarm.
Order of magnitude (indicative only — compare against your own baseline): three consecutive days of temperature rise running 5-8 °C above its own baseline justify a physical inspection.
2. The ramp gets steeper
If the curve from load-up to steady state sharpens — say from 12 minutes to reach 85 °C from 70 °C down to 6 minutes — that points directly at degraded heat exchange or insufficient oil injection, and it usually appears before steady-state temperature does.
3. Oil filter differential pressure creeps up
Differential pressure across the oil filter is the most direct read on oil circuit health. Many machines alarm or prompt a change around 0.5-0.8 bar. A climb from 0.2 to 0.5 bar over a few days is the classic signature of a filter loading up. The reverse also matters: if differential pressure suddenly drops, suspect that the bypass valve has opened — the filter is still clogged, you just can no longer see it.
4. Separator differential pressure rises
A rising differential across the oil separator raises energy consumption, restricts oil return and increases oil carry-over. You see it as higher oil consumption, shorter life from downstream filters, and a discharge temperature that follows it upward.
5. Cooling-side temperature differential changes (the most overlooked one)
Water-cooled units: watch inlet versus outlet cooling water temperature. With healthy heat exchange, that delta sits in a stable band. If the delta narrows while outlet temperature does not rise, water flow is the likely culprit — scaling, a blocked strainer, a partly closed valve. If both inlet and outlet get hotter, the problem is the cooling tower or make-up water temperature.
Air-cooled units: watch discharge air temperature against ambient. Fouled fins, a slipping fan belt or a reversed fan will all move that delta.
6. Load factor unchanged, motor current up
Once grid voltage variation is ruled out, current running 3%-8% above baseline at a constant load factor usually means rising mechanical resistance or growing internal leakage. The counterpart is specific power in kW per m³/min climbing — more electricity for the same cubic metre of air. It is the clearest efficiency degradation indicator there is, and the one a plant owner reads most easily.
7. Shorter load/unload cycles, more frequent cycling
Growing leakage on the demand side, an excessively narrow pressure band or an undersized receiver all cause frequent cycling. Every load-up is a thermal shock; frequent cycling lifts average oil temperature and pushes the whole temperature rise baseline upward.
8. More frequent oil top-ups, darker oil samples
If top-up volume has clearly increased over the past week, or the sample looks dark and smells burnt, the oil has already degraded at temperature. Keep running and the rotor clearances and bearings take the damage next.
Four misjudgements that make things worse
The right troubleshooting order: four quadrants
| Quadrant | What to check first | Common root causes |
|---|---|---|
| Measurement | Sensor location, sensor drift | Reading differs depending on whether the probe sits at the separator outlet or the discharge port |
| Cooling | Inlet/outlet water delta and flow, fouled cooler, fan rotation and belt | Water-side scaling, fouled fins, slipping belt |
| Oil circuit | Oil filter DP, separator DP, oil level, oil condition | Clogged filter, degraded oil, restricted return |
| Air path and load | Inlet filter DP, minimum pressure valve, load factor, demand-side leakage | Inlet restriction, internal leakage, downstream air leaks |
Order matters: rule out measurement and cooling first — cheap and high probability — then move to the oil and air paths, and only then consider the airend itself.
Seven steps you can check yourself
None of this needs a shutdown. Two people can finish it in half a day, and at the end you will know whether the temperature is genuinely high or the cooling simply cannot keep up.
The maths: what one thermal trip costs
Worked example: a 75 kW screw compressor supplying a main production line trips on high temperature in summer (indicative estimate, not a commitment — verify against your own site):
| Item | Indicative amount | Note |
|---|---|---|
| Production loss | Thousands to tens of thousands per event | Depends on how many lines lose air and whether a backup unit can take over |
| Emergency repair | ¥1,000-5,000 | Overtime and subcontractors included |
| Oil change | ¥1,500-6,000 | Usually required once the oil has degraded thermally |
| Parts | ¥2,000-10,000 | Thermostatic valve, oil filter, separator |
| Hidden cost | Hard to quantify | Airend life lost to carbonised oil |
The point is not the size of this one invoice. It is the comparison: had the temperature rise signal been visible seven days earlier, this repair could have been a planned weekend service — filters changed, cooler cleaned, cost mostly parts and labour, production loss close to zero. With one you pick the time; with the other, the time picks you.
Next steps
Seeing a temperature rise drift seven days early and repairing after a trip are two completely different costs. With one you choose the time. With the other, the time chooses you.